arXiv subjects

Robert A. Buhrman

Publications and source records attributed to Robert A. Buhrman.

At least 19 recordsLinked to original sources

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↗

arXiv2021

Interfacial Dzyaloshinskii-Moriya interaction and spin-orbit torque in Au1-xPtx/Co bilayers with varying interfacial spin-orbit coupling

The quantitative roles of the interfacial spin-orbit coupling (SOC) in Dzyaloshinskii-Moriya interaction (DMI) and dampinglike spin-orbit torque (τDL) have remained unsettled after a decade of intensive study. Here, we report a conclusive experiment evidence that, because of the critical role of the interfacial orbital hybridization, the interfacial DMI is not necessarily a linear function of the interfacial SOC, e.g. at Au1-xPtx/Co interfaces where the interfacial SOC can be tuned significantly via strongly composition (x)-dependent spin-orbit proximity effect without varying the bulk SOC and the electronegativity of the Au1-xPtx layer. We also find that τDL in the Au1-xPtx/Co bilayers varies distinctly from the interfacial SOC as a function of x, indicating no important τDL contribution from the interfacial Rashba-Edelstein effect.

cond-mat.mtrl-sci↗

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

Absence of significant spin current generation in Ti/FeCoB bilayers with strong interfacial spin-orbit coupling

After one decade of the intensive theoretical and experimental explorations, whether interfacial spin-orbit coupling (ISOC) at metallic magnetic interfaces can effectively generate a spin current has remained in dispute. Here, utilizing the Ti/FeCoB bilayers that are unique for the negligible bulk spin Hall effect and the strong tunable ISOC, we establish that there is no significant charge-to-spin conversion at magnetic interfaces via spin-orbit filtering effect or Rashba-Edelstein(-like) effect even when the ISOC is stronger than that of a typical Pt/ferromagnet interface and can promote strong perpendicular magnetic anisotropy. We also find a minimal orbital Hall effect in 3d Ti.

cond-mat.mes-hall↗

arXiv2020

Observation of strong bulk damping-like spin-orbit torque in chemically disordered ferromagnetic single layers

Strong damping-like spin-orbit torque (τDL) has great potential for enabling ultrafast energy-efficient magnetic memories, oscillators, and logic. So far, the reported τDL exerted on a thin-film magnet must result from an externally generated spin current or from an internal non-equilibrium spin polarization in noncentrosymmetric GaMnAs single crystals. Here, we for the first time demonstrate a very strong, unexpected τDL from current flow within ferromagnetic single layers of chemically disordered, face-centered-cubic CoPt. We establish that the novel τDL is a bulk effect, with the strength per unit current density increasing monotonically with the CoPt thickness, and is insensitive to the presence or absence of spin sinks at the CoPt surfaces. This τDL most likely arises from a net transverse spin polarization associated with a strong spin Hall effect (SHE), while there is no detectable long-range asymmetry in the material. These results broaden the scope of spin-orbitronics and provide a novel avenue for developing single-layer-based spin-torque memory, oscillator, and logic technologies.

cond-mat.mtrl-sci↗

arXiv2020

Transverse and Longitudinal Spin-Torque Ferromagnetic Resonance for Improved Measurements of Spin-Orbit Torques

Spin-torque ferromagnetic resonance (ST-FMR) is a common method used to measure spin-orbit torques (SOTs) in heavy metal/ferromagnet bilayer structures. In the course of a measurement, other resonant processes such as spin pumping (SP) and heating can cause spin current or heat flows between the layers, inducing additional resonant voltage signals via the inverse spin Hall effect (ISHE) and Nernst effects (NE). In the standard ST-FMR geometry, these extra artifacts exhibit a dependence on the angle of an in-plane magnetic field that is identical to the rectification signal from the SOTs. We show experimentally that the rectification and artifact voltages can be quantified separately by measuring the ST-FMR signal transverse to the applied current (i.e., in a Hall geometry) in addition to the usual longitudinal geometry. We find that in Pt (6 nm)/CoFeB samples the contribution from the artifacts is small compared to the SOT rectification signal for CoFeB layers thinner than 6 nm, but can be significant for thicker magnetic layers. We observe a sign change in the artifact voltage as a function of CoFeB thickness that we suggest may be due to a competition between a resonant heating effect and the SP/ISHE contribution.

cond-mat.mes-hall↗

arXiv2019

Nanosecond Reversal of Three-Terminal Spin Hall Effect Memories Sustained at Cryogenic Temperatures

We characterize the nanosecond pulse switching performance of the three-terminal magnetic tunnel junctions (MTJs), driven by the spin Hall effect (SHE) in the channel, at a cryogenic temperature of 3 K. The SHE-MTJ devices exhibit reasonable magnetic switching and reliable current switching by as short pulses as 1 ns of $<10^{12}$ A/m$^{2}$ magnitude, exceeding the expectation from conventional macrospin model. The pulse switching bit error rates reach below $10^{-6}$ for < 10 ns pulses. Similar performance is achieved with exponentially decaying pulses expected to be delivered to the SHE-MTJ device by a nanocryotron device in parallel configuration of a realistic memory cell structure. These results suggest the viability of the SHE-MTJ structure as a cryogenic memory element for exascale superconducting computing systems.

physics.app-ph↗

arXiv2019

Effective spin-mixing conductance of heavy-metal-ferromagnet interfaces

The effective spin-mixing conductance (G_eff) of a heavy metal/ferromagnet (HM/FM) interface characterizes the efficiency of the interfacial spin transport.Accurately determining G_eff is critical to the quantitative understanding of measurements of direct and inverse spin Hall effects. G_eff is typically ascertained from the inverse dependence of magnetic damping on the FM thickness under the assumption that spin pumping is the dominant mechanism affecting this dependence.Here we report that, this assumption fails badly in many in-plane magnetized prototypical HM/FM systems in the nm-scale thickness regime. Instead, the majority of the damping is from two-magnon scattering at the FM interface, while spin-memory-loss scattering at the interface can also be significant.If these two effects are neglected, the results will be an unphysical "giant" apparent G_eff and hence considerable underestimation of both the spin Hall ratio and the spin Hall conductivity in inverse/direct spin Hall experiments.

cond-mat.mtrl-sci↗

arXiv2019

Cryogenic Memory Architecture Integrating Spin Hall Effect based Magnetic Memory and Superconductive Cryotron Devices

One of the most challenging obstacles to realizing exascale computing is minimizing the energy consumption of L2 cache, main memory, and interconnects to that memory. For promising cryogenic computing schemes utilizing Josephson junction superconducting logic, this obstacle is exacerbated by the cryogenic system requirements that expose the technology's lack of high-density, high-speed and power-efficient memory. Here we demonstrate an array of cryogenic memory cells consisting of a non-volatile three-terminal magnetic tunnel junction element driven by the spin Hall effect, combined with a superconducting heater-cryotron bit-select element. The write energy of these memory elements is roughly 8 pJ with a bit-select element, designed to achieve a minimum overhead power consumption of about 30%. Individual magnetic memory cells measured at 4 K show reliable switching with write error rates below $10^{-6}$, and a 4x4 array can be fully addressed with bit select error rates of $10^{-6}$. This demonstration is a first step towards a full cryogenic memory architecture targeting energy and performance specifications appropriate for applications in superconducting high performance and quantum computing control systems, which require significant memory resources operating at 4 K.

physics.app-ph↗

arXiv2019

Intrinsic nature of the giant spin Hall conductivity of Pt

More than a decade after the first theoretical and experimental studies of the spin Hall conductivity (SHC) of Pt, both its dominant origin and amplitude remain in dispute. Resolving these questions is of fundamental importance for advancing understanding of very strong spin-orbit effects in conducting systems and for maximizing the spin Hall effect for energy-efficient spintronics applications. Here, we report the experimental determination of the rapid variation of the intrinsic SHC of Pt with the carrier lifetime (τ) in the dirty-metal regime by incorporating finely dispersed MgO inter-site impurities into the Pt while maintaining the essential elements of its band structure (face-centered-cubic order). This findings conclusively validate the theoretical prediction that the SHC in Pt in the dirty-metal regime should be dominated by the intrinsic Berry curvature contribution and should decrease rapidly with shortening τ. This also establishes the limit to which the spin Hall ratio θSH of pure Pt can be increased by shortening τ. When the spin backflow at the Pt/ferromagnet interface due to the finite interfacial spin-mixing conductance is taken into account, the amplitude of the intrinsic SHC of Pt in the clean limit is found to be at least 1.6x10^6 (h_bar/2e) Ω-1 m-1, more than 3.5 times greater than the available theoretical predictions. Our work also establishes a compelling spin Hall metal Pt0.6(MgO)0.4 that combines a giant θSH (0.73) with a moderate resistivity (74 μΩ cm), a strong Dzyaloshinskii-Moriya interaction, easy growth, and good integration compatibility for spintronics technology.

cond-mat.mtrl-sci↗

arXiv2019

Layer-dependent spin-orbit torques generated by the centrosymmetric transition metal dichalcogenide $β$-MoTe$_2$

Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in WTe$_2$/ferromagnet heterostructures. Here, we present measurements of spin-orbit torques produced by the low-symmetry material $β$-MoTe$_2$, which unlike WTe$_2$ retains bulk inversion symmetry. We measure spin-orbit torques on $β$-MoTe$_2$/Permalloy heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and MoTe$_2$ thickness down to the monolayer limit. We observe an out-of-plane antidamping torque with a spin torque conductivity as strong as 1/3 of that of WTe$_2$, demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the $β$-MoTe$_2$ -- the out-of-plane antidamping torque is present in MoTe$_2$/Permalloy heterostructures when the $β$-MoTe$_2$ is a monolayer or trilayer thick, but goes to zero for devices with bilayer $β$-MoTe$_2$.

cond-mat.mes-hall↗

arXiv2019

Enhancement of spin transparency by interfacial alloying

We report that atomic-layer alloying (intermixing) at a Pt/Co interface can increase, by approximately 30%, rather than degrade the interfacial spin transparency, and thereby strengthen the efficiency of the dampinglike spin-orbit torque arising from the spin Hall effect in the Pt. At the same time, this interfacial alloying substantially reduces fieldlike spin-orbit torque. Insertion of an ultrathin magnetic alloy layer at heavy-metal/ferromagnet interfaces represents an effective approach for improving interfacial spin transparency that may enhance not only spin-orbit torques but also the spin detection efficiency in inverse spin Hall experiments.

cond-mat.mtrl-sci↗

arXiv2019

Strong damping-like spin-orbit torque and tunable Dzyaloshinskii-Moriya interaction generated by low-resistivity Pd$_{1-x}$Pt$_x$ alloys

Despite their great promise for providing a pathway for very efficient and fast manipulation of magnetization at the nanoscale, spin-orbit torque (SOT) operations are currently energy inefficient due to a low damping-like SOT efficiency per unit current bias, and/or the very high resistivity of the spin Hall materials. Here, we report an advantageous spin Hall material, Pd1-xPtx, which combines a low resistivity with a giant spin Hall effect as evidenced through the use of three independent SOT ferromagnetic detectors. The optimal Pd0.25Pt0.75 alloy has a giant internal spin Hall ratio of >0.47 (damping-like SOT efficiency of ~ 0.26 for all three ferromagnets) and a low resistivity of ~57.5 μΩ cm at 4 nm thickness. Moreover, we find the Dzyaloshinskii-Moriya interaction (DMI), the key ingredient for the manipulation of chiral spin arrangements (e.g. magnetic skyrmions and chiral domain walls), is considerably strong at the Pd1-xPtx/Fe0.6Co0.2B0.2 interface when compared to that at Ta/Fe0.6Co0.2B0.2 or W/Fe0.6Co0.2B0.2 interfaces and can be tuned by a factor of 5 through control of the interfacial spin-orbital coupling via the heavy metal composition. This work establishes a very effective spin current generator that combines a notably high energy efficiency with a very strong and tunable DMI for advanced chiral spintronics and spin torque applications.

cond-mat.mtrl-sci↗

arXiv2019

Current-Induced Torques with Dresselhaus Symmetry Due to Resistance Anisotropy in 2D Materials

We report measurements of current-induced torques in heterostructures of Permalloy (Py) with TaTe$_2$, a transition-metal dichalcogenide (TMD) material possessing low crystal symmetry, and observe a torque component with Dresselhaus symmetry. We suggest that the dominant mechanism for this Dresselhaus component is not a spin-orbit torque, but rather the Oersted field arising from a component of current that flows perpendicular to the applied voltage due to resistance anisotropy within the TaTe$_2$. This type of transverse current is not present in wires made from a single uniform layer of a material with resistance anisotropy, but will result whenever a material with resistance anisotropy is integrated into a heterostructure with materials having different resistivities, thereby producing a spatially non-uniform pattern of current flow. This effect will therefore influence measurements in a wide variety of heterostructures incorporating 2D TMD materials and other materials with low crystal symmetries.

cond-mat.mes-hall↗

arXiv2018

Spin-orbit torques acting upon a perpendicularly-magnetized Py layer

We show that Py, a commonly-used soft ferromagnetic material with weak anisotropy, can become perpendicularly-magnetized while depositing on Ta buffer layer with Hf or Zr insertion layers (ILs) and MgO capping layer. By using two different approaches, namely harmonic voltage measurement and hysteresis loop shift measurement, the dampinglike spin-orbit torque (DL-SOT) efficiencies from Ta/IL/Py/IL/MgO magnetic heterostructures with perpendicular magnetic anisotropy are characterized. We find that though Ta has a significant spin Hall effect, the DL-SOT efficiencies are small in systems with the Ta/Py interface compared to that obtained from the control sample with the traditional Ta/CoFeB interface. Our results indicate that the spin transparency for the Ta/Py interface is much less than that for the Ta/CoFeB interface, which might be related to the variation of spin mixing conductance for different interfaces.

cond-mat.mtrl-sci↗

arXiv2018

Highly efficient spin current generation by the spin Hall effect in Au$_{1-x}$Pt$_x$

We report very efficient spin current generation by the spin Hall effect in the alloy Au0.25Pt0.75, which, as determined by two different direct spin-orbit torque measurements, exhibits a giant internal spin Hall ratio of > 0.58 (anti-damping spin-orbit torque efficiency of ~ 0.35 in bilayers with Co), a relatively low resistivity of ~ 83 uOhm cm, an exceptionally large spin Hall conductivity of > 7.0x10^5 ohm^-1 m^-1, and a spin diffusion length of 1.7 nm. This work establishes Au0.25Pt0.75 as a milestone spin current generator that provides greater energy efficiency than that yet obtained with other heavy metals or with the topological insulators Bi2Se3 and (Bi,Se)2Te3. Our findings should advance spin-orbit torque-based fundamental research and benefit the development of new fast, efficient spin-orbit torque-driven magnetic memories, skyrmion and chiral domain wall devices, and microwave and terahertz emitters.

cond-mat.mes-hall↗

arXiv2017

Reorientable Spin Direction for Spin Current Produced by the Anomalous Hall Effect

We show experimentally that the spin direction of the spin current generated by spin-orbit interactions within a ferromagnetic layer can be reoriented by turning the magnetization direction of this layer. We do this by measuring the field-like component of spin-orbit torque generated by an exchange-biased FeGd thin film and acting on a nearby CoFeB layer. The relative angle of the CoFeB and FeGd magnetic moments is varied by applying an external magnetic field. We find that the resulting torque is in good agreement with predictions that the spin current generated by the anomalous Hall effect from the FeGd layer depends on the FeGd magnetization direction $\hat{m}_{FeGd}$ according to $\vecσ\propto\left ( \hat{y}\cdot \hat{m}_{FeGd} \right )\hat{m}_{FeGd}$, where $\hat{y}$ is the in-plane direction perpendicular to the applied charge current. Because of this angular dependence, the spin-orbit torque arising from the anomalous Hall effect can be non-zero in a sample geometry for which the spin Hall torque generated by non-magnetic materials is identically zero.

cond-mat.mtrl-sci↗

arXiv2017

Thickness dependence of spin-orbit torques generated by WTe2

We study current-induced torques in WTe2/permalloy bilayers as a function of WTe2 thickness. We measure the torques using both second-harmonic Hall and spin-torque ferromagnetic resonance measurements for samples with WTe2 thicknesses that span from 16 nm down to a single monolayer. We confirm the existence of an out-of-plane antidamping torque, and show directly that the sign of this torque component is reversed across a monolayer step in the WTe2. The magnitude of the out-of-plane antidamping torque depends only weakly on WTe2 thickness, such that even a single-monolayer WTe2 device provides a strong torque that is comparable to much thicker samples. In contrast, the out-of-plane field-like torque has a significant dependence on the WTe2 thickness. We demonstrate that this field-like component originates predominantly from the Oersted field, thereby correcting a previous inference drawn by our group based on a more limited set of samples.

cond-mat.mes-hall↗