Time Window in the Dynamic Excitation of Magnetic Nanoparticles

E DE BIASI; ENIO LIMA JR.; C RAMOS; R D ZYSLER; A BUTERA

Manizales, Colombia

Workshop; LAW3M 2010 – 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. Conversely to 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 on our model, including the time and the temperature dependence of the minima population. This model converges accurately to the limits of 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 FMR signal of the high uniaxial anisotropy approximations, as results 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. The sweep rate of the external magnetic field dependence is also studied. Experiments using magnetite (Fe3O4) nanoparticles systems corroborate our model.