Magnetic relaxation in epitaxial films with in-plane and out-of-plane anisotropies

Nanomagnetic materials are needed for increasing data storage capacity and suited for enhancing the performance of permanent magnets. However, their performance is controlled by magnetic switching, which is driven by a competition between thermal activation energies and anisotropy energies. Here, we elucidate the magnetic switching process in epitaxial films with in-plane and out-of-plane magnetic anisotropies. While in both media the magnetization obeys a logarithmic decay over time, a drastic difference is revealed in their magnetic viscosities. The relaxation logarithmic law is a consequence of the epitaxy itself under which the film growth is initiated through random nucleation followed by islands growth and their coalescence, leading to non-uniform structural domains. These structural domains behave like magnetic domains due to the presence of antiphase boundaries where exchange coupling is disrupted. The activation volume for both media is found to match the average size of the structural domains. The very slow relaxation process under out-of-plane anisotropy is linked to the demagnetizing field, which drastically weakens the irreversible magnetic susceptibility. A simple analytical model was developed and found to well predict and corroborate the experimental findings. This study was conducted on CoFe2O4 films epitaxially grown on (100) and (110) MgO substrates.