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Register a new userResilience of the Spin-Orbit Torque against Geometrical Backscattering
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We show in this paper that the technologically relevant field-like spin-orbit torque shows resilience against the geometrical effect of electron backscattering. As device grows smaller in sizes, the effect of geometry on physical properties like spin torque, and hence switching current could place a physical limit on the continued shrinkage of such device -- a necessary trend of all memory devices (MRAM). The geometrical effect of curves has been shown to impact quantum transport and topological transition of Dirac and topological systems. In our work, we have ruled out the potential threat of line-curves degrading the effectiveness of spin-orbit torque switching. In other words, spin-orbit torque switching will be resilient against the influence of curves that line the circumferences of defects in the events of electron backscattering, which commonly happen in the channel of modern electronic devices.
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Precession and relaxation predominantly characterize the real-time dynamics of a spin driven by a magnetic field and coupled to a large Fermi sea of conduction electrons. We demonstrate an anomalous precession with frequency higher than the Larmor frequency or with inverted orientation in the limit where the electronic motion adiabatically follows the spin dynamics. For a classical spin, the effect is traced back to a geometrical torque resulting from a finite spin Berry curvature.
Spin-transfer torque magnetoresistive random access memory (STT-MRAM) is an attractive alternative to current random access memory technologies due to its non-volatility, fast operation and high endurance. STT-MRAM does though have limitations including the stochastic nature of the STT-switching and a high critical switching current, which makes it unsuitable for ultrafast operation at nanosecond and sub-nanosecond regimes. Spin-orbit torque (SOT) switching, which relies on the torque generated by an in-plane current, has the potential to overcome these limitations. However, SOT-MRAM cells studied so far use a three-terminal structure in order to apply the in-plane current, which increases the size of the cells. Here we report a two-terminal SOT-MRAM cell based on a CoFeB/MgO magnetic tunnel junction pillar on an ultrathin and narrow Ta underlayer. In this device, an in-plane and out-of-plane current are simultaneously generated upon application of a voltage, and we demonstrate that the switching mechanism is dominated by SOT. We also compare our device to a STT-MRAM cell built with the same architecture and show that critical write current in the SOT-MRAM cell is reduced by more than 70%.
We study the generation of propagating spin waves in Ta/CoFeB waveguides by spin-orbit torque antennas and compare them to conventional inductive antennas. The spin-orbit torque was generated by a transverse microwave current across the magnetic waveguide. The detected spin wave signals for an in-plane magnetization across the waveguide (Damon-Eshbach configuration) exhibited the expected phase rotation and amplitude decay upon propagation when the current spreading was taken into account. Wavevectors up to about 6 rad/$mu$m could be excited by the spin-orbit torque antennas despite the current spreading, presumably due to the non-uniformity of the microwave current. The relative magnitude of generated anti-damping spin-Hall and Oersted fields was calculated within an analytic model and it was found that they contribute approximately equally to the total effective field generated by the spin-orbit torque antenna. Due to the ellipticity of the precession in the ultrathin waveguide and the different orientation of the anti-damping spin-Hall and Oersted fields, the torque was however still dominated by the Oersted field. The prospects for obtaining a pure spin-orbit torque response are discussed, as are the energy efficiency and the scaling properties of spin-orbit torque antennas.
Current-induced torques in ultrathin Co/Pt bilayers were investigated using an electrically driven FMR technique. The angle dependence of the resonances, detected by a rectification effect as a voltage, were analysed to determine the symmetries and relative magnitudes of the spin-orbit torques. Both anti-damping (Slonczewski) and field-like torques were observed. As the ferromagnet thickness was reduced from 3 to 1 nm, the sign of the field-like torque reversed. This observation is consistent with the emergence of a Rashba spin orbit torque in ultra-thin bilayers.
Using type-x spin-orbit torque (SOT) switching scheme, in which the easy axis (EA) of the ferromagnetic (FM) layer and the charge current flow direction are collinear, is possible to realize a lower-power-consumption, higher-density, and better-performance SOT magnetoresistive random access memory (SOT-MRAM) as compared to the conventional type-y design. Here, we systematically investigate type-x SOT switching properties by both macrospin and micromagnetic simulations. The out-of-plane external field and anisotropy field dependence of the switching current density ($J_{sw}$) is first examined in the ideal type-x configuration. Next, we study the FM layer canting angle ($phi_{EA}$) dependence of $J_{sw}$ through macrospin simulations and experiments, which show a transformation of switching dynamics from type-x to type-y with increasing $phi_{EA}$. By further integrating field-like torque (FLT) into the simulated system, we find that a positive FLT can assist type-x SOT switching while a negative one brings about complex dynamics. More crucially, with the existence of a sizable FLT, type-x switching mode results in a lower critical switching current than type-y at current pulse width less than ~ 10 ns, indicating the advantage of employing type-x design for ultrafast switching using materials systems with FLT. Our work provides a thorough examination of type-x SOT scheme with various device/materials parameters, which can be informative for designing next-generation SOT-MRAM.
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