Spin Current Generation Using Amorphous Materials

The efficient generation of spin currents is critical to numerous low-energy electronic devices, including the attojoule logic gate and spin orbit torque magnetoresistive random access memory (SOT-MRAM). Efforts to identify materials that act as good spin current sources have focused primarily on crystalline systems. In this talk, it will be shown that amorphous materials are potential candidates to generate spin currents. First, the talk will examine the anomalous Hall angle (AHA) in a series of ferromagnetic amorphous transition metal thin films M_xY_1-x (M=Fe, Co; Y=Si, Ge; x=0.40-0.71). It will be shown that the AHA (=σ_xy/σ_xx) is as large as 5%, which is substantial even for crystalline systems. In the amorphous materials it was found that the AHA increases with increasing Hall conductivity (σ​​​​​​​​​​​​​​_xy). This trend, which is opposite to that which occurs in crystalline systems, is attributed to low σ​​​​​​​_xx, while σ​​​​​​​_xy and M remain high. The talk will then report observation of a large spin-orbit torque in an amorphous non-magnetic Fe_xSi_1-x/cobalt bilayer via spin-torque ferromagnetic resonance and harmonic Hall measurements. The origins of this SOT will be discussed.