SMEARED SPIN-FLOP TRANSITION IN RANDOM ANTIFERROMAGNETIC ISING CHAIN
Timonin P.N.

Received: December 7, 2011

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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 the ground-state thermodynamics of this random AF chain can be described exactly for an arbitrary distribution P(J) of AF bonds. Moreover, the site magnetizations of finite chains can be found analytically in this model. We consider a continuous P(J) that is zero above some -J₁ and behaves near it as (-J₁-J)^λ, λ>-1. In this case, the ferromagnetic phase emerges continuously in a field H>H_c= 2J₁. At 0>λ>-1, it has the usual second-order anomalies near H_c with the critical indices obeying the scaling relation and depending on λ. At λ>0, higher-order transitions occur (third, fourth, etc.), marked by a divergence of the corresponding nonlinear susceptibilities. In the chains with an even number of spins, the intermediate ``bow-tie'' phase with linearly modulated AF order exists between the AF and ferromagnetic phases at J<sub>1</sub>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 the ``bow-tie'' phase in low-T neutron diffraction experiments is discussed.