Enhancement of magnetoresistance and ferromagnetic coupling in the complex perovskites CaCu3(Mn4−xAlx)O12 (x = 0, 0.2, 0.4, and 0.6): A neutron diffraction study

New compounds of the series CaCu3(Mn4−xAlx)O12 have been prepared under high pressure conditions (2 GPa), in the presence of KClO4 as oxidizing agent to stabilize Mn3+,4+ mixed valence. The polycrystalline samples have been characterized by x-ray diffraction, neutron powder diffraction (NPD), magnetic, and magnetotransport measurements. All the samples are cubic, space group Im-3. These oxides adopt a superstructure of ABO3 perovskite given by the long-range 1:3 ordering of Ca2+ and Cu2+ ions at the A sublattice. The NPD study for x = 0.4 shows that Al3+ ions are statistically distributed at the octahedral positions, being the (Mn,Al)O6 octahedra strongly tilted, with superexchange (Mn,Al)-O-(Mn,Al) angles of 142.1°. Also, neutron data clearly show that some Mn3+ ions (0.65(2) per formula) are located together with Cu2+ at the square-planar 6b positions. Regarding the magnetic properties, all the compounds present a spontaneous increase of the magnetization below TC, typical of ferro-or ferrimagnetic materials, with TC decreasing upon Al introduction. The magnetic structure determined from low-temperature NPD data unveils a ferromagnetic coupling between (Cu2+, Mn3+)6b spins and Mn8c spins at octahedral positions; this is in contrast with the ferrimagnetic structure observed for RCu3Mn4O12 and CaCu3Mn4O12, where an AFM coupling is observed between both magnetic sublattices. Interestingly, an enhancement of the magnetoresistance effect is observed for x = 0.2, well beyond that found for the parent compound. This effect, in materials subtly doped with non-magnetic elements at the Mn positions, may be of interest for applications.
We thank the financial support of the Spanish Ministry of Economy and Competitiveness to the projects MAT2013-41099-R and RyC-2010-06276.