A general model of MnSi-like helical magnets

Hall, Kyle (2020) A general model of MnSi-like helical magnets. Masters thesis, Memorial University of Newfoundland.

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Abstract

MnSi and other magnets belonging to the B20 designation are known to assume exotic magnetic structures with subtle features. Of these, the most notable is the appearance of long-wavelength helical structures. Previous analyses of these materials considered oft-used and non-specific models to describe these systems. I will present a general, classical model with only nearest neighbour exchange interactions constructed through symmetry considerations. This model is complete up to the determination of the relative strengths of the coupling constants and the inclusion of other interactions. Comparison to other models will reveal a general relationship between this model and those used in previous analyses. Further comparison with experimentally observed features is used to produce magnetic order parameters of the structure and a relationship between their complex values and structure observables. Also presented are the results of computational simulations using the Effective Field Method. These simulations are conducted with specific anisotropic and Zeeman interactions introduced to the model. Periodic boundary conditions are not used to maintain the incommensurate helical structure. The results of these simulations are analyzed to extract several lattice structure parameters and the action of individual exchange constants is considered. Additionally, the relation between this model and others is discussed and the introduction of isolated skyrmions is observed. Finally, the results of preliminary simulations with applied magnetic fields oriented along the helix wavevector are presented. These final results demonstrate the appearance of a conical phase and illuminate the effect of specific anisotropic terms. The critical field Bc₂, i.e., the critical field between the conical and field-induced ferromagnetic states, and the relationship between canting and field strength are also reported.

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