Effective spin description of crystal field effects in NdAl2

We propose a new analysis of the paramagnetic susceptibility and the spontaneous magnetization of the ferromagnet NdAl2 (electronic configuration 4I9/2). As a consequence of a strong crystal field, the observed saturation magnetic moment of m=2.47μB per Nd is considerably smaller than the free Nd3+ value of 3.27 μB. On the other hand, the effective magnetic moment of the high temperature Curie–Weiss susceptibility conforms well to the theoretical value of the Nd3+ ion. We show that the number of relevant states, N  , is the important quantity for the thermodynamics. This number is always an integer. The character of the states seems to be less important. Far above TC=77K all states of the Nd3+ are relevant, i.e. N=2J+1=10N=2J+1=10. For T→0T→0, the effective spin is only Jeff=3Jeff=3, i.e. N=7N=7. The decrease of N is in steps of unity. Each decrease of N is associated with a crossover phenomenon that can be either in the paramagnetic susceptibility or in the spontaneous magnetization. The thermodynamics can therefore be considered as quantized. Depending on whether N   is even or odd, the temperature dependence of the spontaneous magnetization for T→0T→0 is according to different universality classes.