Magnetic properties of mixed-valence iron phosphate glasses

Magnetic properties of mixed-valence iron phosphate glasses, where there coexist Fe2+ and Fe3+ ions, have been investigated. The molar fraction of Fe3+ with respect to the total iron ion, [Fe3+]/[Fe-total], can be controlled by melting the glass at varied temperatures. Experiments of magnetic aging and memory effects as well as dynamic and static scaling analyses of relaxation time and nonlinear magnetic susceptibility have been performed to get insight into the nature of low-temperature magnetic phase of the glass system. The experimental results reveal that the iron phosphate glasses undergo paramagnet-spin-glass transitions at low temperatures. Temperature dependence of magnetic specific heat suggests that as the temperature is lowered, the magnetic moments start to be frozen at a temperature significantly higher than the spin-glass transition temperature accompanied by a deviation in magnetic susceptibility from Curie-Weiss law. The ratio of the absolute value of Weiss temperature to spin-glass transition temperature increases as the ratio [Fe3+]/[Fe-total] becomes larger. This behavior is explainable in terms of the difference in single-ion anisotropy between Fe3+ and Fe2+ ions.