Crystalline-structure-dependent magnetoresistance in ferromagnetic metal/conducting amorphous oxide heterostructures

Abstract

We have studied the transport and structural properties of heterostructures based on a conducting transparent oxide and a ferromagnetic metal $(\mathrm{FM}=\mathrm{Co}$, CoFe, CoFeB) layer. An amorphous indium tungsten oxide (IWO) was used to study possible generation of spin current from a conducting transparent oxide. The resistance of the heterostructure was studied as a function of external magnetic field to evaluate the magnetoresistance (MR) in the $yz$ plane that provides information on the spin Hall magnetoresistance (SMR). We found $yz$-plane magnetoresistance of \ensuremath{\sim}\ensuremath{-}0.2% in IWO/Co and IWO/FeCo heterostructures whereas the effect was nearly zero for IWO/CoFeB bilayers. The MR vanished when a thin (\ensuremath{\sim}1 nm thick) Cu layer was inserted in between the IWO and Co layers. Cross-sectional transmission electron microscopy images and nanobeam diffraction patterns of the films suggested that the Co and FeCo layers formed crystal grains with preferential crystal orientation whereas the Co layer grown on Cu showed grains with random orientation. The CoFeB layer was found to be amorphous. These results show a strong correlation between the crystalline structure of the FM layer and the appearance of $yz$-plane MR, suggesting that FMs with random and/or amorphous structure are suitable for SMR measurements to avoid contamination of the $yz$-plane MR.