Structure and magnetism of skyrmion hosting materials

The discovery of magnetic skyrmions has created significant interest in the field of magnetism due to their novel properties and potential applicability in devices. These vortex-like magnetic structures have been found in an increasing number of materials to-date but there are still very few known skyrmion hosts. This thesis explores a selection of materials in which skyrmions form, examining the underlying crystal structure of these materials and how this influences the magnetism that they create. These magneto-structural relationships have been investigated in two main ways: through solid solutions of known skyrmion hosts, and geometrical confinement of skyrmion hosts on the scale of the size of a skyrmion. In addition to these experimental investigations, a Python code has been developed to assist in simulating the results of experiments based on the magnetic structure in an attempt to further bridge the fields of computational and experimental magnetism in condensed matter physics. There are a select number of powerful techniques for studying the underlying magnetic structure of materials. If the length scale of the magnetic structure is large enough, techniques such as Lorentz transmission electron microscopy, magnetic force microscopy, x-ray holography, and small angle neutron scattering can be used to probe them. This thesis details a Python code which has been created to simulate the results of these experimental techniques given any arbitrary three-dimensional magnetic structure, such as one created by micromagnetic simulations. The structural and magnetic properties of the Néel skyrmion hosts GaV4S8 and GaV4Se8 have been examined through techniques such as dc and ac magnetometry along with x-ray and neutron diffraction. These investigations reveal an interesting behaviour of these materials along with identification of a ferromagnetic-like state in GaV4S8 . Solid solutions of GaV4S8 and GaV4Se8 have been used to create a new GaV4S8-ySey family of materials covering the range 0 ≤ y ≤ 8. The suitability of these materials as skyrmion hosts has been examined. Structural studies reveal that increasing small amounts of substitution at both the GaV4S8 and GaV4Se8 ends reduces the temperature of the structural transition, which is present in both the parent materials, until it no longer exists and they stay in a cubic structure. Magnetic investigations reveal that for small levels of substituent the skyrmion phase only persists for 0 < y < 0.2 and 7.8 < y < 8, with higher substitution levels showing glassy-like magnetism. The magnetic properties of these substituted materials are affected by the substitution altering exchange pathways and results in the creation of increasingly disordered magnetic states. Finally, the geometric confinement of skyrmions has been investigated in nanoparticles of Cu2OSeO3 . A precipitation method followed by thermal treatment technique has been employed, allowing the size of nanoparticles to be tuned dependent on the synthesis temperature. Nanoparticles of sizes on the order of a few nm to µm have been produced and the magnetic properties investigated. Smaller nanoparticles have been seen to exhibit a distinct magnetic phase diagram from the bulk, however, there is further scope for investigations of the dependence of the magnetic structures on the nanoparticle size