Microstructure and magnetism: structure-properties correlations in two-phase Co–S nanocrystalline films

Microstructure, magnetic hysteresis, magnetic anisotropy and magnetic domain structures of Co–S alloy films were investigated in as-prepared state and after annealing. SEM and TEM investigations show that as-prepared films consist of columnar Co nanocrystallites (3–4.5 nm), nanofibrous structure, grown in an amorphous or highly disordered Co–S matrix. For as-prepared films, the hysteresis loops indicate that magnetization is approximately linear with the applied magnetic field up to saturation and both coercivity and remanence are very low. Films displayed long-range, wavy stripe-like, magnetic domain structure and magnetic percolation. The stripe-like domains are closure domains, which transport the magnetic flux of the underlying domains. The domain structure is the result of the competition between perpendicular magnetic anisotropy (PMA), on one side, and dipolar exchange interaction mediated by the soft ferromagnetic amorphous matrix (indirect exchange interaction), on the other side. The structural origin of PMA is mainly attributed to the magnetoelastic anisotropy associated with an in-plane internal biaxial tensile stress and negative magnetostriction. Magnetic properties evolve with annealing temperature as a result of the microstructure change and of the exchange magnetic interaction increase in the films. The results suggest that a nanocrystalline Co–S film behave as a two-phase heterogeneous system which shares some features of both continuous and discontinuous magnetic systems.