Effects of composition gradients on temperature dependent magnetization and coercivity in CoGd alloys

A great deal of attention has been given to ferrimagnetic alloys as potential building blocks of spintronic devices. Studies have recently shown the emergence of bulk Dzyaloshinskii-Moriya interactions and spin-orbit torques in vertically inhomogeneous single layer ferrimagnetic films. In this work, we utilize atomistic spin simulations to explore how the local magnetization varies in CoGd alloys, both due to the decreased coordination number at surfaces as well as due to vertical inhomogeneities, and how this in turn effects film behavior such as coercivities of selected films. We find that, while compositional modulation varies the local compensation point through the film thickness, it has no significant effect on the net compensation temperature of the alloy if the average composition stays the same, even with variations as large as 50% from top to bottom interfaces. However, even minor variations in composition can drastically reduce the out-of-plane coercivity of the films, or even preclude perpendicular anisotropy entirely. This provides new insights into limitations of ferrimagnetic materials with non-uniform properties for the incorporation into spintronic devices.