Sign reversal of field like spin-orbit torque in ultrathin Chromium/Nickel bilayer

Abstract

In this work report unconventional sign change of field like spin orbit torque in ultra-thin Chromium(1.5nm-5nm)/Nickel(8nm) bi-layer. We performed standard spin-torque ferromagnetic resonance (ST-FMR) experiment in Cr/Ni bi-layer by passing radio frequency current and measuring DC voltage. We observe that when thickness of Cr layer is critically low (<6nm) spin orbit torque by Cr on Ni significantly increases. Most importantly the sign of field like torque is opposite to the Oersted field generated torque. To verify interracial nature of this torque, 2 nm thin Cu is inserted between Cr and Ni and field like torque behaves same as Oersted field induced torque. Hence possible origin of such unconventional sign change of field like torque could be inter-facial Rashba like spin orbit interaction which is present between Cr and Ni but vanishes in Cr/Cu/Ni hetero-structure. From our experiment we can estimate that approximately 35 Oe of effective Rashba like magnetic field is created on 8 nm thicker Ni layer, when 1E12 A/m^2 current flows through Cr layer. All experiments are done at room temperature. So Cr thin film is expected to behave like paramagnet (Neel temperature of bulk Cr is 311K). Hence Cr can be a good choice as a heavy metal to employ large spin orbit torque combining bulk spin Hall effect and inter-facial Rashba interaction.