Journal of Magnetism and Magnetic Materials 262 (2003) 235–241 Size and anisotropy determination by ferromagnetic resonance in dispersed magnetic nanoparticle systems E. de Biasi, C.A. Ramos*, R.D. Zysler 1 Centro At ! omico Bariloche and Instituto Balseiro, (8400) San Carlos de Bariloche, R! ıo Negro, Argentina Abstract We present results for the FMR line shape modelling of non-interacting magnetic nanoparticle systems. We compare the results of the Smit and Beljers formalism and the usual linear-model, where the effective anisotropy field, H eff A ; in the superparamagnetic regime is considered as a perturbation to the Zeeman interaction and added to the applied field, H: While the difference between these approaches is negligible for small H eff A (high temperature regime), it becomes more pronounced when H eff A EH: We show how these results influence the determination of the parameters characterizing an array of random particles. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic nanoparticles; Superparamagnetism; Magnetic resonance; Magnetic anisotropy; Ferromagnetic resonance 1. Introduction Recently it has increased noticeably the use of ferromagnetic resonance (FMR) to characterize magnetic nanoparticle systems [1–5]. One of the advantages of FMR over conventional magnetiza- tion measurements is that it yields information on the dynamics of the system. As an example: a mixture of microscopic phases containing a ferromagnetic contribution and a paramagnetic one is not easy to discriminate in a DC magnetiza- tion measurement, while it should be clearly distinguished in an FMR experiment. One of the first contributions to understand the temperature variation of the FMR spectra came from the work of de Biasi and Devezas [3]. More recently Raikher and Stepanov [6] developed a formalism to consider superparamagnetic reso- nances in the limit of large Zeeman interaction as compared to the anisotropy energy. In this limit the effective field acting on the nanoparticle is the linear addition of the applied field, H; and anisotropy fields. Their results indicate that in the superparamagnetic regime the effective aniso- tropy decreases as the temperature increases, and the rate of decrease is determined by the ratio of Zeeman to thermal energy. They also point out that the shape anisotropy has the same functional dependence as an uniaxial anisotropy term, which is different from what was originally predicted [3]. Yet, only recently [4,5,7] these theories have been applied to obtain, from the line shape of the FMR data, the physical parameters that describe the particles (anisotropy, magnetic moment). In particular S ! anchez et al. [5] applied this theory on *Corresponding author. E-mail address: cramos@cab.cnea.gov.ar (C.A. Ramos). 1 Member of the Consejo Nacional de Investigaciones Cient- ! ıficas y T! ecnicas, Argentina. 0304-8853/03/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-8853(02)01496-8