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57Fe-Mössbauer and Magnetic Susceptibility Studies of Iron Phosphate Glass Prepared by Sol-Gel Method

Oxide glasses shows some characteristic behavior at low temperature region. However, there are only few reports about the chemical states of magnetic ions effecting the magnetic properties of oxide glass because structure of oxide glass is amorphous. In this study, the relationship between the local structure and magnetic properties of iron phosphate glass (50Fe2O3·50P2O5 glass, composition in mass%) prepared by sol-gel method was investigated by measuring 57Fe Mössbauer spectra, X-ray diffraction pattern (XRD), dc and ac magnetic susceptibility. Only a halo patterns were detected in the XRD diffractograms of 50Fe2O3·50P2O5 glass annealed (HT) at 200, 300 and 400 oC for 2 h. After the isothermal annealing at 500 oC for 2 h, in contrast, diffraction peaks due to α-Fe2O3 and FePO4 were detected. Mössbauer spectra of 50Fe2O3·50P2O5 glass, recorded before the annealing (Fig. 1(a)), consisted of a doublet with isomer shift (IS) and quadrupole splitting (QS) of 0.33 and 0.63 mm s-1, respectively, reflecting the presence of distorted FeIIIO4 tetrahedra. On the other hand, Mössbauer spectra of 50Fe2O3·50P2O5 glass annealed at higher than 200 oC for 2 h (Fig. 1(b)-(d)) consisted of two doublets. In case of the glass HT at 200 oC, one doublet with IS and QS of 1.0 and 2.6 mm s-1, respectively, was ascribed to FeIIO6 octahedra, and the other with IS and QS of 0.31 and 0.76 mm s-1, respectively, was ascribed to FeIIIO4 tetrahedra. The Mössbauer parameters of the glass HT at 300 oC were similar to that of the glass HT at 200 oC. However, the FeIII/FeII ratio of the glass HT at 200 oC was higher than that of the glass HT at 300 oC. From the temperature dependency of dc magnetic susceptibility under zero field cooling, the value of magnetic moment was the highest around 5 K. In measurements of ac magnetic susceptibility, this peak shifted towards higher temperatures with increasing in frequency of magnetic field. From the relationship between cusp temperature and relaxation time, the value of microscopic relaxation time and the dynamical exponent were estimated 0.22 ns and 13.9, respectively, in the case of the glass heat treated at 200 oC, when the magnetic property was ascribed to spin glass[1]. On the other hand, the value of the dynamic exponent decreased with heat treatment at higher temperature, of which the magnetic properties changed spin glass to superparamagnetism with heat treatment at higher temperature.

iron phosphate glass, sol-gel method, 57Fe Mössbauer spectroscopy, X-ray diffraction

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