Field-like Spin Torque Research Articles

Exchange bias toggling in amine-ended single-molecule magnetic junctions by contact geometry

The molecular scale magnetic proximity effect is proposed in single-molecule magnetic junctions (SMMJs) consisting of a dissociated amine-ended 1,4-benzenediamine (BDA) molecule coupled to two ferromagnetic Co electrodes. Our self-developed JunPy + Landau-Lifshitz-Gilbert simulation combined with first-principles calculation is employed to investigate the role of contact geometry in the magnetotransport properties of SMMJs with the choice of top, bridge, and hollow contact sites. The strong spinterface effect gives rise to distinct angular dependence of equilibrium field-like spin torque (FLST), asymmetric magnetic hysteresis loop and tunable exchange bias. From the analytical derivation of nonequilibrium Keldysh formalism, we believe that a promising way forward is to activate the multi-reflection process via the so-called molecular spinterface that will allow us to conquer as-yet unexplored magnetotransport properties of organic-based spintronics.

Underlying mechanism for exchange bias in single-molecule magnetic junctions

Magnetic proximity has been observed in a variety of solid-state magnetic devices, but has been less discussed at the molecular scale. In this study, the magnetotransport calculation is carried out using the generalized Landau-Lifshitz-Gilbert (LLG) equation combined with density functional theory (DFT) and our self-developed junpy calculated spin-torque effect. Except for the current driven spin torque, which is a promising approach for magnetization switch in magnetic random access memory, the equilibrium fieldlike spin torque also plays a crucial role in the strain-controlled exchange bias with current-controlled magnetic coercivity in single-molecule magnetic junctions. The tight-binding model is further employed to clarify the critical role of the interfacial spin filter effect arising from the hybridization between the linker and Co apex. These multidisciplinary DFT+junpy+LLG results may provide important and practical implications in the dual control of magnetic proximity and magnetization switching in molecular spintronics at low temperature, either by tensile strain or via smaller applied current density of the order of $\mathrm{MA}/{\mathrm{cm}}^{2}$.

Large fieldlike torque in amorphous Ru2Sn3 originated from the intrinsic spin Hall effect

We investigated temperature dependent current driven spin-orbit torques in magnetron sputtered Ru2Sn3 (4 and 10 nm) /Co20Fe60B20 (5 nm) layered structures with in-plane magnetic anisotropy. The room temperature damping-like and field-like spin torque efficiencies of the amorphous Ru2Sn3 films were measured to be 0.14 +- 0.008 (0.07 +- 0.012) and -0.03 +- 0.006 (-0.20 +- 0.009), for the 4 (10 nm) films respectively, by utilizing the second harmonic Hall technique. The large field-like torque in the relatively thicker Ru2Sn3 (10 nm) thin film is unique compared to the traditional spin Hall materials interfaced with thick magnetic layers with in-plane magnetic anisotropy which typically have dominant damping-like and negligible field-like torques. Additionally, the observed room temperature field-like torque efficiency in Ru2Sn3 (10 nm)/CoFeB (5 nm) is up to three times larger than the damping-like torque (-0.20 +- 0.009 and 0.07 +- 0.012, respectively) and thirty times larger at 50 K (-0.29 +- 0.014 and 0.009 +- 0.017, respectively). The temperature dependence of the field-like torques show dominant contributions from the intrinsic spin Hall effect while the damping-like torques show dominate contributions from the extrinsic spin Hall effects, skew scattering and side jump. Through macro-spin calculations, we found that including field-like torques on the order or larger than the damping-like torque can reduce the switching critical current and decrease magnetization procession for a perpendicular ferromagnetic layer.

Interface-induced field-like optical spin torque in a ferromagnet/heavy metal heterostructure

Abstract The manipulation of magnetization in a metallic ferromagnet by using optical helicity has been much attracted attention for future opto-spintronic devices. The optical helicity–induced torques on the magnetization, optical spin torque, have been observed in ferromagnetic thin films recently. However, the interfacial effect of the optical spin torque in ferromagnet/nonmagnetic heavy metal heterostructures have not been addressed so far, which are widely utilized to efficiently control magnetization via electrical means. Here, we studied optical spin torque vectors in the ferromagnet/nonmagnetic heavy metal heterostructures and observed that in-plane field-like optical spin torque was significantly increased with decreasing ferromagnetic layer thicknesses. The interfacial field-like optical spin torque was explained by the optical Rashba–Edelstein effect caused by the structural inversion symmetry breaking. This work will aid in the efficient optical manipulation of thin film nanomagnets using optical helicity.

From Fieldlike Torque to Antidamping Torque in Antiferromagnetic Mn2Au

Efficient electrical switching of antiferromagnets (AFMs) is key to their use in high-density, ultrafast, nonvolatile spintronic memory. Mn${}_{2}$Au, an AFM with opposite spin sublattices, is a unique metallic material, in that fieldlike spin torque can switch its AFM moments. However, switching induced by antidamping torque remains to be verified in metallic AFMs. Here the authors demonstrate current-induced switching of AFM moment in both a (103)-oriented Mn${}_{2}$Au single layer and a Mn${}_{2}$Au/Pt heterojunction by fieldlike torque and antidamping torque respectively. The simultaneous realization of both torque types in metallic Mn2Au makes it a promising candidate for AFM spintronics.

Manipulation of Giant Field-Like Spin Torque in Amine-Ended Single-Molecule Magnetic Junctions

We propose a comprehensive theoretical investigation for the angular, external bias and mechanical strain dependence of the giant field-like spin torque (FLST) effect for amine-ended Co/BDA/Co single-molecule magnetic junctions in the non-collinear magnetic configuration, by employing our newly developed nonequilibrium Green’s function method within the framework of density functional theory. In sharp contrast to the conventional Co/vacuum/Co magnetic tunnel junction, the hard–hard coupling between the amine linker and the Co tip atom plays an active role in the interfacial spin filter to emerge the strong interlayer exchange coupling between two Co electrodes and the anomalous strain-enhanced FLST effect. These intriguing features may pave a novel way to engineer the magnetization switching via either external bias or mechanical strain to achieve the writing process in next-generation multifunctional organic FLST-MRAMs with lower power consumption.

Analytic expression for the giant fieldlike spin torque in spin-filter magnetic tunnel junctions

We propose analytic expressions for fieldlike, ${T}_{\ensuremath{\perp}}$, and spin-transfer, ${T}_{\ensuremath{\parallel}}$, spin torque components in the spin-filter-based magnetic tunnel junction (SFMTJ), by using the single-band tight-binding model with the nonequilibrium Keldysh formalism. In consideration of multireflection processes between noncollinear magnetization of the spin-filter (SF) barrier and the ferromagnetic (FM) electrode, the central spin-selective SF barrier plays an active role in the striking discovery ${T}_{\ensuremath{\perp}}\ensuremath{\gg}{T}_{\ensuremath{\parallel}}$, which can be further identified by the unusual barrier thickness dependence of giant ${T}_{\ensuremath{\perp}}$. Our general expressions reveal the sinusoidal angular dependence of both spin torque components, even in the presence of the SF barrier.

Observation of thermally driven field-like spin torque in magnetic tunnel junctions

We report the thermally driven giant field-like spin-torque in magnetic tunnel junctions (MTJ) on application of heat current from top to bottom. The field-like term is detected by the shift of the magneto-resistance hysteresis loop applying temperature gradient. We observed that the field-like term depends on the magnetic symmetry of the MTJ. In asymmetric structures, with different ferromagnetic materials for free and fixed layers, the field-like term is greatly enhanced. Our results show that a pure spin current density of the order of 109 A/m2 can be produced by creating a 120 mK temperature difference across 0.9 nm thick MgO tunnelling barrier. Our results will be useful for writing MTJ and domain wall-based memories using thermally driven spin torque.

Dual Control of Giant Field-like Spin Torque in Spin Filter Tunnel Junctions.

We predict a giant field-like spin torque, , in spin-filter (SF) barrier tunnel junctions in sharp contrast to existing junctions based on nonmagnetic passive barriers. We demonstrate that has linear bias behavior, is independent of the SF thickness, and has odd parity with respect to the SF’s exchange splitting. Thus, it can be selectively controlled via external bias or external magnetic field which gives rise to sign reversal of via magnetic field switching. The underlying mechanism is the interlayer exchange coupling between the noncollinear magnetizations of the SF and free ferromagnetic electrode via the nonmagnetic insulating (I) spacer giving rise to giant spin-dependent reflection at the SF/I interface. These findings suggest that the proposed field-like-spin-torque MRAM may provide promising dual functionalities for both ‘reading’ and ‘writing’ processes which require lower critical current densities and faster writing and reading speeds.

Oscillation characteristics of zero-field spin transfer oscillators with field-like torque

We theoretically investigate the influence of the field-like spin torque term on the oscillation characteristics of spin transfer oscillators, which are based on MgO magnetic tunnel junctions (MTJs) consisting of a perpendicular magnetized free layer and an in-plane magnetized pinned layer. It is demonstrated that the field-like torque has a strong impact on the steady-state precession current region and the oscillation frequency. In particular, the steady-state precession can occur at zero applied magnetic field when the ratio between the field-like torque and the spin transfer torque takes up a negative value. In addition, the dependence of the oscillation properties on the junction sizes has also been analyzed. The results indicate that this compact structure of spin transfer oscillator without the applied magnetic field is practicable under certain conditions, and it may be a promising configuration for the new generation of on-chip oscillators.

Experimental demonstration of the coexistence of spin Hall and Rashba effects inβ−tantalum/ferromagnetbilayers

We have measured the spin torques of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{tantalum}/\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ bilayers versus Ta thickness at room temperature using a ferromagnetic resonance (FMR) technique. A significant fieldlike spin torque originating from Ta was identified, which is constant and independent of Ta thickness. Because of this constant torque, the spin Hall coefficient ${\ensuremath{\theta}}_{\mathrm{SH}}$ needs to be calculated from the ratio of the symmetric component of the FMR signal to the slope of the antisymmetric component with Ta thickness, from which a value of $\ensuremath{-}0.11\ifmmode\pm\else\textpm\fi{}0.01$ was determined. The saturation magnetization of the CoFeB layers for samples deposited with Ta was found to be smaller than that of a single CoFeB layer, with values of $1.84\ifmmode\pm\else\textpm\fi{}0.01$ and $1.92\ifmmode\pm\else\textpm\fi{}0.01$ T, respectively. The origin of the fieldlike torque is ascribed to an interface spin-orbit coupling, or Rashba effect, due to the strength and constancy of the torque with Ta thickness. From fitting measured data to a semiclassical diffusion model that includes interface spin-orbit coupling, we have determined the spin diffusion length for \ensuremath{\beta}-tantalum to be 2.5 nm.

Thermoelectric spin accumulation and long-time spin precession in a noncollinear quantum dot spin valve

We explore thermoelectric spin transport and spin dependent phenomena in a non-collinear quantum dot spin valve set up. Using this set up, we demonstrate the possibility of a thermoelectric excitation of single spin dynamics inside the quantum dot. Many-body exchange fields generated on the single spins in this set up manifest as effective magnetic fields acting on the net spin accumulation in the quantum dot. We first identify generic conditions by which a zero bias spin accumulation in the dot may be created in the thermoelectric regime. The resulting spin accumulation is then shown to be subject to a field-like spin torque due to the effective magnetic field associated with either contact. This spin torque that is generated may yield long-time precession effects due to the prevailing blockade conditions. The implications of these phenomena in connection with single spin manipulation and pure spin current generation are then discussed.

Stability analysis of perpendicular magnetic trilayers with a field-like spin torque

We have analytically studied the magnetization dynamics in magnetic trilayers with perpendicular anisotropy for both free and pinned layers. By linear stability analysis, we obtain the phase diagram parameterized by the current, magnetic field and relative strength of the field-like spin torque to Slonczewski torque. Under the control of the current and external magnetic field, several magnetic states, such as quasi-parallel and quasi-antiparallel stable states, out-of-plane precession, and bistable states can be realized. The precession frequency can be expressed as a function of the current and external magnetic field. In addition, the presence of field-like spin torque can change the switching current and precession frequency.

Effect of the field-like spin torque on the switching current and switching speed of magnetic tunnel junction with perpendicularly magnetized free layers

We have studied the effect of the fieldlike spin torque term bJ, present in magnetic tunneling junctions (MTJs) with perpendicular magnetic anisotropy in the free layer, on switching characteristics of the devices. We find that bJ has a strong impact on the switching current density and switching speed. The theoretical limit of the switching current density can be significantly reduced and the theoretical limit of the switching time will be lowered compare to the spin-valve (SV) devices. These results strongly suggest that the spin transfer torque random access memory based on the MTJs with perpendicularly magnetized free layer will likely have the multiple benefits of much larger intrinsic signal, smaller switching current, and faster switching speed than the SV devices.

Micromagnetic study of fieldlike spin torque effect on the magnetization switching in tunnel junctions with perpendicular anisotropy

Micromagnetic simulation is employed to study the effect of fieldlike spin torque in a perpendicularly magnetized tunnel junction. We find that the effect of fieldlike spin torque strongly depends on the current direction, showing the torque significantly reduces the switching time for the electrons flowing from free layer to pinned layer while has less effect for the opposite current. This asymmetric behavior is consistent with the theoretical prediction on the bias dependence of the fieldlike spin torque. The observed absence of preswitching and postswitching oscillations can be attributed to the opposite precession phase of local magnetic moments within the free layer.

Field-like spin torque in magnetic tunnel junctions

We show that the exchange splitting asymmetry between the left and right ferromagnetic leads in non-collinear magnetic tunnel junctions (MTJ) tunes the bias behavior of the field-like spin torque, T⊥. These results can be understood by our recently derived general expression, which relates the non-collinear T⊥ to the algebraic sum of four independent non-equilibrium interlayer exchange couplings (IEC) solely in collinear configurations.

Controlling the Nonequilibrium Interlayer Exchange Coupling in Asymmetric Magnetic Tunnel Junctions

We predict an oscillatory bias behavior of the fieldlike spin torque, T(perpendicular), in magnetic tunnel junctions, which can be selectively controlled via the asymmetry in band filling between the ferromagnetic leads. This can lead to a linear or quadratic low-bias behavior, including tuning the bias-induced reversal of T(perpendicular). These findings reconcile the apparently contradictory experimental results recently reported in the literature. The underlying mechanism for the nonequilibrium interlayer exchange coupling (IEC) of noncollinear configurations is the interplay of four independent IEC for the majority- and minority-spin bands of the leads solely in the ferromagnetic configuration.