Influence of the biquadratic interaction to magnetic surface reconstruction

The surface spin ordering is studied in a ferromagnetic crystal where spins interact both by bilinear and biquadratic interaction. The surface spin co nfiguration is unstable with respect to ferromagnetic alignment for certain values of parameters leading to so-called surface magnetic reconstruction It is shown that biquadratic interaction enhances the reconstruction. The e nergy of surface excitations is calculated and it is shown to be much lower than the energy of bulk excitations in presence of surface reconstruction, while the surface Goldstone mode appears only in the case of vanishing ext ernal field and anisotropy. Mean field calculation is presented for the stu dy of the behavior of surface magnetization. [S0163-1829(99)00126-5].