Low-temperature properties of two-dimensional ideal ferromagnets

• Published in: Physical review. B, Condensed matter and materials physics Condensed matter and materials physics, 2012-08, Vol.86 (5), Article 054409
• Main Author: Hofmann, Christoph P
• Format: Article
• Language: English
• Subjects: Condensed matter; Density; Ferromagnetism; Free energy; Mathematical models; Partitions; Three dimensional; Two dimensional
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.86.054409

The manifestation of the spin-wave interaction in the low-temperature series of the partition function has been investigated extensively over more than seven decades in the case of the three-dimensional ferromagnet. Surprisingly, the same problem regarding ferromagnets in two spatial dimensions, to the best of our knowledge, has not been addressed in a systematic way so far. In the present paper the low-temperature properties of two-dimensional ideal ferromagnets are analyzed within the model-independent method of effective Lagrangians. The low-temperature expansion of the partition function is evaluated up to two-loop order and the general structure of this series is discussed, including the effect of a weak external magnetic field. Our results apply to two-dimensional ideal ferromagnets which exhibit a spontaneously broken spin rotation symmetry O(3) arrow right O(2) and are defined on a square, honeycomb, triangular, or kagome lattice. Remarkably, the spin-wave interaction only sets in at three-loop order. In particular, there is no interaction term of order T super(3) in the low-temperature series for the free energy density. This is the analog of the statement that, in the case of three-dimensional ferromagnets, there is no interaction term of order T super(4) in the free energy density. We also provide a careful discussion of the implications of the Mermin-Wagner theorem in the present context and thereby put our low-temperature expansions on safe grounds.