NONCOLLINEAR ALIGNMENT OF THE SURFACE AND BULK MAGNETIC-MOMENT IN LOCALIZED FERROMAGNETS

A model for noncollinear alignment between the surface-atomic-layer ma gnetic moment and bulk magnetic moment is proposed. It takes place due to the competition between ferromagnetic and antiferromagnetic exchan ge interactions of atomic layer with the nearest atomic layer and next -nearest atomic layer in the surface region. The criterion of the stab ility of surface state with collinear surface to bulk alignment is der ived. On the basis of this criterion the phase diagram of surface magn etic states corresponding to a range of surface to bulk alignments at zero temperature is presented. We show that within this model the nonc ollinear surface to bulk alignment leads to a spiral magnetic structur e in the surface region of a bulk ferromagnet. In the framework of thi s model a temperature-induced surface spin-reorientation transition ta kes place due to the change in the balance between exchange energies i n the surface region with temperature. A self-consistent solution of t he magnetization profile determination problem for any number of subsu rface layers considered to be perturbed by the surface is used. In con trast to previous theoretical results we show that the increase in eff ective magnetic moment of a surface with temperature observed in exper iments with Gd(0001), Tb(0001), and FeN3 surfaces does not necessarily imply antiparallel alignment of surface and bulk magnetic moment at z ero temperature. We demonstrate that this phenomenon is consistent wit h parallel surface to bulk alignment at low temperature as demonstrate d in recent experiments on the Gd(0001) surface.