Stable magnetic universality classes for T→0

In previous experimental investigations we have observed that a single power function of absolute temperature according to 1-mS(T)=c×Tε describes the deviations of the magnetic order parameter, mS(T), from saturation of unity at T=0 better than classical power series expansions if an appropriate empirical exponent ε is chosen. Six universality classes represented by different power functions could be established. It is evident that the Tε function holds exactly only if the pre-factor c is a constant. We present experimental evidence that the pre-factor c assumes only discrete values meaning that the universal c×Tε power function is well stabilized against various weakly temperature-dependent non-relevant parameters. Discrete changes of either c or ε which we have called amplitude crossover and exponent crossover, respectively, occur only after changes of the magnetic interactions beyond some threshold value. The thermodynamics of the order parameter can therefore be considered as quantized. In particular, cubic magnets with a pure spin moment belong per definition to the isotropic three-dimensional (3D) universality class. As a consequence, ε is constant and only amplitude crossovers are possible if the thermal variation of the interaction parameters is sufficiently strong. For materials with a smaller variation of the magnetic interactions which, as a consequence, exhibit no amplitude crossover no information on the temperature dependence of the magnetic interactions can be obtained from an analysis of mS(T).