Superparamagnetic effect in the rotatable anisotropy of nanoparticles and films

- Kinetic theory of ferromagnetic resonance in exchange bias nanosystems is evolved.
- Rotatable anisotropy manifests itself as an isotropic shift of the resonance field.
- Temperature behavior of the shift - a way to find symmetry of rotatable anisotropy.

The exchange bias effect in nano-objects is modelled by adding to the magnetic energy two terms allowing for two kinds of anisotropy, viz. a fixed-axis and a rotatable one. The signature of the second contribution is an isotropic shift HRA of the ferromagnetic resonance field. The temperature dependence of HRA that is due to the superparamagnetic theory is obtained in the framework of kinetic theory. Our analysis predicts that the temperature behaviors of HRA are qualitatively different depending on the possible symmetry type of the rotatable anisotropy energy term: unidirectional or uniaxial.