First-Order Phase Transition in Itinerant Antiferromagnetism

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Abstract
The thermodynamical properties of the itinerant antiferromagnetism near the Néel temperature are studied theoretically. Use is made of the two-band model where the Fermi surfaces of the electron band and of the hole band are spherical but different in size. A particular attention is paid to the incommensurate spin density wave (ISDW) state, which corresponds to actual chromium. In the ISDW state, the spatial dependence of the order parameter is extremely important. Several types of the spin density wave are examined. It is shown that the sinusoidal spin density wave state gives rise to the lowest free energy and that the transition at the Néel temperature is of first order at least in the vicinity of the triple point, which divides the CSDW (commensurate spin density wave) and the ISDW regions. Thermodynamical quantities at the Néel temperature are calculated, which are in semi-quantitative agreement with those observed for pure chromium. The relevance of the present result to superconductivity in strong exchange fields are also discussed.

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