Comment on compressible spin systems

Abstract

The zero-temperature phase diagram for the linear compressible chain of classical Heisenberg spins is obtained. In the space of appropriate coupling constants, the spin-flop phase is separated from the aligned phase by a surface along which runs a line of tricritical points. Also, the molecular-field approximation for coupled spin-lattice systems is examined. With classical variables, there are two ways of proceedings, and both have been used previously. The procedures are discussed on a variational basis, and it is pointed out that one gives a lower upper bound on the free energy than the other.