Enhancement of magnetoresistance and ferromagnetic coupling in the complex perovskites CaCu_3(Mn_(4-x)Al_x)O_(12) (x = 0, 0.2, 0.4, and 0.6): A neutron diffraction study

摘要

New compounds of the series CaCu_3(Mn_(4-x)Al_x)O_(12) have been prepared under high pressure conditions (2 GPa), in the presence of KC1O_4 as oxidizing agent to stabilize Mn~(3+, 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 ABO_3 perovskite given by the long-range 1:3 ordering of Ca~(2+) and Cu~(2+) ions at the A sublattice. The NPD study for x = 0.4 shows that Al~(3+) ions are statistically distributed at the octahedral positions, being the (Mn,Al)O_6 octahedra strongly tilted, with superexchange (Mn,Al)-O-(Mn,Al) angles of 142.1°. Also, neutron data clearly show that some Mn~(3+) ions (0.65(2) per formula) are located together with Cu~(2+) at the square-planar 6b positions. Regarding the magnetic properties, all the compounds present a spontaneous increase of the magnetization below T_C, typical of ferro-or ferrimagnetic materials, with T_C decreasing upon Al introduction. The magnetic structure determined from low-temperature NPD data unveils a ferromagnetic coupling between (Cu~(2+), Mn~(3+))_(6b) spins and Mn_(8c) spins at octahedral positions; this is in contrast with the ferrimagnetic structure observed for RCu_3Mn_4O_(12) and CaCu_3Mn_4O_(12), 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.