Time window in the dynamic excitation of magnetic nanoparticles

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

Manizales

Congreso; X Latin American Workshop on Magnetism, Magnetic Materials and their Applications (LAW3M2010); 2010

In this work we present a new theoretical treatment for the ferromagnetic resonance (FMR) in a system of anisotropic magnctic nanoparticles. We have studied the thermal effects on the effective magnetization and anisotropy fields covering a wide range of anisotropy energies. Conversely to previously reported rnodels[l], we have assumed that the experimental time window is associated to the sweep rate of the externa1 field value, and is not related to the microwave excitation frequency. The high anisotropy case is considered on our model, including the time and 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 [2,3,4]) as well as to the low temperature limit, for which the ferromagnetic formalism is naturally recovered. Our results explain the loss of FMR signal of the high uniaxial anisotropy approximation as a result of the increase of the effective anisotropy field at low temperatures. In the blocked regime the model predicts an important dependence on the magnetic history of the sample. We have also studied the dependence of the  magnetic response with the sweep rate of the externa1 magnetic field. FMR experiments using Fe304 nanoparticles systems corroborate our model.