Crystal field effects in the 3d transition metal compounds

For most of the ionic 3d transition metal compounds the observed saturation magnetic moment is fairly consistent with the theoretical spin-only value. Verification of the spin quantum number, S, by means of the observed saturation moment is, however, ambiguous because it is not known how much the Landé g  -factor deviates from the spin-only value of g=2.00g=2.00. We use the universality of the magnetic order parameter at the stable fixed point T=0T=0 to determine the value of S for T  →0 consistently. As we have shown earlier, for integer and half-integer spin values different universality classes hold at T=0T=0. It is therefore possible to get information on whether the spin is integer or half-integer from the temperature power function by which the order parameter approaches saturation. Only for Cr2O3, CoF2 and MnO we have observed that the universality class at T=0T=0 is not consistent with the theoretical spin only value. This is explained by a relevant crystal field which reduces the number of states, N=2S+1N=2S+1, by ΔN=1ΔN=1. The discrete changes of N are associated with a crossover to another universality class, i.e. to another temperature power function of the order parameter.