Neutron diffraction, Moessbauer and magnetotransport study of Fe-substituted derivatives of CaCu3Mn4O12 perovskite with colossal magnetoresistance

The effect of replacing Mn for Fe in the ferrimagnetic perovskite CaCu3Mn4O12 has been studied in the series of nominal composition CaCu3(Mn4-xFex)O12 (x = 0, 0.5, 1.0). These materials have been prepared in polycrystalline form under moderate pressure conditions of 2 GPa, in the presence of KClO4 as oxidizing agent. The x = 1.0 sample has been studied by neutron powder diffraction and Moessbauer spectroscopy to unravel some structural peculiarities. These oxides crystallize in the cubic space group Im3-bar (No. 204) and contain Ca2+ and (Cu2+, Mn3+) at the A sublattice of the ABO3 perovskite, arranged in a 1:3 ordered array. Neutron diffraction suggests that Mn4+ and Fe3+ occupy at random the octahedral B positions of the perovskite structure, whereas the Moessbauer study adds a singular feature: 89% of Fe3+ is, effectively, in an octahedral environment, whereas 11% of Fe3+ is located at square-planar positions together with Cu2+. The materials have also been characterized by magnetic and magnetotransport measurements. All the samples are ferromagnetic and show a progressive decrease of TC as the Fe contents increases. Also the saturation magnetization and Weiss temperatures diminish in the Fe-doped oxides, which suggest an increment of the antiferromagnetic interactions at the (Mn, Fe) octahedral sublattice. The magnetoresistance also decreases upon Fe doping, although a sharp low-field response is observed below TC