Lattice distortions and short-range magnetic order in classical magnetoelastic Heisenberg chains

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Abstract
Lattice distortions of a two-dimensional array of one-dimensional classical Heisenberg spin chains with magnetoelastic coupling are studied in the limit of strong interchain elastic interactions. Exact integration over the spin degrees of freedom leads to a temperature-dependent free-energy functional of the elastic variables only. The equilibrium lattice structure is obtained from the ground state of this free-energy functional and the phase diagram indicates the presence of a tricritical point. The necessity of performing the calculations at constant pressure is stressed and it is shown that a dimerized phase could be reached by applying pressure to certain magnetic materials, provided that there exist positive second-neighbor elastic interactions. The consequences of lattice distortions on the short-range magnetic order are studied by calculating the wave-number-dependent magnetic susceptibility. It is found that the dominant short-range magnetic order is of period four in the dimerized lattice phase.