Relaxational behaviour and spin freezing in ferrates with iron in a pyramidal oxygen coordination

The crystal structure of both Bi0.5Sr1.5YCu1.5Fe0.5O6 and Sr2Pb0.5Y0.5CuFeO6 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 range 4.2-380 K was undertaken. The Mossbauer spectra of both compounds studied 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 drastically 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 in the basal planes of the quadratic pyramidal layers and of the Fe-Cu superexchange. The relaxational character of the Mossbauer spectra offers evidence for a gradual freezing of the Fe spins, reminiscent of the behaviour of concentrated spin glasses.