Mechanisms of spin-flipping and metal-insulator transition in nano-Fe3O4

Abstract

Fe3O4 is a half-metallic ferrimagnet with K exhibiting metal-insulator transition (MIT) at ∼120 K. In bulk form, the saturation magnetization is 0.6 Tesla (∼471 emu cm-3). A recent experimental study has shown that the saturation magnetization of nano-Fe3O4 thin films can achieve up to ∼760 emu cm-3, attributed to spin-flipping of Fe ions at tetrahedral sites assisted by oxygen vacancies (V O). Such a system has shown to have higher MIT temperature (∼150 K). The spin-flipping is a new phenomenon in Fe3O4, while the MIT is a long-standing one. Here, we propose a model and calculations to investigate the mechanisms of both phenomena. Our results show that, for the system without V O, the ferrimagnetic configuration is energetically favorable. Remakably, upon inclusion of V O, the ground-state configuration switches into ferromagnetic. As for the MIT, by proposing temperature dependences of some hopping integrals in the model, we demonstrate that the system without and with V O undergo the MIT in slightly different ways, leading to higher MIT temperature for the system with V O, in agreement with the experimental data. Our results also show that the MIT in both systems occur concomitantly with the redistribution of electrons among the three Fe ions in each Fe3O4 formula unit. As such temperature dependences of hopping integrals may arise due to dynamic Jahn-Teller effects, our phenomenological theory may provide a way to reconcile existing theories relating the MIT to the structural transition and the charge ordering. © 2017 IOP Publishing Ltd.