Surface magnetic phase transitions in films

Abstract

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 T s and the saturation magnetization M s . In the case of γ > 0 , M s and T s decrease from the bulk values as the film thickness is reduced. These theoretical results are in nice agreement with the experimental data on superconducting MgB 2 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 NiFe 2 O 4 ultra-thin films, where a very strong enhancement of the saturation magnetization is observed.