RELAXATIONAL BEHAVIOR AND SPIN FREEZING IN FERRATES WITH IRON IN A PYRAMIDAL OXYGEN COORDINATION

The crystal structure of both Bi0.5Sr1.5YCu1.5Fe0.5O6 and Sr2Pb0.5Y0.5 CuFeO6 is basically composed of double layers of (Cu,Fe)O-5 pyramids. In order to better understand the magnetic properties of this type of compound, a Fe-57 Mossbauer spectroscopy study in the temperature rang e 4.2-380 K was undertaken. The Mossbauer spectra of both compounds st udied present a strong relaxational character. The lines, magnetically split at 4.2 K, become broader and more asymmetrical with increasing temperature. The amount of the paramagnetic phase Increases drasticall y and the magnetic splitting vanishes at 340 K for Sr2Pb0.5Y0.5CuFeO6 and at 295 K for Bi0.5Sr1.5YCu1.5Fe0.5O6. The results are discussed in terms of the superexchange interactions between the Fe atoms placed i n the basal planes of the quadratic pyramidal layers and of the Fe-Cu superexchange. The relaxational character of the Mossbauer spectra off ers evidence for a gradual freezing of the Fe spins, reminiscent of th e behaviour of concentrated spin glasses.