Miklos Kis-Varga

Thermal stability of amorphous and crystalline multilayers produced by magnetron sputtering

Abstract For the demonstration of the high diversity of different applications of the magnetron sputtering technique in the fabrication of multilayers the production, characterisation and investigations on the thermal stability of epitaxial metallic Mo\V as well as amorphous Si\Ge semiconductor multilayers are presented. It is demonstrated, that combination of low and high angle X-ray diffraction as well as TEM is a very essential tool for the characterisation of the modulated structures and it is also very useful in the investigation of the thermal stability. The experimental results are also compared with numerical calculations made by finite element method for the diffusional homogenisation, controlled by bulk diffusion. It was shown, that in the case of the amorphous Si\Ge system the intermixing is possible in amorphous state, and diffusional stresses do not cause a significant curvature on the thermal decay curves of the first order peak of the low angle Bragg reflection (ln(I\I0) vs t), but they play an important role in the strong porosity formation in the silicon side. Furthermore it was illustrated, that the initial curvature on the ln(I\I0) vs t curve is due to the strong concentration dependence of the intrinsic diffusion coefficients. In epitaxial Mo\V multilayers the thermal behaviour is different. There is a fast transformation of the layered structure into a heterogeneous partly reacted structure, which consists of regions of a reacted Mo(V) alloy and the remaining Mo\V layers. This can not be interpreted by bulk diffusion, but rather by the formation of grain-boundaries inside the Mo layer and probably by a grain-boundary motion induced alloying.

Nanoscale investigations of shift of individual interfaces in temperature induced processes of Ni–Si system by secondary neutral mass spectrometry

We describe a method for measurement of nanoscale shift of interfaces in layered systems by a combination of secondary neutral mass spectrometry and profilometer. We demonstrate it by the example of the investigation of interface shifts during the solid state reaction in Ni/amorphous-Si system. The kinetics of the shrinkage of the initial nanocrystalline Ni film and the amorphous Si layer as well as the average growth kinetics of the product phases were determined at 503 K. The results show that nanoscale resolution can be reached and the method is promising for following solid state reactions in different thin film systems.

Photostructural transformations in amorphous chalkogenide nanolayered films produced by thermal vapour deposition

Photo- and thermoinduced changes of As2S3 \SexTe1−x (0.1 < x < 0.5) multilayer structures were investigated by optical and electrical methods as well as by low angle X-ray diffraction (LAXD). It was shown that samples with total thicknesses up to 0.7–0.8 μm of 4–20 nm thick alternating layers with good periodicity, may be prepared by cyclic thermal vapour deposition. The structure of this multilayer may be changed by thermal treatment or illumination with focused laser beam due to the interdiffusion. The corresponding photoinduced changes of optical parameters (absorption, refraction) are useful for optical reading.

Effect of residual strain, grain size and Fe impurities on magnetic properties of nanocrystalline Ni(Fe) alloys

Magnetic properties of ball-milled and relaxation heat-treated Ni have been investigated. Two stages of energy relaxation during isochronal annealing have been distinguished. It has been shown that the changes in the magnetic parameters can rather be interpreted by the presence of Fe clusters produced by the contamination and their dissolution, than by the random anisotropy model.

Correlation between the grain-shape and magnetic properties in nanocrystalline iron

Abstract Correlation between magnetic properties and time evolution of size (d) and shape of grains of pure nanocrystalline iron produced by ball-milling were investigated by Barkhausen-noise and lowfield magnetization measurements. Strong texture and elongated grain structure were developed after some hours milling time, and the thickness of elongated grains was in accordance with the grain size obtained from X-ray line broadening. After prolonged milling, disruption of the elongated grains and development of equiaxed grain structure was observed. There was a maximum in the Barkhausen-noise when the above lamellar structure was the most typical. It is shown that the coercive force (Hc) is not sensitive to the formation of the grain structure, but the course of the Hc(d) curve strongly depends on the initial and preparation conditions. In accordance with our previous results, the saturation magnetization was independent of the grain size.

THERMAL STABILITY OF MO-V EPITAXIAL MULTILAYERS

By use of transmission electron microscopy and x-ray diffraction the degradation of the structure of epitaxial crystalline Mo–V multilayers has been studied during annealing between 1000 and 1250 K. In low vacuum (10−4 mbar), first, a polygonization (driven by the elastic mismatch due to the oxidation of V) took place, which led to a grain size in the order of the thickness of the individual layers (i.e., 1–2 nm). At longer annealing times, a partial recrystallization of Mo—parallel with a grain-boundary assisted discontinuous structural transformation—was observed. The (200) texture was preserved during the degradation process. At low temperatures and in low vacuum, this process was fast, while with improving the vacuum (up to 10−7 mbar) the polygonization was slower and above 1200 K the bulk diffusion controlled intermixing was observed with a final state of completely homogeneous solid solution.

Mechanical Alloying and Magnetic Properties of Fe90Sb10 and Fe15Sb85 Systems

Fe-Sb powder mixtures were ball milled at compositions Fe 90 Sb 10 and Fe 15 Sb 85 X-ray diffraction was used to study the phase transitions and to determine the particle size. The final microstructure of the Fe 15 Sb 85 sample was investigated by TEM. The ball milling of Fe 90 Sb 10 resulted in the formation of Fe-based solid solution and amorphous and/or nanocrystalline Sb. A heterogeneous nanocrystalline granular mixture of Sb, Fe and Sb 2 Fe intermetallic was obtained after 400 h milling of Fe 15 Sb 85 , where the antimony matrix contains small, single domain Fe and Sb 2 Fe particles. The temperature dependences of magnetization and coercivity indicate superparamagnetic behaviour of this sample.

Effect of Substrate Temperature on the Different Diffuseness of Subsequent Interfaces in Binary Multilayers

Epitaxial, coherent Mo/V multilayers were deposited by magnetron sputtering on (001) oriented MgO substrates at 873K (sample MoV-T), 923K (sample MoV-U) and 973K (sample MoV-V), respectively. In order to estimate the concentration profiles in our multilayers, a superlattice refinement modelling procedure has been used on high-angle XRD symmetric scans. The Mo/V interfaces were always sharper than V/Mo ones (in this notation the order of element reflects the sequence of deposition: e.g. the Mo/V interface was formed by the deposition of the V on the Mo surface). Furthermore the interface diffuseness was only slightly different at the lowest substrate temperature, but the difference increased with increasing temperature and an abrupt concentration jump could be observed at the Mo/V interface in the sample, sputtered at the 973 K. This indicates that during deposition the interfacial mixing by impact exchange events is important and thermally activated processes (surface diffusion and/or jumps driven by segregation) are less effective. With increasing substrate temperature the thickness of the V/Mo interfaces were unchanged while the Mo/V interface became shaper and sharper i.e. thermally activated jumps were more active during deposition of V atoms. Thus in forming Mo/V interfaces the segregation tendency of V to the Mo surface results in enhanced exchanges between V atoms (buried in the near surface layers of the Mo substrate) and surface Mo atoms, leading to more sharper interface with increasing temperature. On the other hand during the formation of the V/Mo interfaces the chemical thickness of the interface, provided again by impact exchanges, was practically unchanged.

Investigating Interdiffusion in Mo/V Multilayers from X-Ray Scattering and Kinetic Simulations

In order to investigate interdiffusion in Mo/V multilayers, we present a method that combines both kinetic mean-field simulations and calculations of x-ray scattered intensity with disorder effects. Considering preliminary experimental data obtained on Mo(4nm)/V(2nm) multilayers, we show that the angular shift of spectra observed after annealing, should take place for both Fickian and asymmetric interdiffusion modes. In contrast, clear signatures of the interdiffusion modes are found from the simulation of the peak intensities. These results offer interesting perspectives for a full analysis of the interdiffusion in these systems.

Magnetic and Structural Evolution of Finemet during Special Recrystallization

Astract. Structural and magnetic properties of finemet under various heating conditions, but always without magnetic field, were investigated. Rapid nanocrystallization took place near below 773K and the amorphous fraction of the fully transformed sample was about 30%. The final state was not sensitive to the heating rate but depended on the maximum temperature. μmax was 760.000 at 50Hz exiting field and the DC permeability was over 1.200.000, which is the largest permeability published in literature for heating in absence of magnetic field. On the basis of this result, preparation of materials with such large permeabilities and with magnetic properties insensitive to temperature and mechanical conditions is possible.