Surface magnetic phase transitions in films

Using a simple Landau model, we discuss the different possibilities of generating magnetic effects at a second-order transition for films. Varying the sample size d and/or surface coupling γ one can decrease or increase substantially the surface critical temperature Ts and the saturation magnetization Ms. In the case of γ>0, Ms and Ts decrease from the bulk values as the film thickness is reduced. These theoretical results are in nice agreement with the experimental data on superconducting MgB2 thin films. By contrast, for γ<0, an enhancement of both quantities is expected. This extraordinary transition has rarely been observed experimentally and, usually, the situation is far from being clear. We analyze a new experiment on NiFe2O4 ultra-thin films, where a very strong enhancement of the saturation magnetization is observed.