Effects of pressure on maghemite nanoparticles with a core/shell structure

The magnetic property and intraparticle structure of the γ phase of Fe2O3 (maghemite) nanoparticles with a diameter (D) of 5.1±0.5 nm were investigated through AC and DC magnetic measurements and powder X-ray diffraction (XRD) measurements at pressures (P) up to 27.7 kbar. Maghemite originally exhibits ferrimagnetic ordering below 918 K, and has an inverse-spinel structure with vacancies. Maghemite nanoparticles studied here consist of a core with structural periodicity and a disordered shell without the periodicity, and core shows superparamagnetism. The DC and AC susceptibilities reveal that the anisotropy energy barrier (ΔE/kB) and the effective value of the core moment decrease against the initial pressure (P≤3.8 kbar), recovering at P≥3.8 kbar. The change of ΔE/kB with P is qualitatively identical with that of the core moment, suggesting a down-and-up fluctuation of the number of Fe3+ ions constituting the core at the pressure threshold of about 4 kbar. This phenomenon was confirmed by the analysis of the XRD measurement using Scherrer’s formula. The core volume decreased for P≤2.5 kbar, whereas at higher pressure the core was restructured. For 2.5≤P≤10.7 kbar, the volume shrinkage of particle hardly occurs. There, ΔE/kB is approximately proportional to the volume associated to the ordered fraction of the nanoparticles as seen from XRD, Vcore. From this dependence it is possible to separate the core/shell contribution to ΔE/kB and estimate core and surface anisotropy constants. As for the structural experiments, similar experimental data have been obtained for D=12.8±3.2 nm as well.