Determination of the effective anisotropy of magnetite/maghemite nanoparticles from Mossbauer effect spectra

The stoichiometry and the effective anisotropy constant (K-eff) of several samples of iron-oxide nanoparticles with sizes between 12 and 51 nm, showing spheroidal and cubic shapes, were studied by means of Mossbauer spectroscopy at room temperature (RT). Mossbauer spectra were analyzed and fitted considering two magnetically split subspectra due to Fe3+ and Fe2.5+ ions in tetrahedral and octahedral sites of a non-stoichiometric magnetite, respectively. In order to take into account the collective fluctuations of the NPs magnetic moments, the measured Mossbauer spectra were fitted including a distribution of the hyperfine interactions associated with the NPs volume (V) distribution. To determine the K-eff value, a direct comparison between the size distribution obtained from structural characterization techniques and the KeffV product obtained from Mossbauer spectra fitting was made. In this regard, the K-eff value was determined as the one that maximizes the coincidence between the mean NPs sizes obtained by both methods. Finally, an overall surface anisotropy constant K-s value was obtained by means of a linear relation between the mean NP size, the experimentally determined K-eff and a weighted arithmetic mean magnetic anisotropy related with the bulk magnetite and maghemite anisotropy constants at RT. An evident relation between the KeffV values through the parameter lambda = KeffV/k(B)T and the magnetite/maghemite fraction in each sample was also observed.