ZhETF, Vol. 142,
No. 6,
p. 1164 (December 2012)
(English translation  JETP,
Vol. 115, No. 6,
p. 1020,
December 2012
available online at www.springer.com
)
SMEARED SPINFLOP TRANSITION IN RANDOM ANTIFERROMAGNETIC ISING CHAIN
Timonin P.N.
Received: December 7, 2011
At T=0 and a sufficiently large field, the nearestneighbor antiferromagnetic Ising chain undergoes a firstorder spinflop transition into the ferromagnetic phase. We consider its smearing under the randombond disorder such that all independent random bonds are antiferromagnetic (AF). It is shown that the groundstate 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 J_{1} and behaves near it as (J_{1}J)^{λ}, λ>1. In this case, the ferromagnetic phase emerges continuously in a field H>H_{c}= 2J_{1}. At 0>λ>1, it has the usual secondorder anomalies near H_{c} with the critical indices obeying the scaling relation and depending on λ. At λ>0, higherorder 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 ``bowtie'' phase with linearly modulated AF order exists between the AF and ferromagnetic phases at J_{1}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 formdependent order in a number of other systems with infinite correlation length. The possibility to observe the signs of the ``bowtie'' phase in lowT neutron diffraction experiments is discussed.

