Iron mixed-valence compounds, BaSm(Cu_0.5+xFe_0.5-x)₂O_5+δ. II: the Fe^2.5+ state and magnetic properties

Recently, a fluctuating mixed-valence state formally denoted as Fe^2.5+ was observed in BaSmFe₂O_5+δ double-perovskite samples, having δ < 0.5 [J. Linden, P. Karen, A. Kjekshus, J. Miettinen, T. Pietari, M. Karppinen, Phys. Rev. B 60 (1999) 15251.]. Upon cooling such samples to the Verwey-type transition temperature (T_V ≈ 200 K), a mixing-separation transition of the valence state occurs. In order to study in detail the formation of the mixed-valence state BaSm(Cu_0.5+xFe_0.5-x)₂O_5+δ, samples with a certain amount of Fe (0-10% and 40-50%) replaced by Cu have been characterized by ⁵⁷Fe Mossbauer spectroscopy and magnetization measurements. All samples were found to be magnetically ordered at room temperature. The amount of Fe entering the Fe^2.5+ state decreased rapidly as the Cu doping level was increased, as judged by the Mossbauer spectra. Increasing the Cu concentration also led to an increase of T_V.