Theoretical treatment of the mixed ferro-ferrimagnets composed of ternary-metal Prussian blue analogs in a paramagnetic region

Our objective in the series of the present work is to obtain a type of magnet in which the weight average of ferromagnetic and ferrimagnetic characters is precisely controlled by fine tuning of the composition. For this attempt, we have prepared the (NiₓIIMn_1-xII)1.5[CrIII(CN)₆]·zH₂O system, incorporating both ferromagnetic NiII-CrIII (JNiCr) and antiferromagnetic MnII-CrIII (JMnCr) exchange interactions. In this work, its magnetic properties in a paramagnetic region, such as magnetic susceptibility ({χ}) and Weiss temperature (θc), were studied. In general, the χ^-1 value for typical ferromagnets and ferrimagnets should be zero at Curie temperature (Tc). However, the observed χ^-1 value for $x=0.30$ of this system did not drop off at Tc. The theoretical analysis based on a molecular-field theory considering both ferromagnetic (JNiCr>0) and antiferromagnetic (JMnCr<0) exchange interactions showed that this phenomenon occurred because a compensation temperature is close to Tc. In addition, it showed that the variation of the θc value is not due to changes in the magnitudes of superexchange interactions but due to changes in the weighted average of JNiCr and JMnCr as a function of x.