Beta (β) tungsten thin films: Structure, electron transport, and giant spin Hall effect

We use a simple magnetron sputtering process to fabricate beta (beta) tungsten thin films, which are capable of generating giant spin Hall effect. As-deposited thin films are always in the metastable beta-W phase from 3.0 to 26.7 nm. The beta-W phase remains intact below a critical thickness of 22.1 nm even after magnetic thermal annealing at 280 degrees C, which is required to induce perpendicular magnetic anisotropy (PMA) in a layered structure of beta-W/Co40Fe40B20/MgO. Intensive annealing transforms the thicker films (> 22.1 nm) into the stable alpha-W phase. We analyze the structure and grain size of both beta- and alpha-W thin films. Electron transport in terms of resistivity and normal Hall effect is studied over a broad temperature range of 10K to at least 300K on all samples. Very low switching current densities are achieved in beta-W/Co40Fe40B20/MgO with PMA. These basic properties reveal useful behaviors in beta-W thin films, making them technologically promising for spintronic magnetic random access memories and spin-logic devices. (C) 2015 AIP Publishing LLC.