A TEM Structural Study of the Origin of Perpendicular Magnetic Anisotropy in Ultra-thin CoFeB Film

Perpendicular magnetic anisotropy (PMA) at ferro-magnetic transition metal/insulator interfaces has attracted intensive interest in the context of developing various spintronic devices. Magnetized out-ofplane magnetic tunnel junctions (p-MTJ) are now developed for spin transfer torque (STT) magnetic random access memories (STT-MRAM) applications where the strong perpendicular anisotropy originating from the CoFe/MgO interface allows to maintain the thermal stability of the storage layer magnetization down to at least 20nm technological node. MTJ stack possessing ferromagnetic layer with a large PMA is expected to provide novel features such as high thermal stability and low switching energy consumption. Ultra-thin CoFeB (CFB) films in contact with MgO show a large PMA, which is originated from the interface integration, and have been recently studied in MTJs’ stack [1]. Several studies tried to optimize the PMA and to study its dependence on the thickness of CFB layer [1], on the buffer layer [2] as well as the oxidation condition at the interface and the annealing temperature. Although PMA has been widely studied, the origin of its very large value in CFB/MgO system has not yet been fully unveiled. In this work, we have conducted a detailed TEM characterization for experimentally clarifying the PMA origin in ultra-thin CFB layer on the top of MTJs. Such study would certainly deepen our understanding of the mechanism of PMA in CFB thin film in MTJ stacks.