Magnetic Phase Transition of the Mixed Antiferromagnets Ni1−xMxCl2·2H2O (M=Co, Mn)

Mixed antiferromagnets Ni1−xMxCl2·2H2O (M=Co, Mn) were prepared. The crystal structure of NiCl2-2H2O is a little different from that of CoCl2·2H2O and MnCl2·2H2O. In order to examine how Co or Mn spins in NiCl2·2H2O crystal structure behave, we determined precisely the phase transition temperatures by measuring the specific heats and have obtained the concentration dependence of the phase transition temperature. Substitution of Co for Ni increases a little the transition temperature and contrary to this the substitution of Mn decreases the transition temperature rapidly. The results are discussed on the basis of molecular field theory. In the case of M=Co, the concentration dependence of the phase transition temperature is well explained by molecular field theory. But, in the case of M=Mn, the molecular field theory cannot explain it sufficiently. Thus Mn spins in NiCl2·2H2O crystal show the peculiar behavior. We suppose that this may be attributed to a kind of the instability of Mn spins.