Abstract
Further investigations on magnets with pure spin moments are presented showing that the temperature dependence of the order parameter can accurately be described over a large temperature range by a single T ε power term. The exponent ε is found to be independent of the spin order type but it depends on whether the spin quantum number is integral or half-integral and, of course, on the dimensionality of the magnetic interactions. The six empirical spin wave exponents ε defined in this way are 9/2, 2 and 3 for isotropic, anisotropic and axial interactions and integral spin quantum number but 2, 3/2 and 5/2 for isotropic, anisotropic and axial interactions and half-integral spin quantum number. Thermodynamic crossover between neighbouring exponents is frequently observed: antiferromagnetic NiO having S =1 is cubic above T N but undergoes a progressive trigonal lattice distortion with decreasing temperature. For this material a crossover from isotropic ( ε =9/2) to anisotropic ( ε =2) interactions is observed as a function of decreasing temperature (see Table tI). The hexagonal ferromagnet gadolinium having S =7/2 exhibits a crossover from ε =3/2 to ε =5/2 indicative for a gradual change from anisotropic to predominantly axial interactions with decreasing temperature. Also the itinerant ferromagnets Fe, Ni and Co show the same exponents ε as insulators with S =1/2. While cubic Fe and Ni exhibit ε =2 the same crossover from ε =3/2 to ε =5/2 as for hexagonal Gd is observed for hcp cobalt.
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