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

The magnetic property and intraparticle structure of the gamma phase of Fe2O3 (maghemite) nanoparticles with adiameter(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 918K, 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 (Delta E/ k(B)) 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 Delta E/k(B) 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, Delta E/k(B) is approximately proportional to the volume associated to the ordered fraction of the nanoparticles as seen from XRD, V-core. From this dependence it is possible to separate the core/shell contribution to DE/k(B) 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. (C) 2010 Elsevier B.V. All rights reserved.