Structure and optical properties of ZnSe/SiO<Subscript>2</Subscript> layered nanocomposites

Structure and optical properties of ZnSe/SiO₂ layered nanocomposites obtained using microwave magnetron sputtering have been studied. The nanocomposites are X-ray amorphous at relatively small thicknesses of the zinc selenide layers. When the thickness of the zinc selenide layers exceeds 20 Å, ZnSe/SiO₂ films contain SiO₂ amorphous phase and zinc selenide cubic nanocrystallites. It has been demonstrated that the thickness of zinc selenide layers affects the microstresses, refractive index, and band gap.     

Magnetic properties of ultrathin Ni films

The magnetic properties of Au/Ni/Si(100) films with Ni thicknesses of 8–200 Å are studied at T=77 K using a scanning magnetic microscope with a thin-film high-temperature dc SQUID. It is found that the Ni films, with an area of 0.6×0.6 mm, which are thicker than 26 Å have a single-domain structure with the magnetic moment oriented in the plane of the film and a saturation magnetization close to 0.17 MA/m. For films less than 26 Å thick, the magnetization of the film is found to drop sharply.     

Transition from antiferromagnetic interlayer ordering to superparamagnetic state in Fe/Cr nanostructures by varying Fe thickness

Transition from antiferromagnetic exchange coupling of the Fe layers tosuperparamagnetic state of the Fe/Cr nanostructures is studiedexperimentally and theoretically. The experimental study are performed bymeans of magnetoresistance and magnetization measurements as well asMössbauer spectroscopy for the nanostructures with Fe layers thicknessesfrom 1.4 Å up to 16 Å alternating by 10 Å of Cr layers. It isshown that Fe layers in the nanostructures with thicknesses less than2 Å are not continuous but consist of separate ferromagnetic clusters.Such cluster-layered nanostructures exhibit Langevin paramagnetism of thesuperparamagnetic clusters and the Kondo-like behavior of the resistance.For the considered nanostructures, a modeling of Fe and Cr atoms randomdeposition for the interface layers and self-consistent calculations of themagnetic moments distribution in Periodic Anderson model are carried out. Itis shown that, for nanostructures with extremely thin Fe layers, the Feclusters with lateral size in the range of 11 to 20 Å areself-organized. Calculations of magnetic moments distribution result inhistograms which coincide with the hyperfine field distributions obtained byMössbauer spectroscopy.     

Ellipsometric measurements on vapor deposited organic films

It has been demonstrated that ellipsometry can be used to study the kinetic properties of organic materials deposited, under vacuum, as thin films on optical surfaces. The organic films were deposited in a controlled manner using a vapor effusion source. Evaporation rate measurements on a DC-704 silicone oil film over a thickness range of 140 to 10 Å have shown that the film exists as two distinct layers with a transition thickness of ~18 Å. The second layer (d > 18 Å) exhibits an linear evaporation rate corresponding to that of the bulk fluid at 300 K while the first layer has a value two orders of magnitude smaller. The first layer was interpreted as a single monolayer having a thickness equal to the helix diameter of the silicone molecule. Additional measured kinetic properties were the sticking coefficients and vapor pressure tor various substrate temperatures and the molecular binding energy. The measured values of Δ and ψ were used to determine the optical constants of both DC-704 and DC-705 silicone oil films as N = 1.5 l — i0.00 and 1.48 — i0.00, respectively, where N = n — ik.     

Magnetic-dichroism study of iron silicides formed at the Fe/Si(100) interface

The interplay between the phase composition, electronic structure, and magnetic properties of the Fe/Si(100)2×1 interface has been studied at the initial stages of its formation (at Fe doses up to 8 Å). The experiments were carried out in ultra high vacuum by using high-resolution photoelectron spectroscopy with synchrotron radiation. The interface magnetic properties were examined in terms of magnetic linear dichroism in angle-resolved Fe 3p core-level photoemission. It was found that at room temperature a disordered Fe–Si solid solution is formed at the first stage of Fe deposition (≤3.4 Å). In the coverage range of 3.4–4.3 Å the solid solution transforms into Fe₃Si. However, the in-plane ferromagnetic ordering of the silicide occurs only at 6.8 Å Fe that demonstrates the thickness dependence of the magnetic properties of Fe₃Si. The subsequent sample annealing to 150°C transforms Fe₃Si to ε-FeSi, leading to the disappearance of ferromagnetic behavior.     

Magnetism of Co layers in a Co/Si multilayer film

The magnetic properties of Co/Si multilayer films produced through rf ion sputtering were studied in the temperature range 4.2–300 K. The dependences of the spontaneous magnetization and hysteresis characteristics of the films on the thicknesses of the magnetic layers and nonmagnetic spacers are established. It is shown that these dependences are determined to a large extent by interlayer interfaces, in which the effective magnetic moment of the Co atoms and the exchange interaction decrease and magnetic-anisotropy dispersion arises. A probable cause of the interface formation is interlayer mixing (which is estimated to penetrate to a depth of 15 Å) and the strong effect of Si on the Co electronic structure.     

Dispersion of surface plasmons in dielectric-metal coatings on concave diffraction gratings

Anomalies in the intensity ofp-polarized light from concave diffraction gratings (Wood's anomalies) have been used to obtain surface plasmon dispersion curves for dielectric-metal layers on the grating surface. These include a 350 Å MgF₂ layer and Al₂O₃ layers varying from a few angstroms to over 800 Å on an Al substrate. The wavelength range of the incident and diffracted light is from the visible to the vacuum ultraviolet (7,500?500 Å). Anomalous polarization peaks for Al₂O₃ layers in the vacuum ultraviolet (at ～ 1,600 Å) are shown to shift significantly to longer wavelengths for only a few angstroms (up to 50 Å) of oxide thickness, while for thicknesses greater than ～ 50 Å any further shift is small. In the visible region (> 4,000 Å), on the other hand, the wavelength shifts are small for dielectric thicknesses up to ～ 50 Å, but are large for thicknesses of several hundred angstroms. These results are in substantial agreement with the theoretical dispersion curves for these cases. Also considered are some of the effects of diffusion pump oil.     

Scattering of conduction electrons in Fe(t x , Å)/Cr(10 Å) superlattices with ultrathin iron layers

IR magnetoreflection spectra, diagonal σ _xx and off-diagonal σ _xy components of the effective optical conductivity tensor, and magnetic properties of Fe(t _x , Å)/Cr(10 Å) superlattices have been studied. The abrupt decrease in the amplitude of dissipative function ?ωImσ _xy (ω) (ω is the cyclic frequency of light wave) in the superlattices with ultrathin Fe layers (t _Fe = 3.2, 2.6, 2.1 Å) has been analyzed. It has been found that the magnetorefractive effect in nanostructures with ultrathin iron layers is due to scattering of conduction electrons by magnetic interfacial layers formed in the Cr matrix with complete consumption of deposited iron atoms. The parameters of the interfacial scattering of electrons in the spin-up (└) and spin-down (┌) conduction channels have been discussed.     

Magnetic aftereffect in Co-P trilayer films

This paper reports on the results of experimental investigations of the magnetic properties of a trilayer system that consists of high-coercive polycrystalline and low-coercive amorphous magnetic Co-P layers and a nonmagnetic amorphous Ni-P interlayer. It has been established that the coercive force and the bias field of the hysteresis loop of the low-coercive layer undergo anomalous changes in the range of small thicknesses of the hard magnetic layer. In the same range of thicknesses, a magnetic aftereffect is found to manifest itself, which is unusual for this type of magnetic structures. The observed features are associated primarily with the kinetics of formation of a crystal structure of the high-coercive layer.     

Thickness dependent magnetic and structural properties of Co films grown on GaAs (100)

The structural, magnetic and transport properties measurements carried out on Co thin films deposited by electron beam evaporation on GaAs substrate as a function of layer thickness ranging from 50 Å to 1000 Å are presented here. Structural measurements show the film to be amorphous in nature at lower thickness which becomes crystalline at higher thickness. Magnetic measurements show an increase in saturation magnetization (M_S) with film thickness. M_S values are found to vary from 521 emu/cm³ to 1180 emu/cm³ for thicknesses ranging from 50 Å to 1000 Å. The coercivity and saturation field value shows a systematic decrease up to 600 Å thickness and increase thereafter. Various microstructural parameters were also calculated using GIXRR technique. A clear grain growth is observed in AFM technique with film thickness and its influence on transport properties was also seen. Different surface morphology and magnetic domain structures were obtained on different thin film samples by AFM and MFM techniques, respectively. XPS measurements reveal formation of CoAs phase at the interface between Co and GaAs. All these results are discussed and interpreted in detail in this communication.     

Exhibition of surface steps by high resolution electron microscopy of monoclinic rare earth sesquioxides

On high resolution structure images, the existence of a superperiod has been observed, linked with the apparition of extraspots in the diffraction diagram. The specimen (B-Sm₂O₃) can be described as composed of layers of thickness 6 Å, nearly perpendicular to the incident electron beam. These layers are of 3 different kinds. We have shown that the apparition of the superperiod can be linked with steps of height 6 or 12 Å (i.e. 1 or 2 layers): computed images, using this hypothesis exhibit the observed superperiod, even for thicknesses around 230 Å. This allows atomic steps to be imaged with a high resolution structure image.     

Magnetic properties and structures of Fe/MgF2 multilayered films

Multilayers composed of Fe and MgF₂ with layer thicknesses between 9 Å and 100 Å and of 30 Å, respectively, were prepared with an ultrahigh-vacuum deposition technique. Medium-angle X-ray data show that the Fe layers in the BCC phase have considerable (1 1 0) texture. Periodicity due to multilayered structures was confirmed by a small-angle X-ray diffraction study and cross-section transmission electron microscope for films with Fe layer thicknesses >45 Å. In an Fe/MgF₂(9 Å/30 Å) sample, an island structure for the Fe layers was suggested by the existence of superparamagnetism in a film. At 4.2 K, enhancements of both magnetization and hyperfine field were observed in films having Fe layers thinner than 40 Å. The maxima in the magnetization (233 emu/g of Fe) and in the average hyperfine field (390 kOe) at 4.2 K were found in an Fe/MgF₂(9 Å/30 Å) film and were approximately 105% and 115% that of the bulk α-Fe, respectively. The thickness dependence suggests a 12% enhancement in the magnetic moment of interface Fe atoms. No exchange bias was found in the films, implying that antiferromagnetic fluorides are not formed at the interface, which is different from the case of Fe/LiF and Fe/CaF₂ multilayers.     

Magnetization of layered Mn-Ni and Mn-Co thin films

We report magnetization hysteresis loops for compositionally modulated films of Ni-Mn and Co-Mn. The films, prepared by evaporation, have nominal layer thicknesses of 5 to 25 Å and total thickness of 1 μm. Measurements were made on a SQUID magnetometer at 5 and 50 K with a maximum applied field of 10 kOe. The magnetic behavior observed ranges from ferromagnetism to spin-glassiness. Marked field cooling effects were observed for films with nominal layer thicknesses of 6 Å Ni-6 Å Mn, 13 Å Ni-13 Å Mn, 25 Å Ni-25 Å Mn and 5 Å Co-15 Å Mn.     

Magnetic phase diagram of the spin-valve structure with an antiferromagnetic oxide layer

The magnetic phase diagram of a three-layer ferromagnet-antiferromagnet-ferromagnet structure with an antiferromagnetic oxide layer of variable thickness has been investigated. It has been predicted that there are three ranges of thicknesses in which domain walls created by the frustration of the exchange interaction between the layers have different structures. The structure of these domain walls induced by edges of atomic steps at the interfaces between the layers has been described. The predictions of the theory have been compared with the available experimental results, in particular, with the data indicating the existence of the nanodomain state.     

Experimental discovery of the anisotropy of phonon-plasmon modes in GaAs/AlAs(100) superlattices

Doped (n-type) GaAs/AlAs superlattices with thicknesses of the GaAs and AlAs layers from 1.7 to 6.8 Å and 13.6 Å, respectively, have been studied by means of Raman spectroscopy. The use of a microattachment for Raman backscattering studies has allowed for the observation of the modes with the wave vector directed both across and along the superlattice layers (in the scattering from the side face of the superlattice). The theoretically predicted anisotropy of mixed phonon-plasmon modes caused by the anisotropy of the electron effective mass in the type II superlattices has been experimentally discovered.     

Readdressing the issue of low-temperature resistivity minimum in La0.7Ca0.3MnO3 thin films

We have investigated the origin of low-temperature resistivity minima observed in epitaxial thin films of La_0.7Ca_0.3MnO₃ (thicknesses—300 Å and 3000 Å) using electrical and magneto-transport property measurements. We observe considerably smaller hysteresis in the magnetoresistance measurements for the thicker film than the thinner film. 300 Å film shows meta-stability in the resistivity measurements at low temperature and for this film the sample current ‘I’ shows large effect on the resistivity and its minima temperature. These observations suggest that the strain induces electronic intra grain inhomogeneity in these samples and these inhomogeneities consist of regions of different resistive phases. It appears that the high resistive phase prevents the transport of charge carriers between two low resistive regions thus giving rise to the resistivity minimum in these samples.     

Digital magnetic heterostructures based on Si and Fe

This paper reports on the results of ab initio calculations of the electronic and magnetic properties of Si digital heterostructures doped with a Fe monolayer of the substitutional or interstitial type. It has been revealed that, after the structural relaxation, heterostructures of both types exhibit a two-dimensional metallic behavior and a ferromagnetic ordering in the range of Si spacer thicknesses up to 19 atomic layers. The magnetization and spin polarization at the Fermi level for the heterostructures with an Fe monolayer of the substitutional type are two times higher than those for the system with an Fe monolayer of the interstitial type.     

Structural and magnetic properties of La/Fe multilayers

The structural and magnetic properties of La/Fe multilayers were investigated by X-ray diffraction, RHEED, magnetometry and⁵⁷Fe Mössbauer spectroscopy. Comparison is made with previous results obtained for Ce/Fe multilayers. Remarkably sharp interfaces are found, with roughness between 2 and 2.5 Å. The magnetic interface in the Fe sublayers resulting from the distribution of magnetic hyperfine fields distinctly exceeds the extension of the structural interface and points to a magnetic proximity effect. This is discussed in relation to a strong 3d-5d hybridization recently found in measurements of magnetic circular X-ray dichroism. Both the structural and magnetic La/Fe interface is less extended than the interface in Ce/Fe multilayers. Below a thickness of about 25 Å, the individual Fe layers grow in an amorphous structure on the La layers. In this case, Curie temperatures are below 200 K and the Fe-layer saturation magnetization is reduced up to 50%, and there is evidence of a non-collinear spin structure. It is argued that this mainly reflects the properties of pure amorphous Fe.     

57Fe Mössbauer study of Fe/Bi multilayers

Fe/Bi artificially structured films (ASF's) have been prepared by alternate depositions of Fe and Bi in an ultrahigh vacuum. X-ray diffraction measurements in the small angle range confirmed the formation of periodic structures in all the samples prepared on glass substrates cooled down to about 125K. the CEMS at room temperature indicated that the samples are ferromagnetic except the one with 2Å-thick Fe layers. The CEM spectra also indicated that the structure of Fe layers is amorphous when the Fe layer thicknesses are less than 15Å. Magnetization measurement and CEM spectrum at 6K show that Fe monolayers in the Fe/Bi ASF are ferromagnetic.     

Solid Fabry-Perot etalons for X-rays

Solid Fabry-Perot etalons for X-rays have been constructed using sputter deposition techniques, each etalon consisting of two Layered Synthetic Microstructures (LSM) Bragg diffraction structures separated by a carbon spacer. The individual LS mirrors contain fifteen tungsten layers (t_w = 8.5 Å) separated by carbon layers (t_c = 19.1 Å. The thick carbon spacers act as resonant cavities; for the structures reported on here the spacer thicknesses, t_sp, are 496.6 Å and 981 Å. The structures were characterized at grazing incidence in reflection using Cu Kα (λ = 1.5418 Å) radiation. The measured response of the etalons agrees well with calculation. Observed reflection efficiencies for Cu K_α were approximately 50 percent of that calculated. This discrepancy is believed to be the result of the interfacial roughness (～3.25 Å) between component layers and the sensitivity of the etalon response to the divergence of the incident X-ray beam.