Spin glass-like antiferromagnetic interactions in iron phosphate glasses

The phenomenon of spin freezing at low temperatures in iron-containing oxide glasses resulting from antiferromagnetic interactions has been previously reported for several oxide glass systems. The temperature dependence of the DC magnetic susceptibility has been measured with a SQUID magnetometer for a series of four iron oxide–phosphorus pentoxide glasses, containing between 30 and 44 mol% Fe2O3, and prepared so as to have an Fe3+ fraction of ∼0.8. Well defined cusps are observed in the susceptibilities at temperatures between 5 K and 8 K for the four samples, cooled in zero field and measured at 0.025 T. As for classical metallic spin glass alloys, the susceptibilities measured after cooling from high temperature in this field are almost constant below the cusp temperatures, corresponding to spins freezing into a complicated configuration. Similar measurements at 0.5 T also demonstrate the freezing transition but the cusps in the susceptibility after zero-field cooling are broadened. The distribution and environments of iron ions within the samples are discussed in the light of the temperature-dependent magnetic structure factors and spin correlation functions, observed in neutron diffraction experiments on the same samples, along with the nuclear real space total correlation functions.