It is shown that the phase transition in low-Tc clean itinerant ferromagnets is generically of first order, due to correlation effects that lead to a nonanalytic term in the free energy. A tricritical point separates the line of first order transitions from Heisenberg critical behavior at higher temperatures. Sufficiently strong quenched disorder suppresses the first order transition via the appearance of a critical end point. A semiquantitative discussion is given in terms of recent experiments on MnSi, and predictions for other experiments are made.
D. Belitz et al., "First Order Transitions and Multicritical Points in Weak Itinerant Ferromagnets," Physical Review Letters, vol. 82, no. 23, pp. 4707-4710, American Physical Society (APS), Jun 1999.
The definitive version is available at https://doi.org/10.1103/PhysRevLett.82.4707
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