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

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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.
©2019 American Chemical Society This work was supported by grants from the Spanish Ministry of Science and Innovation MAT2015-67557 -C2 -1-P