Multiphase region of helimagnetic superlattices at low temperature in an extended six-state clock model

摘要

The variety of magnetic phases observed in rare-earth heterostructures at lowtemperatures \cite{Jehan}, such as Ho/Y, may be elucidated by an ANNNI-likemodel Hamiltonian. In previous work modelling bulk Ho \cite{Seno}, such aHamiltonian with a one-dimensional parameter space produced a single multiphasepoint. In contrast, the parameter space of the heterostructure model isthree-dimensional, and instead of an isolated multiphase point, we findtwo-dimensional multiphase regions. In an example of Villain's ``order fromdisorder'' \cite{Villain80a,Villain91}, an infinitesimal temperature breaks theground-state degeneracy. In first order of a low-temperature expansion, we findthat the degeneracy is broken everywhere in a multiphase region except on aline. A segment of the line appears to remain multiphase to all orders in alow-temperature expansion when the number $L$ of magnetic layers betweennon-magnetic spacers is 4 but not for other values of $L$. For L=4, thehierarchy of phases more closely resembles that in the ANNNI model than in thebulk six-state clock model on which the present model is based.