Beta-tungsten

While the commonly existing stable alpha-tungsten (α-W) has a body-centered cubic (A2) structure, β-W adopts the topologically close-packed A15 structure containing eight atoms per unit cell,[1][2] and it irreversibly transforms to the stable α phase through thermal annealing of up to 650 °C.

[3] It has been found that β-W possesses the giant spin Hall effect, wherein the applied charge current generates a transverse spin current, and this leads to potential applications in magnetoresistive random access memory devices.

[4] β-W was first observed by Hartmann et al. in 1931 as part of the dendritic metallic deposit formed on the cathode after electrolysis of phosphate melts below 650°C.

While the initial interest in β-W thin films was driven by its superconducting properties at low temperatures,[11] the discovery of giant spin Hall effect in β-W thin films by Burhman et al. in 2012 has generated new interest in the material for potential applications in spintronic magnetic random access memories and spin-logic devices.

[12] Two key properties of β-W have been well-established: the high electrical resistivity and the giant spin Hall effect.

Although the exact value depends on the preparation conditions, β-W has an electrical resistivity of at least five to ten times higher than that of α-W (5.3 μΩ.cm),[1][13][14][15] and this high conductivity[contradictory] will remain almost unchanged in a temperature range of 5 to 380 K,[16] making β-W a potential thin film resistor while α-W is a thin film conductor.

In contrast, α-W exhibits a much smaller spin Hall angle of less than 0.07 and a comparable spin-diffusion length.

[2] While there have been some reports about preparing β-W with chemical methods such as hydrogen reduction reaction,[6][17] almost all the reported β-W in the recent thirty years are prepared through sputter deposition, an atom-by-atom physical vapor deposition (PVD) technique.