Smeared spin-flop transition in random antiferromagnetic Ising chain

Abstract

At T = 0 and a sufficiently large field, the nearest-neighbor antiferromagnetic Ising chain undergoes a first-order spin-flop transition into the ferromagnetic phase. We consider its smearing under the random-bond disorder such that all independent random bonds are antiferromagnetic (AF). It is shown that it can be described exactly for arbitrary distribution of AF bonds P(J). Moreover, the site magnetizations of finite chains can be found analytically in this model. We consider continuous P(J) which is zero above some -J_1 and behaves near it as (-J_1 - J)^λ, λ> -1. In this case ferromagnetic phase emerges continuously in a field H > H_c = 2J_1. At 0 > λ> -1 it has usual second-order anomalies near H_c with critical indices obeying the scaling relation and depending on λ. At λ> 0 the higher-order transitions appear (third, fourth etc.) marked by the divergence of corresponding nonlinear susceptibilities. In the chains with even number of spins the intermediate "bow-tie" phase with linearly modulated AF order exists between AF and ferromagnetic ones at J_1 < H < H_c. Its origin can be traced to the infinite correlation length of the degenerate AF phase from which it emerges. This implies the existence of similar inhomogeneous phases with size- and form-dependent order in a number of other systems with infinite correlation length. The possibility to observe the signs of "bow-tie" phase in low-T neutron diffraction experiments is discussed.