Effective spin description of crystal field effects in NdAl 2

Abstract

We propose a new analysis of the paramagnetic susceptibility and the spontaneous magnetization of the ferromagnet NdAl 2 (electronic configuration 4I 9/2). As a consequence of a strong crystal field, the observed saturation magnetic moment of m = 2.47 μ B per Nd is considerably smaller than the free Nd 3+ value of 3.27 μ B. On the other hand, the effective magnetic moment of the high temperature Curie–Weiss susceptibility conforms well to the theoretical value of the Nd 3+ 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 T C = 77 K all states of the Nd 3+ are relevant, i.e. N = 2 J + 1 = 10 . For T → 0 , the effective spin is only J eff = 3 , i.e. N = 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 → 0 is according to different universality classes.