Time window in the dynamic excitation of magnetic nanoparticles.

EMILIO DE BIASI; ENIO LIMA JR.; CARLOS RAMOS; ROBERTO ZYSLER; ALEJANDRO BUTERA

Workshop; IX Latin American Workshop on Magnetism, Magnetic Materials and Their Applications; 2010

In this work we present a new theoretical treatment for the ferromagnetic resonance response (FMR) in a system of anisotropic magnetic nanoparticles. We have studied the thermal effects on the effective magnetization and the anisotropy field covering a wide range of the anisotropy energy. At variance with previously reported models, we have assumed that the experimental time window is associated to the sweep rate of the external field value, and is not related with the frequency of the microwave excitation. The high anisotropy case is considered in our model, including the time and the  temperature dependence of the minima population. The model converges accurately to the limits of the low anisotropy approximation (for which there exists well-known analytical models), as well as the low temperature limit, in which the ferromagnetic formalism is naturally obtained. Our results explain the loss of the FMR signal in the high uniaxial anisotropy approximation, as a result of the increase of the effective anisotropy field at low temperatures. In addition, it predicts an important dependence of the magnetic history of the sample in the blocked regime. We have also studied the dependence of the magnetic response with the sweep rate of the external magnetic field. Experiments using magnetite (Fe3O4) nanoparticles systems corroborate our model