Magnetocrystalline and Surface Anisotropy in CoFe2O4 Nanoparticles.

Valentina Mameli,María Salvador,Franco D’Orazio,Gaspare Varvaro,Valeria Rodionova,Anna Musinu,Carla Cannas,Montserrat Rivas,Davide Peddis,Dino Fiorani,Alexander Omelyanchik

doi:10.3390/nano10071288

Abstract

The effect of the annealing temperature Tann on the magnetic properties of cobalt ferrite nanoparticles embedded in an amorphous silica matrix (CoFe2O4/SiO2), synthesized by a sol-gel auto-combustion method, was investigated by magnetization and AC susceptibility measurements. For samples with 15% w/w nanoparticle concentration, the particle size increases from ~2.5 to ~7 nm, increasing Tann from 700 to 900 °C. The effective magnetic anisotropy constant (Keff) increases with decreasing Tann, due to the increase in the surface contribution. For a 5% w/w sample annealed at 900 °C, Keff is much larger (1.7 × 106 J/m3) than that of the 15% w/w sample (7.5 × 105 J/m3) annealed at 700 °C and showing comparable particle size. This indicates that the effect of the annealing temperature on the anisotropy is not only the control of the particle size but also on the core structure (i.e., cation distribution between the two spinel sublattices and degree of spin canting), strongly affecting the magnetocrystalline anisotropy. The results provide evidence that the magnetic anisotropy comes from a complex balance between core and surface contributions that can be controlled by thermal treatments.

Highlights

Within the last few years, magnetic nanoparticles have contributed to the development of a variety of cutting edge technologies in fields such as ferrofluids [1], microwave devices [2], biomedicine [3,4], or catalysis [5,6]
The results provide evidence that the relative surface and core contributions to the effective magnetic anisotropy and saturation magnetization of CoFe2O4 nanoparticles embedded in a silica matrix can be controlled by the annealing temperature Tann
For samples with 15% w/w of nanoparticles, the value of the effective magnetic anisotropy constant Keff increases and the saturation magnetization decreases by decreasing Tann from 900 to 700 ◦C, with a decrease in particle size, showing a dominant role of the disordered surface

Summary

Introduction

Within the last few years, magnetic nanoparticles have contributed to the development of a variety of cutting edge technologies in fields such as ferrofluids [1], microwave devices [2], biomedicine [3,4], or catalysis [5,6]. The growing interest that magnetic nanoparticles attract demands a fundamental understanding of their properties, which are very different from their bulk counterparts In this context, spinel ferrites are excellent candidates thanks to their tunable physico-chemical properties [7]. The atomic arrangement corresponds to a face-centered cubic structure of the oxygen atoms, with Fe3+ and M2+ occupying the tetrahedral (Td) and octahedral (Oh) sites [7] Such a structure makes magnetic spinel nanoparticles attractive. It provides a tool to tailor their magnetic properties (e.g., magnetic crystalline anisotropy and saturation magnetization) by the variation of the cation distribution between the two sublattices This can be done by changing the chemical composition, the preparation method, and thermal treatments [8,9,10]

Methods

Results

Conclusion