Hyperfine Fields in Ultrathin Fe and Feau Alloy Films on Au(111)
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HYPERFINE FIELDS IN ULTRATHIN Fe AND FeAu ALLOY FILMS ON Au(ll1)
M. BROCKMANN, L. PFAU, G. LUGERT AND G. BAYREUTHER Universitit Regensburg, Institut for Angewandte Physik, UniversititsstraBe 31, D-8400 Regensburg, Germany
ABSTRACT
Magnetic properties of ultrathin Fe and FewAu 40 alloy films on Au(1 11) were studied by SQUID magnetometry and conversion electron M6ssbauer spectroscopy. In order to get information on the influence of interdiffusion, iron films with thin alloy zones at the interfaces to Au have been prepared by co-evaporation of iron and gold and compared with iron films with presumably sharp interfaces. It was found that the presence of an 0.5 ML (mass coverage in monolayer) alloy zone reduces the effective magnetic interface anisotropy field and affects the growth mode of a subsequently deposited iron film such that the film is more sensitive to annealing. Groundstate magnetic moments and hyperfine fields are significantly enhanced in Fe/Au(1 11) (tFQ< 4 ML) and Fe6oAu 40 films, compared to bulk Fe.
1. INTRODUCTION
Ultrathin epitaxial Fe films grown on Au(1 11) show remarkable magnetic properties like enhanced groundstate magnetic moments and a pronounced magnetic interface anisotropy with the easy axis perpendicular to the film planet . While for an iron thickness tFe > 10 ML it is well known that iron grows in a bcc lattice with the Fe(1 10) plane parallel to the Au(l 11) plane, the lattice structure for films with tFe < 10 ML is controversial. Some authors state that Fe grows in the bcc(l 10) phase from the first layer on2, while others find a pseudomorphic growth of fcc Fe up to 3-4 ML where a transition to the bcc phase sets in3. In any case it is to be expected that the growth mode is affected by the preparation conditions and should manifest itself in the magnetic properties of the sample. In our Fe/Au(1 11) films we found a significant dependence of the effective magnetic surface anisotropy field, Hs, on the substrate temperature during film growth, Ts. In epitaxial films grown at Ts = 310 K we observe the largest value of IH.1 = 40 ± 3 kOe. This value is large enough to turn the magnetization perpendicular to the film plane if the thickness is smaller than 3 ML. For Ts = 450 K we obtain IHsI = 13 + 5 kOe. This reduction of the effective magnetic interface anisotropy field can be explained in terms of an interdiffusion of Fe and Au at the interface or by a more three dimensional growth mode at higher substrate temperatures. In order to study the influence of interdiffusion on the magnetic properties we prepared epitaxial ultrathin iron films with thin (0.5 or 2 ML) FeAu alloy zones at the interfaces by co-deposition of Fe and Au. We found IHsI = 17 + 5 kOe (for Ts = 310 K). In addition, for the identification of an alloy component and its contribution to the magnetic behaviour of the film pure FeAu alloy films have also been investigated. SQUID magnetometry was used to determine integral magnetic properties (average magnetic moment and Mat. Res. Soc. Symp. Proc. Vol. 313. t1993 Materials
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