Classical Heisenberg antiferromagnet away from the pyrochlore lattice limit: Entropic versus energetic selection

The stability of the disordered ground state of the classical Heisenberg pyrochlore antiferromagnet is studied within extensive Monte Carlo simulations by introducing an additional exchange interaction J(') that interpolates between the pyrochlore lattice (J(')=0) and the face-centered cubic lattice (J(')=J). It is found that for J(')/J as low as J(')/Jgreater than or equal to0.01, the system is long-range ordered: the disordered ground state of the pyrochlore antiferromagnet is unstable to very small deviations from the pure J(')=0 limit. Furthermore, it is found that the selected phase is a collinear state energetically greater than the incommensurate phase suggested by a mean field analysis. To the best of our knowledge this is the first example in magnetic systems where entropic selection prevails over the energetic one.