Magnetic properties of mixed ferro-ferrimagnets composed of Prussian blue analogs

We have succeeded in controlling the saturation magnetization (I-s), the Weiss temperature (theta), and the coercive field (H-c) using compounds in the series ((NixMnI-xII)-Mn-II)(1.5)[Cr-III(CN)(6)] as model compounds. The key to this strategy is to manipulate both ferromagnetic (J>0) and antiferromagnetic (J<0) exchange interactions by incorporating the appropriate molar ratios of the transition-metal ions. Minimum values of I-s were found for x values close to 3/7 (0.429), just at the point where parallel spins (Cr-III and Ni-II) and antiparallel spins (Mr(II)) should completely cancel out. The theta(c) values increased monotonically from negative to positive with increasing x, indicating that the predominant interaction mode was shifting from antiferromagnetic to ferromagnetic. The highest coercive field was observed for a compound with an x value close to 3/7. The magnetization vs temperature curves below T-c exhibited various types of behavior, depending on x. For example, the curves for x=0 and x=1 exhibited monotonic increases in magnetization below T-c with decreasing T, while the curve for x=0.45 exhibited a single maximum, and that for x=0.38 exhibited two maxima in a field of 1000 G. Of particular interest is the fact that the compound for which x was 0.38 exhibited negative values of magnetization in a field of 10 G below approximately 39 K and positive values above this temperature, showing that the magnetic pole can be inverted. We analyzed these temperature dependences using molecular-field theory with three types of sublattice (Ni, Mn, Cr) sites. This phenomenon is observed because the negative magnetization due to the Mn-II sublattice and the positive magnetizations due to the Ni-II and Cr-III sublattices have different temperature dependences. [S0163-1829(97)05042-X].