Time window in the dynamic excitation of magnetic nanoparticles

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

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

In this work we preserit a new theoretical treatment for the ferromagnetic resonarice (FMR) in a system of anisotropic magnctic nanoparticles. We have studied the thermal effects on the effective magnetization and anisotropy fields covcring a wide range of anisotropy energies. Conversely to prcviously reported rnodels[l], we have assumed that the expcrimental tirne window is associated to the sweep rate of the externa1 field value, and is nnt related to the microwave excitation frequency. The high anisotropy case is considered on our model, including the time and ten~perature 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 ferromagnctic 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 Measuring techniques ond instrumentorion magnetic response with the sweep rate of the externa1 magnetic field. FMR experiments using Fe304 nanoparticles systems corroborate our 111odel Re f2rrnce.s [I]R. S. de Biasi. W. S. D. Folly. Phys. B. Corid. Mater. 321, 117 (2001). [2]R. S. de Biasi. and T. C. Devezas, J. Appl. Phys. 49, 2466 ( 1978). [3]Yu. L. Raikhcr, and V. l. Stepanov. Sov. Phys. JETP. 75, 764 (1 992). [4]E. De Biasi, C. A. Ramos, and R. D. Zyslcr. J. Magri. Magn. Mater. 262,235 (2003)