Apparent diamagnetic response of an inhomogeneous ferromagnet

We present magnetization measurements on a weakly ferromagnetic Pd 0.5 at. % Fe alloy (Tc=15 K). Due to the preparation technique for the sample, it has a thin surface layer with slightly enhanced Tc. In fields above 200 mG, the magnetization is typical of a ferromagnet. However, when cooling in very small fields (H25 mG), the magnetization reverses its direction at low temperatures, apparently becoming diamagnetic. The effect is very similar, but of opposite sign, to that observed in some high-Tc superconducting samples where the magnetization becomes paramagnetic on field cooling (paramagnetic Meissner effect, PME). Whereas the origin of the PME in superconductors is controversial, the effect in our ferromagnetic sample is explained in terms of dipolar polarization of the interior of the sample by the surface layer with enhanced Tc. Removing the surface layer eliminates this anomalous effect and the sample behaves like an ordinary ferromagnet, down to the lowest fields.