ALBERT

Notes: Ferromagnetic (FC) and antiferromagnetic coupling (AFC) of Co layers across a metastable fcc Fe spacer layer has been observed. Room-temperature-grown Fe on Co/Cu(100) was chosen as a spacer layer because it exhibits three distinct structural and magnetic phases depending on the thickness range: fct and ferromagnetic (region I), fcc and nonferromagnetic (region II), bcc and ferromagnetic (region III) (listed in order of increasing thickness). Co/Fe/Co sandwiches were grown on Cu(100) by molecular beam epitaxy with a base pressure of ∼2×10−10 Torr, and characterized by low-energy electron diffraction and reflection high-energy electron diffraction. The magnetic properties were studied in situ using surface magneto-optic Kerr effect. Using a wedged Fe spacer layer, we investigated the magnetic coupling between Co films across many thicknesses of Fe. We found FC in region I, strong AFC at the boundary between regions I and II, and weak AFC in region II. We also studied the effect of just the Co overlayer on the metastable fcc Fe. We find that Co/Fe/Cu(100) differs qualitatively from Fe/Co/Cu(100). Finally, we find an oscillation in the AFC with a periodicity of ∼12 A(ring) by artificially increasing the thickness range of region II. © 1996 American Institute of Physics.

Notes: A rich variety of magnetic and structural properties have been found in fcc Fe films grown on Cu(100). In order to better comprehend the relation between the magnetic and structural properties of fcc Fe, we investigated fcc Fe films grown by molecular beam epitaxy on fcc Co(100). Structural characterization by low-energy electron diffraction and reflection high-energy electron diffraction indicate that the structural properties of fcc Fe films grown on Co(100) at room temperature are very similar to those of fcc Fe on Cu(100), exhibiting three distinct regions (fct, fcc, and bcc), with characteristic reconstructions at the boundaries. Magnetic measurements with in situ surface magneto-optic Kerr effect (SMOKE) reveal in-plane magnetization at room temperature for the three regions. Regions I and III are ferromagnetic, while region II has a small Kerr signal which is constant throughout the region. Cusps in the coercivity of the SMOKE loops are found to correspond to transitions between the three regions. Oxygen absorption experiments performed at room temperature revealed no change in the magnetization of region II, suggesting that the live layers responsible for the magnetic signal in this region are not at the surface. © 1996 American Institute of Physics.

Notes: An overview is provided of recent efforts to explore magnetic and related structural issues for ultrathin Fe films grown epitaxially as wedge structures onto Ag(100) and Cu(100). Experiments were carried out utilizing the surface magneto-optic Kerr effect. Ordinary bcc Fe is lattice matched to the primitive unit cell of the Ag(100) surface. Fe wedges on Ag(100) can be fabricated whose thick end has in-plane magnetic easy axes due to the shape anisotropy, and whose thin end has perpendicular easy axes due to the surface magnetic anisotropy. A spin-reorientation transition can thus be studied in the center of the wedge where the competing anisotropies cancel. The goal is to test the Mermin–Wagner theorem which states that long-range order is lost at finite temperatures in an isotropic two-dimensional Heisenberg system. Fe wedges on Cu(100) can be studied in like manner, but the lattice matching permits fcc and tetragonally distorted fcc phases to provide structural complexity in addition to the interplay of competing magnetic anisotropies. The results of these studies are new phase identifications that help both to put previous work into perspective and to define issues to pursue in the future.

Notes: Fe/Mo/Fe and Co/Cu/Co sandwiches were grown by molecular beam epitaxy onto Mo(100) and Cu(100) single crystals, respectively, and characterized by high- and low-energy electron diffraction and in situ surface magneto-optic Kerr-effect measurements. The spacer layer in both case was fabricated to have a wedged shape in order to create a continuous change of the spacer-layer thickness. Oscillatory behavior between ferromagnetic and antiferromagnetic coupling was found; and is shown to originate at the interface between the magnetic layer and the spacer layer. For Fe/Mo/Fe, short-period oscillations are observed with a periodicity of ∼3 ML of Mo. Hysteresis loops for antiferromagnetically coupled cases are calculated from a simple model, and the results reproduce the general characteristics observed experimentally.

Notes: Fe wedges epitaxially grown on Cu(100) have been employed to investigate the interplay between magnetic and structural instabilities. 2–4 monolayer (ML) clean Fe films grown at room temperature are ferromagnetic with perpendicular easy axes. bcc Fe films(approximately-greater-than)11 ML thick are ferromagnetic with in-plane easy axes. Most importantly, 6–11 ML fcc Fe films are antiferromagnetic and have a ferromagnetic surface. Films grown below 200 K and annealed to room temperature do not exhibit the antiferromagnetic phase, but remain ferromagnetic and undergo a spin-reorientation transition from perpendicular to in plane at ∼6 ML. A new phase diagram for Fe/Cu(100) is proposed as a function of thickness and growth temperature. In addition, an impurity-stabilized layer-by-layer growth that persists to 30–40 ML Fe is also reported.

Notes: Fe(110)/Ag(111) heterostructures, composed of fixed 3 monolayer (ML) Fe bilayers and variable-thickness Ag bilayers, were grown by molecular-beam epitaxy and investigated by transmission Mössbauer spectroscopy in the temperature range from 4.2 to 300 K. We found that as the Ag layer is thick enough ((approximately-greater-than)17 ML) to magnetically isolate the neighboring Fe layers, a quasilinear temperature dependence of the Mössbauer hyperfine field results due to the two-dimensional spin-wave excitations. As the Ag layer is reduced to 4 ML, a dimensional crossover in the spin-wave excitations is induced by the magnetic interaction between neighboring Fe layers which makes the magnetic temperature dependence change from a two-dimensional T-linear relation to a three-dimensional T3/2 dependence.

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: 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: Reported reductions of Curie temperature and a linear temperature dependence of magnetization in quasi-two-dimensional magnetic films has generated significant interest in the past decade. We have made several Fe(110)/Ag(111) superlattices by molecular-beam epitaxy with the Fe component 2 monolayers thick and the Ag component of various thicknesses. By taking Mössbauer spectroscopy measurement in an external field, we found that a central feature in the Mössbauer spectrum at high temperature may not relate to the reduction of Curie temperature, but instead comes from the thermal relaxation of small islands in the Fe film. The magnetic hyperfine field has a linear temperature dependence, and this is explained by a simple relaxation model based on the island structure. This model is also supported by studying superlattices grown with different conditions.

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.