Ab initio study of magnetite (Fe304). The verwey transition, the mossbauer spectrum, the half metallic character vs the semiconducting nature and the role of the symmetry

MEDINA CHANDUVÍ H. H.; MUDARRA NAVARRO A. M.; RODRÍGUEZ TORRES C. E.; GIL REBAZA A. V.; ERRICO L. A.; SALCEDO RODRÍGUEZ K. L.; MELO QUINTERO J. J.

Brasov

Conferencia; International Conference on the Applications of the Mössbauer Effect; 2021

Institute of Materials Physics (NIMP)

Ferrites (XFe2O4) are a family of magnetic oxides that, due to their numerous technological applications and intriguing magnetic properties, becomes one of the most extensively studied magnetic materials. Ferrites crystallize in the spinel face-centered cubic structure and are characterized by an atomic arrangement of two sites for the cations: sites A (tetrahedral oxygen coordination) and sites B (octahedral oxygen coordination). Two types of ferrites can be distinguished, normal and inverted. In the first case, Fe2+ ions occupy the A sites and the Fe3+ ions occupy the B sites. In the case of the completely inverted ferrites, the A sites are populated by Fe3+ ions and Fe3+ and Fe2+ ions occupy the B sites in the same proportion. There are also cases of partial inversion.Iron-ferrite (Fe3O4, magnetite, space group Fd3m) is a magnetic material with many technological applications (computer memory, magnetic recording, spintronic devices, among others) and has been extensively studied for the past half-century. But, the issue of the charge order below the so-called Verwey transition temperature (TV, in the order of 120 K) has not been understood. Above TV magnetite presents a half metallic behavior. Below TV the conductivity of the system decreases abruptly and a semiconducting character (with a small band gap) is reported. This first-order metal-insulator transition is associated with a reduction of the symmetry of magnetite form the cubic Fd3m structure to a monoclinic Cc structure. The transition has been viewed as an order-disorder transition in relation to the arrangement of cations on the octahedral sites of the inverse spinel structure.In this work we present ab initio calculations in order to determine the electronic, magnetic and hyperfine properties of Fe3O4. Calculations have been performed using the Full Potential Linearized Augmented Plane-Wave (FP-LAPW) methods. GGA+U and state-of-theart hybrid exchange-correlation functionals were employed (Heyd-Scuceria-Ernserhof, HSE06, and the Tran-Blaha modified Becke-Johnson, TB-mBJ). We will show here that even in the cubic structure, the reduction of the symmetry of the system generates the half-metallic-semiconducting transition. A minimum band gap of 0.2 eV is predicted. We also predict that the reduction of the symmetry split the B sublattice in mainly two groups of crystallographically equivalent characterised by 2+ and 3+ oxidation states, slightly different magnetic moments and different hyperfine parameters. Our results are in agreement with Mössbauer experimental results and confirms calculations reported previously but now in the framework of extremely well converged calculations and considering state-of-the art exchange and correlation potentials.