Mapping magnetization orientation within compositionally graded rare-earth transition-metal alloy thin films for spintronics

Abstract Compositionally-graded amorphous rare earth:transition metal ferrimagnetic thin films offer exciting potential for enabling field-free spin-orbit torque (SOT) switching in spintronic devices. Vertical compositional gradients have previously been developed to enhance SOT efficiency via a proposed transition from out-of-plane (OOP) to in-plane (IP) magnetization to induce Dzyaloshinskii-Moriya interaction that breaks inversion symmetry within the layer. Here, we directly mapped the magnetization orientations within compositionally-graded GdCoFe layers by probing the depth-dependent local effective Gd-magnetization or the local net magnetization using X-ray and neutron reflectivity (PNR), respectively. These methods showed contrast between IP and OOP magnetization zones through the film thicknesses and PNR showed the magnetization compensation point within a graded GdCoFe layer. Transitions from IP to OOP magnetization depended on the local composition, and it was observed that some of these changes do not agree with expectations from the study of films with uniform composition. These results give the spatial distribution of magnetization and hence the anisotropy, essential for understanding and optimising field-free SOT switching.