Giant magnetoresistance of novel ferromagnets AMg4Mn6O15 (A=K, Rb, and Cs) with highly symmetric structure

A novel family of cubic manganese oxides, AMg4Mn6O15 (A=K, Rb, and Cs), were discovered. In this type of structure, the MnO6 octahedra share edges, constructing a highly symmetric framework where Mn atoms form a three-dimensional network of truncated octahedra. The crystallographic site of Mn is unique and the average oxidation state of Mn is 3.5, indicating a mixed-valence electronic state where Mn3+ and Mn4+ sites are completely indistinguishable. These compounds become ferromagnetic with fully polarized magnetic moments of Mn ions. The ferromagnetic transition temperature TC is 170K, that is considerably high for manganese oxides. The electric resistivity of KMg4Mn6O15 is about 105Ωcm at 300K and exhibits a non-metallic temperature dependence. It reveals a large negative magnetoresistance; about 40% of the resistivity is suppressed by 5T of magnetic field at TC.