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

OROZCO-HENAO, J M; D. MURACA; SÁNCHEZ, F H; MENDOZA ZÉLIS, P

JOURNAL OF PHYSICS - D (APPLIED PHYSICS)

IOP PUBLISHING LTD

Lugar: Londres; Año: 2022 vol. 55

0022-3727

The stoichiometry and the effective anisotropy constant (Keff) of several samples of iron-oxide nanoparticles with sizes between 12 and 51 nm, showing spheroidal and cubic shapes, were studied by means of Mössbauer spectroscopy at room temperature (RT). Mössbauer spectra were analyzed and fitted considering two magnetically split subspectra due to $Fe^{3+}$ and $Fe^{2.5+}$ 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 Mössbauer spectra were fitted including a distribution of the hyperfine interactions associated with the NPs volume (V) distribution. To determine the Keff value, a direct comparison between the size distribution obtained from structural characterization techniques and the $K_{eff}V$ product obtained from Mössbauer spectra fitting was made. In this regard, the Keff 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 Ks value was obtained by means of a linear relation between the mean NP size, the experimentally determined Keff and a weighted arithmetic mean magnetic anisotropy related with the bulk magnetite and maghemite anisotropy constants at RT. An evident relation between the $K_{eff}V$ values through the parameter $lambda = K_{eff}V/k_BT$ and the magnetite/maghemite fraction in each sample was also observed.