Evolution of the magnetic structure with chemical composition in spinel iron oxide nanoparticles

Abstract

Magnetic properties of iron oxide nanoparticles with spinel structure are strictly related to a complex interplay between cationic distribution and the presence of a non-collinear spin structure (spin canting). With the aim to gain better insight into the effect of the magnetic structure on magnetic properties, in this paper we investigated a family of small crystalline ferrite nanoparticles of the formula CoxNi1-xFe2O4 (0 ≤x≤ 1) having equal size (≈4.5 nm) and spherical-like shape. The field dependence of magnetization at low temperatures indicated a clear increase of magnetocrystalline anisotropy and saturation magnetization (higher than the bulk value for CoFe2O4: ∼130 A m(2) kg(-1)) with the increase of cobalt content. The magnetic structure of nanoparticles has been investigated by Mössbauer spectroscopy under an intense magnetic field (8 T) at a low temperature (10 K). The magnetic properties have been explained in terms of an evolution of the magnetic structure with the increase of cobalt content. In addition a direct correlation between cationic distribution and spin canting has been proposed, explaining the presence of a noncollinear spin structure in terms of superexchange interaction energy produced by the average cationic distribution and vacancies in the spinel structure.

The interplay between single particle anisotropy and interparticle interactions in ensembles of magnetic nanoparticles

“…It was evaluated as the field at which the difference between the two branches of the M(H) curves was reduced down to 1% of their maximum value. 23 The larger anisotropy energy of sample CFO can be ascribed to its higher Co content. Cobalt ions have a marked magneto-crystalline anisotropy far above that of nickel and iron, indeed its orbital magnetic moment is not quenched by the crystal field, with a particular strong spin-orbit coupling for Co 2+ ions in octahedral sites.…”

“…[47][48][49] The difference in saturation magnetization among CFO and CNFO can be explained by the different magnetic moments of Co and Ni and an unusual cationic distribution, as discussed in ref. 23.…”

“…20 In this context, the magnetic behavior of an ensemble of nanoparticles can be described as a complex interplay between the single particle anisotropy (SPA) and the interparticle interactions (II). The effect of SPA 12,[21][22][23][24] and II 20,25-27 on the magnetic properties of the nanoparticles has been thoroughly discussed in the literature. Nevertheless, a clear approach to separate and quantify the individual contribution of SPA and II in the dense ensembles of the nanoparticles is still lacking, since, due to the overlapping of the two contributions, the collective order is usually hidden by the SPM blocking behavior.…”

On the magnetic anisotropy and nuclear relaxivity effects of Co and Ni doping in iron oxide nanoparticles

“…This is in agreement with the values reported in the literature, both for bulk ferrites 22,23 and nanoparticles. 24 The fluctuation rate 1/s N of the magnetization between the energy minima slows down when the temperature decreases. At a certain temperature T B , defined as blocking temperature, the N eel relaxation time s N becomes comparable to the experimental measuring time s m .…”

Temperature-dependent dynamic correlations in suspensions of magnetic nanoparticles in a broad range of concentrations: a combined experimental and theoretical study

“…Secondly, one can modify the internal structure of the particles varya Ural Federal University, Lenin av. 51 34,35 or creating core-shell particles with various types of agglomerates serving as a core, thus providing particle internal anisotropy [36][37][38][39][40][41] . High demand for specific magnetic building blocks stems from the potential for diagnostics, therapy and in-vitro analysis 2,42-45 .…”