In order to study the effects of free layer thickness on MR ratio and switching field, we changed the free layer material and its thickness. In the regime of an extremely thin free layer, both MR ratio and switching field decreased with decreasing free layer thickness. There is a critical thickness, at which no MR behavior was found. The MTJs with NiFe and CoFe free layers show no MR ratio below 1.5 nm and 1 nm, respectively. No MR behavior is associated with discontinuous film formation below a certain thickness. The difference of the critical thickness is associated with the different wettability of the free layer material on the aluminum oxide surface.