Demonstration of Nanosecond Operation in Stochastic Magnetic Tunnel Junctions.

Magnetic tunnel junctions operating in the superparamagnetic regime are promising devices in the field of probabilistic computing, which is suitable for applications like high-dimensional optimization or sampling problems. Further, random number generation is of interest in the field of cryptography. For such applications, a device's uncorrelated fluctuation time-scale can determine the effective system speed. It has been theoretically proposed that a magnetic tunnel junction designed to have only easy-plane anisotropy provides fluctuation rates determined by its easy-plane anisotropy field and can perform on a nanosecond or faster time-scale as measured by its magnetoresistance's autocorrelation in time. Here, we provide experimental evidence of nanosecond scale fluctuations in a circular-shaped easy-plane magnetic tunnel junction, consistent with finite-temperature coupled macrospin simulation results and prior theoretical expectations. We further assess the degree of stochasticity of such a signal.

Planar Hall Driven Torque in a Ferromagnet/Nonmagnet/Ferromagnet System.

An important goal of spintronics is to covert a charge current into a spin current with a controlled spin polarization that can exert torques on an adjacent magnetic layer. Here we demonstrate such torques in a two ferromagnet system. A CoNi multilayer is used as a spin current source in a sample with structure CoNi/Au/CoFeB. Spin torque ferromagnetic resonance is used to measure the torque on the CoFeB layer. The response as a function of the applied field angle and current is consistent with the symmetry expected for a torque produced by the planar Hall effect originating in CoNi. We find the strength of this effect to be comparable to that of the spin Hall effect in platinum, indicating that the planar Hall effect holds potential as a spin current source with a controllable polarization direction.

Spin-orbit torque driven by a planar Hall current.

Spin-orbit torques (SOTs) in multilayers of ferromagnetic (FM) and non-magnetic (NM) metals can manipulate the magnetization of the FM layer efficiently. This is employed, for example, in non-volatile magnetic memories for energy-efficient mobile electronics1,2 and spin torque nano-oscillators3-7 for neuromorphic computing8. Recently, spin torque nano-oscillators also found use in microwave-assisted magnetic recording, which enables ultrahigh-capacity hard disk drives9. Most SOT devices employ spin Hall10,11 and Rashba12 effects, which originate from spin-orbit coupling within the NM layer and at the FM/NM interfaces, respectively. Recently, SOTs generated by the anomalous Hall effect in FM/NM/FM multilayers were predicted13 and experimentally realized14. The control of SOTs through crystal symmetry was demonstrated as well15. Understanding all the types of SOTs that can arise in magnetic multilayers is needed for a formulation of a comprehensive SOT theory and for engineering practical SOT devices. Here we show that a spin-polarized electric current known to give rise to anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) in a FM16 can additionally generate large antidamping SOTs with an unusual angular symmetry in NM1/FM/NM2 multilayers. This effect can be described by a recently proposed magnonic mechanism17. Our measurements reveal that this torque can be large in multilayers in which both spin Hall and Rashba torques are negligible. Furthermore, we demonstrate the operation of a spin torque nano-oscillator driven by this SOT. These findings significantly expand the class of materials that exhibit giant SOTs.