The influence of an ultra-high resistivity Ta underlayer on perpendicular magnetic anisotropy in Ta/Pt/Co/Pt heterostructures

Abstract

Thin films with perpendicular magnetic anisotropy (PMA) play an essential role in the development of technologies due to their excellent thermal stability and potential application in devices with high density, high stability, and low energy consumption. Many studies have focused on the relationship between the resistivity of heavy metals and the PMA of the neighbouring magnetic metals in magnetic multi-layered films. However, reports on the effects of heavy metals non-adjacent to the magnetic metals on the PMA are rare. Herein, we demonstrate the influence of the heavy metal Ta underlayer non-adjacent to the magnetic Co layer on the PMA and thermal stability in the Ta/Pt/Co/Pt heterostructures. A type of amorphous Ta film having an ultra-high resistivity (ρ_max = 3.9 × 10⁵ μΩ cm) was optimized by DC sputtering at a high sputtering Ar pressure, low sputtering power, and large target-to-substrate distance. The value of resistivity is three orders of magnitude higher than that of the β-Ta underlayer. We found that this special Ta underlayer can effectively improve the PMA and thermal stability of the magnetic Co layer based on the anomalous Hall and planar Hall effect measurements. The maximum magnetic anisotropic field reaches 1.1 T at a low temperature. It is very likely that the ultra-high resistivity leads to the increase in the additional electron scattering in the Ta/Pt interface, while the latter results in the enhancement of the PMA and thermal stability in the structure. These results reveal the inherent relationship between the resistivity of the heavy metal underlayer and PMA, and provide a novel approach to improve the PMA and thermal stability of heavy metal/magnetic metal multi-layered films.