M ay 2 00 6 Multiphase region of helimagnetic superlattices at low temperature in an extended six - state clock model

The variety of magnetic phases observed in rare-earth heterostructures at low temperatures [1], such as Ho/Y, may be elucidated by an ANNNI-like model Hamiltonian. In previous work modelling bulk Ho [2], such a Hamiltonian with a one-dimensional parameter space produced a single multiphase point. In contrast, the parameter space of the heterostructure model is three-dimensional, and instead of an isolated multiphase point, we find two-dimensional multiphase regions. In an example of Villain's " order from disorder " [3, 4], an infinitesimal temperature breaks the ground-state degeneracy. In first order of a low-temperature expansion, we find that the degeneracy is broken everywhere in a multiphase region except on a line. A segment of the line appears to remain multiphase to all orders in a low-temperature expansion when the number L of magnetic layers between non-magnetic spacers is 4 but not for other values of L. For L = 4, the hierarchy of phases more closely resembles that in the ANNNI model than in the bulk six-state clock model on which the present model is based.