Investigation of High-Performance Skyrmions in Ferrimagnetic Thin Films

The demand for devices with high-speed operations, high-density data storage, and low energy consumption is continuously increasing due to the rapid development of information technologies. The magnetic skyrmion-based device has received considerable interest as one of the candidates for next-generation devices that meet those requirements. Although many magnetic materials can host magnetic skyrmions, which are stabilized by the Dzyaloshinskii-Moriya Interaction (DMI), not all are capable of satisfying the size and stability requirements necessary for application. Theoretical calculations have indicated that ferrimagnetic materials are preferable for hosting small skyrmions as a result of their low saturation magnetization and intrinsic perpendicular magnetic anisotropy (PMA) at large thicknesses. For example, simulations and experiments have confirmed small (10–30nm) skyrmions in GdCo thin films at room temperature.
This study systematically investigated the Dzyaloshinskii-Moriya Interaction (DMI) of Pt/GdCo/W_x Pt_1-x. A magnetron sputtering system deposited the ferrimagnetic amorphous GdCo alloy thin films on a Pt seed layer. The compositions of GdCo thin films were adjusted to achieve a low saturation magnetization and perpendicular magnetic anisotropy. Different compositions of a WPt layer were used to cap the GdCo layers. These GdCo samples' DMIs were measured using Brillouin light scattering (BLS). It showed that the DMI in GdCo thin films could be adjusted by varying the capping layer composition and the ferrimagnetic layer thickness. The study of the dependence of the DMI thickness confirmed its interfacial nature and indicated an exhibit of intrinsic bulk DMI.
GdCo is an ideal material for hosting small skyrmions at room temperature due to its magnetic properties, except for its poor thermal stability, which is a significant materials processing drawback. Therefore, alternative materials that have better thermal stability than GdCo are required. Mn4N film is one of the alternative materials that have the potential to overcome the thermal stability challenge.
In this study, ferrimagnetic Mn4N thin films were grown epitaxially on MgO(100) substrates by reactive sputtering. An annealing process decreased the surface roughness of the MgO substrate and enhanced the PMA of the Mn4N films. At room temperature, the optimal films had a comparable saturation magnetization and magnetic anisotropy energy compared to GdCo thin films. Additionally, their remanent magnetization to saturation magnetization ratio (Mr/Ms) was nearly one. The interfaces of MgO/Mn4N/CuPt were examined by measuring X-ray photoelectron spectroscopy (XPS) and polarized neutron reflectometry (PNR). XPS indicated that mixing regions of about 5 nm thickness are present at both interfaces. PNR showed that these interfaces have essential magnetization gradients at room temperature.
Moreover, Magnetic force microscopy (MFM) observed magnetic skyrmions with tunable sizes in Mn4N capped with Cu_x Pt_1-x layer. The size of the skyrmion decreased from 300nm to 50nm as the concentration of Cu increased. The average DMI of MgO/Mn4N/Cu_x Pt_1-x, extracted from the anomalous Hall effect with various tilted angles, was based on magnetic droplet theory with DMI effects. When the concentration of Cu in the CuxPt1-x capping layer increased from 0 to 1, the DMI of MgO/Mn4N/Cu_x Pt_1-x decreased from 0.267 mJ/m^2 to 0.011 mJ/m^2 with non-linear tendencies. Furthermore, a solid solution model is developed to analyze the interfacial mixing layer effects on DMI.