Enhanced Torques in Co/Pt Systems via Orbital Current Contribution

Abstract

In the last decade, charge to spin conversion and spin-orbit torques have been extensively studied to manipulate the magnetization for efficient data storage devices functionalities. The charge-spin conversion is governed by spin-orbit interactions such as spin Hall effect in the bulk of heavy metals and Rashba effect at their interfaces[1]. Recently, substantial increase in the magnetic torques has been observed with Cu/CuOx or Cu/Al2O3 light element interfaces. The observation of magnetic torques in the absence of spin-orbit interaction can be explained in the framework of orbital currents at Cu/CuOx interface[2-3]. Furthermore, as the orbital effects are believed to be one order larger than the spin-orbital coupling effects, we expect larger torques efficiency by utilizing the orbital currents.To aim this, we have measured the damping-like (HDL) and field-like (HFL) effective fields in SiOx/Co(2)/Pt(x)/Cu/CuOx(3) and SiOx/Co2)Pt(x)/AlOx(1) samples (and where x=2-6 nm) using second harmonic Hall measurement technique. Here, all the samples possess an in-plane magnetic anisotropy. As the orbital current does not carry spin (S=0), it is important to convert them into spin-current to interact with magnetization via s-d exchange interactions to promote the torques. Therefore, Pt is used to convert the orbital current into the spin current. Although the HFL is found to be negligible in all the samples, one observes that the HDL increases linearly with the current density in Pt, like expected for the current induced torque. Fig. 1 shows HDL for 107 A/cm2 current density in Pt. However, we do not observe significant change for AlOx, a two-fold increase is observed in Co(2)/Pt(4)/Cu/CuOx(3) compared to our reference Co(2)/Pt(4) sample, and where the exact spin current profile has been analyzed. As proposed in the recent literature [2], we attribute this enhancement in torques to the generation of orbital current at Cu/CuOx interface.  Fig1. Comparison of HDL in various capping layers as a function of Pt thickness. The Blue (Black) arrow shows the increase in HDL in Co/Pt/Cu/CuOx (Co/Pt/Cu/AlOx) samples due to the orbital current at Cu/CuOx (Cu/AlOx) interface.