Experimental temperature dependence of the magnetization of surface layers of Fe at interfaces with nonmagnetic materials

摘要

Theoretical studies have investigated the temperature dependence of the magnetization at the surface of ferromagnets. It has become clear that surface anisotropy can significantly influence this temperature dependence. For isotropic exchange uniform throughout the sample the temperature dependence of the magnetization varies as M(T)/M(0)≊(1-BT3/2) with a B factor at least twice as great as the bulk value. For cases in which the exchange interaction is weaker in the surface layer, or in which there is a significant surface exchange anisotropy a linear temperature dependence may result. These theoretical results have seen little experimental test. We report here measurements made on epitaxial (110) Fe films in which different temperature dependences are found depending upon the interfacial material chosen as a covering layer. The ferromagnetic films were made of 56Fe, permitting Mössbauer spectroscopic measurements of the magnetic hyperfine field of the surface alone by growing a surface layer of 57Fe. These hyperfine measurements were made at a number of temperatures from 4.2 to 300 K. For Ag as the interfacial covering material a T3/2 dependence was seen. For MnF2 a linear T dependence was noted. Covering materials such as MgO have also been used. Results will be discussed in terms of expected surface anisotropy.