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

We have succeeded in controlling the saturation magnetization ${(I}_S),$ the Weiss temperature (θ), and the coercive field ${(H}_c)$ using compounds in the series $\left({\mathrm{Ni}}_x^{\mathrm{II}}{\mathrm{Mn}}_{1-x}^{\mathrm{II}}{)}_{1.5}{[\mathrm{Cr}}^{\mathrm{III}}\right(\mathrm{C}\mathrm{N}{)}_6]$ as model compounds. The key to this strategy is to manipulate both ferromagnetic $\mathrm{}(J>\mathrm{}0\mathrm{})\mathrm{}$ and antiferromagnetic $\mathrm{}(J<\mathrm{}0\mathrm{})\mathrm{}$ 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 (${\mathrm{Cr}}^{\mathrm{III}}$ and ${\mathrm{Ni}}^{\mathrm{II}}$) and antiparallel spins $\left({\mathrm{Mr}}^{\mathrm{II}}\right)$ 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 $x.\mathrm{}$ For example, the curves for $x=0\mathrm{}$ and $x=1\mathrm{}$ exhibited monotonic increases in magnetization below $T_c$ with decreasing $T,$ while the curve for $x=0.45\mathrm{}$ exhibited a single maximum, and that for $x=0.38\mathrm{}$ 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 ${\mathrm{Mn}}^{\mathrm{II}}$ sublattice and the positive magnetizations due to the ${\mathrm{Ni}}^{\mathrm{II}}$ and ${\mathrm{Cr}}^{\mathrm{III}}$ sublattices have different temperature dependences.