Generic Skyrmion Phase Diagram in Chiral Magnetic Films

Abstract

This thesis investigates the phase diagrams of isolated and condensed skyrmions in chiral ferromagnetic films under a perpendicular magnetic field and in chiral ferrimagnetic films in the absence of an external magnetic field. In chiral magnetic films, skyrmions exist either as isolated phase confined within the sample or as condensed phase occupying the entire system. The stability of magnetic skyrmions in perpendicularly magnetized chiral ferromagnetic films is fully determined by the dimensionless parameters k and k^', which measure the relative strengths of interactions favoring curling and collinear magnetic structures rather than individual material parameters. In the absence of magnetic crystalline anisotropy and in the presence of a perpendicular magnetic field, |k^'| = 4 distinguishes isolated skyrmions, including circular and polygonal skyrmions, from condensed skyrmions, including skyrmion crystal and helical states. This condition aligns with κ = 1, which separates these phases in the absence of a perpendicular magnetic field where κ solely determines the (meta)stable structures. Based on these results, this thesis establishes a comprehensive skyrmion phase diagram in the kk^' -plane across the full parameter space. This framework is extended to ferrimagnetic systems with two antiferromagnetically coupled sublattices. The role of the inter-sublattice exchange coupling J is investigated through the dimensionless parameter ζ_eff, which characterizes the crossover between strong and weak inter-sublattice locking. In the strongcoupling regime, the system can be described by a ferromagnetic material with thickness-weighted effective parameters, where k_eff = 1 separates isolated and condensed skyrmion phases, leading to a phase diagram in the k_eff k_2eff-plane. As the coupling decreases, the sublattice ℓ (ℓ = 1, 2) becomes independently governed by kℓ = 1, naturally leading to a phase diagram in the k₁k₂-plane with distinct combinations of isolated and condensed skyrmion phases. Overall, unified and physically transparent frameworks are established for understanding skyrmion phase stability in both chiral ferromagnetic and ferrimagnetic systems.