The Effect of Strain in MnSi Thin Films and the Stability of In-Plane Skyrmions

Abstract

Magnetic skyrmions represent a topologically stable field configuration that can arise in magnetic materials that lack centres of inversion symmetry. These solitons have potential applications in digital memory storage because of their ability to be manipulated by currents and because of the fact that their topological stability can imply energetic stability or metastability.

MnSi is a system that has no centre of inversion symmetry and so it is possible to host skyrmions. In MnSi(111)/Si(111) thin films, an epitaxial mismatch induces an in-plane tensile stress which results in a hard-axis magnetocrystalline anisotropy. For in-plane fields, there is controversy surrounding the phase diagram. Yokouchi et al. have used planar Hall effect measurements to claim that a skyrmionic phase does not exist at temperatures of 25 K and fields of 500 mT, which contradicts previous SQUID magnetometry measurements. The first aim of this thesis is to resolve the controversy surrounding the existence or absence of skyrmions using a less indirect technique.

Polarized neutron reflectometry (PNR) is sensitive to magnetic spatial variation along the depth of the film and so can be used to resolve the presence or absence of skyrmions. A new type of skyrmion lattice in MnSi, one with its axis of symmetry pointing within the plane of the film was uncovered using PNR in conjunction with computational micromagnetic modelling. It was found that the spacing in the lattice was 22 nm, in good agreement with small angle neutron scattering. At lower fields of 300 mT, the system transitions into a metastable state. This state consists of elliptically extended skyrmions. At higher fields of 700 mT it was shown that the system exists in a twisted ferromagnetic state.

The viability of SiC(0001) substrates to reverse the sign of the anisotropy was also investigated. The mismatch between MnSi(111) and SiC(0001) is such that the anisotropy might be reversed and result in an easy-axis anisotropy. Reversing the sign of this could result in the stability of out-of-plane skyrmions. The magnetic state of the films was probed using Hall effect measurements and the morphological state was probed using atomic force microscopy. It was found that dewetting instabilities during the growth of MnSi(111)/SiC(0001) result in an in-plane tensile strain and a hard-axis uniaxial anisotropy. Though the dewetting produced an unexpected magnetocrystalline anisotropy, it also produced unusual morphological geometries that could be explored in the future.