Publications

In-plane and out-of-plane magnetoresistivity (MR) measurements were performed on the same antiferromagnetic (AF)YBa2Cu3O6.25 single crystal using a six-lead configuration, in magnetic fields H applied along the ab plane. We identified three terms contributing to both MRs. The first term is anisotropic with respect to the in-plane field orientation, with a twofold symmetry for the in-plane MR and a fourfold symmetry for the out-of-plane MR. We show that these anisotropic features of the magnetoresistivity tensor can be understood in terms of the coupling between the spin and the elastic degrees of freedom. The second term in both MRs is negative and isotropic upon in-plane H rotation. We ascribe this term to charge-carrier scattering on the AF domain walls. Finally, the third term is positive, quadratic in H and seems to correlate with the AF phase transition.

Results of x-ray powder diffraction and magnetic measurements performed on RFe11.3W0.7 compounds, where R=Dy, Ho, Er, and Lu, are presented. The Curie temperature, saturation magnetization, and spin reorientation transitions are discussed in relation to several RFe12-xTx systems, where T=Ta, Nb, Mo, Ti, and the x-substituent concentration is within a narrow range x=0.5-1.0. In these systems, the dependence of the Curie temperature versus the unit cell volume, for a given R partner, suggests combined size and electronic effects produced by the T atom. The strengths of the rare-earth-iron and iron-iron exchange interactions are evaluated in the mean-field framework, and analyzed with reference to RFe12-xTx compounds, with T=Ta, Re, Nb, Mo, Ti, Cr, and V. Based on (i) the phenomenological mean-field model accounting for the sublattice of conduction band electrons, (ii) Brooks' reinterpretation of Campbell's formulation of the intersublattice exchange, and (iii) Wigner-Seitz cell analysis, we explore the role played by the conduction band electrons, particularly the nd-type electrons (n=3, 4, 5), on the intersublattice exchange coupling in the iron-rich RFe12-xTx class of compounds. It is proposed that the variation in the conduction-electron density, by varying the T element type and concentration, leads to a modification of the 5d band occupancy number and consequently, of the R-Fe intersublattice exchange coupling.

The crystallization behaviour of amorphous melt-spun SmxFe80-xB20 ribbons is investigated by differential scanning calorimetry, X-ray diffraction and Mossbauer spectrometry. On increasing the Sm content. the crystallization temperature reaches a maximum for a critical Sm content (x(c) approximate to 8) and the main crystallization products are different below and above x,. This is attributed to a change in the short-range local order in the amorphous alloy. (C) 2002 Elsevier Science B.V. All rights reserved.

The continuity between the thermally driven first-order vortex lattice melting observed at high temperatures and the disorder-driven transition appearing in the low-temperature region of the vortex phase diagram of YBa2Cu3O7-delta (YBCO) is still controversial. Pal et al. have reported a rich vortex phase diagram for relatively clean YBCO crystals, in which, in qualitative agreement with a recent theoretical proposal, the end point of the vortex lattice melting is located well above the disorder-driven transition line. However, the results presented by Pal et al. suggest the existence of domains of different quenched disorder degree in the investigated crystal, and in these conditions the global magnetic measurements cannot lead to definite conclusions about the vortex phase diagram of high-temperature superconductors.

The fuzzy-logic process control system has been adapted to the unbalanced magnetron reactive sputter deposition of Ti1-xAlxN coatings. This made possible the dynamic control of the deposition rate, chemical composition and structure of the coatings and provided stable working conditions inside the hysteresis loop of the reactive sputtering also. By applying this control system, multilayers of micro- and nanocrystalline composite structures were prepared with compositions of Ti39Al61N and Ti56Al44N of TiN-AlN cubic solid solution phases. (C) 2002 Elsevier Science B.V. All rights reserved.

Advanced electroceramics have played a critical role in the development of new and modem technologies such as computers, telecomunications and aerospace and they will continue to play the leading role in the technology of the future. The present report presents and discusses a number of advanced electroceramic materials with special references to their specific properties that make them indispensable in a large area of applications. The materials discussed include cobalt-mangan oxides used as negative temperature coefficient thermistors, semiconducting doped barium titanate used for positive temperature coefficient thermistors with large applications in thermal protections and piezoelectric electroceramics of PZT type, which find new application as ceramic transducers and sensors in biomedical and aerospace industries or as simply buzzers, filters, igniters, ultrasonic cleaners, towed array sonars, medical imaging system, ink-jet printing heads, camera shuttlers, positioners for optical systems, micromotors and actuators. Such advanced electroceramic materials are extremely useful for the future sophisticated technologies such as heads for magnetic recording, scanning tunneling microscope, sensors for antilock braking systems, ultrasonic bone-healing devices and some other biomedical applications.

TiN, TiC and Ti(C,N) hard coatings were deposited on Si substrates by a cathodic are system. Various investigation techniques (ERDA, XPS, and XRD) were used to analyze the microchemical and microstructural characteristics of the films (surface chemistry, chemical composition, texture) prepared under different deposition conditions.

A method to produce composition modulated Bi2-xSbxTe3 alloys by electrodeposition has been developed. The electrolyte, which consists in a HCl 6M (pH = 0) aqueous solution, allows for the co-deposition of bismuth, antimony and tellurium to be accomplished at room temperature on glassy carbon, platinum or nickel electrode. The composition of the films, their crystal structures and morphology were studied as function of electrochemical parameters and bath composition. It is shown that the electrodeposits are monophasic and exhibit a polycrystalline state. The electrical resistivity was of the order of 7-10 mOmegacm, whereas the electrodeposition rate value was 30-50 mum/h for the films with good thermoelectric properties.

Compact two-pole and four-pole passband filters in 900 MHz frequency band using novel square microstrip resonators are presented. The measured filters' responses demonstrate the feasibility of selective filters using these new resonators.

The magnetic properties of interfacial regions between ferromagnetic nanograins and the residual matrix in nanocrystalline materials, are modelled using Monte Carlo simulation. The exchange coupling between matrix and nanograin influences differently the nanograin surface and the interface between nanograin and matrix. This behaviour has been explained in terms of a polarisation mechanism of the interfacial zones leading to magnetic correlation between spins of nanograin surface and core.