arXiv subjects

Daniel C. Ralph

Publications and source records attributed to Daniel C. Ralph.

At least 37 records · Page 2Linked to original sources

arXiv2023

Exchange bias between van der Waals materials: tilted magnetic states and field-free spin-orbit-torque switching

Magnetic van der Waals heterostructures provide a unique platform to study magnetism and spintronics device concepts in the two-dimensional limit. Here, we report studies of exchange bias from the van der Waals antiferromagnet CrSBr acting on the van der Waals ferromagnet Fe3GeTe2 (FGT). The orientation of the exchange bias is along the in-plane easy axis of CrSBr, perpendicular to the out-of-plane anisotropy of the FGT, inducing a strongly tilted magnetic configuration in the FGT. Furthermore, the in-plane exchange bias provides sufficient symmetry breaking to allow deterministic spin-orbit torque switching of the FGT in CrSBr/FGT/Pt samples at zero applied magnetic field. A minimum thickness of the CrSBr greater than 10 nm is needed to provide a non-zero exchange bias at 30 K.

cond-mat.mes-hall↗

arXiv2023

Manipulating chiral-spin transport with ferroelectric polarization

A collective excitation of the spin structure in a magnetic insulator can transmit spin-angular momentum with negligible dissipation. This quantum of a spin wave, introduced more than nine decades ago, has always been manipulated through magnetic dipoles, (i.e., timereversal symmetry). Here, we report the experimental observation of chiral-spin transport in multiferroic BiFeO3, where the spin transport is controlled by reversing the ferroelectric polarization (i.e., spatial inversion symmetry). The ferroelectrically controlled magnons produce an unprecedented ratio of up to 18% rectification at room temperature. The spin torque that the magnons in BiFeO3 carry can be used to efficiently switch the magnetization of adja-cent magnets, with a spin-torque efficiency being comparable to the spin Hall effect in heavy metals. Utilizing such a controllable magnon generation and transmission in BiFeO3, an alloxide, energy-scalable logic is demonstrated composed of spin-orbit injection, detection, and magnetoelectric control. This observation opens a new chapter of multiferroic magnons and paves an alternative pathway towards low-dissipation nanoelectronics.

physics.app-ph↗

arXiv2022

Thermally-generated spin current in the topological insulator Bi$_2$Se$_3$

We complete measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents in the topological insulator Bi$_2$Se$_3$ by quantifying the efficiency with which thermal gradients can generate transverse spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnesistance for bilayers of Bi$_2$Se$_3$/CoFeB. We find that Bi$_2$Se$_3$ does generate substantial thermally-driven spin currents. A lower bound for the ratio of spin current to thermal gradient is $J_s/\nabla_x T$ = (4.9 $\pm$ 0.9) $\times$ 10$^{6}$ ($\hbar/2e$) A m$^{-2}$ / K $μ$m$^{-1}$, and a lower bound for the magnitude of the spin Nernst ratio is $-$0.61 $\pm$ 0.11. The spin Nernst ratio for Bi$_2$Se$_3$ is the largest among all materials measured to date, 2-3 times larger compared to previous measurements for the heavy metals Pt and W.

cond-mat.mes-hall↗

arXiv2022

Anisotropic Gigahertz Antiferromagnetic Resonances of the Easy-Axis van der Waals Antiferromagnet CrSBr

We report measurements of antiferromagnetic resonances in the van der Waals easy-axis antiferromagnet CrSBr. The interlayer exchange field and magnetocrystalline anisotropy fields are comparable to laboratory magnetic fields, allowing a rich variety of gigahertz-frequency dynamical modes to be accessed. By mapping the resonance frequencies as a function of the magnitude and angle of applied magnetic field we identify the different regimes of antiferromagnetic dynamics. The spectra show good agreement with a Landau-Lifshitz model for two antiferromagnetically-coupled sublattices, accounting for inter-layer exchange and triaxial magnetic anisotropy. Fits allow us to quantify the parameters governing the magnetic dynamics: at 5 K, the interlayer exchange field is $μ_0 H_E =$ 0.395(2) T, and the hard and intermediate-axis anisotropy parameters are $μ_0 H_c =$ 1.30(2) T and $μ_0 H_a =$ 0.383(7) T. The existence of within-plane anisotropy makes it possible to control the degree of hybridization between the antiferromagnetic resonances using an in-plane magnetic field.

cond-mat.mtrl-sci↗

arXiv2022

Gate-tunable anomalous Hall effect in a 3D topological insulator/2D magnet van der Waals heterostructure

We demonstrate advantages of samples made by mechanical stacking of exfoliated van der Waals materials for controlling the topological surface state of a 3-dimensional topological insulator (TI) via interaction with an adjacent magnet layer. We assemble bilayers with pristine interfaces using exfoliated flakes of the TI BiSbTeSe2 and the magnet Cr2Ge2Te6, thereby avoiding problems caused by interdiffusion that can affect interfaces made by top-down deposition methods. The samples exhibit an anomalous Hall effect (AHE) with abrupt hysteretic switching. For the first time in samples composed of a TI and a separate ferromagnetic layer, we demonstrate that the amplitude of the AHE can be tuned via gate voltage with a strong peak near the Dirac point. This is the signature expected for the AHE due to Berry curvature associated with an exchange gap induced by interaction between the topological surface state and an out-of-plane-oriented magnet.

cond-mat.mes-hall↗

arXiv2022

A puzzling insensitivity of magnon spin diffusion to the presence of 180$^\circ$ domain walls in a ferrimagnetic insulator

We present room-temperature measurements of magnon spin diffusion in epitaxial ferrimagnetic insulator MgAl$_{0.5}$Fe$_{1.5}$O$_{4}$ (MAFO) thin films near zero applied magnetic field where the sample forms a multi-domain state. Due to a weak uniaxial magnetic anisotropy, the domains are separated primarily by 180$^\circ$ domain walls. We find, surprisingly, that the presence of the domain walls has very little effect on the spin diffusion -- nonlocal spin transport signals in the multi-domain state retain at least 95% of the maximum signal strength measured for the spatially-uniform magnetic state, over distances at least five times the typical domain size. This result is in conflict with simple models of interactions between magnons and static domain walls, which predict that the spin polarization carried by the magnons reverses upon passage through a 180$^\circ$ domain wall.

cond-mat.mtrl-sci↗

arXiv2022

Giant bulk spin-orbit torque and efficient electrical switching in single ferrimagnetic FeTb layers with strong perpendicular magnetic anisotropy

Efficient manipulation of antiferromagnetically coupled materials that are integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is of great interest for low-power, fast, dense magnetic storage and computing. Here, we report a distinct, giant bulk damping-like spin-orbit torque in strong-PMA ferrimagnetic Fe100-xTbx single layers that are integration-friendly (composition-uniform, amorphous, sputter-deposited). For sufficiently-thick layers, this bulk torque is constant in the efficiency per unit layer thickness, ξ_DL^j/t, with a record-high value of 0.036nm-1, and the dampinglike torque efficiency ξ_DL^j achieves very large values for thick layers, up to 300% for 90 nm layers. This giant bulk torque by itself switches tens of nm thick Fe100-xTbx layers that have very strong PMA and high coercivity at current densities as low as a few MA/cm2. Surprisingly, for a given layer thickness, ξ_DL^j shows strong composition dependence and becomes negative for composition where the total angular momentum is oriented parallel to the magnetization rather than antiparallel. Our findings of giant bulk spin torque efficiency and intriguing torque-compensation correlation will stimulate study of such unique spin-orbit phenomena in a variety of ferrimagnetic hosts. This work paves a promising avenue for developing ultralow-power, fast, dense ferrimagnetic storage and computing devices.

cond-mat.mtrl-sci↗

arXiv2022

Origins of transverse voltages generated by applied thermal gradients and applied electric fields in ferrimagnetic-insulator/heavy-metal bilayers

We compare thermal-gradient-driven transverse voltages in ferrimagnetic-insulator/heavy-metal bilayers (Tm3Fe5O12/W and Tm3Fe5O12/Pt) to corresponding electrically-driven transverse resistances at and above room temperature. We find for Tm3Fe5O12/W that the thermal and electrical effects can be explained by a common spin-current detection mechanism, the physics underlying spin Hall magnetoresistance (SMR). However, for Tm3Fe5O12/Pt the ratio of the electrically-driven transverse voltages (planar Hall signal/anomalous Hall signal) is much larger than the ratio of corresponding thermal-gradient signals, a result which is very different from expectations for a SMR-based mechanism alone. We ascribe this difference to a proximity-induced magnetic layer at the Tm3Fe5O12/Pt interface.

cond-mat.mes-hall↗

arXiv2022

Strong, Temperature-Dependent Spin-Orbit Torques in Heavy Fermion YbAl$_3$

The use of current-generated spin-orbit torques[1] to drive magnetization dynamics is under investigation to enable a new generation of non-volatile, low-power magnetic memory. Previous research has focused on spin-orbit torques generated by heavy metals[2-8], interfaces with strong Rashba interactions[9,10] and topological insulators [11-14]. These families of materials can all be well-described using models with noninteracting-electron bandstructures. Here, we show that electronic interactions within a strongly correlated heavy fermion material, the Kondo lattice system YbAl$_{3}$, can provide a large enhancement in spin-orbit torque. The spin-torque conductivity increases by approximately a factor of 4 as a function of decreasing temperature from room temperature to the coherence temperature of YbAl$_{3}$ ($T^* \approx 37$ K), with a saturation at lower temperatures, achieving a maximum value greater than any heavy metal element. This temperature dependence mimics the increase and saturation at $T^*$ of the density of states at the Fermi level arising from the ytterbium 4$f$-derived heavy bands in the Kondo regime, as measured by angle-resolved photoemission spectroscopy[15]. We therefore identify the many-body Kondo resonance as the source of the large enhancement of spin-orbit torque in YbAl$_{3}$. Our observation reveals new opportunities in spin-orbit torque manipulation of magnetic memories by engineering quantum many-body states.

cond-mat.mes-hall↗

arXiv2021

Separation of Artifacts from Spin-Torque Ferromagnetic Resonance Measurements of Spin-Orbit Torque for the Low-Symmetry van der Waals Semi-Metal ZrTe$_\textbf{3}$

We measure spin-orbit torque generated by exfoliated layers of the low-symmetry semi-metal ZrTe$_3$ using the spin-torque ferromagnetic resonance (ST-FMR) technique. When the ZrTe$_3$ has a thickness greater than about 10 nm, artifacts due to spin pumping and/or resonant heating can cause the standard ST-FMR analysis to overestimate the true magnitude of the torque efficiency by as much as a factor of 30, and to indicate incorrectly that the spin-orbit torque depends strongly on the ZrTe$_3$ layer thickness. Artifact-free measurements can still be achieved over a substantial thickness range by the method developed recently to detect ST-FMR signals in the Hall geometry as well as the longitudinal geometry. ZrTe$_3$/Permalloy samples generate a conventional in-plane anti-damping spin torque efficiency $ξ_{||}^{\text{DL}}$ = 0.014 $\pm$ 0.004, and an unconventional in-plane field-like torque efficiency $|ξ_{||}^{\text{FL}}|$ = 0.003 $\pm$ 0.001. The out-of-plane anti-damping torque is negligible. We suggest that artifacts similarly interfere with the standard ST-FMR analysis for other van der Waals samples thicker than about 10 nm.

cond-mat.mtrl-sci↗

arXiv2021

Sagnac interferometry for high-sensitivity optical measurements of spin-orbit torque

We adapt Sagnac interferometry for magneto-optic Kerr effect measurements of spin-orbit-torque-induced magnetic tilting in thin-film magnetic samples. The high sensitivity of Sagnac interferometry permits for the first time optical quantification of spin-orbit torque from small-angle magnetic tilting of samples with perpendicular magnetic anisotropy (PMA). We find significant disagreement between Sagnac measurements and simultaneously-performed harmonic Hall (HH) measurements of spin-orbit torque on Pt/Co/MgO and Pd/Co/MgO samples with PMA. The Sagnac results for PMA samples are consistent with both HH and Sagnac measurements for the in-plane geometry, so we conclude that the conventional analysis framework for PMA HH measurements is flawed. We suggest that the explanation for this discrepancy is that although magnetic-field induced magnetic tilting in PMA samples can produce a strong planar Hall effect, when tilting is instead generated by spin-orbit torque it produces negligible change in the planar Hall signal. This very surprising result demonstrates an error in the most-popular method for measuring spin-orbit torques in PMA samples, and represents an unsolved puzzle in understanding the planar Hall effect in magnetic thin films.

cond-mat.mes-hall↗

arXiv2021

Tilted spin current generated by the collinear antiferromagnet RuO2

We report measurements demonstrating that when the Neel vector of the collinear antiferromagnet RuO2 is appropriately canted relative to the sample plane, the antiferromagnet generates a substantial out of plane damping-like torque. The measurements are in good accord with predictions that when an electric field, E is applied to the spin split band structure of RuO2 it can cause a strong transverse spin current even in the absence of spin-orbit coupling. This produces characteristic changes in all three components of the E induced torque vector as a function of the angle of E relative to the crystal axes, corresponding to a spin current with a well defined tilted spin orientation s approximately (but not exactly) parallel to the Neel vector, flowing perpendicular to both E and S. This angular dependence is the signature of an antiferromagnetic spin Hall effect with symmetries that are distinct from other mechanisms of spin-current generation reported in antiferromagnetic or ferromagnetic materials.

cond-mat.mes-hall↗

arXiv2021

Maximizing Spin-Orbit Torque Generated by the Spin Hall Effect of Pt

Efficient generation of spin-orbit torques (SOTs) is central for the exciting field of spin-orbitronics. Platinum, the archetypal spin Hall material, has the potential to be an outstanding provider for spin-orbit torques due to its giant spin Hall conductivity, low resistivity, high stabilities, and the ability to be compatible with CMOS circuits. However, pure clean-limit Pt with low resistivity still provides a low damping-like spin-orbit torque efficiency, which limits its practical applications. The efficiency of spin-orbit torque in Pt-based magnetic heterostructures can be improved considerably by increasing the spin Hall ratio of Pt and spin transmissivity of the interfaces. Here we reviews recent advances in understanding the physics of spin current generation, interfacial spin transport, and the metrology of spin-orbit torques, and summarize progress towards the goal of Pt-based spin-orbit torque memories and logic that are fast, efficient, reliable, scalable, and non-volatile.

cond-mat.mes-hall↗

arXiv2021

Resonant Measurement of Non-Reorientable Spin-Orbit Torque from a Ferromagnetic Source Layer Accounting for Dynamic Spin Pumping

Using a multilayer structure containing (cobalt detector layer)/(copper spacer)/(Permalloy source layer), we show experimentally how the non-reorientable spin-orbit torque generated by the Permalloy source layer (the component of spin-orbit torque that does not change when the Permalloy magnetization is rotated) can be measured using spin-torque ferromagnetic resonance (ST-FMR) with lineshape analysis. We find that dynamic spin pumping between the magnetic layers exerts torques on the magnetic layers as large or larger than the spin-orbit torques, so that if dynamic spin pumping is neglected the result would be a large overestimate of the spin-orbit torque. Nevertheless, the two effects can be separated by performing ST-FMR as a function of frequency. We measure a non-reorientable spin torque ratio $ξ_{\text{Py}} = 0.04 \pm 0.01$ for spin current flow from Permalloy through an 8 nm Cu spacer to the Co, and a strength of dynamic spin pumping that is consistent with previous measurements by conventional ferromagnetic resonance.

cond-mat.mes-hall↗

arXiv2021

Unveiling the mechanism of bulk spin-orbit torques within chemically disordered Fe$_x$Pt$_{1-x}$ single layers

Recent discovery of spin-orbit torques (SOTs) within magnetic single-layers has attracted attention in the field of spintronics. However, it has remained elusive as to how to understand and how to tune the SOTs. Here, utilizing the single layers of chemically disordered Fe$_x$Pt$_{1-x}$, we unveil the mechanism of the "unexpected" bulk SOTs by studying their dependence on the introduction of a controlled vertical composition gradient and on temperature. We find that the bulk damping like SOT arises from an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation. The torque can be strong only in the presence of a vertical composition gradient and the SOT efficiency per electric field is insensitive to temperature but changes sign upon reversal of the orientation of the composition gradient, which are in analogue to behaviors of the strain. From these characteristics we conclude that the imbalanced internal spin current originates from a bulk spin Hall effect and that the associated inversion asymmetry that allows for a non-zero net torque is most likely a strain non-uniformity induced by the composition gradient. The fieldlike SOT is a relatively small bulk effect compared to the dampinglike SOT. This work points to the possibility of developing low-power single-layer SOT devices by strain engineering.

cond-mat.mes-hall↗

arXiv2021

Anisotropic Magnon Spin Transport in Ultra-thin Spinel Ferrite Thin Films -- Evidence for Anisotropy in Exchange Stiffness

We report measurements of magnon spin transport in a spinel ferrite, magnesium aluminum ferrite $\mathrm{MgAl_{0.5}Fe_{1.5}O_4}$ (MAFO), which has a substantial in-plane four-fold magnetic anisotropy. We observe spin diffusion lengths $> 0.8$ $\mathrm{μm}$ at room temperature in 6 nm films, with spin diffusion length 30% longer along the easy axes compared to the hard axes. The sign of this difference is opposite to the effects just of anisotropy in the magnetic energy for a uniform magnetic state. We suggest instead that accounting for anisotropy in exchange stiffness is necessary to explain these results.

cond-mat.mtrl-sci↗

arXiv2021

Resolving Discrepancies in Spin-Torque Ferromagnetic Resonance Measurements: Lineshape vs. Linewidth Analyses

When spin-orbit torques are measured using spin-torque ferromagnetic resonance (ST-FMR), two alternative ways of analyzing the results to extract the torque efficiencies -- lineshape analysis and analysis of the change in linewidth versus DC current -- often give inconsistent results. We identify a source for these inconsistencies. We show that fits of ST-FMR data to the standard analysis framework leave significant residuals that we identify as due to (i) current-induced excitations of a small volume of magnetic material with magnetic damping much larger than the bulk of the magnetic layer, that we speculate is associated with the heavy-metal/magnet interface and (ii) oscillations of the sample magnetization at the modulation frequency due to heating. The dependence of the residual signals on DC current can interfere with an accurate extraction of spin-torque efficiencies by the linewidth method. We show that the discrepancies between the two types of analysis can be largely eliminated by extrapolating the window of magnetic fields used in the linewidth fits to small values so as to minimize the influence of the residual signals.

cond-mat.mes-hall↗

arXiv2020

Direct Visualization of Trimerized States in 1T'-TaTe$_{2}$

Transition-metal dichalcogenides containing tellurium anions show remarkable charge-lattice modulated structures and prominent interlayer character. Using cryogenic scanning transmission electron microscopy (STEM), we map the atomic-scale structures of the high temperature (HT) and low temperature (LT) modulated phases in 1T'-TaTe$_{2}$. At HT, we directly show in-plane metal distortions which form trimerized clusters and staggered, three-layer stacking. In the LT phase at 93 K, we visualize an additional trimerization of Ta sites and subtle distortions of Te sites by extracting structural information from contrast modulations in plan-view STEM data. Coupled with density functional theory calculations and image simulations, this approach opens the door for atomic-scale visualizations of low temperature phase transitions and complex displacements in a variety of layered systems.

cond-mat.mtrl-sci↗