Synthesis, characterization and superparamagnetic resonance studies of ZnFe2O4 nanoparticles.

Superparamagnetic nanoparticles of zinc ferrite (ZnFe2O4) were produced by a microwave induced combustion synthesis method. XRD, FT-IR, SEM, VSM and ESR were used for the structural, morphological, and magnetic investigation of the product, respectively. Average particle size of the nanoparticles was estimated by the Schérrer equation using the full-width at half maximum (FWHM) of the most intense XRD peak and found as 41 nm. Magnetization measurements have shown that the samples have a blocking temperature of 72 K which indicates a superparamagnetic behavior. Superparamagnetic resonance (SPR) spectra at room temperature show a broad line with a Landé g-factor, g(eff) approximately 2. We used a theoretical formalism based on a distribution of diameters of the nanoparticles following lognormal proposed by Berger et al. The nanoparticles behave as single magnetic domains with random orientations of magnetic moments which are subject to thermal fluctuations. A Landau-Lifshitz line shape function presents adequate results which are in good agreement with the experimental ones. At high temperatures, the SPR line shape is governed by the core anisotropy and the thermal fluctuations. By decreasing the temperature, the magnetic susceptibility of shell spins increases. As a result of this, the surface spins produce an effective field on the core leading to a decrease of resonance field, B(r). Also, the effective anisotropy increases as the shell spins begin to order. So, the results are interpreted by a simple model, in which each single-domain nanoparticle is considered as a core-shell system, with magneto-crystalline anisotropy on the core and surface anisotropy on the shell.