Structural, magnetic and electronic properties of Fe1+xGa2−xO4 nanoparticles synthesized by the combustion method

Abstract

The combustion method was used to prepare a precursor powder of an iron-gallium oxide compound which was further heat-treated in order to obtain a set of Fe_1+xGa_2−xO₄ nanoparticles. All samples have a cubic spinel-type structure (space group Fdm) and the particle size varies from 1.8 to 28.0 nm depending on the treatment conditions. From the comparative analysis by XRD, EDS, and Raman and Mössbauer spectroscopy the creation of a new spinel phase γ-FeGaO₃, which was mainly located on the particle surface, was established. As a result, the composition consists of a FeGa₂O₄ core covered by a FeGaO₃ shell. The relative content of FeGa₂O₄/FeGaO₃ compounds in the composites can be varied by heat treatment. The maximum in the ZFC magnetization curves appeared in all samples at about 20–30 K corresponding to the spin-freezing temperature T_sg, which is much higher than in the bulk compound with a pure inverse spinel structure (Ga)[FeGa]O₄. The values of effective Curie temperature Θ_C for the Fe_1+xGa_2−xO₄ nanoparticles are rather high and positive, indicating a ferromagnetic interaction between iron ions. The high values of the magnetic frustration parameter f = Θ_C/T_sg (up to 7) indicate a high degree of magnetic frustration. The low temperature Mössbauer data reveal the magnetic ordering of Fe ions in all samples with the magnetic transition at about 20–26 K depending on the particle size. The specific features of the Mössbauer parameters indicate the properties of non-homogeneous magnetic systems with frustrated interactions specific to spin-glasses. The magnetic system behaves as a spin-glass below T_sg and it is superparamagnetic above T_sg. Such a system is called a “super-spin-glass”. The anisotropy energy E_anis strongly depends on the content of Fe²⁺ and Fe³⁺ ions which contribute to the magnetocrystalline E_cryst and exchange E_ex anisotropies, respectively. The anisotropy energy can be tuned by variation of the content of the (FeGaO₃)–(FeGa₂O₄) phases in these complex composites.