Roadmap on nanoscale magnetic resonance imaging.

The field of nanoscale magnetic resonance imaging (NanoMRI) was started 30 years ago. It was motivated by the desire to image single molecules and molecular assemblies, such as proteins and virus particles, with near-atomic spatial resolution and on a length scale of 100 nm. Over the years, the NanoMRI field has also expanded to include the goal of useful high-resolution nuclear magnetic resonance (NMR) spectroscopy of molecules under ambient conditions, including samples up to the micron-scale. The realization of these goals requires the development of spin detection techniques that are many orders of magnitude more sensitive than conventional NMR and MRI, capable of detecting and controlling nanoscale ensembles of spins. Over the years, a number of different technical approaches to NanoMRI have emerged, each possessing a distinct set of capabilities for basic and applied areas of science. The goal of this roadmap article is to report the current state of the art in NanoMRI technologies, outline the areas where they are poised to have impact, identify the challenges that lie ahead, and propose methods to meet these challenges. This roadmap also shows how developments in NanoMRI techniques can lead to breakthroughs in emerging quantum science and technology applications. .

Strong on-Chip Microwave Photon-Magnon Coupling Using Ultralow-Damping Epitaxial Y3Fe5O12 Films at 2 K.

Y3Fe5O12 is arguably the best magnetic material for magnonic quantum information science (QIS) because of its extremely low damping. We report ultralow damping at 2 K in epitaxial Y3Fe5O12 thin films grown on a diamagnetic Y3Sc2Ga3O12 substrate that contains no rare-earth elements. Using these ultralow damping YIG films, we demonstrate for the first time strong coupling between magnons in patterned YIG thin films and microwave photons in a superconducting Nb resonator. This result paves the road toward scalable hybrid quantum systems that integrate superconducting microwave resonators, YIG film magnon conduits, and superconducting qubits into on-chip QIS devices.

Enhancing Perpendicular Magnetic Anisotropy in Garnet Ferrimagnet by Interfacing with Few-Layer WTe2.

Engineering magnetic anisotropy in a ferro- or ferrimagnetic (FM) thin film is crucial in a spintronic device. One way to modify the magnetic anisotropy is through the surface of the FM thin film. Here, we report the emergence of a perpendicular magnetic anisotropy (PMA) induced by interfacial interactions in a heterostructure comprised of a garnet ferrimagnet, Y3Fe5O12 (YIG), and a low-symmetry, high spin-orbit coupling (SOC) transition metal dichalcogenide, WTe2. At the same time, we also observed an enhancement in Gilbert damping in the WTe2-covered YIG area. Both the magnitude of interface-induced PMA and the Gilbert damping enhancement have no observable WTe2 thickness dependence down to a single quadruple layer, indicating that the interfacial interaction plays a critical role. The ability of WTe2 to enhance the PMA in FM thin film, combined with its previously reported capability to generate out-of-plane damping like spin torque, makes it desirable for magnetic memory applications.

Spin-Orbit Torque Nano-oscillators by Dipole-Field-Localized Spin Wave Modes.

We demonstrate a high-quality spin-orbit torque nano-oscillator comprised of spin wave modes confined by the magnetic field by the strongly inhomogeneous dipole field of a nearby micromagnet. This approach enables variable spatial confinement and systematic tuning of magnon spectrum and spectral separations for studying the impact of multimode interactions on auto-oscillations. We find these dipole-field-localized spin wave modes exhibit good characteristic properties as auto-oscillators─narrow line width and large amplitude─while persisting up to room temperature. We find that the line width of the lowest-lying localized mode is approximately proportional to temperature in good agreement with theoretical analysis of the impact of thermal fluctuations. This demonstration of a clean oscillator with tunable properties provides a powerful tool for understanding the fundamental limitations and line width contributions to improve future spin-Hall oscillators.

Nonlocal Uniform-Mode Ferromagnetic Resonance Spin Pumping.

Nonlocal spin transport using lateral structures is attractive for spintronic devices. Typically, a spin current is generated by a ferromagnetic (FM) or a heavy metal (HM) electrode in a nonlocal structure, which can be detected by another FM or HM electrode. Here, we report a new nonlocal spin injection scheme using uniform-mode ferromagnetic resonance (FMR) spin pumping in Pt/Y3Fe5O12 (YIG) lateral structures. This scheme is enabled by well-separated resonant fields of Pt/YIG and bare YIG due to substantial change of anisotropy in YIG films induced by a Pt overlayer, allowing for clearly distinguishable local and nonlocal spin pumping. Our results show that the spin decay length of nonlocal uniform-mode spin pumping in 20 nm YIG films is 2.1 µm at room temperature.

Fundamental Spin Interactions Underlying the Magnetic Anisotropy in the Kitaev Ferromagnet CrI_{3}.

We lay the foundation for determining the microscopic spin interactions in two-dimensional (2D) ferromagnets by combining angle-dependent ferromagnetic resonance (FMR) experiments on high quality CrI_{3} single crystals with theoretical modeling based on symmetries. We discover that the Kitaev interaction is the strongest in this material with K∼-5.2 meV, 25 times larger than the Heisenberg exchange J∼-0.2 meV, and responsible for opening the ∼5 meV gap at the Dirac points in the spin-wave dispersion. Furthermore, we find that the symmetric off-diagonal anisotropy Γ∼-67.5 µeV, though small, is crucial for opening a ∼0.3 meV gap in the magnon spectrum at the zone center and stabilizing ferromagnetism in the 2D limit. The high resolution of the FMR data further reveals a µeV-scale quadrupolar contribution to the S=3/2 magnetism. Our identification of the underlying exchange anisotropies opens paths toward 2D ferromagnets with higher T_{C} as well as magnetically frustrated quantum spin liquids based on Kitaev physics.

Interfacial Rashba-Effect-Induced Anisotropy in Nonmagnetic-Material-Ferrimagnetic-Insulator Bilayers.

Interfacial magnetic anisotropy in magnetic insulators has been largely unexplored. Recently, interface-induced skyrmions and electrical control of magnetization have been discovered in insulator-based heterostructures, which demand a thorough understanding of interfacial interactions in these materials. We observe a substantial, tunable interfacial magnetic anisotropy between Tm_{3}Fe_{5}O_{12} epitaxial thin films and fifteen nonmagnetic materials spanning a significant portion of the periodic table, which we attribute to Rashba spin-orbit coupling. Our results show a clear distinction between nonmagnetic capping layers from the d block and the p block. This work offers a new path for controlling magnetic phases in magnetic insulators for low-loss spintronic applications.

Spin-Hall Topological Hall Effect in Highly Tunable Pt/Ferrimagnetic-Insulator Bilayers.

Electrical detection of topological magnetic textures such as skyrmions is currently limited to conducting materials. Although magnetic insulators offer key advantages for skyrmion technologies with high speed and low loss, they have not yet been explored electrically. Here, we report a prominent topological Hall effect in Pt/Tm3Fe5O12 bilayers, where the pristine Tm3Fe5O12 epitaxial films down to 1.25 unit cell thickness allow for tuning of topological Hall stability over a broad range from 200 to 465 K through atomic-scale thickness control. Although Tm3Fe5O12 is insulating, we demonstrate the detection of topological magnetic textures through a novel phenomenon: "spin-Hall topological Hall effect" (SH-THE), where the interfacial spin-orbit torques allow spin-Hall-effect generated spins in Pt to experience the unique topology of the underlying skyrmions in Tm3Fe5O12. This novel electrical detection phenomenon paves a new path for utilizing a large family of magnetic insulators in future skyrmion technologies.

Electron Paramagnetic Resonance of a Single NV Nanodiamond Attached to an Individual Biomolecule.

Electron paramagnetic resonance (EPR), an established and powerful methodology for studying atomic-scale biomolecular structure and dynamics, typically requires in excess of 10(12) labeled biomolecules. Single-molecule measurements provide improved insights into heterogeneous behaviors that can be masked in ensemble measurements and are often essential for illuminating the molecular mechanisms behind the function of a biomolecule. Here, we report EPR measurements of a single labeled biomolecule. We selectively label an individual double-stranded DNA molecule with a single nanodiamond containing nitrogen-vacancy centers, and optically detect the paramagnetic resonance of nitrogen-vacancy spins in the nanodiamond probe. Analysis of the spectrum reveals that the nanodiamond probe has complete rotational freedom and that the characteristic timescale for reorientation of the nanodiamond probe is slow compared with the transverse spin relaxation time. This demonstration of EPR spectroscopy of a single nanodiamond-labeled DNA provides the foundation for the development of single-molecule magnetic resonance studies of complex biomolecular systems.

Nanofiber-based paramagnetic probes for rapid, real-time biomedical oximetry.

EPR (electron paramagnetic resonance) based biological oximetry is a powerful tool that accurately and repeatedly measures tissue oxygen levels. In vivo determination of oxygen in tissues is crucial for the diagnosis and treatment of a number of diseases. Here, we report the first successful fabrication and remarkable properties of nanofiber sensors for EPR-oximetry applications. Lithium octa-n-butoxynaphthalocyanine (LiNc- BuO), an excellent paramagnetic oxygen sensor, was successfully encapsulated in 300-500 nm diameter fibers consisting of a core of polydimethylsiloxane (PDMS) and a shell of polycaprolactone (PCL) by electrospinning. This core-shell nanosensor (LiNc-BuO-PDMS-PCL) shows a linear dependence of linewidth versus oxygen partial pressure (pO2). The nanofiber sensors have response and recovery times of 0.35 s and 0.55 s, respectively, these response and recovery times are ~12 times and ~218 times faster than those previously reported for PDMS-LiNc-BuO chip sensors. This greater responsiveness is likely due to the high porosity and excellent oxygen permeability of the nanofibers. Electrospinning of the structurally flexible PDMS enabled the fabrication of fibers having tailored spin densities. Core-shell encapsulation ensures the non-exposure of embedded LiNc-BuO and mitigates potential biocompatibility concerns. In vitro evaluation of the fiber performed under exposure to cultured cells showed that it is both stable and biocompatible. The unique combination of biocompatibility due to the PCL 'shell,' the excellent oxygen transparency of the PDMS core, and the excellent oxygen-sensing properties of LiNc-BuO makes LiNc-BuO-PDMS-PCL platform promising for long-term oximetry and repetitive oxygen measurements in both biological systems and clinical applications.

Nanoscale scanning probe ferromagnetic resonance imaging using localized modes.

The discovery of new phenomena in layered and nanostructured magnetic devices is driving rapid growth in nanomagnetics research. Resulting applications such as giant magnetoresistive field sensors and spin torque devices are fuelling advances in information and communications technology, magnetoelectronic sensing and biomedicine. There is an urgent need for high-resolution magnetic-imaging tools capable of characterizing these complex, often buried, nanoscale structures. Conventional ferromagnetic resonance (FMR) provides quantitative information about ferromagnetic materials and interacting multicomponent magnetic structures with spectroscopic precision and can distinguish components of complex bulk samples through their distinctive spectroscopic features. However, it lacks the sensitivity to probe nanoscale volumes and has no imaging capabilities. Here we demonstrate FMR imaging through spin-wave localization. Although the strong interactions in a ferromagnet favour the excitation of extended collective modes, we show that the intense, spatially confined magnetic field of the micromagnetic probe tip used in FMR force microscopy can be used to localize the FMR mode immediately beneath the probe. We demonstrate FMR modes localized within volumes having 200 nm lateral dimensions, and improvements of the approach may allow these dimensions to be decreased to tens of nanometres. Our study shows that this approach is capable of providing the microscopic detail required for the characterization of ferromagnets used in fields ranging from spintronics to biomagnetism. This method is applicable to buried and surface magnets, and, being a resonance technique, measures local internal fields and other magnetic properties with spectroscopic precision.

A strong ferroelectric ferromagnet created by means of spin-lattice coupling.

Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism. Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics. Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today. Recently, however, a new route to ferroelectric ferromagnets was proposed by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTiO(3), was predicted to exhibit strong ferromagnetism (spontaneous magnetization, approximately 7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, approximately 10 microC cm(-2)) simultaneously under large biaxial compressive strain. These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high-temperature manifestations of this spin-lattice coupling mechanism. Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics.

Antiferromagnonic spin transport from Y3Fe5O12 into NiO.

We observe highly efficient dynamic spin injection from Y3Fe5O12 (YIG) into NiO, an antiferromagnetic (AF) insulator, via strong coupling, and robust spin propagation in NiO up to 100-nm thickness mediated by its AF spin correlations. Strikingly, the insertion of a thin NiO layer between YIG and Pt significantly enhances the spin currents driven into Pt, suggesting exceptionally high spin transfer efficiency at both YIG/NiO and NiO/Pt interfaces. This offers a powerful platform for studying AF spin pumping and AF dynamics as well as for exploration of spin manipulation in tailored structures comprising metallic and insulating ferromagnets, antiferromagnets, and nonmagnetic materials.

Damping of confined modes in a ferromagnetic thin insulating film: angular momentum transfer across a nanoscale field-defined interface.

We observe a dependence of the damping of a confined mode of precessing ferromagnetic magnetization on the size of the mode. The micron-scale mode is created within an extended, unpatterned yttrium iron garnet film by means of the intense local dipolar field of a micromagnetic tip. We find that the damping of the confined mode scales like the surface-to-volume ratio of the mode, indicating an interfacial damping effect (similar to spin pumping) due to the transfer of angular momentum from the confined mode to the spin sink of ferromagnetic material in the surrounding film. Though unexpected for insulating systems, the measured intralayer spin-mixing conductance g_↑↓=5.3×10(19) m(-2) demonstrates efficient intralayer angular momentum transfer.

Histone H3 and H4 N-terminal tails in nucleosome arrays at cellular concentrations probed by magic angle spinning NMR spectroscopy.

Chromatin is a supramolecular assembly of DNA and histone proteins, organized into nucleosome repeat units. The dynamics of chromatin organization regulates DNA accessibility to eukaryotic transcription and DNA repair complexes. Yet, the structural and dynamic properties of chromatin at high concentrations characteristic of the cellular environment (>∼200 mg/mL) are largely unexplored at the molecular level. Here, we apply MAS NMR to directly probe the dynamic histone protein regions in (13)C,(15)N-enriched recombinant nucleosome arrays at cellular chromatin concentrations and conditions designed to emulate distinct states of DNA condensation, with focus on the flexible H3 and H4 N-terminal tails which mediate chromatin compaction. 2D (1)H-(13)C and (1)H-(15)N spectra reveal numerous correlations for H3 and H4 backbone and side-chain atoms, enabling identification of specific residues making up the dynamically disordered N-terminal tail domains. Remarkably, we find that both the H3 and H4 N-terminal tails are overall dynamic even in a highly condensed state. This significant conformational flexibility of the histone tails suggests that they remain available for protein binding in compact chromatin states to enable regulation of heterochromatin. Furthermore, our study provides a foundation for quantitative structural and dynamic investigations of chromatin at physiological concentrations.

Control of magnetocrystalline anisotropy by epitaxial strain in double perovskite Sr(2)FeMoO(6) films.

We demonstrate tuning of magnetocrystalline anisotropy in high-quality Sr(2)FeMoO(6) epitaxial films over a range of several thousand Gauss using strain induced by epitaxial growth on substrates of varying lattice constants. Spectroscopic measurements reveal a striking, linear dependence of the out-of-plane anisotropy on the strain-induced tetragonal distortion of the Sr(2)FeMoO(6) lattice. This anisotropy can be tuned from +2000 to -3300 Oe, a range sufficient to rotate the easy axis from in plane to out of plane. Combined with its half-metallicity and high Curie temperature, this result implies a broad range of scientific and technological applications for this novel spintronic material.

Broadband multi-magnon relaxometry using a quantum spin sensor for high frequency ferromagnetic dynamics sensing.

Development of sensitive local probes of magnon dynamics is essential to further understand the physical processes that govern magnon generation, propagation, scattering, and relaxation. Quantum spin sensors like the NV center in diamond have long spin lifetimes and their relaxation can be used to sense magnetic field noise at gigahertz frequencies. Thus far, NV sensing of ferromagnetic dynamics has been constrained to the case where the NV spin is resonant with a magnon mode in the sample meaning that the NV frequency provides an upper bound to detection. In this work we demonstrate ensemble NV detection of spinwaves generated via a nonlinear instability process where spinwaves of nonzero wavevector are parametrically driven by a high amplitude microwave field. NV relaxation caused by these driven spinwaves can be divided into two regimes; one- and multi-magnon NV relaxometry. In the one-magnon NV relaxometry regime the driven spinwave frequency is below the NV frequencies. The driven spinwave undergoes four-magnon scattering resulting in an increase in the population of magnons which are frequency matched to the NVs. The dipole magnetic fields of the NV-resonant magnons couple to and relax nearby NV spins. The amplitude of the NV relaxation increases with the wavevector of the driven spinwave mode which we are able to vary up to 3 × 106 m-1, well into the part of the spinwave spectrum dominated by the exchange interaction. Increasing the strength of the applied magnetic field brings all spinwave modes to higher frequencies than the NV frequencies. We find that the NVs are relaxed by the driven spinwave instability despite the absence of any individual NV-resonant magnons, suggesting that multiple magnons participate in creating magnetic field noise below the ferromagnetic gap frequency which causes NV spin relaxation.

Magnetic force microscopy of superparamagnetic nanoparticles.

The use of magnetic force microscopy (MFM) to detect probe-sample interactions from superparamagnetic nanoparticles in vitro in ambient atmospheric conditions is reported here. By using both magnetic and nonmagnetic probes in dynamic lift-mode imaging and by controlling the direction and magnitude of the external magnetic field applied to the samples, it is possible to detect and identify the presence of superparamagnetic nanoparticles. The experimental results shown here are in agreement with the estimated sensitivity of the MFM technique. The potential and challenges for localizing nanoscale magnetic domains in biological samples is discussed.

Voltage-driven, local, and efficient excitation of nitrogen-vacancy centers in diamond.

Magnetic sensing technology has found widespread application in a diverse set of industries including transportation, medicine, and resource exploration. These uses often require highly sensitive instruments to measure the extremely small magnetic fields involved, relying on difficult-to-integrate superconducting quantum interference devices and spin-exchange relaxation-free magnetometers. A potential alternative, nitrogen-vacancy (NV) centers in diamond, has shown great potential as a high-sensitivity and high-resolution magnetic sensor capable of operating in an unshielded, room-temperature environment. Transitioning NV center-based sensors into practical devices, however, is impeded by the need for high-power radio frequency (RF) excitation to manipulate them. We report an advance that combines two different physical phenomena to enable a highly efficient excitation of the NV centers: magnetoelastic drive of ferromagnetic resonance and NV-magnon coupling. Our work demonstrates a new pathway that combine acoustics and magnonics that enables highly energy-efficient and local excitation of NV centers without the need for any external RF excitation and, thus, could lead to completely integrated, on-chip, atomic sensors.