Small-angle X-ray Reflectivity Research Articles

Perpendicular magnetic anisotropy and microstructure properties of nanoscale Co/Au multilayers

We investigated the role of microstructure and Co layer thickness on the perpendicular magnetic anisotropy of as-deposited and annealed Ta (5 nm)/[Co (tCo)/Au (2 nm)] × N = 20 multilayers with 1 ⩽ tCo ⩽ 2 nm prepared using dc-magnetron sputtering. These multilayers were characterized using a vibrating sample magnetometer, a p-MOKE magnetometer and a microscopy magnetometer, small angle x-ray reflection (XRR), and wide angle x-ray diffraction (XRD) analysis. These multilayers demonstrated strong perpendicular magnetic anisotropy with their saturation magnetization close to the bulk magnetization of Co. Magnetization and magnetic anisotropy increased with annealing and this increase is directly linked to the strain relaxation and sharpening of the interfaces after annealing. Using XRR analysis before and after annealing, and fitting these XRR data, the multilayer periodicities are extracted and the refined layer thickness and surface roughness are determined. Using XRD analysis and fitting these XRD spectra, information regarding both the average lattice spacing of atoms and the strain developed on an individual layer were determined.

Non-constant diffusion characteristics of nanoscopic Mo–Si interlayer growth

In situ small-angle X-ray reflection and wide angle X-ray diffraction of synthetic, Mo–Si based multilayer structures were used to study layer interdiffusion dynamics at temperatures between 250 and 300°C. The in situ reflection measurements revealed information on non-constant interdiffusion characteristics during the interlayer growth. The activation energy for interdiffusion was found to gradually increase with growing Si-on-Mo interlayer thickness, towards a saturation level of approximately 2.5eV. Contrary, the activation energy for interdiffusion at the Mo-on-Si interlayer was almost constant at a value around 2.6eV. Wide angle X-ray diffraction at different stages in the annealing cycle further showed the evolution of Mo crystallites. Evolution of these crystallites was found to be strongly correlated to the change in period thickness of the multilayer structures.

Reactive diffusion in Sc/Si multilayer X-ray mirrors with CrB2 barrier layers

Processes undergoing in Sc/Si multilayer X-ray mirrors (MXMs) with periods of ∼27 nm and barrier layers of CrB20.3- and 0.7-nm thick within the temperature range of 420–780 K were studied by methods of small-angle X-ray reflectivity (λ=0.154 nm) and cross-sectional transmission electron microscopy. All layers with the exception of Sc ones are amorphous. Barrier layers are stable at least up to a temperature of 625 K and double the activation energy of diffusional intermixing at moderate temperatures. Introduction of barriers improves the thermal stability of Sc/Si MXMs at least by 80 degrees. Diffusion of Si atoms through barrier layers into Sc layers with formation of silicides was shown to be the main degradation mechanism of MXMs. A comparison of the stability for Sc/Si MXMs with different barriers published in the literature is conducted. The ways of further improvement of barrier properties are discussed.

High temperature annealing stability of magnetic properties in MgO-based perpendicular magnetic tunnel junction stacks with CoFeB polarizing layer

We studied Co/Pd multilayers (MLs) and CoFeB spin polarizing layer for MgO-based perpendicular magnetic tunnel junctions (MTJs) by engineering the exchange coupling between CoFeB and Co/Pd MLs and the interface morphology of Co and Pd in Co/Pd MLs. One of the key challenges of maintaining perpendicular anisotropy in both the fixed and free layers was achieved by tuning the energy balance between the Co/Pd MLs and the CoFeB layers. A perpendicular squareness ratio of near unity in M–H loops of full stack structures clearly indicated excellent perpendicular anisotropy even after annealing at 350 °C for 1 h in vacuum. Very low intermixing with sharp Co/Pd interfaces confirmed by the small angle x-ray reflectivity measurements was believed to be the key to high temperature annealing stability of magnetic properties. Our results shed light on the mechanisms resulting in low TMR (tunneling magnetoresistance) for Co/M (M=Pd, Pt, Ni) multilayer-based MTJs in this study as well as in previously published reports.

Single crystalline Tm 2O 3 films grown on Si (0 0 1) by atomic oxygen assisted molecular beam epitaxy

Ultrathin single crystalline Tm 2O 3 films have been grown on Si (0 0 1) substrate by molecular beam epitaxy using metallic Tm source at a substrate temperature of 600 °C and an atomic oxygen ambient pressure of 2×10 −7 Torr. The epitaxial relationship between the Tm 2O 3 films and the Si substrates is Tm 2O 3 (1 1 0)//Si (0 0 1), Tm 2O 3 [0 0 1]//Si [1 1 0] or Tm 2O 3 [1 −1 0]//Si [1 1 0]. Higher oxygen pressure would change the preferential growth orientations from (1 1 0) to (1 1 1) with the growth mode from epitaxy to non-epitaxy. After annealing in O 2 ambience at 450 °C for 30 min, the single crystalline films exhibit a dielectric constant of 10.8 and a leakage current density of 2×10 −3 A/cm 2 at an electric field of 1 MV cm −1 with an equivalent oxide thickness of 2.3 nm. Small angle X-ray reflectivity measurements were carried out to investigate the annealing effect in the improvement of electrical properties of the films.

Role of the uncompensated interface spins in polycrystalline exchange-biased systems

This work reports on the evolution of the exchange-bias (EB) field and the shape of the magnetization curves with the thickness of the non-magnetic spacer layer (SL, either Cu or Al2O3) of polycrystalline Co/IrMn/SL/Co films as well as their modifications caused by different post-deposition annealing treatments. Conventional x-ray diffractometry, small-angle x-ray reflectivity and cross-section transmission electron microscopy were used for the structural characterization. The hysteresis loops traced on the as-made films without SL present a pattern of two oppositely displaced subloops; it was observed that the positively shifted subloop gradually vanishes with the increase in the SL thickness. Our study points out that the significant decrease in the EB field of the top-pinned Co layers after magnetic annealing could be attributed to relaxation of the bottom interfacial IrMn spin structure caused by the heating. This relaxation mechanism might be considered as an alternative to the commonly accepted IrMn and Co interdiffusion and defect creation at the interface. Models for the uncompensated spins' configurations at each of the ferromagnet (FM)/antiferromagnet (AF) and AF/FM interfaces are proposed in order to explain the modifications of the magnetic behaviour with the SL thickness and with the annealing.

Annealing-Induced Changes in Double-Brush Langmuir−Blodgett Films of α-Helical Diblock Copolypeptides

The effect of annealing on the structure and the helix orientation in Langmuir-Blodgett (LB) monolayers of diblock copolymers (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) with unidirectional helix orientation deposited on hydrophilic silicon substrates was characterized by means of small-angle X-ray reflectivity, transmission Fourier transform infrared spectroscopy, and atomic force microscopy. Upon annealing at 100 degrees C for 24 h, the alpha-helices became less tilted toward the substrate surface normal. Surface area shrinkage accompanied the change in tilt, indicated by an increase in both film thickness and electron density, resulting in more compact and uniform films. The enhancement of the helix orientation by thermal annealing was greater for the PMLGSLG block and for the diblock copolymers with the shorter block lengths. For these diblock copolymers, annealing resulted in postorientation of the PMLGSLG block helices almost perpendicular to the substrate surface. This effect originates from a considerable increase in intermolecular packing of the PLGA block caused by hydrogen bonds between the carboxylic groups upon annealing, as well as the high mobility of the PMLGSLG block helices for rearrangement favored by the melted side chain mantle at elevated temperatures.

Influence of nano-oxide layer on the giant magnetoresistance and exchange bias of NiMn/Co/Cu/Co spin valve sensors

NiMn is an interesting material for achieving a high exchange bias in spin valve systems. We investigated the influence of a nano-oxide layer (NOL) inserted in the pinned Co layer on the magnetotransport properties of NiMn/Co/Cu/Co spin valve sensors. The samples were annealed at 350 °C for 10 min to achieve the antiferromagnetic L10 ordered structure of NiMn. The NOL has been characterized by small angle x-ray reflectivity, transmission electron microscopy (TEM), and energy filtered TEM. The inclusion of the NOL leads to an increase in the giant magnetoresistance (GMR) by 20 % indicating a high degree of specular reflection at the NOL. For NOL positions close to the NiMn/Co interface, a decrease in the exchange bias field (Hex) is observed. The best combination of high GMR value and large Hex was found when the NOL was inserted in the center of the pinned Co layer.

Multilayer films consisted of azulene-based dye molecules and polyelectrolyte: Preparation, characterization and photoluminescent property

Ultrathin multilayer films of two azulene-based (Az-based) dye molecules ( Az), 3-methylazulene-1-carboxylic acid hydrazide ( Az-1) and 5-(4-phenylamine)-2-(3-methylazulene-1-yl)-1,3,4-oxadiazoles ( Az-2), and poly(sodium 4-styrenesulfonate) (PSS) have been prepared by layer-by-layer self-assembly and characterized by UV–vis spectroscopy, fluorescence spectroscopy, small-angle X-ray reflectivity measurements and atomic force microscopy (AFM) imaging. UV–vis spectra show that the characteristic absorbance values of the multilayer films increase almost linearly with the number of PSS/ Az bilayers, suggesting that the deposition process is regular and highly reproducible from layer to layer. Average thicknesses for the PSS/ Az-1 and PSS/ Az-2 bilayers of the multilayer films are ca. 0.9 and 1.4 nm, respectively. AFM images provide the surface morphology of the PSS/ Az films, indicating that the film surface is relatively uniform and smooth. The occurrence of photoluminescent activity conforms the potential for creating luminescent multilayer films with Az-based dye molecules.

Competition between interlayer exchange and Zeeman energies on the way to saturation of magnetization in Fe/Cr multilayers

We have studied the field and temperature dependence of magnetization on ion-beam sputtered Fe/Cr multilayers. The samples were characterized by small angle x-ray reflectivity and grazing incidence x-ray diffraction techniques. The temperature dependence of the magnetization at the saturation field (Hsat) is interpreted in terms of the spin-wave theory with an additional paramagnetic contribution from the nanometer size Cr. At moderately high magnetic fields but below Hsat, an unusual increase in the magnetization with temperature, for samples of higher Cr thickness, was observed. The presence of an anomalous peak, especially for the highest Cr thickness sample at around room temperature and at magnetic fields as high as 2 T, is believed to appear as a result of an intricate competition between the Fe–Fe interlayer exchange coupling and the Zeeman energy of the outer Fe spins for the requirement of the minimum energy state.

Poly(vinyl alcohol) as Emulsifier Stabilizes Solid Triglyceride Drug Carrier Nanoparticles in the α-Modification

Colloidal dispersions of solid lipids are under intensive investigation as drug delivery systems. In the present study, poly(vinyl alcohol) (PVA) was tested as an alternative stabilizer for triglyceride nanoparticles. The dispersions contained 10% triglyceride (trimyristin or tristearin) and 5% PVA and were prepared by high pressure melt homogenization. The nanoparticle dispersions were investigated for their thermal behavior and storage stability with special regard to the polymorphic transitions of the triglyceride matrix, including effects of storage temperature and the incorporation of model drugs (diazepam, ubidecarenone) using photon correlation spectroscopy, differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. The release of the model drug diazepam from a selected nanoparticle dispersion was investigated with differential pulse polarography. Triglyceride nanoparticles prepared with PVA displayed an unusually high stability of the metastable alpha-modification depending on the type of triglyceride and the storage conditions. In tristearin nanoparticles, the alpha-polymorph was stable for at least 9 months at refrigerator temperature and the particles exhibited a spherical shape in electron microscopic investigations. Moreover, the alpha-form in PVA-stabilized tristearin nanoparticles seemed to be highly disordered, as it did not lead to a pronounced small-angle X-ray reflection. Storage at higher temperatures led to a transformation of the particles into the beta-modification, which usually was accompanied by an increase in particle size. Incorporation of the two model drugs did not change the crystal modification of the particle matrix to a large extent. After dilution into a large volume of release medium, a large fraction of the model drug diazepam was released immediately but there was no further release over several hours. The high stability of PVA-stabilized tristearin nanoparticles with regard to particle size and alpha-modification makes them suitable as a model for investigations on the influence of the polymorphic form (e.g., in comparison with nanoparticles in the more stable beta-modification) on pharmaceutically important parameters such as drug load and drug release.

Effect of negative substrate bias on HWCVD deposited nanocrystalline silicon (nc-Si) films

The influence of the negative substrate bias on the interfacial and microstructural characteristics of nanocrystalline silicon (nc-Si) thin films was deposited by hot wire chemical vapor deposition (HWCVD). Structural characterization of nc-Si films was performed by small angle X-ray diffraction (SAXRD), Raman spectroscopy, X-ray reflectivity (XRR) and field emission scanning electron microscopy (FESEM). Crystalline fraction and crystallite size increases from 61.31 to 74.13% and 13.3 to 21.6 nm, respectively, with an increasing negative bias from 0 to −200 V. Furthermore, the deposition rate of nc-Si films increases from 25 to 68 nm/min by increase of negative substrate bias from 0 to −200 V.

Thickness dependence of microstructures in La 0.9Sr 0.1MnO 3 thin films grown on exact-cut and miscut SrTiO 3 substrates

The thickness dependence of microstructures of La 0.9Sr 0.1MnO 3 (LSMO) thin films grown on exact-cut and miscut SrTiO 3 (STO) substrates, respectively, was investigated by high-angle X-ray diffraction (HXRD), X-ray small-angle reflection (XSAR), X-ray reciprocal space mapping and atomic force microscopy (AFM). Results show that the LSMO films are in pseudocubic structure and are highly epitaxial [0 0 1]-oriented growth on the (0 0 1) STO substrates. The crystalline quality of the LSMO film is improved with thickness. The epitaxial relationship between the LSMO films and the STO substrates is [0 0 1] LSMO∥[0 0 1] EXACT-STO, and the LSMO films have a slight mosaic structure along the q x direction for the samples grown on the exact-cut STO substrates. However, an oriented angle of about 0.24° exists between [0 0 1] LSMO and [0 0 1] MISCUT-STO, and the LSMO films have a mosaic structure along the q z direction for that grown on the miscut STO substrates. The mosaic structure of both groups of the samples tends to reduce with thickness. The diffraction intensity of the (0 0 4) peaks increases with thickness of the LSMO film. The XSAR and AFM observations show that for both groups, the interface is sharp and the surface is rather smooth. The mechanism was discussed briefly.

The structure of Mo/Si multilayers prepared in the conditions of ionic assistance

The influence of a negative substrate-applied bias potential on the structure of periodic Mo/Si multilayer compositions has been investigated by means of cross-sectional electron microscopy, small-angle X-ray reflectivity, X-ray diffraction and by modeling the small-angle spectra. It is known that the crystalline structure of molybdenum layers is the main source of interface roughness. In the absence of a bias potential application, the interface roughness tends to develop from the substrate towards the surface of a Mo/Si multilayer composition. A negative bias potential (up to -200 V) applied to a substrate during silicon layer deposition leads to smoother interfaces and improves the layer morphology. After increasing the bias potential over -200 V a considerable growth of an amorphous interlayer transition zone can be observed at Si-on-Mo interfaces. By raising the bias potential during the deposition of Mo layers a development of roughness at Mo-on-Si interfaces as well as growing interlayer thicknesses were found.

Self-assembled multilayer films of poor water-soluble copper(II) complexes constructed from dipyrido[3,2- d:2′,3′- f]quinoxaline (Dpq) ligand as well as their fluorescent properties

New multilayer films were prepared by alternating adsorption of poly(sodium 4-styrenesulfonate) (PSS) and a new complex of [Cu 2(Dpq) 2(Ac) 2(H 2O) 2] (ClO 4) 2·H 2O ( 1) (Dpq=dipyrido[3,2- d:2′,3′- f]quinoxaline, Ac=acetate) or a related complex [Cu(Dpq) 2(H 2O)] (ClO 4) 2·H 2O ( 1a) by electrostatic layer-by-layer self-assembly technique, respectively. Compounds 1 and 1a have been synthesized and structurally characterized by elemental analyses, IR spectroscopy, and single-crystal X-ray diffraction analyses. Single-crystal X-ray analyses show that complexes 1 and 1a possess a dinuclear and a mononuclear structure, respectively, which are further extended into layered frameworks by π– π stacking and hydrogen-bonding interactions. The multilayer films were characterized by UV–vis spectroscopy, fluorescence spectroscopy, small-angle X-ray reflectivity measurements, and atomic force microscopy (AFM) imaging. UV spectroscopy shows that the deposition process is regular and highly reproducible from layer to layer. AFM image indicates that the film surface is uniform and smooth. The fluorescent properties of the films were studied and the results showed that the forming condition of the films had great influence on their properties.

Structure of interfaces in thin-film metallic multilayer heterostructures

It is demonstrated that the structure of interfaces in thin-film metallic multilayers can be predicted from the phase diagrams of binary alloys of metals constituting the multilayers. Experimental data on the small-angle X-ray reflectivity and magnetization of the samples are used for characterizing the structure of the interfaces.

Thickness dependence of microstructures in La0.8Ca0.2MnO3 thin films

The thickness dependence of microstructures of La0.8Ca0.2MnO3 (LCMO)∕SrTiO3 (STO) thin films was investigated by high-resolution x-ray diffraction, small angle x-ray reflection, grazing incidence x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The results show that all the LCMO films are well oriented in (00l) direction perpendicular to the substrate surface. Self-organized crystalline grains with a tetragonal shape are uniformly distributed on the film surface, indicating the deposition condition being of benefit to the formation of the crystalline grains. With increasing the film thickness, the crystalline quality of the LCMO film is improved, while the surface becomes rougher. There exists a nondesigned cap layer on the upper surface of the LCMO layer for all the samples. The mechanism is discussed briefly.

First Direct Synthesis of Highly Ordered Bifunctionalized Mesoporous Silica Thin Films

Optically transparent and highly ordered mesoporous organosilica thin films functionalized with two different organic groups in various proportions were synthesized by templated-directed cocondensation of tetraethylorthosilicate (TEOS) and a mixture of two distinct and functional organotriethoxysilanes [NC(CH2)3Si(OEt)3 and O=P(OEt)2(CH2)3Si(OEt)3]. The mesostructured films obtained by evaporation induced self-assembly (EISA) approach were deposited on glass or silicon substrates by dip-coating. They were characterized by Grazing Incidence Small Angle X-ray Scattering (GISAXS) and X-ray reflectivity. We showed that whatever the proportion in organic groups, only 2D hexagonal phase having p6m symmetry was observed for all the materials indicating a good compatibility between the organic groups. The bi-functionalization of the internal pores surface by the organotriethoxysilanes groups was clearly evidenced by using micro-Raman spectroscopy.

The charge transfer structure and effective energy transfer in multiplayer assembly film

Charge transfer multiplayer films have been prepared by layer-by-layer self-assembly technique. The films incorporate the rare-earth-containing polyoxometalate K11[Eu{PW11O39}2]·nH2O and the rich electron polyelectrolyte poly(3-viny-1-methyl-pyridine) quaternary ammonium and display a linear increase in the absorption and film thickness with the number of deposition cycles. Ultraviolet and visible absorption spectra, atomic force micrographs, small-angle X-ray reflectivity measurements, and photoluminescence spectra were used to determine the structure of films. Linear and regular multilayer growth was observed. We can observe the formation of charge transfer complex compound in multiplayer by layer-by-layer assembly method. Most importantly, the luminescence spectra show the charge transfer band in assembly films, which suggest that energy could be effectively transferred to rare earth ions in assembly multiplayer films.

Microstructural studies of YB 2Cu 3O 7− δ/Nd 2CuO 4/YB 2Cu 3O 7− δ Josephson junctions with a Nd 2CuO 4 buffer layer grown on YSZ substrate

The effect of Nd 2CuO 4 (NCO) buffer layer on the microstructures of YB 2Cu 3O 7− x /Nd 2CuO 4/YB 2Cu 3O 7− x Josephson junctions grown on a YSZ substrate was investigated by high-resolution X-ray diffraction, small angle X-ray reflection, X-ray reciprocal space maps, atomic force microscopy and scanning electron microscopy. The results show that the YBCO films were well oriented in the [0 0 L] direction perpendicular to the substrate surface. No intermediate layer was formed between the YBCO and NCO films. The FWHM of the YBCO film with a NCO buffer was smaller comparing to that of the YBCO film deposited directly onto YSZ substrates. Moreover, its surface morphology was more compact and the average size of grains on the surface was also much smaller, forming a solid base for fabrication of YBCO/NCO/YBCO multilayer Josephson junctions.