Magnetic properties and triple valence fluctuations of iron in PrSr2Fe3O9-delta

The compound PrSr2Fe3O9-delta (delta(similar to) < 0.1) has been studied by magnetization and Fe-57 Mossbauer spectroscopy measurements, The magnetiza tion studies show magnetic hysteresis phenomena at low temperatures and thr ee 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 m agnetization curves. Mossbauer studies confirm the T-N and T-v values. Well below and above T-N, the spectra are composed of two well-defined sub-spec tra (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 i n the magnetic hyperfine field (469 and 269 kOe) whereas above T-N the diff erence in the IS is only 0.24 mm/s. For 90 K < T < T-N the spectra ape comp lex: and cannot be interpreted in terms of well defined (2 or even 3) sub-s pectra. All spectra, in the entire temperature range can be well explained and reproduced by a triple valence fluctuation model, assuming that Fe appe ar in three valence stares. (a) 2Fe(3+) + Fe5+ at low temperatures, (b) Fe3 + and 2Fe(4+) above T-N and (c) at 90 K < T < T-N triple valence fluctuatio ns occur: Fe3+ <-> Fe5+ <-> Fe5+. An electron is transferred from Fe3+ to F e5+ to form two nonmagnetic Fe4+ ions, This process dilutes the magnetic io ns concentration to a point of collapse of the magnetic order. (C) 1999 Els evier Science B.V. All rights reserved.