Temperature dependence of the hyperfine field and magnetization in ultrathin epitaxial Fe films

Conversion-electron Mössbauer spectroscopy and superconducting quantum interference device magnetometry have been used to measure the magnetic hyperfine field and the spontaneous magnetization of ultrathin Fe(110) films grown epitaxially on Ag(111) substrate films as a function of temperature. For the first time, hyperfine-field data with monolayer resolution and magnetization data are obtained from the same sample consisting of four Fe(110) atomic layers. A comparison of the present results to previous experiments and theories leads to the following conclusions: (1) the ground-state hyperfine field at the Fe(110)/Ag(111) interface is enhanced relative to bulk in agreement with other experiments, but in disagreement with recent band calculations; this discrepancy may be explained by considering dipolar field contributions at the interface; (2) the relative spin deviation from saturation (T=0) measured by the hyperfine field is the same as measured by the spontaneous magnetization for the interface atoms as well as for the center of the film; (3) the spin deviation, ΔM(T), is well described by a $T^{3/2}$ law in all layers of an ultrathin Fe film; hence, a linear M-T relation is not a characteristic feature of a two-dimensional ferromagnet, as often quoted in the literature; (4) ultrathin Fe films show pronounced surface and size effects of the thermally excited spin deviation; no theory exists at present which includes layer-dependent ground-state moments, surface and bulk spin waves; and (5) the spin-wave parameter, B, differs in different experiments for similar film systems. It is suggested that the structure and roughness of the interface in real films play an important role.