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

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.