Surface spin-flop and discommensuration transitions in antiferromagnets

TL;DR

This paper models the phase diagrams of antiferromagnets with surface and discommensuration transitions using a 1D XY spin chain, revealing complex surface behaviors and critical points.

Contribution

It introduces a detailed phase diagram analysis for surface and discommensuration transitions in antiferromagnets using classical XY spin models.

Findings

Surface spin-flop phase exists for all anisotropy D.

First-order surface state transitions end in critical points.

Discommensurations occur in the surface restructuring process.

Abstract

Phase diagrams as a function of anisotropy $D$ and magnetic field $H$ are obtained for discommensurations and surface states for an antiferromagnet in which $H$ is parallel to the easy axis, by modeling it using the ground states of a one-dimensional chain of classical XY spins. A surface spin-flop phase exists for all $D$, but the interval in $H$ over which it is stable becomes extremely small as $D$ goes to zero. First-order transitions, separating different surface states and ending in critical points, exist inside the surface spin-flop region. They accumulate at a field $H'$ (depending on $D$) significantly less than the value $H_{SF}$ for a bulk spin-flop transition. For $H' < H < H_{SF}$ there is no surface spin-flop phase in the strict sense; instead, the surface restructures by, in effect, producing a discommensuration infinitely far away in the bulk. The results are used to explain in detail the phase transitions occurring in systems consisting of a finite, even number of layers.