X-ray photoelectron spectroscopy investigation of Ta/CoFeB/TaOx heterostructures

Abstract

In-depth understanding of structural and magnetic property correlation in technologically important heavy metal/ferromagnet/oxide heterostructures is of great interest due to its potential applications in magnetic-memory devices. Here, we investigate the structural and magnetic properties of the as-deposited and annealed ultrathin Ta/CoFeB/TaOx stack in which zero field magnetization switching, reasonably large Dzyaloshinskii-Moriya interaction and stabilization of skyrmions has been reported in recent time. Interestingly, nearly two times increase in the saturation magnetization and a reduction in average surface roughness by half is observed in 300 °C annealed film stack in comparison to the as-deposited film. To get further insight into the mechanism resulting in such drastic change in saturation magnetization, surface sensitive X-ray photoelectron spectroscopy has been used. In the annealed film stack, the analysis of Ta peaks revealed that B has intermixed with Ta layers thus forming new chemical state TaB. Additionally, annealing caused Boron diffusion out of CoFeB layer and a subsequent exposure to atmosphere resulting in the formation of BOx. Furthermore, the signature of metallic Fe0 2p1/2 peak in Fe 2p spectra has been evidenced in the annealed film stack which plays a crucial role in increasing the saturation magnetization. We believe, these results will provide a useful insight in understanding interfacial effects which are key to application of such ultrathin film heterostructures in modern spintronics devices.