Electron paramagnetic resonance and magnetic properties of α-BiNb1-xFexO4-δ

Iron-containing ceramic materials based on BiNbO4 of orthorhombic modification were obtained by solid-phase synthesis. Solid solutions alpha-BiNb1-xFexO4-delta were synthesized in the narrow concentration interval of x <= 0.03. The single-phase structure of the samples prepared was determined by X-ray diffraction and SEM- analyses. In the electron paramagnetic resonance (EPR) spectra of BiNb1-xFexO4-delta, a wide band with the center at g = 2.16 - 2.37 was observed, and a low-intensity signal with g-factor of about 4.3 was recorded on its low-field wing. Along with the broad spectrum component, a noise-like signal formed by a large number of narrow lines related to the integral signal of ferromagnetic resonance (FMR) and its fine structure was observed in the magnetic field range of 190 - 400 mT. Based upon measurements of magnetic susceptibility, the paramagnetic components and the effective magnetic moment values of iron atoms at different temperatures and for different concentrations of solid solutions were calculated. The isotherms of the paramagnetic component of the magnetic susceptibility of iron atoms in BiNb1-xFexO4-delta were typical of antiferromagnets. The value of the effective magnetic moment of iron atoms increased with the rise in temperature from 5.84 mu B at 90 K to 6.43 mu B at 320 K. This can be explained by the presence of exchange-bonded aggregates of Fe (III) atoms with antiferromagnetic and ferromagnetic types of exchange in the solid solutions. The magnetic behavior of BiNb1-xFexO4-delta was investigated by theoretical calculation of magnetic susceptibility within the framework of the model of diluted solid solutions taking into account the presence of possible types of clusters of iron atoms. The best agreement between the experimental and calculated data was obtained for the dimer parameters of J(dim )=-17 cm(-1) and J(dim) = 45 cm(-1) for the antiferromagnetic and ferromagnetic types of exchange.