Marek Biskup, Lincoln Chayes and Zohar Nussinov
Orbital ordering in transition-metal compounds: I. The 120-degree model
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ABSTRACT.  We study the classical version of the 120-degree model. This is an attractive nearest-neighbor system in three dimensions with XY (rotor) spins and interaction such that only a particular projection of the spins gets coupled in each coordinate direction. 
Although the Hamiltonian has only discrete symmetries, it turns out that every constant field is a ground state. 
Employing a combination of spin-wave and contour arguments we establish the existence of long-range order at low temperatures. 
This suggests a mechanism for a type of ordering in certain models of transition-metal compounds where the very existence of long-range order has heretofore been a matter of some controversy.