Magnetic phase diagram of nanostructured zinc ferrite as a function of inversion degree delta

Abstract

Magnetic properties of spinel zinc ferrites are strongly linked to the synthesis method and the processing route since they control the microstructure of the resulting material. In this work, ZnFe_2O_4 nanoparticles were synthesized by the mechanochemical reaction of stoichiometric ZnO and alpha-Fe2O3, and single-phase ZnFe_2O_4 was obtained after 150 h of milling. The as-milled samples, with a high inversion degree, were subjected to different thermal annealings up to 600 ºC to control the inversion degree and, consequently, the magnetic properties. The as-milled samples, with a crystallite size of 11 nm and inversion degree delta = 0.57, showed ferrimagnetic behavior even above room temperature, as shown by Rietveld refinements of the X-ray diffraction pattern and superconducting quantum interference device magnetometry. The successive thermal treatments at 300, 400, 500, and 600 degrees C decrease delta from 0.15 to 0.18, affecting the magnetic properties. A magnetic phase diagram as a function of delta can be inferred from the results: for delta < 0.25, antiferromagnetism, ferrimagnetism, and spin frustration were observed to coexist; for 0.25 < delta < 0.5, the ferrimagnetic clusters coalesced and spin glass behavior vanished, with only a pure ferrimagnetic phase with a maximum magnetization of M_s = 3.5 mu_B remaining. Finally, for delta > 0.5, a new antiferromagnetic order appeared due to the overpopulation of nonmagnetic Zn on octahedral sites that leads to equally distributed magnetic cations in octahedral and tetrahedral sites.