Anticollinear magnetic order induced by impurities in the frustrated   Heisenberg model of pnictides
  We present Monte Carlo simulations for a classical antiferromagnetic Heisenberg model with both nearest ($J_1$) and next-nearest ($J_2$) exchange couplings on the square lattice in the presence of non-magnetic impurities. We show that the order-by-disorder entropy selection, associated with the Ising-like phase transition that appears for $J_2/J_1>1/2$ in the pure spin model, is quenched at low temperature due to the presence of non-magnetic impurities. Evidences that a new competing order is stabilized around the impurities, and in turn induces a re-entrance phase transition are reported. Implications for local magnetic measurement of the parent compound of iron pnictides are briefly discussed. 
C\'edric Weber, Fr\'ed\'eric Mila
cond-mat.str-el cond-mat.dis-nn cond-mat.stat-mech cond-mat.supr-con