Interface structure of CoFeB/MgO magnetic tunnel junctions from hard x-ray photoelectron spectroscopy

Present sensors in hard drives rely on tunnel magnetoresistance (TMR) in CoFeB/MgO/CoFeB structures. The device fabrication has been refined to meet strict demands. Despite this, fundamental understanding of the optimization process, i.e. post-annealing, is missing. In particular, boron diffusion has been suggested to be integral to the creation of a textured CoFe alloy with boron diffused either into the MgO tunnel barrier, forming boron oxides, or into a seed layer. Such diffusion would thus indirectly be essential for a large MR in the device. We have used hard x-ray photoelectron spectroscopy (HAXPES), to investigate a series of CoFeB/MgO/CoFeB structures. By systematically studying the modifications of chemical state of various constituents for different structures and post-annealing conditions, we are able to provide a detailed geometric interpretation of how elements diffuse and modify the structure. In particular we show that at the annealing temperatures required for achieving optimal MR, boron diffusion is limited to a very thin (sub-nm) region at the interface and does not progress beyond this point.