Anomalous behavior of spin wave resonances in Ga_{1-x}Mn_{x}As thin films

We report ferromagnetic and spin wave resonance absorption measurements on high quality epitaxially grown Ga_{1-x}Mn_{x}As thin films. We find that these films exhibit robust ferromagnetic long-range order, based on the fact that up to seven resonances are detected at low temperatures, and the resonance structure survives to temperatures close to the ferromagnetic transition. On the other hand, we observe a spin wave dispersion which is linear in mode number, in qualitative contrast with the quadratic dispersion expected for homogeneous samples. We perform a detailed numerical analysis of the experimental data and provide analytical calculations to demonstrate that such a linear dispersion is incompatible with uniform magnetic parameters. Our theoretical analysis of the ferromagnetic resonance data, combined with the knowledge that strain-induced anisotropy is definitely present in these films, suggests that a spatially dependent magnetic anisotropy is the most likely reason behind the anomalous behavior observed.Comment: 9 pages, including 6 figure

Exotic thin films made from cobalt ferrite

Epitaxial CoFe2O4 thin films have been grown by PLD on (100) MgO substrate. Two types of spin-reorientation have been observed in such films upon annealing or increasing the film-thickness. In the as-deposited layers and at low thickness the easy axis is confined to the normal to the film plane whereas at large thickness the film plane becomes the preferential direction of the magnetization. On the other hand annealing induces a reorientations is explained in term of competition between stress and magnetocrystalline anisotropies. In the as-deposited state the tensile stress induces a huge perpendicular anisotropy, dominating the in-plane magnetocrystalline component. However annealing or increasing the film thickness releases the stress and allows the in-plane magnetocrystalline anisotropy to take the lead