Thin film epitaxial [111] Co$_{50}$Pt$_{50}$: Structure, magnetisation, and spin polarisation

TL;DR

This study investigates the structural, magnetic, and spin-polarisation properties of epitaxial Co50Pt50 thin films with different crystal structures, revealing intrinsic perpendicular magnetic anisotropy and a high spin polarisation suitable for spintronics applications.

Contribution

It provides the first comprehensive comparison of the $L1_1$ and $L1_0$ crystal structures of Co50Pt50, highlighting their magnetic anisotropy orientations and spin polarisation characteristics.

Findings

$L1_1$ structure exhibits perpendicular magnetic anisotropy.

$L1_0$ structure shows anisotropy perpendicular to the [100] plane.

Ballistic spin polarisation is approximately 47%.

Abstract

Ferromagnetic films with perpendicular magnetic anisotropy are of interest in spintronics and superconducting spintronics. Perpendicular magnetic anisotropy can be achieved in thin ferromagnetic multilayer structures, when the anisotropy is driven by carefully engineered interfaces. Devices with multiple interfaces are disadvantageous for our application in superconducting spintronics, where the current perpendicular to plane is affected by the interfaces. Robust intrinsic PMA can be achieved in certain Co$_x$Pt$_{100-x}$ alloys and compounds at any thickness, without increasing the number of interfaces. Here, we grow equiatomic Co$_{50}$Pt$_{50}$ and report a comprehensive study on the structural, magnetic, and spin-polarisation properties in the $L1_1$ and $L1_0$ ordered compounds. Primarily, interest in Co$_{50}$Pt$_{50}$ has been in the $L1_0$ crystal structure, where layers of Pt and Co are stacked alternately in the [100] direction. There has been less work on $L1_1$ crystal structure, where the stacking is in the [111] direction. For the latter $L1_1$ crystal structure, we find magnetic anisotropy perpendicular to the film plane. For the former $L1_0$ crystal structure, the magnetic anisotropy is perpendicular to the [100] plane, which is neither in-plane or out-of-plane in our samples. We obtain a value for the ballistic spin polarisation of the $L1_1$ and $L1_0$ Co$_{50}$Pt$_{50}$ to be $47\pm3\%$.