Spin & magnetic sensing

The nitrogen-vacancy (NV) center in diamond is a prominent quantum-sensing platform. Combining readout at the quantum projection noise limit with strong dipolar interactions promises substantial gains in sensitivity. However, experimentally accessing this regime has remained a longstanding challenge. In this work, we demonstrate quantum-projection-noise-resolved readout of a strongly-interacting, two-dimensional ensemble of NV centers. Our approach leverages repetitive readout via the NV's intrinsic ¹⁵N nuclear memory at a moderate magnetic field (∼ 0.3 T), improving the readout fidelities by nearly an order of magnitude. This enables us to directly resolve the quantum projection noise of a coherent spin state and to watch the ensemble's intrinsic dipolar interactions shear this noise into an anisotropic profile. Our results open the door to direct measurements of spin squeezing and entanglement-enhanced sensing in the solid state.

The unitary synthesis problem (Aaronson-Kuperberg, CCC 2007) asks whether every $n$-qubit unitary $U$ is computable by efficient quantum circuits relative to some classical oracle f = fU depending on $U$. Recently, Lombardi-Ma-Wright (STOC 2024) proved that Haar-random unitaries cannot be efficiently synthesized by algorithms that make 1 query (or poly$(n)$ parallel queries) to an arbitrary classical oracle. In this work, we prove several results about the hardness (and easiness!) of variants of unitary synthesis. Our results include: (1) 1-query vs. 2-query unitary synthesis: we prove 1-query lower bounds for synthesizing random permutation unitaries P x⟩ = π(x)⟩, as well as random alternating-basis phase unitaries F2 · H n · F1. This gives 1-query lower bounds for "explicit" families of unitaries that have efficient (even 2-query) synthesis algorithms. (2) Upper bound for complex phase unitaries: we also consider complex phase unitaries x⟩ αx x⟩, which have a clean 2-query synthesis algorithm with no obvious 1-query algorithm. In this case, we prove an upper bound: there are 1-query algorithms (relative to binary phase oracles) that constant-approximate these unitaries in diamond distance. In order to prove our lower bounds, we introduce and analyze two new cryptographic games: the oracle state search game and the oracle Choi state game. Compared to prior work, our framework is mathematically simple, more flexible in what it can prove, and more accurately captures the hardness of synthesizing unitaries that are not "fully random". Finally, we also use the search game to prove a new hardness-of-approximation result for quantum programs (synthesizing unitaries relative to quantum advice) for phase unitaries, giving a sharper separation between 1-query unitary synthesis and quantum programs.

Identifying reliable, nondestructive indicators of electrolyte degradation is essential for advancing the diagnostics and lifetime assessment of lithium-ion batteries. In this study, we performed multinuclear nuclear magnetic resonance spectroscopy at various time points during systematic degradation of battery electrolyte components undergoing thermal cycling. We highlight a strong correlation between electrolyte degradation state and the 7Li nuclear spin polarization lifetime, while other nuclei did not respond as strongly. A mechanistic rationale for this phenomenon is provided using molecular dynamics simulations. The remarkable sensitivity of 7Li nuclear spin lifetime to electrolyte health underscores its potential value as a nondestructive, field-independent probe suited for diagnosing defects and evaluating the overall health of lithium-ion batteries.

Nuclear-magnetic-resonance experiments can interrogate a broad spectrum of molecular-tumbling regimes and can accurately measure interatomic distances in solution with sub-nanometer resolution. In the zero- to ultralow-field (ZULF) regime, population and coherence decay reveal nontrivial behavior due to strong coupling between nuclear spins. We note, in particular, the surprising effects that different resonances show different relaxation rates, depending on (i) the (pre)polarizing magnet field and the shuttling trajectory to the detection region at nano- and microtesla fields, (ii) the strength of the measurement field, (iii) the detection method (single-channel or quadrature), and even (iv) the nutation angle induced by the excitation pulse. We describe herein experimental data of relaxation rates measured for a 13C-labeled formic acid sample, with an atomic-magnetometer-based ZULF setup, and develop a theoretical framework to explain the detected effects and extract molecular properties. The observed effects could be used for spectral assignment, for the establishment of specific motional regimes, for image contrast, and for the characterization of relaxation processes at nano- to microtesla magnetic fields.

Systems of spins with strong dipolar interactions and controlled dimensionality enable new explorations in quantum sensing and simulation. In this work, we investigate the creation of strong dipolar interactions in a two-dimensional ensemble of nitrogen-vacancy (NV) centers generated via plasma-enhanced chemical vapor deposition on (111)-oriented diamond substrates. We find that diamond growth on the (111) plane yields high incorporation of spins, both nitrogen and NV centers, where the density of the latter is tunable via the miscut of the diamond substrate. Our process allows us to form dense, preferentially aligned, 2D NV ensembles with volume-normalized ac sensitivity down to η ac = 810 pT μ m 3 / 2 Hz − 1 / 2. Furthermore, we show that (111) affords maximally positive dipolar interactions among a 2D NV ensemble, which is crucial for leveraging dipolar-driven entanglement schemes and exploring new interacting spin physics.

Spin squeezed states provide a seminal example of how the structure of quantum mechanical correlations can be controlled to produce metrologically useful entanglement. Such squeezed states have been demonstrated in a wide variety of artificial quantum systems ranging from atoms in optical cavities to trapped ion crystals. By contrast, despite their numerous advantages as practical sensors, spin ensembles in solid-state materials have yet to be controlled with sufficient precision to generate targeted entanglement such as spin squeezing. In this work, we present the first experimental demonstration of spin squeezing in a solid-state spin system. Our experiments are performed on a strongly-interacting ensemble of nitrogen-vacancy (NV) color centers in diamond at room temperature, and squeezing (-0.5 ± 0.1 dB) is generated by the native magnetic dipole-dipole interaction between NVs. In order to generate and detect squeezing in a solid-state spin system, we overcome a number of key challenges of broad experimental and theoretical interest. First, we develop a novel approach, using interaction-enabled noise spectroscopy, to characterize the quantum projection noise in our system without directly resolving the spin probability distribution. Second, noting that the random positioning of spin defects severely limits the generation of spin squeezing, we implement a pair of strategies aimed at isolating the dynamics of a relatively ordered sub-ensemble of NV centers. Our results open the door to entanglement-enhanced metrology using macroscopic ensembles of optically active spins in solids.

The negatively charged boron vacancy (V_{B}^{-}) in hexagonal boron nitride (hBN) has garnered significant attention among defects in two-dimensional materials. This owes, in part, to its deterministic generation, well-characterized atomic structure, and optical polarizability at room temperature. We investigate the latter through extensive measurements probing both the ground and excited state polarization dynamics. We develop a semiclassical model based on these measurements that predicts a near-unity degree of spin polarization, surpassing other solid-state spin defects under ambient conditions. Building upon our model, we include the presence of nuclear spin degrees of freedom adjacent to the V_{B}^{-} and perform a comprehensive set of Lindbladian numerics to investigate the hyperfine-induced polarization of the nuclear spins. Our simulations predict a number of important features that emerge as a function of magnetic field which are borne out by experiment.

mechanisms of kinetic and potential spin-exchange, allowing for future functional design of magnetic organic molecules. The role of active space choice on VTE accuracy and predicted magnetic coupling is additionally explored.

We introduce a novel protocol, which enables Heisenberg-limited quantum-enhanced sensing using the dynamics of any interacting many-body Hamiltonian. Our approach - dubbed butterfly metrology - utilizes a single application of forward and reverse time evolution to produce a coherent superposition of a "scrambled" and "unscrambled" quantum state. In this way, we create metrologically-useful long-range entanglement from generic local quantum interactions. The sensitivity of butterfly metrology is given by a sum of local out-of-time-order correlators (OTOCs) - the prototypical diagnostic of quantum information scrambling. Our approach broadens the landscape of platforms capable of performing quantum-enhanced metrology; as an example, we provide detailed blueprints and numerical studies demonstrating a route to scalable quantum-enhanced sensing in ensembles of solid-state spin defects.

Spin squeezing is a form of entanglement that reshapes the quantum projection noise to improve measurement precision. Here, we provide numerical and analytic evidence for the following conjecture: any Hamiltonian exhibiting finite-temperature easy-plane ferromagnetism can be used to generate scalable spin squeezing, thereby enabling quantum-enhanced sensing. Our conjecture is guided by a connection between the quantum Fisher information of pure states and the spontaneous breaking of a continuous symmetry. We demonstrate that spin squeezing exhibits a phase diagram with a sharp transition between scalable squeezing and non-squeezing. This transition coincides with the equilibrium phase boundary for XY order at a finite temperature. In the scalable squeezing phase, we predict a sensitivity scaling that lies between the standard quantum limit and the scaling achieved in all-to-all coupled one-axis twisting models. A corollary of our conjecture is that short-ranged versions of two-axis twisting cannot yield scalable metrological gain. Our results provide insights into the landscape of Hamiltonians that can be used to generate metrologically useful quantum states. Generating highly squeezed states for quantum sensing requires precise entanglement properties, which makes it a hard task. Now a conjecture identifies a realistic regime of magnetic order at finite temperatures that enables scalable spin squeezing.

Dynamic critical fluctuations in magnetic materials encode important information about magnetic ordering in the associated critical exponents. Using nitrogen-vacancy centers in diamond, we implement T2 (spin-decoherence) noise magnetometry to study critical dynamics in a 2D Van der Waals magnet CrSBr. By analyzing NV decoherence on time scales approaching the characteristic correlation time τc of critical fluctuations, we extract the critical exponent $ν$ for the correlation length. Our result deviates from the Ising prediction and highlights the role of long-range dipolar interactions in 2D CrSBr. Furthermore, analyzing the divergence of the correlation length suggests the possibility of 2D-XY criticality in CrSBr in a temperature window near TC where static magnetic domains are absent. Our work provides a first demonstration of T2 noise magnetometry to quantitatively analyze critical scaling behavior in 2D materials.

Hexagonal boron nitride (hBN) is attracting increasing attention for quantum information science due to its ability to host carbon-related color centers that perform as robust, high-temperature quantum light sources, as well as native defects such as charged V B – centers with optically addressable spin states toward spin-based quantum applications. So far, achieving isolation of individual V B – defects out of ensemble emission remains difficult due to their low quantum yield, warranting the search for other candidates. Here, we show that oxygen annealing creates individual quantum emitters characterized by a phonon-sideband-free emission in the 800 nm band. The emission is strongly linearly polarized (79%), with a characteristic emission dipole rotation of 20° and a saturation intensity around 1 MHz, comparable to the brightest hBN quantum emitters reported previously. Their time dynamics appear free of spectral diffusion or blinking, implying excellent room-temperature photostability without additional passivation techniques. Detailed cryogenic studies estimate zero pump power line width down to 87.5 μeV, which is several orders of magnitude narrower compared to carbon-related or V B – defect center emission under nonresonant excitation. The demonstrated photophysical properties of oxygen-related quantum emitters are promising to advance free-space quantum communication and might enable indistinguishable single photons or interfacing with 87 Rb vapor-based quantum memory technologies in future work.

Uniformly random unitaries, i.e. unitaries drawn from the Haar measure, have many useful properties, but cannot be implemented efficiently. This has motivated a long line of research into random unitaries that "look" sufficiently Haar random while also being efficient to implement. Two different notions of derandomisation have emerged: $t$-designs are random unitaries that information-theoretically reproduce the first $t$ moments of the Haar measure, and pseudorandom unitaries (PRUs) are random unitaries that are computationally indistinguishable from Haar random. In this work, we take a unified approach to constructing $t$-designs and PRUs. For this, we introduce and analyse the "$PFC$ ensemble", the product of a random computational basis permutation $P$, a random binary phase operator $F$, and a random Clifford unitary $C$. We show that this ensemble reproduces exponentially high moments of the Haar measure. We can then derandomise the $PFC$ ensemble to show the following: (1) Linear-depth $t$-designs. We give the first construction of a (diamond-error) approximate $t$-design with circuit depth linear in $t$. This follows from the $PFC$ ensemble by replacing the random phase and permutation operators with their $2t$-wise independent counterparts. (2) Non-adaptive PRUs. We give the first construction of PRUs with non-adaptive security, i.e. we construct unitaries that are indistinguishable from Haar random to polynomial-time distinguishers that query the unitary in parallel on an arbitary state. This follows from the $PFC$ ensemble by replacing the random phase and permutation operators with their pseudorandom counterparts. (3) Adaptive pseudorandom isometries. We show that if one considers isometries (rather than unitaries) from $n$ to n + ω(n) qubits, a small modification of our PRU construction achieves general adaptive security.

Systems of spins with strong dipolar interactions and controlled dimensionality enable new explorations in quantum sensing and simulation. In this work, we investigate the creation of strong dipolar interactions in a two-dimensional ensemble of nitrogen-vacancy (NV) centers generated via plasma-enhanced chemical vapor deposition (PECVD) on (111)-oriented diamond substrates. We find that diamond growth on the (111) plane yields high incorporation of spins, both nitrogen and NV centers, where the density of the latter is tunable via the miscut of the diamond substrate. Our process allows us to form dense, preferentially aligned, 2D NV ensembles with volume-normalized AC sensitivity down to ηAC = 810 pT um3/2 Hz-1/2. Furthermore, we show that (111) affords maximally positive dipolar interactions amongst a 2D NV ensemble, which is crucial for leveraging dipolar-driven entanglement schemes and exploring new interacting spin physics.

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena. The megabar regime represents an exciting frontier, where recent discoveries include novel high-temperature superconductors, as well as structural and valence phase transitions. However, at such high pressures, many conventional measurement techniques fail. Here, we demonstrate the ability to perform local magnetometry inside of a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach utilizes a shallow layer of Nitrogen-Vacancy (NV) color centers implanted directly within the anvil; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the NV center to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH9. By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: local diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping the Meissner effect and flux trapping, we directly image the geometry of superconducting regions, revealing significant inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed loop optimization of superhydride materials synthesis.

Magnetic tunnel junctions (MTJs), which are the fundamental building blocks of spintronic devices, have been used to build true random number generators (TRNGs) with different trade-offs between throughput, power, and area requirements. MTJs with high-barrier magnets (HBMs) have been used to generate random bitstreams with 200~Mb/s throughput and pJ/bit energy consumption. A high temperature sensitivity, however, adversely affects their performance as a TRNG. Superparamagnetic MTJs employing low-barrier magnets (LBMs) have also been used for TRNG operation. Although LBM-based MTJs can operate at low energy, they suffer from slow dynamics, sensitivity to process variations, and low fabrication yield. In this paper, we model a TRNG based on medium-barrier magnets (MBMs) with perpendicular magnetic anisotropy. The proposed MBM-based TRNG is driven with short voltage pulses to induce ballistic, yet stochastic, magnetization switching. We show that the proposed TRNG can operate at frequencies of about 500~MHz while consuming less than 100~fJ/bit of energy. In the short-pulse ballistic limit, the switching probability of our device shows robustness to variations in temperature and material parameters relative to LBMs and HBMs. Our results suggest that MBM-based MTJs are suitable candidates for building fast and energy-efficient TRNG hardware units for probabilistic computing.

The most direct approach for characterizing the quantum dynamics of a strongly interacting system is to measure the time evolution of its full many-body state. Despite the conceptual simplicity of this approach, it quickly becomes intractable as the system size grows. An alternate approach is to think of the many-body dynamics as generating noise, which can be measured by the decoherence of a probe qubit. Here we investigate what the decoherence dynamics of such a probe tells us about the many-body system. In particular, we utilize optically addressable probe spins to experimentally characterize both static and dynamical properties of strongly interacting magnetic dipoles. Our experimental platform consists of two types of spin defects in nitrogen delta-doped diamond: nitrogen-vacancy colour centres, which we use as probe spins, and a many-body ensemble of substitutional nitrogen impurities. We demonstrate that the many-body system's dimensionality, dynamics and disorder are naturally encoded in the probe spins' decoherence profile. Furthermore, we obtain direct control over the spectral properties of the many-body system, with potential applications in quantum sensing and simulation.

Systems of spins engineered with tunable density and reduced dimensionality enable a number of advancements in quantum sensing and simulation. Defects in diamond, such as nitrogen-vacancy (NV) centers and substitutional nitrogen (P1 centers), are particularly promising solid-state platforms to explore. However, the ability to controllably create coherent, two-dimensional spin systems and characterize their properties, such as density, depth confinement, and coherence, is an outstanding materials challenge. We present a refined approach to engineer dense (≳1 ppm ⋅ nm), 2D nitrogen, and NV layers in diamond using delta-doping during plasma-enhanced chemical vapor deposition epitaxial growth. We employ both traditional materials techniques, e.g., secondary ion mass spectrometry, alongside NV spin decoherence-based measurements to characterize the density and dimensionality of the P1 and NV layers. We find P1 densities of 5–10 ppm ⋅ nm, NV densities between 1 and 3.5 ppm ⋅ nm tuned via electron irradiation dosage, and depth confinement of the spin layer down to 1.6 nm. We also observe high (up to 0.74) ratios of NV to P1 centers and reproducibly long NV coherence times, dominated by dipolar interactions with the engineered P1 and NV spin baths.

The rational design of nanoscopic DNA tiles has yielded highly ordered crystalline matter in 2D and 3D. The most well-studied 3D tile is the DNA tensegrity triangle, which is known to self-assemble into macroscopic crystals. However, contemporary rational design parameters for 3D DNA crystals nearly universally invoke integer numbers of DNA helical turns and Watson-Crick (WC) base pairs. In this study, 24-bp edges are substituted into a previously 21-bp (two helical turns of DNA) tensegrity triangle motif to explore whether such unconventional motif can self-assemble into 3D crystals. The use of noncanonical base pairs in the sticky ends results in a cubic arrangement of tensegrity triangles with exceedingly high symmetry, assembling a lattice from winding helical axes and diamond-like tessellation patterns. Reverting this motif to sticky ends with Watson-Crick pairs results in a trigonal hexagonal arrangement, replicating this diamond arrangement in a hexagonal context. These results showcase that the authors can generate unexpected, highly complex, pathways for materials design by testing modifications to 3D tiles without prior knowledge of the ensuing symmetry. This study expands the rational design toolbox for DNA nanotechnology; and it further illustrates the existence of yet-unexplored arrangements of crystalline soft matter.

Spin Mixing in Ferromagnets RevealedExperiments with thin magnetic films show that mixing between spin states has a larger than expected effect on spectroscopic measurements used to probe magnetic interactions in materials.By

The anomalous Hall effect, a hallmark of broken time-reversal symmetry and spin-orbit coupling, is frequently observed in magnetically polarized systems. Its realization in non-magnetic systems, however, remains elusive. Here, we report on the observation of anomalous Hall effect in nominally non-magnetic KTaO3. Anomalous Hall effect emerges in reduced KTaO3 and shows an extrinsic to intrinsic crossover. A paramagnetic behavior is observed in reduced samples using first principles calculations and quantitative magnetometry. The observed anomalous Hall effect follows the oxygen vacancy-induced magnetization response, suggesting that the localized magnetic moments of the oxygen vacancies scatter conduction electrons asymmetrically and give rise to anomalous Hall effect. The anomalous Hall conductivity becomes insensitive to scattering rate in the low temperature limit (T<5 K), implying that the Berry curvature of the electrons on the Fermi surface controls the anomalous Hall effect. Our observations describe a detailed picture of many-body interactions, triggering anomalous Hall effect in a non-magnetic system.

Numerous conceptually important quantum algorithms rely on a blackbox device known as an oracle, which is typically difficult to construct without knowing the answer to the problem that the algorithm is intended to solve.A notable example is Grover's search algorithm.Here we propose a Grover search for solutions to a class of NP-complete decision problems known as subset sum problems, including the special case of number partitioning.Each problem instance is encoded in the couplings of a set of qubits to a central spin or boson, which enables a realization of the oracle without knowledge of the solution.The algorithm provides a quantum speedup across a known phase transition in the computational complexity of the partition problem, and we identify signatures of the phase transition in the simulated performance.Whereas the naive implementation of our algorithm requires a spectral resolution that scales exponentially with system size for NP-complete problems, we also present a recursive algorithm that enables scalability.We propose and analyze implementation schemes with cold atoms, including Rydberg-atom and cavity-QED platforms.

Sensitive and accurate diagnostic technologies with magnetic sensors are of great importance for identifying and localizing defects of rechargeable solid batteries using noninvasive detection. We demonstrate a microwave-free alternating current (AC) magnetometry method with negatively charged NV centers in diamond based on a cross-relaxation feature between nitrogen-vacancy (NV) centers and individual substitutional nitrogen (P1) centers occurring at 51.2 mT. We apply the technique to non-destructively image solid-state batteries. By detecting the eddy-current-induced magnetic field of the battery, we distinguish a defect on the external electrode and identify structural anomalies within the battery body. The achieved spatial resolution is μμμ360μm. The maximum magnetic field and phase shift generated by the battery at the modulation frequency of 5 kHz are estimated as 0.04 mT and 0.03 rad respectively.

The most direct approach for characterizing the quantum dynamics of a strongly-interacting system is to measure the time-evolution of its full many-body state. Despite the conceptual simplicity of this approach, it quickly becomes intractable as the system size grows. An alternate framework is to think of the many-body dynamics as generating noise, which can be measured by the decoherence of a probe qubit. Our work centers on the following question: What can the decoherence dynamics of such a probe tell us about the many-body system? In particular, we utilize optically addressable probe spins to experimentally characterize both static and dynamical properties of strongly-interacting magnetic dipoles. Our experimental platform consists of two types of spin defects in diamond: nitrogen-vacancy (NV) color centers (probe spins) and substitutional nitrogen impurities (many-body system). We demonstrate that signatures of the many-body system's dimensionality, dynamics, and disorder are naturally encoded in the functional form of the NV's decoherence profile. Leveraging these insights, we directly characterize the two-dimensional nature of a nitrogen delta-doped diamond sample. In addition, we explore two distinct facets of the many-body dynamics: First, we address a persistent debate about the microscopic nature of spin dynamics in strongly-interacting dipolar systems. Second, we demonstrate direct control over the spectral properties of the many-body system, including its correlation time. Our work opens the door to new directions in both quantum sensing and simulation.

The N\'eel order of an antiferromagnet subject to a spin torque can undergo precession in a circular orbit about any chosen axis. To orient and stabilize the motion against the effects of magnetic anisotropy, the spin polarization should have components in-plane and normal to the plane of the orbit, where the latter must exceed a threshold. For biaxial antiferromagnets, the precessional motion is described by the equation for a damped-driven pendulum, which has hysteresis as a function of the spin current with a critical value where the period diverges. The fundamental frequency of the motion varies inversely with the damping and as (xp-1)1/p, with the drive-to-criticality ratio $x$ and the parameter $p>2$. An approximate closed-form result for the threshold spin current is presented, which depends on the minimum cutoff frequency the orbit can support. Precession about the hard axis has zero cutoff frequency and the lowest threshold, while the easy axis has the highest cutoff. A device setup is proposed for electrical control and detection of the dynamics, which is promising to demonstrate a tunable terahertz nano-oscillator.

Entanglement entropy is an essential metric for characterizing quantum many-body systems, but its numerical evaluation for neural network representations of quantum states has so far been inefficient and demonstrated only for the restricted Boltzmann machine architecture. Here we estimate generalized R\'enyi entropies of autoregressive neural quantum states with up to $N=256$ spins using quantum Monte Carlo methods. A naive ``direct sampling'' approach performs well for low-order R\'enyi entropies but fails for larger orders when benchmarked on a one-dimensional Heisenberg model. We therefore propose an improved ``conditional sampling'' method exploiting the autoregressive structure of the network ansatz, which outperforms direct sampling and facilitates calculations of higher-order R\'enyi entropies in both one- and two-dimensional Heisenberg models. Access to higher-order R\'enyi entropies allows for an approximation of the von Neumann entropy as well as extraction of the single-copy entanglement. Both methods elucidate the potential of neural network quantum states in quantum Monte Carlo studies of entanglement entropy for many-body systems.

Solid-state battery technology is motivated by the desire to deliver flexible power storage in a safe and efficient manner. The increasingly widespread use of batteries from mass production facilities highlights the need for a rapid and sensitive diagnostic tool for identifying battery defects. We demonstrate the use of atomic magnetometry to measure the magnetic fields around miniature solid-state battery cells. These fields encode information about battery manufacturing defects, state of charge, and impurities, and they can provide important insights into battery aging processes. Compared with SQUID-based magnetometry, the availability of atomic magnetometers, however, highlights the possibility of constructing a low-cost, portable, and flexible implementation of battery quality control and characterization technology.

Using an ensemble of atoms in an optical cavity, we engineer a family of nonlocal Heisenberg Hamiltonians with continuously tunable anisotropy of the spin-spin couplings. We thus gain access to a rich phase diagram, including a paramagnetic-to-ferromagnetic Ising phase transition that manifests as a diverging magnetic susceptibility at the critical point. The susceptibility displays a symmetry between Ising interactions and XY (spin-exchange) interactions of the opposite sign, which is indicative of the spatially extended atomic system behaving as a single collective spin. Images of the magnetization dynamics show that spin-exchange interactions protect the coherence of the collective spin, even against inhomogeneous fields that completely dephase the noninteracting and Ising systems. Our results underscore prospects for harnessing spin-exchange interactions to enhance the robustness of spin squeezing protocols.

Abstract Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics, and magnetic resonance spectroscopy. The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics. After photoexcitation of the chromophore, pentacene–radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing. Under the right conditions, the triplet spin polarization, arising from mS-selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling. Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene–trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene–trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals.

An important goal of spintronics is to covert a charge current into a spin current with a controlled spin polarization that can exert torques on an adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet system. A CoNi multilayer is used as a spin current source in a sample with structure CoNi/Au/CoFeB. Spin torque ferromagnetic resonance is used to measure the torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torque produced by the planar Hall effect originating in CoNi. We find the strength of this effect to be comparable to that of the spin Hall effect in platinum, indicating that the planar Hall effect holds potential as a spin current source with a controllable polarization direction.

The ever-increasing demand for high-capacity rechargeable batteries highlights the need for sensitive and accurate diagnostic technology for determining the state of a cell, for identifying and localizing defects, and for sensing capacity loss mechanisms. Here, we leverage atomic magnetometry to map the weak induced magnetic fields around Li-ion battery cells in a magnetically shielded environment. The ability to rapidly measure cells nondestructively allows testing even commercial cells in their actual operating conditions, as a function of state of charge. These measurements provide maps of the magnetic susceptibility of the cell, which follow trends characteristic for the battery materials under study upon discharge. In particular, hot spots of charge storage are identified. In addition, the measurements reveal the capability to measure transient internal current effects, at a level of μA, which are shown to be dependent upon the state of charge. These effects highlight noncontact battery characterization opportunities. The diagnostic power of this technique could be used for the assessment of cells in research, quality control, or during operation, and could help uncover details of charge storage and failure processes in cells.

The lithium-ion cell is the leading electric energy-storage technology reducing the environmental impact of fossil fuels. Their high energy density makes Li-ion batteries inherently dangerous due to the possibility of thermal runaway. Prerelease battery diagnostics incorporated at the final stages of cell manufacturing can improve personal and public safety and reduce the environmental impact of failures. An innovative diagnostic technique termed inside-out magnetic resonance imaging (io-MRI) measures magnetic field (MF) perturbations outside the battery’s conductive casing. Weak magnetic fields produced by an operating cell during charge–discharge events can reveal early signs of the battery’s malfunction and shed light on the electrochemical phenomena occurring at the electrodes. Since commercial cells are strongly magnetic, and magnetic objects are known to bring about failures of MRI methodology, the choice of MRI technique becomes central to the analysis. Single-point ramped imaging with T 1 enhancement (SPRITE) is immune to notorious “susceptibility” artefacts due to its fully phase-encoded three-dimensional (3D) acquisition. SPRITE enables rapid distortion-free visualization of magnetic field patterns associated with a cell’s mechanical integrity, state of charge, and operating mode. Io-SPRITE in combination with a solid-state detection-medium device achieved sensitive and safe measurements of current-induced magnetic fields very close to the surface of the cell.

We study the dynamics of the Neel order in biaxial antiferromagnets under the influence of an antidamping-like torque exerted by a spin current. Large spin currents can produce steady-state precession in a plane perpendicular to spin polarization. When the spin-current is polarized along the hard-anisotropy axis, the Neel order revolves in the most stable `easy plane' with dynamical equation of a damped-driven pendulum. The two-dimensional parameter space of drive and damping is explored to determine the regimes of periodic and stationary solutions, and the hysteretic region is identified. Analytic solutions for oscillations in angular velocity are derived when damping and drive are large, and a model for the fundamental mode is conceived. The oscillations are coherent for large drive and spiky for large damping near the threshold. We also propose experimental setup for electrical control and detection of terahertz auto-oscillations in thin films. The in-plane antiferromagnet overlaid with lateral spin-valve structure and the perpendicular antiferromagnet overlaid with spin-Hall structure form compatible geometries. The current drive after initiating oscillations can be lowered by exploiting the hysteresis to mitigate excessive Joule heating in the metal and to tune the oscillations to detectable frequencies. Room-temperature antiferromagnetic insulators such as in-plane NiO and perpendicular Cr2O3 are considered to calculate the current density, oscillation frequency and spin-pumping voltage.

An important goal of spintronics is to covert a charge current into a spin current with a controlled spin polarization that can exert torques on an adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet system. A CoNi multilayer is used as a spin current source in a sample with structure CoNi/Au/CoFeB. Spin torque ferromagnetic resonance is used to measure the torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torques produced by the planar Hall effect originating in CoNi. We find the strength of this effect to be comparable to that of the spin Hall effect in platinum, indicating that the planar Hall effect holds potential as a spin current source with a controllable polarization direction.

We present a study of the transport properties of thermally generated spin currents in an insulating ferrimagnetic-antiferromagnetic-ferrimagnetic trilayer over a wide range of temperature. Spin currents generated by the spin Seebeck effect (SSE) in a yttrium iron garnet (YIG) YIG/NiO/YIG trilayer on a gadolinium gallium garnet (GGG) substrate were detected using the inverse spin Hall effect (ISHE) in Pt. By studying samples with different NiO thicknesses, the spin diffusion length of NiO was determined to be ∼3.8 nm at room temperature. Surprisingly, a large increase of the SSE signal was observed below 30 K, and the field dependence of the signal closely follows a Brillouin function for an S=7/2 spin. The increase of the SSE signal at low temperatures could thus be associated with the paramagnetic SSE from the GGG substrate. Besides, a broad peak in the SSE response was observed around 100 K. These observations are important in understanding the generation and transport properties of spin currents through magnetic insulators and the role of a paramagnetic substrate in spin current generation.

Low-damping magnetic insulators are essential for pure spin current-based electronics as they can generate and transfer spin currents without associated charge currents. Nanometer-thick epitaxial thin films of low-damping magnetic insulators are particularly important in order to control and switch the magnetization via spin transfer torques. We have recently developed films of the ferromagnetic insulator MgAl0.5Fe1.5O4 (MAFO) with a low Gilbert damping parameter (∼0.001). In contrast to Y3Fe5O12 (YIG), MAFO films can be grown on a variety of substrates and have significant in-plane magnetic anisotropy, leading to higher spin-wave frequencies. Here, we demonstrate efficient spin current injection from MAFO into adjacent Pt and β-W layers by ferromagnetic resonance (FMR) broadening and inverse spin Hall effect measurements. Angular dependent magnetoresistance (ADMR) measurements indicate that the proximity effect magnetoresistance is small compared to the spin Hall magnetoresistance associated with spin pumping. FMR and ADMR measurements indicate that MAFO/Pt interfaces have a spin-mixing conductance of ∼2 × 1014 Ω−1 m−2, comparable to that of YIG/Pt. These measurements also show that the spin transport can be described by Dyakonov-Perel spin relaxation combined with an extrinsic spin Hall effect (from skew scattering). These results demonstrate the promise of spinel ferrites for spin current-based spintronics.

Sensing of a few unpaired electron spins, such as in metal ions and radicals, is a useful but difficult task in nanoscale physics, biology, and chemistry. Single negatively charged nitrogen-vacancy (NV –) centers in diamond offer high sensitivity and spatial resolution in the optical detection of weak magnetic fields produced by a spin bath but often require long acquisition times on the order of seconds. Here, we present an approach based on coupled spin and charge dynamics in dense NV ensembles in strongly fluorescent nanodiamonds (NDs) to sense external magnetic dipoles. We apply this approach to various paramagnetic species, including gadolinium complexes, magnetite nanoparticles, and hemoglobin in whole blood. Taking advantage of the high NV density, we demonstrate a dramatic reduction in acquisition time (down to tens of milliseconds) while maintaining high sensitivity to paramagnetic centers. Strong luminescence, high sensitivity, and short acquisition time make dense NV – ensembles in NDs a potentially promising tool for biosensing and bioimaging applications.

Purpose The optimization and analysis of spin ensemble trajectories in the hybrid state—a state in which the direction of the magnetization adiabatically follows the steady state while the magnitude remains in a transient state. Methods Numerical optimizations were performed to find spin ensemble trajectories that minimize the Cramér‐Rao bound for ‐encoding, ‐encoding, and their weighted sum, respectively, followed by a comparison between the Cramér‐Rao bounds obtained with our optimized spin‐trajectories, Look‐Locker sequences, and multi‐spin‐echo methods. Finally, we experimentally tested our optimized spin trajectories with in vivo scans of the human brain. Results After a nonrecurring inversion segment on the southern half of the Bloch sphere, all optimized spin trajectories pursue repetitive loops on the northern hemisphere in which the beginning of the first and the end of the last loop deviate from the others. The numerical results obtained in this work align well with intuitive insights gleaned directly from the governing equation. Our results suggest that hybrid‐state sequences outperform traditional methods. Moreover, hybrid‐state sequences that balance ‐ and ‐encoding still result in near optimal signal‐to‐noise efficiency for each relaxation time. Thus, the second parameter can be encoded at virtually no extra cost. Conclusions We provided new insights into the optimal encoding processes of spin relaxation times in order to guide the design of robust and efficient pulse sequences. We found that joint acquisitions of and in the hybrid state are substantially more efficient than sequential encoding techniques.

The ever-increasing demand for high-capacity rechargeable batteries highlights the need for sensitive and accurate diagnostic technology for determining the state of a cell, for identifying and localizing defects, or for sensing capacity loss mechanisms. Here, we demonstrate the use of atomic magnetometry to map the weak induced magnetic fields around a Li-ion battery cell as a function of state of charge and upon introducing mechanical defects. These measurements provide maps of the magnetic susceptibility of the cell, which follow trends characteristic for the battery materials under study upon discharge. In addition, the measurements reveal hitherto unknown long time-scale transient internal current effects, which were particularly pronounced in the overdischarged regime. The diagnostic power of this technique is promising for the assessment of cells in research, quality control, or during operation, and could help uncover details of charge storage and failure processes in cells.

We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.

Abstract We present a study of precessional magnetization switching in orthogonal spin-torque spin-valve devices at low temperatures. The samples consist of a spin-polarizing layer that is magnetized out-of-the film plane and an in-plane magnetized free and reference magnetic layer separated by non-magnetic metallic layers. We find coherent oscillations in the switching probability, characterized by high speed switching (~200 ps), error rates as low as 10−5 and decoherence effects at longer timescales (~1 ns). Our study, which is conducted over a wide range of parameter space (pulse amplitude and duration) with deep statistics, demonstrates that the switching dynamics are likely dominated by the action of the out-of-plane spin polarization, in contrast to in-plane spin-torque from the reference layer, as has been the case in most previous studies. Our results demonstrate that precessional spin-torque devices are well suited to a cryogenic environment, while at room temperature they have so far not exhibited coherent or reliable switching.

The examination and optimized preparation of nuclear spin singlet order has enabled the development of new types of applications that rely on potentially long-term polarization storage. Lifetimes several orders of magnitude longer than T1 have been observed. The efficient creation of such states relies on special pulse sequences. The extreme cases of very large and very small magnetic equivalence received primary attention, while relatively little effort has been directed towards studying singlet relaxation in the intermediate regime. The intermediate case is of interest as it is relevant for many spin systems, and would also apply to heteronuclear systems in very low magnetic fields. Experimental evidence for singlet-triplet leakage in the intermediate regime is sparse. Here we describe a pulse sequence for efficiently creating singlets in the intermediate regime in a broad-band fashion. Singlet lifetimes are studied with a specially synthesized molecule over a wide range of magnetic fields using a home-built sample-lift apparatus. The experimental results are supplemented with spin simulations using parameters obtained from ab initio calculations. This work indicates that the chemical shift anisotropy (CSA) mechanism is relatively weak compared to singlet-triplet leakage for the proton system observed over a large magnetic field range. These experiments provide a mechanism for expanding the scope of singlet NMR to a larger class of molecules, and provide new insights into singlet lifetime limiting factors.

We propose a robust scheme for generating macroscopic superposition states of spin or motion with the aid of a single photon. Shaping the wave packet of the photon enables high-fidelity preparation of nonclassical states of matter even in the presence of photon loss. Success is heralded by photodetection, enabling the scheme to be implemented with a weak coherent field. We analyze applications to preparing Schrödinger cat states of a collective atomic spin or of a mechanical oscillator coupled to an optical resonator. The method generalizes to preparing arbitrary superpositions of coherent states, enabling full quantum control. We illustrate this versatility by showing how to prepare Dicke or Fock states, as well as superpositions in the Dicke or Fock basis.

We studied the spin depolarization of ensembles of nitrogen-vacancy (NV) centers in nitrogen-rich single-crystal diamonds. We found a strong dependence of the evolution of the polarized state in the dark on the concentration of NV centers. At low excitation power, we observed a simple exponential decay profile in the low-density regime and a paradoxical inverted exponential profile in the high-density regime. At higher excitation power, we observed complex behavior, with an initial sharp rise in the luminescence signal after the preparation pulse followed by a slower exponential decay. Magnetic field and excitation laser power-dependent measurements suggest that the rapid initial increase of the luminescence signal is related to recharging of the nitrogen-vacancy centers (from neutral to negatively charged) in the dark. The slow relaxing component corresponds to the longitudinal spin relaxation of the NV ensemble. The shape of the decay profile reflects the interplay between two mechanisms: the NV charge-state conversion in the dark and the longitudinal spin relaxation. These mechanisms, in turn, are influenced by ionization, recharging, and polarization dynamics during excitation. Interestingly, we found that charge dynamics are dominant in NV-dense samples even at very feeble excitation power. These observations may be important for the use of ensembles of NV centers in precession magnetometry and sensing applications.

Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific and research community, whereas significantly growing interest in magnetism and magnetic materials in relation to energy applications is noticeable, and other technological fields are emerging as well. Also, more and more work is occurring in which complex topologies of magnetically ordered states are being explored, hereby aiming at a technological utilization of the very theoretical concepts that were recognised by the 2016 Nobel Prize in Physics. Given this somewhat shifted scenario, it seemed appropriate to select topics for this Roadmap article that represent the three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism. While many of the contributions in this Roadmap have clearly overlapping relevance in all three fields, their relative focus is mostly associated to one of the three pillars. In this way, the interconnecting roles of having suitable magnetic materials, understanding (and being able to characterize) the underlying physics of their behaviour and utilizing them for applications and devices is well illustrated, thus giving an accurate snapshot of the world of magnetism in 2017. The article consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. Evidently, the depth at which each contribution can describe the subject matter is limited and a full review of their statuses, advances, challenges and perspectives cannot be fully accomplished. Also, magnetism, as a vibrant research field, is too diverse, so that a number of areas will not be adequately represented here, leaving space for further Roadmap editions in the future. However, this 2017 Magnetism Roadmap article can provide a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism.

Purpose: The analysis of optimized spin ensemble trajectories for relaxometry in the hybrid state. Methods: First, we constructed visual representations to elucidate the differential equation that governs spin dynamics in hybrid state. Subsequently, numerical optimizations were performed to find spin ensemble trajectories that minimize the Cramer-Rao bound for T1-encoding, T2-encoding, and their weighted sum, respectively, followed by a comparison of the Cramer-Rao bounds obtained with our optimized spin-trajectories, as well as Look-Locker and multi-spin-echo methods. Finally, we experimentally tested our optimized spin trajectories with in vivo scans of the human brain. Results: After a nonrecurring inversion segment on the southern hemisphere of the Bloch sphere, all optimized spin trajectories pursue repetitive loops on the northern half of the sphere in which the beginning of the first and the end of the last loop deviate from the others. The numerical results obtained in this work align well with intuitive insights gleaned directly from the governing equation. Our results suggest that hybrid-state sequences outperform traditional methods. Moreover, hybrid-state sequences that balance T1- and T2-encoding still result in near optimal signal-to-noise efficiency. Thus, the second parameter can be encoded at virtually no extra cost. Conclusion: We provide insights regarding the optimal encoding processes of spin relaxation times in order to guide the design of robust and efficient pulse sequences. We find that joint acquisitions of T1 and T2 in the hybrid state are substantially more efficient than sequential encoding techniques.

We study the magnetization dynamics of thin-film magnetic elements with in-plane magnetization subject to a spin current flowing perpendicular to the film plane. We derive a reduced partial differential equation for the in-plane magnetization angle in a weakly damped regime. We then apply this model to study the experimentally relevant problem of switching of an elliptical element when the spin polarization has a component perpendicular to the film plane, restricting the reduced model to a macrospin approximation. The macrospin ordinary differential equation is treated analytically as a weakly damped Hamiltonian system, and an orbit-averaging method is used to understand transitions in solution behaviors in terms of a discrete dynamical system. The predictions of our reduced model are compared to those of the full Landau-Lifshitz-Gilbert-Slonczewski equation for a macrospin.

We show that a slowly decaying current pulse can lead to nearly deterministic precessional switching in the presence of noise. We consider a biaxial macrospin, with an easy axis in-plane and a hard axis out-of-plane, typical of thin film nanomagnets patterned into asymmetric shapes. Out-of-plane precessional magnetization orbits are excited with a current pulse with a component of spin polarization normal to the film plane. By numerically integrating the stochastic Landau-Lifshitz-Gilbert-Slonczewski equation we show that thermal noise leads to strong dephasing of the magnetization orbits. However, an adiabatically decreasing pulse amplitude overwhelmingly leads to magnetization reversal, with a final state dependent on the pulse polarity. We develop an analytic model to explain this phenomena and to determine the pulse decay time necessary for adiabatic magnetization relaxation and thus deterministic magnetization switching.

Graphene on hydrogen terminated monolayer nanodiamond heterostructures provides a new way to improve carrier transport characteristics of the graphene, offering up to 60% improvement when compared with similar graphene on SiO2/Si substrates. These heterostructures offers excellent current-carrying abilities whilst offering the prospect of a fast, low cost and easy methodology for device applications. The use of ND monolayers is also a compatible technology for the support of large area graphene films. The nature of the C-H bonds between graphene and H-terminated NDs strongly influences the electronic character of the heterostructure, creating effective charge redistribution within the system. Field effect transistors (FETs) have been fabricated based on this novel herterostructure to demonstrate device characteristics and the potential of this approach.

An understanding of the growth of (110) quantum wells (QWs) is of great importance to spin systems due to the observed long spin relaxation times. In this article, we report on the metamorphic growth and characterization of high mobility undoped InAs (110) QWs on GaAs (110) substrates. A low-temperature nucleation layer reduces dislocation density, results in tilting of the subsequent buffer layer and increases the electron mobility of the QW structure. The mobility varies widely and systematically (4000–16 000 cm2/Vs at room temperature) with deposition temperature and layer thicknesses. Low-temperature transport measurements exhibit Shubnikov de-Haas oscillations and quantized plateaus in the quantum Hall regime.

We present the results of zero temperature macrospin and micromagnetic simulations of spin transfer switching of thin film nanomagnets in the shape of an ellipse with a spin-polarization tilted out of the layer plane. The perpendicular component of the spin-polarization is shown to increase the reversal speed, leading to a lower current for switching in a given time. However, for tilt angles larger than a critical angle, the layer magnetization starts to precess about an out-of-plane axis, which leads to a final magnetization state that is very sensitive to simulation conditions. As the ellipse lateral size increases, this out-of-plane precession is suppressed, due to the excitation of spatially non-uniform magnetization modes.

Correction for 'Singlet lifetime measurements in an all-proton chemically equivalent spin system by hyperpolarization and weak spin lock transfers' by Y. Zhang et al., Phys. Chem. Chem. Phys., 2015, 17, 24370-24375.

We study the magnetization dynamics of spin-torque oscillators in the presence of thermal noise and as a function of the spin-polarization angle in a macrospin model. The macrospin has biaxial magnetic anisotropy, typical of thin film magnetic elements, with an easy axis in the film plane and a hard axis out of the plane. Using a method that averages the energy over precessional orbits, we derive analytic expressions for the current that generates and sustains out-of-plane precessional states. We find that there is a critical angle of the spin polarization necessary for the occurrence of such states and predict a hysteretic response to applied current. This model can be tested in experiments on orthogonal spin-transfer devices, which consist of both an in-plane and out-of-plane magnetized spin polarizers, effectively leading to an angle between the easy and spin-polarization axes.

We present the quasi-static and dynamic switching characteristics of orthogonal spin-transfer devices incorporating an out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve device at cryogenic temperatures. Switching at 12 K between parallel and anti-parallel spin-valve states is investigated for slowly varied current as well as for current pulses with durations as short as 200 ps. We demonstrate 100% switching probability with current pulses 0.6 ns in duration. We also present a switching probability diagram that summarizes device switching operation under a variety of pulse durations, amplitudes, and polarities.

Site-specific spin polarization labeling of a peptide was conducted by homogeneous hydrogenation with parahydrogen. Surprisingly, polarization transfer to a remote alanine residue was observed. The diastereoselectivity of the hydrogenation reaction was determined, and these results show that parahydrogen can be used to enhance signals and elucidate the hydrogenation processes of dehydropeptide units in complex molecules.

Magnetization excitation and reversal induced by spin-transfer torques is described in terms of power received or dissipated in a macrospin system. This approach provides a clear and intuitive understanding of the effect of both applied magnetic fields and injected spin-polarized currents on magnetization reversal. It is illustrated by solving the case of magnetization reversal in a nanopillar spin valve with perpendicular magnetizations although the approach can be applied more generally. The appearance of critical currents below which spin-transfer torque is no longer efficient is explained by a break in the uniaxial symmetry of such structures.

The theoretical basis of two-frequency saturation is given here in the framework of Provotorov theory. The parameters influencing the saturation efficiency are discussed and studied experimentally using a liquid-crystalline test system. It is shown that double-frequency irradiation can be extremely efficient when the irradiation frequencies are placed at opposite sides of the characteristic frequency of the spin system, and that the frequency separation in the double-frequency irradiation can be varied over a large range. Provotorov theory is also shown to provide good insights into the experimental findings, which would otherwise be difficult to obtain from simulations.

Entangled quantum systems can exhibit correlations that cannot be simulated classically. For historical reasons such correlations are called "Bell inequality violations." We give two new two-player games with Bell inequality violations that are stronger, fully explicit, and arguably simpler than earlier work. The first game is based on the Hidden Matching problem of quantum communication complexity, introduced by Bar-Yossef, Jayram, and Kerenidis. This game can be won with probability 1 by a strategy using a maximally entangled state with local dimension $n$ (e.g., n EPR-pairs), while we show that the winning probability of any classical strategy differs from 1/2 by at most O((n)/n). The second game is based on the integrality gap for Unique Games by Khot and Vishnoi and the quantum rounding procedure of Kempe, Regev, and Toner. Here $n$-dimensional entanglement allows the game to be won with probability 1/(n)², while the best winning probability without entanglement is $1/n$. This near-linear ratio is almost optimal, both in terms of the local dimension of the entangled state, and in terms of the number of possible outputs of the two players.

Optimal control theory is applied for designing pulse sequences to optimally excite a spin-3/2 system with residual quadrupolar coupling in the presence of quadrupolar relaxation. A homogeneous form of the master equation is constructed to simulate the dynamics of the spin system, and a general optimization procedure with a homogeneous form of the equation of motion is described. The optimized pulses are tested with (23)Na NMR, and their performance is compared with that of pulses optimized in the absence of relaxation.

Single crystal Ni/Co(111) superlattices have been grown by molecular beam epitaxy. The Ni thickness is 3 ML whereas the Co thickness varies from 0.2 to 4 ML. The superlattices were studied using magnetometry and ferromagnetic resonance spectroscopy and they all exhibit strong perpendicular to the plane magnetic anisotropy. The maximum magnetocrystalline anisotropy is obtained for one cobalt monolayer. Kerr microscopy measurements show the variation of domain pattern as the Co layer thickness changes.

A sensor that integrates high-sensitivity micro-Hall effect magnetometry and high-frequency electron paramagnetic resonance spectroscopy capabilities on a single semiconductor chip is presented. The Hall-effect magnetometer (HEM) was fabricated from a two-dimensional electron gas GaAs∕AlGaAs heterostructure in the form of a cross, with a 50×50μm2 sensing area. A high-frequency microstrip resonator is coupled with two small gaps to a transmission line with a 50Ω impedance. Different resonator lengths are used to obtain quasi-TEM fundamental resonant modes in the frequency range 10–30GHz. The resonator is positioned on top of the active area of the HEM, where the magnetic field of the fundamental mode is largest, thus optimizing the conversion of microwave power into magnetic field at the sample position. The two gaps coupling the resonator and transmission lines are engineered differently—the gap to the microwave source is designed to optimize the loaded quality factor of the resonator (Q⩽150) while the gap for the transmitted signal is larger. This latter gap minimizes losses and prevents distortion of the resonance while enabling measurement of the transmitted signal. The large filling factor of the resonator permits sensitivities comparable to that of high-quality factor resonant cavities. The integrated sensor enables measurement of the magnetization response of micron scale samples upon application of microwave fields. In particular, the combined measurement of the magnetization change and the microwave power under cw microwave irradiation of single crystal of molecular magnets is used to determine of the energy relaxation time of the molecular spin states. In addition, real-time measurements of the magnetization dynamics upon application of fast microwave pulses are demonstrated.

Dilute frozen solutions of the single molecule magnet Ni(4) (S=4) have been studied using 130 GHz electron paramagnetic resonance (EPR). Despite the random orientation of the molecules, well defined EPR absorption peaks are observed due to the strong variation of the splittings between the different spin states on magnetic field. Temperature dependent studies above 4 K and comparison with simulations enable identification of the spin transitions and determination of the Hamiltonian parameters. The latter are found to be close to those of Ni(4) single crystals. No echo was detected from Ni(4) in pulsed experiments, which sets an upper bound of about 50 ns on the spin coherence time.

High-frequency electron paramagnetic resonance (EPR) studies of the antiferromagnetic Mn-[3×3] molecular grid clearly reveal a breaking of the =0 selection rule, providing direct evidence for the mixing of spin wave functions ($S$ mixing) induced by the comparable exchange and magnetoanisotropy energy scales within the grid. This finding highlights the potential utility of EPR for studies of exchange splittings in molecular nanomagnets, which is normally considered the sole domain of inelastic neutron scattering, thereby offering improved sensitivity and energy resolution.

Integrated magnetic sensors that allow simultaneous EPR and magnetization measurements have been developed to study single molecule magnets. A high frequency microstrip resonator has been integrated with a micro-Hall effect magnetometer. EPR spectroscopy is used to determine the energy splitting between the low lying spin states of a Ni4 single crystal, with an S=4 ground state, as a function of applied fields, both longitudinal and transverse to the easy axis at 0.4K. Concurrent magnetization measurements show changes in spin population associated with microwave absorption. Such studies enable determination of the energy relaxation time of the spin system.

The magnetic properties and magnetization dynamics of polycrystalline ultrathin Co layers were investigated using a broadband ferromagnetic resonance technique at room temperature. A variable-thickness (10.3em0exnm100.3em0exnm) Co layer is sandwiched between 10-nm-thick Cu layers (100.3em0exnm Cu0.3em0exCo100.3em0exnm Cu), while materials in contact with the Cu outer interfaces are varied to determine their influence on the magnetization damping. The resonance field and the linewidth were studied for in-plane magnetic fields in field-swept experiments at a fixed frequency, from 40.3em0exto0.3em0ex250.3em0exGHz. The Co layers have a lower magnetization density than the bulk and an interface contribution to the magnetic anisotropy normal to the film plane. The Gilbert damping, as determined from the frequency dependence of the linewidth, increases with decreasing Co layer thickness for films with outer Pt layers. This enhancement is not observed in structures without Pt layers. The result can be understood in terms of a nonlocal contribution to the damping due to spin pumping from Co through the Cu layer and spin relaxation in Pt layers. Pt layers just 1.50.3em0exnm thick are found to be sufficient to enhance the damping and thus act as efficient ``spin sinks.'' In structures with Pt outer layers, this nonlocal contribution to the damping becomes predominant when the Co layer is thinner than 40.3em0exnm.

The symmetry of magnetic quantum tunneling has been studied in the prototype single molecule magnet Mn12-acetate using a micro-Hall effect magnetometer and superconducting high field vector magnet system. An average crystal fourfold symmetry is shown to be due to local molecular environments of twofold symmetry that are rotated by 90 degrees with respect to one another, confirming that disorder which lowers the molecule symmetry is as important to magnetic quantum tunneling. We have studied a subset of these lower (twofold) site symmetry molecules and present evidence for a Berry phase effect consistent with a local twofold symmetry.

We have studied the crossover between thermally assisted and pure quantum\ntunneling in single crystals of high spin (S=10) uniaxial single molecule\nmagnet Mn12-acetate using micro-Hall effect magnetometry. Magnetic hysteresis\nexperiments have been used toinvestigate the energy levels that determine the\nmagnetization reversal as a function of magnetic field and temperature. These\nexperiments demonstrate that the crossover occurs in a narrow (~0.1 K) or broad\n(~1 K) temperature interval depending on the magnitude and direction of the\napplied field. For low external fields applied parallel to the easy axis, the\nenergy levels that dominate the tunneling shift abruptly with temperature. In\nthe presence of a transverse field and/or large longitudinal field these energy\nlevels change with temperature more gradually. A comparison of our experimental\nresults with model calculations of this crossover suggest that there are\nadditional mechanisms that enhance the tunneling rate of low lying energy\nlevels and broaden the crossover for small transverse fields.\n

The crossover between thermally assisted and pure quantum tunneling has been studied in single crystals of high spin (S = 10) uniaxial molecular magnet Mn12 using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation experiments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow (approximately 0. 1 K) or broad (approximately 1 K) temperature interval depending on the magnitude of the field transverse to the anisotropy axis.

The magnetization processes and the domain structure of epitaxial (110) Fe submicron-sized elements are studied experimentally and with micromagnetic simulations. Magnetic force microscopy and Kerr magnetometry results are in good agreement with the simulations, thus allowing the development of a consistent picture of the micromagnetics of these systems.

The use of small scale Hall probes in the study of nanometer scale magnetic structures and high temperature superconductors is discussed. Micron sized Hall probes are a powerful means of studying local magnetic properties and are complementary to other local magnetometry methods such as those based on magnetic force microscopy and superconducting quantum interference device techniques. In addition to presenting an overview of our recent studies on nanofabricated magnetic samples1 and ring shaped superconductors,2 we discuss the factors that determine the spatial and field resolution of this technique. Among the important issues considered are the Hall response to inhomogeneous magnetic fields and the noise characteristics of sensor materials. A variety of materials have been studied including two-dimensional GaAs/GaAlAs heterostructures, three-dimensional InGaAs, and Bi semimetal thin films. Finally, our progress toward the realization of a variable temperatures scanning Hall probe microscope will be reported.

The low temperature magnetic properties of arrays of scanning tunneling microscope (STM) fabricated ferromagnetic particles have been studied as a function of their dimension using a novel high sensitivity Hall magnetometer. Iron deposits with controlled shape and nanometer scale diameters (∼25 nm) are formed using a STM to decompose a metalorganic precursor [Fe(CO)5] in the active area of the measurement device. The hysteresis loops change significantly in going from nearly isotropic to oriented high aspect ratio (6:1 length to diameter) filamentary particles. In particles of intermediate aspect ratio and diameter the largest coercive field of 2.7 kOe is observed. This behavior as well as the characteristics of the Hall magnetometer (spin sensitivity of 10−14 emu/ Hz1/2) are described.