  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.
