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Notes: Gas phase chemical reactions, of importance in the deposition of amorphous semiconductors, were studied in a remote hydrogen plasma reactor with electron spin resonance (ESR). The following reactant pairs (and their observed room-temperature rate coefficients) were characterized: (1) H+SiH4(2.4×1011 cm3 mole−1s−1), (2) D+SiH4 (2.1×1011 cm3 mole−1 s−1), and (3) H+C2H2 (1.2×1010 cm3 mole−1 s−1). The interpretation of these coefficients in terms of primary and secondary gas phase reactions is discussed, and the values are compared where possible with previously published data. In addition, the paramagnetic centers of the silicon-based film that is deposited in situ during the ESR measurement can now be microscopically identified in light of recent studies.

Notes: This paper treats a free surface between a liquid metal and an inert gas in the presence of a magnetic field with arbitrary orientation relative to the free surface. The free surface intersects a perfectly conducting surface at rest and an insulated surface rotating about an axis which is perpendicular to both surfaces and which is far from the liquid-metal region. This problem models free surfaces in liquid-metal sliding electric contacts for motors and generators. There is a primary azimuthal liquid-metal velocity which is driven by the rotation of the insulated surface, and there is a secondary flow which involves radial and axial velocities and which is driven by the centrifugal force due to the primary velocity. The free-surface positions, pressures, and velocities are presented as functions of the magnetic-field orientation and strength.

Notes: 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 kA(ring) Ag(100) grown on polished single-crystal NaCl(001) substrates. Before the Ag base growth, a 200-A(ring) 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-kA(ring) 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.

Notes: Epitaxial Fe(100)/Ni and Fe(110)/Ni heterostructures were grown using a Perkin-Elmer PHI 430B molecular-beam-epitaxy system equipped with (RHEED) and quadrupole mass analysis. The growth system typically achieved a base pressure of less than 5×10−10 Torr, and a growth pressure of less than 3×10−9 Torr. Typical growth rates were 3 A(ring)/min for Fe and 2 A(ring)/min for Ni. For all the heterostructures, the Ni thickness was held at 14 A(ring), the number of repetitions varied between 8 and 15 cycles, and growth always began with the Fe bilayer. Protective Ag covers were grown on all films. Three Fe (100)/Ni heterostructures were grown on 5-kA(ring) single-crystal Ag(100) bases grown on NaCl(001).1 The single-crystal Fe(100) bilayer thicknesses were 3, 8, or 12 monolayers (ML). The substrate growth temperature for this series was ramped from 40 to 80 °C due to radiant heating from the effusion cells. Four Fe(110)/Ni heterostructures were grown with Fe bilayer thicknesses of 2, 4, 8, and 12 ML. These heterostructures were grown on 5-kA(ring) Ag(111) single-crystal bases grown on single-crystal natural muscovite mica.An intervening epilayer of NaCl (150 A(ring)) deposited between the mica and Ag base facilitated film removal from the Fe-contaminated mica for ex situ transmission 57Fe Mössbauer analysis. The substrate growth temperature for this series was held at 180 °C, since this appears to be optimal for Fe(110) growth on Ag(111).2 Note that the resultant Fe(110) growth is mosaic with Fe[001] parallel to Ag〈110〉 (threefold symmetry). The RHEED observation of the growth of Ni on Fe(100) always resulted in the Ni RHEED pattern closely following that of the Fe (100) pattern, with broader Ni RHEED lines apparent. The characteristic behavior of our Ni RHEED patterns mimicked that observed by Heinrich et al. for bcc Ni(100),3 and did not match that of fcc Ni. The Ni-on-Fe(110) growth was analogous in RHEED characteristics to that of the (100) case. The Ni RHEED patterns again closely matched that of Fe(110), the only real difference being the broadening of the Ni RHEED streaks. Note that fcc Ni(111) was seen to grow on Ag(111) under similar growth parameters. It is likely that a metastable bcc Ni(110) structure analogous to bcc Ni(100) was observed. The quality of the Fe/Ni RHEED patterns did not seem to significantly worsen from bilayer to bilayer throughout the growths of either series. Furthermore, the respective Ag cover layers for all films showed excellent RHEED patterns. All the observed Mössbauer spectra for both series of Fe/Ni multilayers show sextets at room temperature, except for the 2-ML Fe(110) film, which exhibited a very small additional single-line central feature. At 4.2 K, the 2-ML Fe(110) film had no change in central feature, ruling out superparamagnetism as a cause. All films exhibited in-plane magnetization, and thinner Fe bilayers exhibited a growing isomer-shifted second sextet-site presence, suggestive of an interfacial Fe site at the Fe/Ni interface.An enhanced hyperfine field is seen for the thinnest Fe bilayer films at 4.2 K. This enhancement is greatest for the Fe(100) system [most enhanced Fe(100) site=365 kOe vs most enhanced Fe(110) site=351 kOe, compared to 341 kOe for bulk]. The thickest Fe bilayer films for both series showed nearly-single-site, bulklike hyperfine-field behavior. The Mössbauer spectra observed for these epitaxial Fe/Ni heterostructures are different than that previously reported for polycrystalline fcc Fe/fcc Ni films.4 More detailed structural and magnetic studies of the novel bcc Ni reported here should be pursued.

Notes: By doping a very small amount of 57Fe into La2CuO4, Mössbauer spectroscopy has been applied to study the magnetic property of the parent compound. From the measurement of the magnetic hyperfine field at the Fe nuclei for various temperatures between 4.2 K and TN, the temperature dependence of the sublattice magnetization for La2CuO4 has been discussed. A theoretical calculation shows that, with temperature increasing a 3D-2D dimensional crossover occurs in the magnetic dynamics of an anisotropic antiferromagnetic, which is indeed confirmed by the data. The best fit to the data using this theory yields J=1600 K and r=0.011.

Notes: 1.5% of Fe has been substituted for Cu in several "2212'' and "2223'' Bi-Sr-Ca-Cu superconductors. All of the samples show a reduction of Tc by about 13 K due to the Fe impurities. Mössbauer measurements at room temperature reveal structural characteristics such as stacking faults and intergrowth of different phases in these Bi-based compounds on the microscopic scale. The suppression of Tc due to Fe doping in the Bi "2212'' or " 2223'' system is comparable to that of the "123'' system, but much smaller than that of the "214'' system. The interplanar correlation existing in the "123'' and the Bi "2212'' and "2223''systems seems to play an important role in sustaining the high-temperature superconductivity and weakening the detrimental effect of impurity elements on superconductivity in these two systems.

Notes: The nature of the interlayer coupling in epitaxial Fe(110)/Ag(111) multilayer structures was investigated using a highly surface-sensitive Mössbauer spectroscopy technique. The films were grown by molecular beam epitaxy and analyzed with x-ray diffraction and in situ RHEED to verify their crystallinity and orientation. All the films took the general form [56Fe3057Fe2Agx]15, where x=3 to 22 monolayers (ML). The entire film is invisible to the Mössbauer effect except for the 2 ML 57Fe layers, which therefore act as a probe of the magnetic environment of the Fe surfaces. Information regarding the surface spin-wave spectrum can then be obtained by measuring the temperature dependence of the hyperfine field at the surface. In the limit of no Ag interlayer, the 57Fe probe layers would be in direct contact with 56Fe on both sides and therefore should display a (1−BT3/2) hyperfine field temperature dependence, with B=5.2×10−6 K−3/2. As the Ag interlayer thickness is increased, we expect the hyperfine field to follow a (1−kBT3/2) temperature dependence, with 1〈k〈3.5 depending on the strength of the interlayer exchange. In addition, we expect an enhancement in H(0), the hyperfine field at 0 K, as the surface exchange decreases. The RHEED and x-ray studies showed all the samples to be single-crystalline and well-oriented, with no Bragg reflections other than Fe(110) and Ag(111) present. As the Ag interlayer thickness increased, the films showed the expected softening of the spin-wave spectrum, with k increasing from 1 at x=0 to 2.2 at x=22. However, a clear oscillation in k was observed at x=6 ML, and a possible second oscillation visible at x=12 ML. This implies an oscillation superimposed upon the monotonically decreasing interlayer exchange. The value of H(0) also shows a similar oscillation at x=6 ML.

Notes: The polarization of photoemitted electrons from thin AlxGa1−xAs layers grown by molecular-beam epitaxy has been studied as a function of Al concentration by varying x in steps of 0.05 from 0.0 to 0.15. As the fraction x is increased, the wavelength dependence of the polarization shifts toward shorter wavelengths, permitting wavelength tuning of the region of maximum polarization. A maximum electron polarization of 42%–43% is obtained for AlxGa1−xAs samples with x≥0.05 while the maximum polarization of GaAs (x=0) samples reaches 49%. To investigate the lower polarization of AlxGa1−xAs, additional samples have been studied, including a short-period superlattice (GaAs)7 - (AlAs)1 .

Notes: It is well known that two-dimensional spin-wave excitations result in a linear temperature dependence of the magnetization in a quasi-two-dimensional ferromagnetic system. However, it has been shown also that magnetic relaxation from small islands inside a film can also result in a similar linear temperature dependence. In this paper, it is found that comparative Mössbauer measurements with and without a weak magnetic field can clearly distinguish these two different mechanisms: The linear temperature dependence of the magnetization is unaffected by the external field if 2D spin-wave excitations are responsible for the linear behavior, while the linear slope of the temperature dependence of the magnetization is reduced by the external field if magnetic relaxation is involved.

Notes: A series of samples of YBa2(Cu1−xFex)3O7 with x ranging from 0 to 0.13 was made and investigated by x-ray diffraction, dc SQUID magnetometry, and Mössbauer spectroscopy. In the samples made by the ordinary process, Fe mainly occupies Cu(1) sites. The orthorhombic-to-tetragonal structural transition occurs at x(approximately-equal-to)0.03, and the superconducting transition temperature is depressed to zero at x(approximately-equal-to)0.17. By annealing the samples in Ar gas at 750 °C for 24 h and then reloading the lost oxygen in O2 gas at 210 °C for 48 h, we were able to draw about 30% of the Fe from Cu(1) sites into Cu(2) sites. This was verified by the enhancement of the component in the Mössbauer spectrum corresponding to Fe in the Cu(2) sites. The structural and superconducting properties of this new series of samples were also investigated. Comparison with the ordinary Fe-doped samples was also made.