arXiv subjects

R. A. Buhrman

Publications and source records attributed to R. A. Buhrman.

At least 37 records · Page 2Linked to original sources

arXiv2016

On the origin of field-like spin-orbit torques in heavy metal-ferromagnet-oxide thin film heterostructures

We report measurements of the thickness and temperature (T) dependencies of current-induced spin-orbit torques, especially the field-like (FL) component, in various heavy metal (HM)/normal metal (NM) spacer/ferromagnet (FM)/Oxide (MgO and HfOx/MgO) heterostructures. The FL torque in these samples originates from spin current generated by the spin Hall effect (SHE) in the HM. For a FM layer sufficiently thin that a substantial portion of this spin current can reach the FM/Oxide interface, T-dependent spin scattering there can yield a strong FL torque that is, in some cases, opposite in sign to that exerted at the NM/FM interface.

cond-mat.mtrl-sci↗

arXiv2016

Control of spin-orbit torques through crystal symmetry in WTe2/ferromagnet bilayers

Recent discoveries regarding current-induced spin-orbit torques produced by heavy-metal/ferromagnet and topological-insulator/ferromagnet bilayers provide the potential for dramatically-improved efficiency in the manipulation of magnetic devices. However, in experiments performed to date, spin-orbit torques have an important limitation -- the component of torque that can compensate magnetic damping is required by symmetry to lie within the device plane. This means that spin-orbit torques can drive the most current-efficient type of magnetic reversal (antidamping switching) only for magnetic devices with in-plane anisotropy, not the devices with perpendicular magnetic anisotropy that are needed for high-density applications. Here we show experimentally that this state of affairs is not fundamental, but rather one can change the allowed symmetries of spin-orbit torques in spin-source/ferromagnet bilayer devices by using a spin source material with low crystalline symmetry. We use WTe2, a transition-metal dichalcogenide whose surface crystal structure has only one mirror plane and no two-fold rotational invariance. Consistent with these symmetries, we generate an out-of-plane antidamping torque when current is applied along a low-symmetry axis of WTe2/Permalloy bilayers, but not when current is applied along a high-symmetry axis. Controlling S-O torques by crystal symmetries in multilayer samples provides a new strategy for optimizing future magnetic technologies.

cond-mat.mes-hall↗

arXiv2016

Strong Spin Hall Effect in the Antiferromagnet PtMn

Effectively manipulating magnetism in ferromagnet (FM) thin film nanostructures with an in-plane current has become feasible since the determination of a 'giant' spin Hall effect (SHE) in certain heavy metal (HM)/FM system. Recently, both theoretical and experimental reports indicate that the non-collinear and collinear metallic antiferromagnet (AF) materials can have both a large anomalous Hall effect (AHE) and a strong SHE. Here we report a systematic study of the SHE in PtMn with several PtMn/FM systems. By using interface engineering to reduce the 'spin memory loss' we obtain a spin torque efficiency as large as 0.24. This is more than twice the previously reported spin torque efficiency for PtMn. We also find that the apparent spin diffusion length in PtMn is surprisingly long (about 2.3nm).

cond-mat.mtrl-sci↗

arXiv2015

Spin torque study of the spin Hall conductivity and spin diffusion length in platinum thin films with varying resistivity

We report measurements of the spin torque efficiencies in perpendicularly-magnetized Pt/Co bilayers where the Pt resistivity $ρ_{Pt}$ is strongly dependent on thickness $t_{Pt}$ . The damping-like spin Hall torque efficiency per unit current density, $ξ^j_{DL}$ , varies significantly with $t_{Pt}$, exhibiting a peak value $ξ^j_{DL}=0.12$ at $t_{Pt} = 2.8 - 3.9$ nm. In contrast, $ξ^j_{DL}/ρ_{Pt}$ increases monotonically with $t_{Pt}$ and saturates for $t_{Pt} > 5$ nm, consistent with an intrinsic spin Hall effect mechanism, in which $ξ^j_{DL}$ is enhanced by an increase in $ρ_{Pt}$ . Assuming the Elliott-Yafet spin scattering mechanism dominates we estimate that the spin diffusion length $λ_s = (0.77 \pm 0.08) \times 10^{-15} Ωm^2 /ρ_{Pt}$.

cond-mat.mes-hall↗

arXiv2015

GPU-accelerated micromagnetic simulations using cloud computing

Highly-parallel graphics processing units (GPUs) can improve the speed of micromagnetic simulations significantly as compared to conventional computing using central processing units (CPUs). We present a strategy for performing GPU-accelerated micromagnetic simulations by utilizing cost-effective GPU access offered by cloud computing services with an open-source Python-based program for running the MuMax3 micromagnetics code remotely. We analyze the scaling and cost benefits of using cloud computing for micromagnetics.

physics.comp-ph↗

arXiv2014

Dependence of the Efficiency of Spin Hall Torque on the Transparency of Pt-Ferromagnetic Layer Interfaces

We report that spin current transport across Pt-ferromagnet (FM) interfaces is strongly dependent on the type and the thickness of the FM layer and on post-deposition processing protocols. By employing both harmonic voltage measurements and spin-torque ferromagnetic resonance measurements, we find that the efficiency of the Pt spin Hall effect in exerting a damping-like spin torque on the FM ranges from < 0.05 to > 0.10 under different interfacial conditions. We also show that the temperature dependence of the spin torque efficiencies for both the damping-like torque and field-like torque is dependent upon the details of the Pt-FM interface. The "internal" spin Hall angle of the Pt thin films used in this study, after taking the interfacial spin transmission factor into account, is estimated to be ~ 0.20. This suggests that a careful engineering of Pt-FM interfaces can improve the spin-Hall-torque efficiency of Pt-based spintronic devices.

cond-mat.mes-hall↗

arXiv2014

Enhancement of Perpendicular Magnetic Anisotropy and Transmission of Spin-Hall-Effect-Induced Spin Currents by a Hf Spacer Layer in W/Hf/CoFeB/MgO Layer

We report that strong perpendicular magnetic anisotropy of the ferromagnetic layer in a W/CoFeB/MgO multilayer structure can be established by inserting a Hf layer as thin as 0.25 nm between the W and CoFeB layers. The Hf spacer also allows transmission of spin currents generated by an in-plane charge current in the W layer to apply strong spin torque on the CoFeB, thereby enabling current-driven magnetic switching. The antidamping-like and field-like components of the spin torque exerted on a 1 nm CoFeB layer are of comparable magnitudes in this geometry. Both components originate from the spin Hall effect in the underlying W layer.

cond-mat.mtrl-sci↗

arXiv2013

Central role of domain wall depinning for perpendicular magnetization switching driven by spin torque from the spin Hall effect

We study deterministic magnetic reversal of a perpendicularly magnetized Co layer in a Co/MgO/Ta nano-square driven by spin Hall torque from an in-plane current flowing in an underlying Pt layer. The rate-limiting step of the switching process is domain-wall (DW) depinning by spin Hall torque via a thermally-assisted mechanism that eventually produces full reversal by domain expansion. An in-plane applied magnetic field collinear with the current is required, with the necessary field scale set by the need to overcome DW chirality imposed by the Dzyaloshinskii-Moriya interaction. Once Joule heating is taken into account the switching current density is quantitatively consistent with a spin Hall angle θ$_{SH}$ ${\approx}$ 0.07 for 4 nm of Pt.

cond-mat.mes-hall↗

arXiv2013

A quasi-linear spin-torque nano-oscillator via enhanced negative feedback of power fluctuations

We report an approach to improving the performance of spin torque nano-oscillators (STNOs) that utilizes power-dependent negative feedback to achieve a significantly enhanced dynamic damping. In combination with a sufficiently slow variation of frequency with power this can result in a quasi-linear STNO, with very weak non-linear coupling of power and phase fluctuations over a range of bias current and field. An implementation of this approach that utilizes a non-uniform spin-torque demonstrates that highly coherent room temperature STNOs can be achieved while retaining a significant tunability.

cond-mat.mes-hall↗

arXiv2012

Gate voltage modulation of spin-Hall-torque-driven magnetic switching

Two promising strategies for achieving efficient control of magnetization in future magnetic memory and non-volatile spin logic devices are spin transfer torque from spin polarized currents and voltage-controlled magnetic anisotropy (VCMA). Spin transfer torque is in widespread development as the write mechanism for next-generation magnetic memory, while VCMA offers the potential of even better energy performance due to smaller Ohmic losses. Here we introduce a 3-terminal magnetic tunnel junction (MTJ) device that combines both of these mechanisms to achieve new functionality: gate-voltage-modulated spin torque switching. This gating makes possible both more energy-efficient switching and also improved architectures for memory and logic applications, including a simple approach for making magnetic memories with a maximum-density cross-point geometry that does not require a control transistor for every MTJ.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic oscillations driven by the spin Hall effect in 3-terminal magnetic tunnel junction devices

We show that direct current in a tantalum microstrip can induce steady-state magnetic oscillations in an adjacent nanomagnet through spin torque from the spin Hall effect (SHE). The oscillations are detected electrically via a magnetic tunnel junction (MTJ) contacting the nanomagnet. The oscillation frequency can be controlled using the MTJ bias to tune the magnetic anisotropy. In this 3-terminal device the SHE torque and the MTJ bias therefore provide independent controls of the oscillation amplitude and frequency, enabling new approaches for developing tunable spin torque nano-oscillators.

cond-mat.mtrl-sci↗

arXiv2012

Spin transfer torque devices utilizing the giant spin Hall effect of tungsten

We report a giant spin Hall effect (SHE) in β-W thin films. Using spin torque induced ferromagnetic resonance with a β-W/CoFeB bilayer microstrip we determine the spin Hall angle to be |θ|=0.30\pm0.02, large enough for an in-plane current to efficiently reverse the orientation of an in-plane magnetized CoFeB free layer of a nanoscale magnetic tunnel junction adjacent to a thin β-W layer. From switching data obtained with such 3-terminal devices we independently determine |θ|=0.33\pm0.06. We also report variation of the spin Hall switching efficiency with W layers of different resistivities and hence of variable (α and β) phase composition.

cond-mat.mtrl-sci↗

arXiv2012

Network analyzer measurements of spin transfer torques in magnetic tunnel junctions

We demonstrate a simple network-analyzer technique to make quantitative measurements of the bias dependence of spin torque in a magnetic tunnel junction. We apply a microwave current to exert an oscillating spin torque near the ferromagnetic resonance frequency of the tunnel junction's free layer. This produces an oscillating resistance that, together with an applied direct current, generates a microwave signal that we measure with the network analyzer. An analysis of the resonant response yields the strength and direction of the spin torque at non-zero bias. We compare to measurements of the spin torque vector by time-domain spin-torque ferromagnetic resonance.

cond-mat.mes-hall↗

arXiv2012

Resonance Measurement of Nonlocal Spin Torque in a Three-Terminal Magnetic Device

A pure spin current generated within a nonlocal spin valve can exert a spin transfer torque on a nanomagnet. This nonlocal torque enables new design schemes for magnetic memory devices that do not require the application of large voltages across tunnel barriers that can suffer electrical breakdown. Here we report a quantitative measurement of this nonlocal spin torque using spin-torque-driven ferromagnetic resonance. Our measurement agrees well with the prediction of an effective circuit model for spin transport. Based on this model, we suggest strategies for optimizing the strength of nonlocal torque.

cond-mat.mes-hall↗

arXiv2012

Spin torque switching with the giant spin Hall effect of tantalum

We report a giant spin Hall effect (SHE) in β-Ta that generates spin currents intense enough to induce efficient spin-transfer-torque switching of ferromagnets, thereby providing a new approach for controlling magnetic devices that can be superior to existing technologies. We quantify this SHE by three independent methods and demonstrate spin-torque (ST) switching of both out-of-plane and in-plane magnetized layers. We implement a three-terminal device that utilizes current passing through a low impedance Ta-ferromagnet bilayer to effect switching of a nanomagnet, with a higher-impedance magnetic tunnel junction for read-out. The efficiency and reliability of this device, together with its simplicity of fabrication, suggest that this three-terminal SHE-ST design can eliminate the main obstacles currently impeding the development of magnetic memory and non-volatile spin logic technologies.

cond-mat.mtrl-sci↗

arXiv2012

Review and Analysis of Measurements of the Spin Hall Effect in Platinum

Several different experimental techniques have been used in efforts to measure the spin Hall conductivity and the spin Hall angle in Pt samples at room temperature, with results that disagree by more than a factor of 20, with spin Hall conductivities from 2.4 x 10^4 to 5.1 x 10^5 [hbar/(2e)] (Ohm-m)^-1 and spin Hall angles from 0.0037 to 0.08. We review this work, and analyze possible reasons for the discrepancies. We explain that the smallest values for the spin Hall angle that have been reported, based on measurements of lateral permalloy/copper/platinum devices, are incorrect because the original analyses did not properly take into account that copper layers in these devices will shunt charge current flowing through adjacent platinum wires, thereby greatly reducing the size of the spin-Hall-related signals. We suggest that differences between the results for the spin Hall angle found by other experimental techniques are primarily a consequence of different assumptions about the value of the spin diffusion length in Pt. We present a new measurement of the spin diffusion length in Pt within sputtered Pt/permalloy bilayer thin films at room temperature, finding 1.4 \pm 0.3 nm, a much smaller value than has generally been assumed previously. With this value for the spin diffusion length, the previously-discordant results can be brought into much better agreement, with the result that the spin Hall conductivities are (1.4 - 3.4) x 10^5 [hbar/(2e)] (Ohm-m)^-1 and the spin Hall angles are greater than 0.05. These values are sufficiently large that the spin Hall effect in Pt can be used to generate spin currents and spin transfer torques strong enough for efficient manipulation of magnetic moments in adjacent ferromagnetic layers.

cond-mat.mes-hall↗

arXiv2011

Magnetic switching by spin torque from the spin Hall effect

The spin Hall effect (SHE) generates spin currents within nonmagnetic materials. Previously, studies of the SHE have been motivated primarily to understand its fundamental origin and magnitude. Here we demonstrate, using measurement and modeling, that in a Pt/Co bilayer with perpendicular magnetic anisotropy the SHE can produce a spin transfer torque that is strong enough to efficiently rotate and reversibly switch the Co magnetization, thereby providing a new strategy both to understand the SHE and to manipulate magnets. We suggest that the SHE torque can have a similarly strong influence on current-driven magnetic domain wall motion in Pt/ferromagnet multilayers. We estimate that in optimized devices the SHE torque can switch magnetic moments using currents comparable to those in magnetic tunnel junctions operated by conventional spin-torque switching, meaning that the SHE can enable magnetic memory and logic devices with similar performance but simpler architecture than the current state of the art.

cond-mat.mtrl-sci↗

arXiv2010

Spin Torque Ferromagnetic Resonance Induced by the Spin Hall Effect

We demonstrate that the spin Hall effect in a thin film with strong spin-orbit scattering can excite magnetic precession in an adjacent ferromagnetic film. The flow of alternating current through a Pt/NiFe bilayer generates an oscillating transverse spin current in the Pt, and the resultant transfer of spin angular momentum to the NiFe induces ferromagnetic resonance (FMR) dynamics. The Oersted field from the current also generates an FMR signal but with a different symmetry. The ratio of these two signals allows a quantitative determination of the spin current and the spin Hall angle.

cond-mat.mes-hall↗