Fe(100)/Ag(100) heterostructures grown on NaCl(001) by molecular beam epitaxy (abstract)

摘要

Three heterostructures were grown, each with 40 monolayer (ML) Ag bilayer thicknesses. The Fe(100) bilayers had thicknesses of 3, 6, and 9 ML. All growths were performed with a Perkinhyphen;Elmer PHI 430B molecularhyphen;beamhyphen;epitaxy (MBE) system equipped with reflection highhyphen;energy electron diffraction (RHEED) and a quadrupole mass analyzer. The growth region consistently achieved a base pressure of less than 5times;10minus;10Torr, and a growth pressure of less than 1.5times;10minus;9Torr. The base for all of our heterostructures consisted of 5 kAring; Ag(100) grown on polished singlehyphen;crystal NaCl(001) substrates. Before the Ag base growth, a 200hyphen;Aring; NaCl epilayer was deposited on the previously outhyphen;gassed NaCl substrate at 250thinsp;deg;C. Excellent Ag(100) RHEED patterns were obtained after a 3hyphen;h posthyphen;growth anneal of the base at 375thinsp;deg;C. Typical heterostructure growth rates were 2 ML/min for Fe and 15 ML/min for Ag. All the heterostructures were capped by a 5hyphen;kAring; Ag protective cover.Our growths experienced a ramped substrate growth temperature between 30 and 75thinsp;deg;C caused by radiant heating from our effusion cells (due to our present inability to cool the substrate). Since earlier work1reported that layerhyphen;byhyphen;layer growth of Fe(100) on Ag(100) (indicated by RHEED oscillations) occurs at substrate temperatures far below room temperature, the growth of optimally flat Fe(100) films was hindered in our work. Removal of the NaCl substrate allowed57Fe transmission Mouml;ssbauer spectroscopy to be performed. Only the 6hyphen; and 9hyphen;ML Fe bilayer films showed sextet features at room temperature (RT). The 9hyphen;ML film spectrum at RT consisted of a broadened sextet with inhyphen;plane bulklike magnetization. The 6hyphen;ML spectrum at RT had a large (65percnt;) singlehyphen;line central feature together with the sextet component. A small external field (5 kOe) applied to the 6hyphen;ML film at RT almost totally removed the central feature in the spectrum revealing a broadened twohyphen;site sextet spectrum. The indicative presence of superparamagnetism is expected from our islandlike growth of Fe(100) at warmer substrate temperatures. The 3hyphen;ML spectrum at RT consisted of two differently isomerhyphen;shifted single lines. At 4.2 K, all of the Mouml;ssbauer spectra consisted only of sextets.A twohyphen;site sextet nature in the 3hyphen; and 6hyphen;ML films was apparent, being more pronounced in the 3hyphen;ML film. The 3hyphen;ML film magnetization was heavily canted out of plane, and was virtually identical in appearance and Mouml;ssbauer fit parameters to the 2.4hyphen;ML Fe(100)/Ag(100) superlattice at 15 K reported by Volkeningetal.2at NRL. Striking differences in the magnetic behavior of ultrathin epitaxial multilayers of the Fe(100)/Ag(100) system and the Fe(110)/Ag(111) system had been previously observed by various groups using Mouml;ssbauer spectroscopy.2ndash;5These differences caused a lively discussion, especially since nosinglegroup had yet studiedbothsystems with Mouml;ssbauer spectroscopy. This work, together with our previous work in the Fe(100)/Ag(111) system, allows our group to be the first to compare these systems firsthyphen;hand with Mouml;ssbauer spectroscopy. Because the Fe(100)/Ag(100) series we studied closely agreed with previous experimental results2despite differences in substrate growth temperature, increased superparamagnetism, and choice of substrate, there is no doubt that the observation of perpendicular magnetization at small Fe(100) thicknesses is a real effect. There appear to be great differences in the growth behavior and hyperfinehyphen;field characteristics between the Fe(100)/Ag(100) and Fe(110)/Ag(111) systems.