Non-magnetic Heavy Metals Research Articles

Spin transport properties of spinel vanadate-based heterostructures

Spin–orbit coupling and breaking of inversion symmetry are necessary ingredients to enable a pure spin current-based manipulation of the magnetization via the spin–orbit torque effect. Currently, magnetic insulator oxides with non-dissipative characteristics are being explored. When combined with non-magnetic heavy metals, known for their large spin–orbit coupling, they offer promising potential for energy-efficient spin-orbitronics applications. The intrinsic electronic correlations characterizing those strongly correlated oxides hold the promises to add extra control-knobs to the desired efficient spin-wave propagation and abrupt magnetization switching phenomena. Spinel vanadate FeV2O4 (FVO) exhibits several structural phase transitions, which are accompanied by an intricate interplay of magnetic, charge, and orbital orderings. When grown as a thin film onto SrTiO3, the compressive strain state induces a perpendicular magnetic anisotropy, making FVO-based heterostructures desirable for spin-orbitronics applications. In this study, we have optimized the deposition of stoichiometric and epitaxial Pt/FVO heterostructures by pulsed laser deposition and examined their spin-related phenomena. From angle-dependent magnetotransport measurements, we observed both anisotropic magnetoresistance and spin Hall magnetoresistance (SMR) effects. Our findings show the SMR component as the primary contributor to the overall magnetoresistance, whose high value of 0.12% is only comparable to properly optimized oxide-based systems.

Easy-plane spin Hall oscillator

Spin Hall oscillators (SHOs) based on bilayers of a ferromagnet (FM) and a non-magnetic heavy metal (HM) are electrically tunable nanoscale microwave signal generators. Achieving high output power in SHOs requires driving large-amplitude magnetization dynamics by a direct spin Hall current. Here we present an SHO engineered to have easy-plane magnetic anisotropy oriented normal to the bilayer plane, enabling large-amplitude easy-plane dynamics driven by spin Hall current. Our experiments and micromagnetic simulations demonstrate that the easy-plane anisotropy can be achieved by tuning the magnetic shape anisotropy and perpendicular magnetic anisotropy in a nanowire SHO, leading to a significant enhancement of the generated microwave power. The easy-plane SHO experimentally demonstrated here is an ideal candidate for realization of a spintronic spiking neuron. Our results provide an approach to design of high-power SHOs for wireless communications, neuromorphic computing, and microwave assisted magnetic recording.

Spin blocking effect at Ni/Pt heterojunction

Spin current, the flow of spin angular momentum, can carry and transport energy and/or information without generating Joule heating, which makes spin-based devices become one of the potential aspects for the next-generation information processing devices. It is important to investigate the generation, transport, and detection of spins for developing spin-based devices, in which the spin transport and its related phenomena attract ongoing interest due to the complex interactions between spins and condensed matter system. Here, spin transport phenomenon is studied at a heterojunction consisting of ferromagnetic metal nickel and nonmagnetic heavy metal platinum, where transport spins are found to be totally blocked. Two series of spin-pumping devices, i.e. the yttrium iron garnet (YIG)/Ni/Pt trilayer devices and the contrastive YIG/Ni bilayer devices, are made in this work. The YIG serves as a substrate and spin-pump layer, on which nickel film and platinum film are deposited by a dc magnetron sputtering system. Spin currents are generated from YIG and injected into nickel layers by spin pumping technology. The voltage signals corresponding to the inverse spin Hall effect are detected and analyzed comparatively for both YIG/Ni/Pt trilayer device and YIG/Ni bilayer device. It is found that the platinum layers in YIG/Ni/Pt trilayer devices act only as charge current shunting but do not contribute to the spin-charge conversion. This implies that the spin current cannot transport through the Ni/Pt interface even when the nickel layer is as thin as 1 nm, in other words, the spin current is blocked at the Ni/Pt interface. Our result proposes a heterojunction that can block transport spins totally, which has never been discussed before, and the present study may expand the views and promote the development of spin-based devices.

Field-driven Modulation of Spin Hall Magnetoresistance in Nanoscale-layered Pt/IrMn₃/NiFe Heterostructures

Spin Hall magnetoresistance (SMR) emerges from both spin Hall effect (SHE) and inverse spin Hall effect (ISHE), which is usually investigated in a nonmagnetic heavy metal (NM) in contact with a ferromagnet (FM). Depending on the orientation of FM magnetization and spin polarization of NM layer, the transmission or reflection of the spin current at the FM\NM interface could occur. SMR was investigated in 1.5 nm NiFe\3.0 nm IrMn₃\3.0 nm Pt magnetic heterostructures. IrMn₃ thin film grown on ferromagnetic NiFe layer exhibits the glassy magnetic behavior and displays spin reorientation transition in the magnetic field-temperature space. Below the Néel temperature of IrMn₃, the angle dependence of MR measurement in the xy and xz plane showed a normal positive SMR. However, a sign change in SMR was observed in the magnetic field applied in the yz plane. The switching from negative to positive SMR occurs at the field above a critical value of 6 kG. Our results provide a potential advantage for the source of spin flow in the signal in multilayer heterostructures and highlight the importance of magnetic structure (Néel vector) in AF, which opens the possible way to manipulate the spin current transportation and AF memory devices.

XMCD and ab initio study of interface-engineered ultrathin Ru/Co/W/Ru films with perpendicular magnetic anisotropy and strong Dzyaloshinskii-Moriya interaction.

Understanding the nature of recently discovered spin-orbital induced phenomena and a definition of a general approach for "ferromagnet/heavy-metal" layered systems to enhance and manipulate spin-orbit coupling, spin-orbit torque, and the Dzyaloshinskii-Moriya interaction (DMI) assisted by atomic-scale interface engineering are essential for developing spintronics and spin-orbitronics. Here, we exploit X-ray magnetic circular dichroism (XMCD) spectroscopy at the L2,3-edges of 5d and 4d non-magnetic heavy metals (W and Ru, respectively) in ultrathin Ru/Co/W/Ru films to determine their induced magnetic moments due to the proximity to the ferromagnetic layer of Co. The deduced orbital and spin magnetic moments agree well with the theoretically predicted values, highlighting the drastic effect of constituting layers on the system's magnetic properties and the strong interfacial DMI in Ru/Co/W/Ru films. As a result, we demonstrate the ability to simultaneously control the strength of magnetic anisotropy and intermixing-enhanced DMI through the interface engineered inversion asymmetry in thin-film chiral ferromagnets, which are a potential host for stable magnetic skyrmions.

Enhancement of Current-Induced Out-of-Plane Spin Polarization by Heavy-Metal-Impurity Doping in Fe Thin Films

Current-induced spin polarization in magnets has recently attracted much attention because out-of-plane spin polarization is allowed, as opposed to polycrystalline nonmagnetic heavy metals. We investigate the $\mathrm{Pt}$-impurity-doping dependence of current-induced out-of-plane spin polarization in $\mathrm{Fe}$ thin films with the use of magnetocircular dichroism. Current-induced out-of-plane spin polarization is clearly observed and attributed to anomalous spin-orbit torque, in which the spin-orbit torque induced by the spin Hall effect in $\mathrm{Fe}$ acts on the magnetic moments of $\mathrm{Fe}$ at the surfaces, generating out-of-plane spin polarization. Notably, current-induced out-of-plane spin polarization is significantly enhanced with increasing $\mathrm{Pt}$ concentration. $\mathrm{Pt}$ impurities with a large spin-orbit interaction induce the extrinsic spin Hall effect of $\mathrm{Fe}$, and consequently, the anomalous spin-orbit torques are enhanced. Our results provide a guiding principle for strengthening current-induced spin polarization in magnets.

Magnetic proximity effect in magnetic-insulator/heavy-metal heterostructures across the compensation temperature

Nonmagnetic heavy metals can develop a magnetic moment due to the proximity of a magnetic layer. Here, the sign of the proximity effect in Pt and W grown on the ferrimagnetic insulator Dy${}_{3}$Fe${}_{5}$O${}_{12}$ (DyIG) is examined. Polarized neutron reflectometry indicates the importance of a clean metal/garnet interface, and shows that the proximity effect in Pt/DyIG is positive above and below the DyIG compensation temperature, indicating a different origin compared to that of Pt adjacent to a ferrimagnetic metal alloy.

Identification of spin-dependent thermoelectric effects in metamagnetic FeRh/heavy-metal bilayers

A vertical flux of heat can bring about hybrid generation of charge and spin currents and eventually convert into the transverse electric voltage in the bilayers composed of metallic magnet and non-magnetic heavy metal (HM). We identified the thermoelectric effects in the sputter-deposited metallic film of CsCl-ordered FeRh/HM throughout its metamagnetic transition from ferromagnetic (FM) to antiferromagnetic (AFM) phase. With the employment of different HMs (Pt, Au) as the spin detective layers, we found that the FM phase allows for hybrid generation of charge and spin currents by heat, respectively, attributed to the anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE), while the AFM phase merely retains the ANE from residual nanoscale FM domains at cryogenic temperatures, which was further confirmed by the control measurement based on the adjustment of spin Hall angle for W during its β to α phase transition. Contribution from the proximity-induced ANE of HM was verified to be negligible compared with that of ANE and SSE of FeRh. Our method opens up more access to quantitatively discern the entangled thermo-charge/spin contributions in metallic magnets, and the combination of thermoelectric effects with metamagnetic phase transition gives impetus to exploiting more versatile and energy-saving thermo-spin logic applications.

Meronlike Spin Textures in In-Plane-Magnetized Thin Films

The interfacing of magnetic materials with nonmagnetic heavy metals with a large spin-orbit coupling, such as $\mathrm{Pt}$, results in an asymmetric exchange interaction at the interface due to the Dzyaloshinskii-Moriya interaction (DMI), which in turn leads to the formation of skyrmions and topological spin structures in perpendicularly magnetized multilayers. Here, we show that out-of-plane spin domains with lateral dimensions from 200 nm to 2 $\ensuremath{\mu}\mathrm{m}$ are stabilized in in-plane magnetized $\mathrm{Ta}$/$\mathrm{Co}$/$\mathrm{Pt}$ trilayers. We show that these spin textures are largely insensitive to the direction of the in-plane magnetization switched by either magnetic fields or electric fields applied across a ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ gate dielectric. The results of micromagnetic simulations indicate that the DMI is required for the stabilization of such out-of-plane domains and that the presence of surface roughness helps to stabilize larger structures, in agreement with experimental results. We identify these spin structures as meronlike topological textures, characterized by a perpendicular spin texture in an uniformly in-plane magnetized system.

Spin-Wave Relaxation by Eddy Currents in Y3Fe5O12/Pt Bilayers and a Way to Suppress It

Because of their record-low intrinsic magnetic damping properties, single-crystal yttrium-iron-garnet (YIG) films serve as an excellent model medium for studying magnon-induced spintronic phenomena such as spin pumping and the spin-orbit torque effect. For this purpose, YIG films are covered with sub-skin-depth layers of nonmagnetic heavy metals with strong spin-orbit coupling. In the present work, we show experimentally and theoretically that ohmic losses of spin-wave-induced microwave eddy currents in the heavy-metal layer deliver a strong contribution to spin-wave damping in these hybrid structures. We demonstrate that this adverse effect can be controlled and largely eliminated by placing a highly conducting metal plate near to the surface of the YIG/$\mathrm{Pt}$ structures. These findings are of value for a proper interpretation of experiments on the magnon spintronic effects and for the design of future magnon spintronic devices.

Observation of carrier concentration dependent spintronic terahertz emission from n-GaN/NiFe heterostructures

The development of terahertz (THz) spintronics has created a paradigm shift in the generation of THz radiation through the combination of ultrafast magnetism and spin-based electronics. However, research in this area has primarily focused on all-metallic devices comprising a ferromagnetic thin film adjacent to a non-magnetic heavy metal. Here, we report the experimental observation of spintronic THz emission from an n-doped wide bandgap semiconductor, n-GaN. We found that the amplitude of THz emission strongly depends on the carrier concentration of the semiconductor layer, which could be attributed to the tunable Rashba state occurring at the n-GaN/ferromagnet interface. Our work offers exciting prospects for pursuing wide bandgap semiconductor-based spintronic THz devices and demonstrating their intriguing spin Hall physics at the ultrafast timescale.

Stack structure and temperature dependence of spin-orbit torques in heterostructures with antiferromagnetic PtMn

We evaluate the stack structure and temperature dependence of the spin–orbit torques (SOTs) in metallic antiferromagnet (AFM)/ferromagnet (FM) PtMn/CoFeB heterostructures using an extended harmonic Hall measurement. We show that PtMn/CoFeB exhibits sizable Slonczewski-like and fieldlike SOTs whose magnitude is comparable to that of heterostructures with nonmagnetic heavy metals like Pt or Ta. We also find that the SOTs in our PtMn/CoFeB structures are virtually constant with temperature. The findings will offer clues to understand the generation mechanism of SOTs in AFM/FM systems and pave the way for nonvolatile memory and neuromorphic computing applications.

Field-Free Switching of Perpendicular Magnetization Through Spin Hall and Anomalous Hall Effects in Ferromagnet–Heavy-Metal–Ferromagnet Structures

Spin-orbit coupling gives rise to the spin Hall effect (SHE) in nonmagnetic heavy metals, and to the anomalous Hall effect (AHE) in ferromagnetic metals. This article proposes a mechanism to switch the perpendicular magnetization in a magnetic trilayer, without an assisting magnetic field, by utilizing the SHE or AHE plus out-of-plane spin polarization. The authors derive the analytic expression for the resulting spin torque, and perform macrospin simulation to validate the field-free switching. The proposed switching mechanism combines well-established spin-valve technology with the spin Hall effect, potentially enhancing switching efficiency in practical devices.

Spin-current driven spontaneous coupling of ferromagnets

A theoretical framework is proposed for the spin-current driven synchronized self-oscillations in ferromagnets in the spin Hall geometry. The spin current generated by the spin Hall effect in a bottom nonmagnetic heavy metal excites a self-oscillation of the magnetization in an attached ferromagnet through spin-transfer effect. The spin current simultaneously creates spin accumulation inside the ferromagnet. Therefore, when the top surfaces of two ferromagnets are connected by a nonmagnetic material having a long spin diffusion length, another spin current flows according to the gradient of the spin accumulations between the ferromagnets. This additional spin current excites an additional spin torque leading to a coupled motion of the magnetizations. This coupling mechanism comes purely from spin degree of freedom in the system without using electric and/or magnetic interactions. The additional spin torque acts as a repulsive force between the magnetizations, and prefers an antiphase synchronization between the oscillators. The phase difference in a synchronized state is determined by the competition between this additional spin torque and spin pumping. Eventually, either an in-phase or antiphase synchronization is spontaneously excited in the individual ferromagnets, depending on the current magnitude. These conclusions are obtained by deriving the theoretical formula of the additional spin torque from the diffusive spin transport theory and solving the equation of motion of the magnetizations both numerically and analytically.

Strong Enhancement of the Spin Hall Effect by Spin Fluctuations near the Curie Point of Fe_{x}Pt_{1-x} Alloys.

Robust spin Hall effects (SHE) have recently been observed in nonmagnetic heavy metal systems with strong spin-orbit interactions. These SHE are either attributed to an intrinsic band-structure effect or to extrinsic spin-dependent scattering from impurities, namely, side jump or skew scattering. Here we report on an extraordinarily strong spin Hall effect, attributable to spin fluctuations, in ferromagnetic Fe_{x}Pt_{1-x} alloys near their Curie point, tunable with x. This results in a dampinglike spin-orbit torque being exerted on an adjacent ferromagnetic layer that is strongly temperature dependent in this transition region, with a peak value that indicates a lower bound 0.34±0.02 for the peak spin Hall ratio within the FePt. We also observe a pronounced peak in the effective spin-mixing conductance of the FM/FePt interface, and determine the spin diffusion length in these Fe_{x}Pt_{1-x} alloys. These results establish new opportunities for fundamental studies of spin dynamics and transport in ferromagnetic systems with strong spin fluctuations, and a new pathway for efficiently generating strong spin currents for applications.

Spin-transfer torque induced by the spin anomalous Hall effect

The anomalous Hall effect in ferromagnets is a well-known phenomenon in which electromotive force is generated in the direction perpendicular to both the electric current and the magnetization. With nonmagnetic heavy metals, the generation of spin current by the spin Hall effect, as well as excitation of magnetization dynamics in an adjacent ferromagnet by spin-transfer torque, has been reported. However, the generation of a spin current in ferromagnets by the anomalous Hall effect has so far not been measured. Here we report the observation of spin current generation by the anomalous Hall effect through spin-transfer torque excitation in CoFeB/Cu/NiFe trilayer films. In this spin anomalous Hall effect, a spin current generated in the CoFeB by the anomalous Hall effect is injected into the NiFe and excites spin-transfer torque in the NiFe. This leads to a measurable modulation in the ferromagnetic resonance linewidth of the NiFe. The spin anomalous Hall angle in the CoFeB is estimated to be −0.14 ± 0.05, which is comparable to the spin Hall angle reported in heavy metals. The generation of a spin current by the anomalous Hall effect is observed by measuring the modulation in the ferromagnetic resonance linewidth of CoFeB/Cu/NiFe trilayer films.

Synchronization of Spin Torque Oscillators through Spin Hall Magnetoresistance

Spin torque oscillators placed onto a nonmagnetic heavy metal show synchronized auto-oscillations due to the coupling originating from spin Hall magnetoresistance effect. Here, we study a system having two spin torque oscillators under the effect of the spin Hall torque, and show that switching the external current direction enables us to control the phase difference of the synchronization between in-phase and antiphase.

Novel Operation of a Multi-Bit SOT Memory Cell Addressed With a Single Write Line

Spin–orbit torque (SOT) originates from the spin–orbit interaction of non-magnetic heavy metals (HMs), allowing for an electrical manipulation of perpendicular magnetization in HM/ferromagnet (FM)/oxide structures. In this paper, we experimentally demonstrate the SOT-induced switching of two FM bits addressed with a single write line. We fabricate a device consisting of two perpendicularly magnetized Ta/CoFeB/MgO structures with a common Ta underlayer, in which the magnetization directions of the two FM bits could be concurrently controlled by injecting a single current pulse. This suggests that multiple bits in SOT-based devices can be written as either “0” or “1” at the same time. Moreover, the selective switching of a specific bit is achieved by differentiating the critical switching currents between the two FM bits, which is crucial in demonstrating multi-level cell SOT memory. Our results provide an efficient writing mechanism, enabling wider applications of SOT-based spintronic devices.

Dynamic coupling of ferromagnets via spin Hall magnetoresistance

The synchronized magnetization dynamics in ferromagnets on a nonmagnetic heavy metal caused by the spin Hall effect is investigated theoretically. The direct and inverse spin Hall effects near the ferromagnetic/nonmagnetic interface generate longitudinal and transverse electric currents. The phenomenon is known as the spin Hall magnetoresistance effect, whose magnitude depends on the magnetization direction in the ferromagnet due to the spin transfer effect. When another ferromagnet is placed onto the same nonmagnet, these currents are again converted to the spin current by the spin Hall effect and excite the spin torque to this additional ferromagnet, resulting in the excitation of the coupled motions of the magnetizations. The in-phase or antiphase synchronization of the magnetization oscillations, depending on the value of the Gilbert damping constant and the field-like torque strength, is found in the transverse geometry by solving the Landau-Lifshitz-Gilbert equation numerically. On the other hand, in addition to these synchronizations, the synchronization having a phase difference of a quarter of a period is also found in the longitudinal geometry. The analytical theory clarifying the relation among the current, frequency, and phase difference is also developed, where it is shown that the phase differences observed in the numerical simulations correspond to that giving the fixed points of the energy supplied by the coupling torque.

Magnetic scattering and spin-orbit coupling induced magnetoresistance in nonmagnetic heavy metal and magnetic insulator bilayer systems

We report on the experimental study of the angular dependent magnetoresistance (MR) of heavy metal/ferromagnetic insulator bilayer structures. Through altering the relative composition in heavy metal $\mathrm{P}{\mathrm{t}}_{\ensuremath{\delta}}\mathrm{T}{\mathrm{a}}_{1--\ensuremath{\delta}}$ alloy, we continuously tune its spin Hall angle from positive, crossing zero, and to negative and study its impact on the MR. Most notably, both spin Hall effect and MR disappear simultaneously in $\mathrm{P}{\mathrm{t}}_{0.32}\mathrm{T}{\mathrm{a}}_{0.68}$ (3 nm)/YIG when the effective spin Hall angle vanishes, evidencing the essential role of spin-orbit coupling in heavy metal for the MR. By introducing Fe impurities, we further identify that magnetic scattering is also essential to induce the MR in Pt/Fe-doped $\mathrm{Si}{\mathrm{O}}_{2}$ at large magnetic field, where the MR ratio increases monotonically with doping level.