Ordering in magnetic films with surface anisotropy

Abstract

Effects of the surface exchange anisotropy on ordering of ferromagnetic films are studied for the exactly solvable classical spin-vector model with D \to \infty components. For small surface anisotropy \eta'_s << 1 (defined relative to the exchange interaction), the shift of T_c in a film consisting of N >> 1 layers behaves as T_c^{\rm bulk} - T_c(N) ~ (1/N)\ln(1/\eta'_s) in three dimensions. The finite-size-scaling limit T_c^{\rm bulk} - T_c(N) \propto 1/(\eta'^{1/2}N^2), which is realized for the model with a bulk anisotropy \eta' << 1 in the range N\eta'^{1/2} >~ 1, never appears for the model with the pure surface anisotropy. Here for N\exp(-1/\eta'_s) >~ 1 in three dimensions, film orders at a temperature above T_c^{\rm bulk} (the surface phase transition). In the semi-infinite geometry, the surface phase transition occurs for whatever small values of \eta'_s (i.e., the special phase transition corresponds to T_c^{\rm bulk}) in dimensions three and lower.