Study of the skyrmion state stability and metastability with SANS in Ni and Zn substituted Cu2OSeO3

Magnetic skyrmions are vortex-like arrangements of magnetic moments and constitute a new topological state of the magnetisation. Skyrmions are characterised by low depinning currents and topological stability. These features make them suitable candidates as information carriers in electronic devices that would be characterised by lower power consumption and a higher data storage density. For technological development, it is crucial to understand skyrmions’ characteristics and behaviour in different conditions to ultimately engineer materials with specifications tailored to their needs. Fundamental research on skyrmion materials is needed to identify new avenues for skyrmionic technological developments. In this thesis, we investigate the properties of the skyrmion lattice in the bulk chiral magnet Cu2OSeO3 and its Ni and Zn chemically substituted compounds. We present a thorough investigation of the effect of hydrostatic pressure on the skyrmion state, with particular attention given to the choice of pressure transmitting medium. Our measurements indicate how pressure can be exploited as a third thermodynamic coordinate to the magnetic phase diagram of a bulk skyrmion host. The stability and structure of the skyrmion state have also been studied related to the macroscopic shape of the sample. Although the skyrmion lattice is imagined to be uniform across bulk materials, we show that demagnetisation effects induce several discontinuities in the lattice affecting its shape and stability. Demagnetisation also affects the decay of the metastable skyrmion state, inducing longer lifetimes in specific regions of the sample. Our measurements highlight the importance of considering demagnetisation effects when studying the skyrmion state. We also present a more detailed work on the decay of the metastable skyrmion state. We extracted the velocity of Bloch points, that mediate the decay, from measurements of the lifetime and mean correlation length of skyrmion tubes. This thesis’s work provides insights on the skyrmion state characteristics and shows how small angle neutron scattering (SANS) technique can be used to provide information on the three dimensional ordering of the skyrmion lattice structure. Moreover, we show how SANS can also provide information on the dynamic of the metastable skyrmion state