Magnetic properties and triple valence fluctuations of iron in PrSr₂Fe₃O_9-δ

The compound PrSr₂Fe₃O_9-δ (δ^~<0.1) has been studied by magnetization and ⁵⁷Fe Mössbauer spectroscopy measurements. The magnetization studies show magnetic hysteresis phenomena at low temperatures and three distinct phase transitions; an antiferromagnetic transition at T_N=190 K, a transition at T_v=170 K associated with change in iron valence and T_irr=270 K, the merging temperature of the zero field and field cooled magnetization curves. Mössbauer studies confirm the T_N and T_v values. Well below and above T_N, the spectra are composed of two well-defined sub-spectra (ratio 2 : 1), though all Fe ions reside in one crystallographic site. At 4.2 K the subspectra differ in their isomer shift (IS) by 0.4 mm/s and in the magnetic hyperfine field (469 and 269 kOe) whereas above T_N the difference in the IS is only 0.24 mm/s. For 90 K<T<T_N the spectra are complex and cannot be interpreted in terms of well defined (2 or even 3) sub-spectra. All spectra, in the entire temperature range can be well explained and reproduced by a triple valence fluctuation model, assuming that Fe appear in three valence states. (a) 2Fe³⁺+Fe⁵⁺ at low temperatures, (b) Fe³⁺ and 2Fe⁴⁺ above T_N and (c) at 90 K<T<T_N triple valence fluctuations occur: Fe³⁺↔Fe⁴⁺↔Fe⁵⁺. An electron is transferred from Fe³⁺ to Fe⁵⁺ to form two nonmagnetic Fe⁴⁺ ions. This process dilutes the magnetic ions concentration to a point of collapse of the magnetic order.