The magnetic state of a CaMnO3−δ crystal with ordered O vacancies was studied, for δ = 0.25, where the numbers of Mn4+ and Mn3+ ions in the manganite were equal to each other), using neutron diffraction. Magnetic scattering in the CaMnO2.75 crystal in the ground state was determined by the wave vector (½,½,½)2π/ac (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappeared at TN = 116K, whereas short-range magnetic order was retained up to 240K. It was shown that the instability of the G-type structure at 60K to TN was associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involved chains with Mn3+-Mn4+ ferromagnetic interaction. A comparison of the results of neutron diffraction investigations, with experimental data on the magnetic characteristics and electrical resistivity, demonstrated that the specific features revealed by the spin system of the CaMnO2.75 crystal were governed directly by competition of the Mn3+-Mn4+ ferromagnetic double exchange with the antiferromagnetic super-exchange between Mn ions.

Magnetic Structure of a CaMnO2.75 Crystal with Ordered Oxygen Vacancies. S.F.Dubinin, N.N.Loshkareva, S.G.Teploukhov, A.V.Korolev, É.A.Neĭfeld, V.E.Arkhipov, V.D.Parkhomenko, Y.P.Sukhorukov, A.M.Balbashov: Physics of the Solid State, 2006, 48[8], 1526-32