Surface spin-flop and discommensuration transitions in antiferromagnets

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 HSF for a bulk spin-hop 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. [S0163-1829(99)02309-7].