All-optical investigation of anisotropic spin pumping in W/CoFeB/W heterostructure

Abstract

Pure spin current generation and transport in magnetic heterostructures comprising of normal metal (NM)/ferromagnet (FM) interfaces are imperative for future energy efficient spintronic devices. The above properties are considered to be equivalent in terms of spin-mixing conductance and spin pumping efficiency. Correspondingly, the enhancement in Gilbert damping should be equal for every interface with same NM and FM layers. However, this equivalency may break down under certain circumstances, giving rise to anisotropy. Here, we have shown by time-resolved magneto-optical Kerr effect measurements that for W/CoFeB interface, the enhancement in Gilbert damping is dissimilar when the position of W and CoFeB are interchanged. The damping coefficient (α) is 0.010 for CoFeB/W, whereas it is 0.0135 for W/CoFeB. Our in-depth investigation reveals that the precessional trajectory for W/CoFeB system is strongly affected by the induced magnetic anisotropy which is not present in CoFeB/W, leading towards the observed directional spin pumping. This is clearly demonstrated in W/CoFeB/W trilayer where the spin current generated by spin pumping diffuses more towards W underlayer in comparison to W overlayer. Our findings add new insights in the role of ferromagnet/nonmagnet interfaces for controlled generation and utilization of pure spin current for application in advanced spintronic devices.