In order to investigate the nature of the spin arrangement in the surface of a semi-infinite helical magnet, we calculate both the surface spin-wave frequency spectra and the classical spin-ordering energies. We present detailed calculations for a (100) surface of a body-centered tetragonal lattice with nearest neighbor and next nearest neighbor exchange interactions (J1 and J2, which are both negative). The possibility that the exchange interactions in the surface (Js2) may differ from those in the bulk (J2) is taken into account. We find the following: In the case 0≳Js2≳J2, the surface spin-wave frequencies are real and positive, and hence the uniform helical spin arrangement is stable. On the other hand, in the case Js 2<J2, there exist soft surface spin-wave modes, which indicate that the uniform helical spin arrangement is not stable. In the latter case, it is shown that a nonuniform spin arrangement, characterized by a rearrangement of spins in the surface, has a lower classical spin-ordering energy than the uniform state and hence is stable.
ASJC Scopus subject areas
- Physics and Astronomy(all)