<i>Ab initio</i> calculation of structural, electronic and magnetic properties and hyperfine parameters at the Fe sites of pristine ZnFe₂O₄

Abstract

In this work we present an ab initio study of structural, electronic, magnetic and hyperfine properties of pristine Zn-ferrite (ZnFe₂O₄, ZFO). Density Functional Theory calculations were performed using the Full-Potential Linearized Augmented Plane Waves (FPLAPW) method in the framework of the Generalized Gradient (GGA) and the GGA+U approximations. In order to discuss the magnetic ordering and the electronic structure of the system we considered different spin arrangements. We found that ZFO presents an energy landscape characterized by a large number of metastable states separated by an energy barrier of about K_BT_F, being K_B the Boltzmann constant and T_F the freezing temperature, indicating that ZFO can be described as an spin-glass system at low temperature (<10.5 K). Our calculations also support the picture that below 10.5 K small ferromagnetic spin-clusters (short-range interactions) surrounded by similar spin-clusters with opposite spin orientations (long-range interactions) coexist. Finally, our calculations predict a band gap of normal ZFO of 2.2 eV and successfully describe the hyperfine properties (isomer shift, magnetic hyperfine field and electric field gradient tensor) at the Fe sites that are seen by Mossbauer Spectroscopy (MS) at 4.2 and 300 K. This comparison enables us to characterize the local spin structure around Fe atoms and to explain the origin of the two hyperfine interactions experimentally observed, giving support to the coexistence of short- and a long-range order below 10.5 K.