ZnFe2O4(001) surface structural study by ab-initio calculation

R. FACCIO; K.L. SALCEDO RODRÍGUEZ; A.V. GIL REBAZA; L. ERRICO; C.E. RODRÍGUEZ TORRES

Simposio; XXIII LatinAmerican Symposium on Solid State Physics - SLAFES XXIII; 2018

The spinel-type compounds (F-3md space group) structure is characterised by two different sites for cations: one with tetrahedral coordination of oxygen atoms (A site) and the other one with an octahedral coordination (B site). The group presents many degree of freedom (cations types and distributions, bonds angles with the anions, etc), giving rise to a large variety of properties and in consequence potential applications in spintronic applications. The magnetic properties of spinel ferrites strongly depend on the distribution of cations (A and B sites). Magnetic coupling in ferrites occurs via super-exchange interactions. Pure bulk zinc ferrite (ZnFe2O4) has only B-O-B interaction because all the A-sites are occupied by Zn atoms, which have no-magnetic moment (normal spinel). This makes ZnFe2O4 paramagnetic at room temperature and antiferromagnetic below 10K. However, when particle size is reduced to the nanoscale, it has been found that some of the iron ions move towards A sites (partially-inverted ferrite) leading to A-O-B coupling that result in ferrimagnetic Fe arrangement and thus enhancing the total magnetic moment of the sample [1]. We present here an ab initio study of the surface and sub-surface structural, electronic and magnetic properties of ZnFe2O4. The study was carried out by means of First Principles calculations based on the Density Functional theory (DFT). In order to solve the Kohn-Sham equations we employed FP-LAPW method as implemented in the Wien2k [2] code. In order to elucidate the degree of inversion and how the inversion depends on the deep position in the surface, we have calculated the necessary energy for an inversion in the bulk material and correspondingly in the surface. We also calculated the X-ray absorption near edge (XANES) spectra at the Zn. Our predictions are compared with the experimental XANES spectra obtained in conventional and grazing incidence configurations, on thin films of zinc ferrite measured in the LNLS (Campinas, Brazil), in order to identify the structural changes associated to the GI-XANES case.