Very Thin (2–200 Å) Ferromagnetic NiFe Films

The magnetization of flat, single‐crystal NiFe (111) films (45% to 50% Ni) was measured for ${70}^{\circ }{\text{≦T≦400}}^{\circ }\mathrm{K}$ . There is good qualitative agreement with spin‐wave theory for the dependence of the magnetization on temperature, at least for films consisting of 3 to 20 layers. Yet there are two characteristic deviations from theoretical expectations: (1) Down to 3 layers (6 Å) the magnetization is independent of the field, within 1%, for $\text{300≦H≦5000\hspace{0.17em}}\mathrm{Oe}$ , while theory predicts considerable dependence on the field. (2) Above 30 layers, the film magnetization at low temperatures is definitely less temperature dependent than the bulk magnetization. The deviations are supposedly connected with strong anisotropies, with the surface normal as easy axis, which are nearly proportional to the reciprocal of the film thickness and may therefore be interpreted as surface anisotropy (about 0.23 erg/cm² at 250°K). Critical temperatures are given for films consisting of up to 6 layers.