Crystal and magnetic structures of the complex perovskites LaCu₃(Mn_4–xFe_x)O₁₂ with colossal magnetoresistance

Abstract

We have studied the series of nominal composition LaCu₃(Mn_4–xFe_x)O₁₂ (x = 0.5, 1) where Mn is replaced by Fe cations in the ferrimagnetic perovskite LaCu₃Mn₄O₁₂. These materials have been synthesized in polycrystalline form under moderate pressure conditions of 2 GPa, in the presence of KClO₄ as oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) at room temperature (RT) in order to study the crystal structure. The sample with nominal x = 1 has also been studied by NPD at low temperature to determine its magnetic structure. These oxides crystallize in the cubic space group Im-3 (No. 204) with general formula A'A₃B₄O₁₂ and contain La³⁺ and (Cu²⁺, Mn³⁺) at the A sublattice of the perovskite, in a 1 : 3 ordered arrangement. NPD suggests that Mn⁴⁺/Mn³⁺ and Fe³⁺ occupy at random the octahedral B positions of the perovskite structure. The materials have also been characterized by magnetic and magnetotransport measurements. All the samples are ferrimagnetic and show a progressive decrease of T_C as the Fe content increases. The replacement of Mn³⁺ cations by Fe³⁺ at the octahedral sublattice give rise to a decrease of T_C since the Fe ions disturb the ferromagnetic interactions within this magnetic sublattice. The introduction of Fe changes the resistivity response from metallic to a semiconductor behavior. However, the magnetoresistance remains unchanged upon Fe doping and a sharp low-field response is observed below T_C.