Synthesis and magnetic properties of Li-Mn-Cu spinel oxides

LiMn2-XCuXO4 (X < 0.5) compounds were synthesized by the citrate gel method. It was found that the specimens with X < 0.3 had a cubic symmetry of Fd3m, while further substitution (X > 0.4) lowered the symmetry from Fd3m to P4(3)32. The Cu2+-substitution led to the enhancement of low-temperature magnetic susceptibility M/H. Simultaneously, the Weiss constant changed its sign from negative to positive. This implied that the dominant exchange interaction varies from antiferromagnetic to ferromagnetic couplings. For the compounds with X = 0.5, the magnetic moment per chemical formula at 4.2K was 3.99 mu B and Curie temperature was approximately 39 K. The value of the magnetic moment was close to the theoretical value, in which a ferrimagnetic collinear spin structure was attained: the spins of the Cu2+ ion were antiferromagnetically coupled with those of the Mn4+ ion. Additionally, the magnetic properties of Li-Mn-Cu spinel oxides were briefly discussed, comparing them with those of Li-Mn-M (M = Ni, Mg) spinel oxides.