Ferromagnetic Transitions and the One-Third-Power Law

Experiments have shown that the reduced magnetization of insulating ferromagnets varies as m(T) = D(1-T/Tc)f for T>0.9Tc, and that f=⅓. We report calculations of the magnetization of the Heisenberg model which give, over a wide temperature range, both the observed exponent f=⅓ and the observed co-efficient D, within 2% of the experimental value. This result is obtained for a two-spin cluster theory; similar but slightly less satisfactory agreement is demonstrated by two forms of Green function theory. Because these theories are not satisfactory in the immediate vicinity of the phase transition, and because the experimental ⅓-power law extends far from the transition [in terms of m(T)], we suggest that the ⅓-power law may be a property of the stable ferromagnetic phase rather than a reflection of the phase transition in the sense of the Landau theory.

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