Fe(100)/Ag(100) heterostructures grown on NaCl(001) by molecular beam epitaxy (abstract)
Authors: Gutierrez C. J. , Qiu Z. Q. , Wieczorek M. D. , Walker J. C. 
Published: July 2002

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 Perkin-Elmer PHI 430B molecular-beam-epitaxy (MBE) system equipped with reflection high-energy electron diffraction (RHEED) and a quadrupole mass analyzer. The growth region consistently achieved a base pressure of less than 5×10−10 Torr, and a growth pressure of less than 1.5×10−9 Torr. The base for all of our heterostructures consisted of 5 kÅ Ag(100) grown on polished single-crystal NaCl(001) substrates. Before the Ag base growth, a 200-Å NaCl epilayer was deposited on the previously out-gassed NaCl substrate at 250 °C. Excellent Ag(100) RHEED patterns were obtained after a 3-h post-growth anneal of the base at 375 °C. Typical heterostructure growth rates were 2 ML/min for Fe and 15 ML/min for Ag. All the heterostructures were capped by a 5-kÅ Ag protective cover.

Our growths experienced a ramped substrate growth temperature between 30 and 75 °C caused by radiant heating from our effusion cells (due to our present inability to cool the substrate). Since earlier work1 reported that layer-by-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 allowed 57Fe transmission Mössbauer spectroscopy to be performed. Only the 6- and 9-ML Fe bilayer films showed sextet features at room temperature (RT). The 9-ML film spectrum at RT consisted of a broadened sextet with in-plane bulklike magnetization. The 6-ML spectrum at RT had a large (65%) single-line central feature together with the sextet component. A small external field (5 kOe) applied to the 6-ML film at RT almost totally removed the central feature in the spectrum revealing a broadened two-site sextet spectrum. The indicative presence of superparamagnetism is expected from our islandlike growth of Fe(100) at warmer substrate temperatures. The 3-ML spectrum at RT consisted of two differently isomer-shifted single lines. At 4.2 K, all of the Mössbauer spectra consisted only of sextets.

A two-site sextet nature in the 3- and 6-ML films was apparent, being more pronounced in the 3-ML film. The 3-ML film magnetization was heavily canted out of plane, and was virtually identical in appearance and Mössbauer fit parameters to the 2.4-ML Fe(100)/Ag(100) superlattice at 15 K reported by Volkening et al.2 at 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 Mössbauer spectroscopy.2–5 These differences caused a lively discussion, especially since no single group had yet studied both systems with Mössbauer 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 first-hand with Mössbauer spectroscopy. Because the Fe(100)/Ag(100) series we studied closely agreed with previous experimental results2 despite 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 hyperfine-field characteristics between the Fe(100)/Ag(100) and Fe(110)/Ag(111) systems.