K. Vad

Magnetic particle hyperthermia: Néel relaxation in magnetic nanoparticles under circularly polarized field.

The mechanism of magnetization reversal in single-domain ferromagnetic particles is of interest in many applications, in most of which losses must be minimized. In cancer therapy by hyperthermia the opposite requirement prevails: the specific loss power should be maximized. Of the mechanisms of dissipation, here we study the effect of Néel relaxation on magnetic nanoparticles unable to move or rotate and compare the losses in linearly and circularly polarized fields. We present exact analytical solutions of the Landau-Lifshitz equation as derived from the Gilbert equation and use the calculated time-dependent magnetizations to find the energy loss per cycle. In frequencies lower than the Larmor frequency, linear polarization is found to be the better source of heat power, at high frequencies (beyond the Larmor frequency) circular polarization is preferable.

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.

Kinetic pathways of diffusion and solid-state reactions in nanostructured thin films

Mass transport and solid-state reactions in nanocrystalline thin films are reviewed. It is illustrated that diffusion along different grain boundaries (GBs) can have important effects on the overall intermixing process between two pure films. These processes can be well characterized by a bimodal GB network, with different (fast and slow) diffusivities. First the atoms migrate along fast GBs and accumulate at the film surface. These accumulated atoms form a secondary diffusion source for back diffusion along slow boundaries. Thus the different GBs of the thin films can be gradually filled up with the diffusing atoms and composition depth profiles reflect the result of these processes. Similar processes can be observed in binary systems with intermetallic layers: instead of nucleation and growth of the reaction layer at the initial interface, the reaction takes place in the GBs and the amount of the product phase grows by the motion of its interfaces perpendicular to the GBs. Thus, the entire layer of the pure parent films can be consumed by this GB diffusion-induced solid-state reaction (GBDIREAC), and a fully homogeneous product layer can be obtained.

Application of Surface Roughness Data for the Evaluation of Depth Profile Measurements of Nanoscale Multilayers

A secondary neutral mass spectrometric (SNMS) depth profile study of electrodeposited Co/Cu multilayers was performed. Depth profile measurements were performed both in the conventional way (i.e., starting the sputtering from the final deposit surface) and in the reverse manner (i.e., detaching the multilayers from the substrate and starting the analysis from the substrate side, which was very smooth as compared to the final deposit surface). The latter method could yield significantly larger intensity fluctuations in the SNMS spectra. Surface roughness data were measured with atomic force microscopy (AFM) for multilayers with different bilayer numbers but otherwise exhibiting the same layer structure as those used for the depth profiling. The experimental AFM surface roughness evolution was used to calculate the result of the depth profile measurements quantitatively. An excellent agreement was obtained between this calculation and the SNMS measurements. It was shown that the decrease in the intensity fluctuations during the depth profile analysis stems mainly from the increase in surface roughness of the samples studied, especially in the conventional sputtering mode. It was also concluded that the thickness fluctuation of the entire multilayer deposit and that of each layer are strongly correlated.

Investigations of diffusion kinetics in Si/Ta/Cu/W and Si/Co/Ta systems by secondary neutral mass spectrometry

Proper understanding of the degradation mechanisms and diffusion kinetics of copper and cobalt interconnections for advanced microelectronics is important from the point of view of fundamental research and technology as well. In this paper Si(substrate)/Ta(10 nm)/Cu(25 nm)/W(10 nm) and Si(substrate)/Co(150 nm)/Ta(10 nm) samples, prepared by DC magnetron sputtering, were in investigated. The samples were annealed at several temperatures ranging from 423 K to 823 K for various times. The composition distributions were detected by means of Secondary Neutral Mass Spectrometry (SNMS). Microstructural characterization of samples was carried out by means of Transmission Electron Microscopy (TEM). It is shown that the changes in the composition profiles were mainly caused by grain boundary, GB, diffusion and the effective GB diffusion coefficients of Ta in Cu were determined both by the ‘‘first appearance’’ and ‘‘centre-gradient’’ methods. The activation energy is 100 kJ/mol. The importance of the Ta penetration into the Cu and its accumulation at the Cu/W interface can lead to an increase of the Ta content in the copper film. This can be an important factor in the change/degradation of the physical parameters (e.g. the electrical resistance) of interconnects. Furthermore a Ta segregation factor in Cu was evaluated. Preliminary results in the Si(substrate)/Co(150 nm)/Ta(10 nm) indicate fast (GB) diffusion of the Si into the Co layer, formation of a cobalt silicide layer at the Co/Si interface and Si accumulation first at the Ta/Co interface and later a retarded accumulation at the free Ta surface. 2010 Elsevier Ltd. All rights reserved.

The effect of Pr substituted in Eu1−xPrxBa2Cu3O7−δ and EuBa2−xPrxCu3O7−δ perovskites

Abstract Eu1−xPrxBa2Cu3O7−δ (x=0.0, 0.1, 0.5, 0.7, 1.0) and EuBa2−xPrxCu3O7−δ (x=0.0, 0.1, 0.3, 0.5, 0.7) perovskites have been synthesized and investigated by means of 151 Eu Mossbauer spectroscopy, X-ray diffractometry, resistivity and susceptibility measurements. We have found that Pr can be introduced into the Ba site in EuBa2Cu3O7−δ high temperature superconductor. Our results show that EuBa2−xPrxCu3O7−δ is a superconductor for x≤0.7.

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.

Investigation of Grain Boundary Diffusion in Thin Films by SNMS Technique

It was shown more recently in our Laboratory [1,2,3] that having a substrate/diffusant/thin-film/cap-layer structure (the thin film was typically several 10 nm thick, with the same order of magnitude of grain size; the refractory metal cap layer was used just to avoid the oxidation), first the diffusant atoms migrated very fast across the thin film and segregated at the film/cap-layer interface. The accumulated atoms at the film/cap layer interface form a secondary diffusion reservoir and atoms diffuse back to the layer. Later on, the thin film was gradually filled up with the diffusing atoms and composition depth profiles, determined by Secondary Neutral Mass Spectroscopy (SNMS), showed a maximum at the cap layer-thin film interface. The accumulated atoms at this interface formed a secondary diffusion reservoir and atoms diffused back to the layer. These observations can be interpreted supposing a bimodal grain boundary structure with different (fast and low) diffusivities. The observed grain boundary diffusion phenomena can be classified as C-type diffusion. The appearance of the peak observed at the cap layer interface can be used as a tool to determine the grain boundary diffusivity along the fast boundaries. Because the fast boundaries were saturated in the first stage of the process, this back-diffusion took place along the low-diffusivity boundaries only. Thus the SNMS depth-profiling is a good method to determine grain boundary diffusivities in a bimodal structure. In addition, from the overall impurity content inside the film the segregation can also be estimated, if the bulk solubility is low and the GB density is known. Numerical simulations of C-type GB diffusion in thin films with a bimodal structure confirmed that the interpretation of the result depicted above is reasonable [4]. In order to estimate roughly the GB diffusion data we determined the fast diffusivity using the first appearance method. The lower diffusivity was determined from the time evolution of the broadening of the diffusant/thin film interface. In addition both (slow and fast) diffusivities were also estimated from fitting numerical solutions obtained in [4] too.

Transverse and secondary voltages in Bi2Sr2CaCu2O8 single crystals

Abstract Multicontact configuration is one of the most powerful arrangements for electrical transport measurements applied to study vortex phase transition and vortex phase dimensionality in strongly anisotropic high- T c superconducting materials. In this paper we present electrical transport measurements using a multiterminal configuration, which prove both the existence of guided vortex motion in Bi 2 Sr 2 CaCu 2 O 8 single crystals near the transition temperature and that secondary voltage in zero external magnetic field is induced by thermally activated vortex loop unbinding. The phase transition between the bound and unbound states of the vortex loops was found to be below the temperature where the phase coherence of the superconducting order parameter extends over the whole volume of the sample. We show experimentally that 3D/2D phase transition in vortex dimensionality is a length-scale-dependent layer decoupling process and takes place simultaneously with the 3D/2D phase transition in superconductivity at the same temperature.

RF susceptibility of magnetron sputtered YBaCuO−x films

Abstract We measured the complex susceptibility of two different quality magnetron sputtered YBa 2 Cu 3 O 7−x films in RF range. Temperature and static magnetic field dependence of complex magnetic susceptibility is a useful indicator of appearance of phase coherence in the film including fluctuation region above T c . The characteristics showed a low frequency type behaviour at loss peak and diamagnetic drop at near transition temperature and normal state non-zero susceptibility. One of our samples displayed an enhanced diamagnetism in the normal state close to T c , which was probably attributed to the combination of the normal skin effect and excess conductivity due to superconductive fluctuations. The low temperature susceptibility was also frequency dependent and qualitatively agreed with the vortex-glass scaling model.