Inter-grain effects on the magnetism of M-type strontium ferrite

By using films as model systems, a combined study based on experiments and micromagnetic simulations has been performed to get further insight into the magnetic behaviour of M-type SrFe12O19 (SrF) system in the nanosized range. Both nanocrystalline SrF quasi-isotropic and highly anisotropic films have been obtained by sputtering. Tuning of the preferred c-axis orientation, either normal or in the film plane, has been possible by varying the growth parameters and, importantly, with no need of any underlayer. Small changes in the oxygen ratio (up to 2%) and in-situ heating lead to different microstructure, which determine the strong variations of the magnetic behaviour observed, including both magnetization curves, easy axis directions, and their different coercive fields, which vary from 1 kOe to 5 kOe within the sample series. Micromagnetic simulations reproduce the experiments with a model based on a mixed population of randomly distributed SrF grains with in-plane and out-of-plane uniaxial anisotropies. The simulations show that intergranular interactions play the major role on the coercive field behaviour of the SrF system. A microstructure comprising largely uncoupled grains leads to improved magnetic performance of highly c-axis oriented SrF nanostructures.