ALBERT

Notizen: Soft x-ray magnetic circular dichroism (SXMCD) of the L2 and L3 edges is used to independently determine magnetic information for single crystal FexCo1−x alloy thin films (0≤x≤1) grown on ZnSe(001). Similar to surface magneto-optical Kerr effect (SMOKE) but with element and site specificity, SXMCD, which is the difference between the absorption of left and right circularly polarized soft x rays, can be used to determine a variety of magnetic parameters including spin-orbit and exchange interaction energies. Measurements made at the National Synchrotron Light Source (NSLS) U4B beamline on the Fe–Co alloy system exhibit unique changes in SXMCD signal as a function of alloy concentration. The SXMCD signal is found to rise linearly with reducing atomic concentration for both the Co and Fe. At the dilute limit of 10% FE (x=0.1), the Fe L3 SXMCD is found to increase by almost a factor of three to near 40%. In some Fe/Co structures, the largest reported SXMCD values in excess of 70% are observed! This example shows the utility of SXMCD in obtaining magnetic information in alloy systems in an element specific and even site specific manner.

Notizen: The temperature dependence of the unusual magnetic 90° coupling in epitaxial Fe/Al/Fe(001) trilayers has been determined. For specific values of applied field and temperature, in-plane anisotropy and the interlayer coupling drive abrupt first-order changes in the temperature-dependent moment of these trilayers. The occurrence of such a first-order transition can be used to determine the value of the interlayer coupling constant, JQ, at the transition temperature. We show that Fe/Al/Fe(001) trilayers also exhibit distinct second-order transitions in the temperature-dependent moment, and that the occurrence of a second-order transition can similarly be used to specify JQ(T). A comparison is made of the values of JQ(T) determined by these two approaches, and the dependence of JQ on T is analyzed.

Notizen: 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.

Notizen: Ferromagnetic alloy films of bcc FexCo1−x(001) were epitaxially grown on ZnSe-epilayered GaAs substrates spanning both the thermodynamically allowed bcc regime (0.25〈x≤1) and the epitaxy-extended metastable bcc regime (0≤x≤0.25). Conversion-electron extended x-ray absorption fine-structure (EXAFS) and reflection high-energy electron-diffraction (RHEED) measurements verify that metastable bcc FexCo1−x films with 0≤x≤0.25 are stabilized by epitaxy on ZnSe. Additionally, the composition-dependent magnetic properties of these films were characterized by ferromagnetic resonance and vibrating sample magnetometry.

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: Trilayers of CoFe/Mn/CoFe(001) have been prepared by molecular beam epitaxy and their magnetic properties measured by magnetometry and ferromagnetic resonance. Very strong near-90° coupling between the CoFe layers, with no evidence for 180° coupling, was found in all but the thickest Mn-layer samples. The coupling energy has the form suggested recently for the case when the interlayer itself is antiferromagnetic. An analysis of the ferromagnetic resonance data indicates that the magnitude of the coupling oscillates with the Mn thickness. © 1996 American Institute of Physics.

Notizen: Recent advances in high-coercivity planar-magnetization film magnets and Bi-YIG (bismuth-substituted yttrium iron garnet) film technology have opened up the possibility of fabricating very small magneto-optic devices. In this article we demonstrate that these two technologies can be brought together to fabricate a high performance waveguide magneto-optic isolator for use in integrated optical circuits. The device operates at 1.55-μm wavelength and consists of a waveguide etched into a Bi-YIG film whose magnetization is saturated by a 22-μm-thick TbCu7-type SmCo magnet with a coercivity of 4 kOe. Isolation ratios of 25 dB have been obtained in the wavelength region between 1490 and 1555 nm.

Notizen: Thin permalloy (Ni80Fe20) single-layer films and multilayers were fabricated onto Ta-buffered oxidized silicon substrates using a prototype dual ion beam sputtering system (Commonwealth Scientific Corporation). The room temperature substrates were placed in a uniform in-plane magnetic field during growth (∼150 Oe) yielding a uniaxial magnetic anisotropy in the permalloy films (typically ∼6 Oe). An examination of single-layer permalloy films with thicknesses from 20–50 nm indicated that the presence of an ∼100 eV ion-beam assist beam focused directly onto the substrate during film growth consistently modified the coercive and remnant characteristics of the films. Typically, the nonion beam assist modified NiFe films exhibited a coercivity ∼1 Oe, while the ion beam assist modified films exhibit a slightly larger coercivity (∼1.5 Oe).Based on this apparent influence of the assist ion beam treatment on the film coercivity, the fabrication of soft spin-valve multilayer structures exploiting differences in the ion beam treatment of chemically identical ferromagnetic layers seemed plausible. To test this idea ,two parallel series of multilayers composed of [2.5 nm permalloy]-[Cu (2.5 nm, 4 nm, or 5.5 nm)] or [0.3 nm Co-1.9 nm permalloy -0.3 nm Co]-[Cu (2.5 nm, 4 nm, or 5.5 nm)] were fabricated. In these 30 cycle multilayers, alternating ferromagnetic layers were subjected to a 150 eV assist ion beam treatment during growth (higher coercivity layer), or to NO assist ion beam treatment during growth (lower coercivity layer). VSM analysis revealed that the simplest uncoupled spin-valve multilayer configuration consisted of the [2.5 nm permalloy]-[4 nm Cu] multilayer with Hc1 ∼0.8 Oe and Hc2 ∼2.2 Oe, and the [0.3 nm Co - 1.9 nm permalloy - 0.3 nm Co] - [4 nm Cu] multilayer with Hc1 ∼ 3 Oe and Hc2 ∼ 15 Oe. Film analysis by low-angle x-ray diffraction and reflectivity, as well as the effect of the Co interface layer on GMR properties will also be reported. © 1996 American Institute of Physics.