Dimitrios Niarchos

Synthesis, thermogravimetric and 57Fe Mössbauer studies of the oxygen deficient perovskite REBaCuFeO5 + x series (RE = Y, Nd, Sm, Gd, Dy, Tm, Lu)

Abstract We present the results of thermogravimetric and 57 Fe Mossbauer studies of the oxygen deficient perovskite REBaCuFeO 5+x series with RE = Y, Nd, Sm, Gd, Dy, Tm, Lu. The presence of the removable content x of oxygen was determined by analysis of TGA measurements in a stream of Ar gas. The amount of removable oxygen was highest for the Nd compound and varied between 0.07 and 0.12 for the other compounds. By means of the Rietveld method we performed the refinement of the structure of the compounds, based on the space group P4mm, after taking into consideration that the Fe ion occupies the lb 1 ( z = 0.27) site and Cu the lb 2 ( z = 0.74) site. From the analysis of the Mossbauer spectra we found that the compounds become magnetically ordered with transition temperatures depending on the oxygen content. The values of the hyperfine parameters in all the samples indicate the ferric iron to be in a high spin state.

High coercivity in boron substituted Sm-Co melt-spun magnets

The structural and magnetic properties of nanocomposite melt-spun Sm(Co/sub 0.74-x/Fe/sub 0.1/Cu/sub 0.12/Zr/sub 0.04/B/sub x/)/sub 7.5/ magnets have been investigated as a function of boron content (x = 0.005 0.05), wheel speed and annealing conditions. The as-spun ribbons are nanocrystalline with fine microstructure and average grain size of 60-100 nm. X-ray diffraction indicates that the as-spun samples have the metastable hexagonal TbCu/sub 7/-type structure phase and fcc-Co as a secondary soft phase. Magnetization at nonsaturating 5 T field is 45-72 emu/g and the reduced remanence (Mr/Ms) is above 0.8. The loop shape exhibits a characteristic step due to the soft magnetic phase. At room temperature, Hc values of 20-28 kOe are obtained for as spun samples, with a record value of 38.5 kOe for x = 0.04. At 380/spl deg/C Hc values higher than 5 kOe are observed. Coercivity and loop shape are strongly dependent on annealing conditions.

Tailoring the magnetic anisotropy of CoFeB/MgO stacks onto W with a Ta buffer layer

The emergence of perpendicular magnetic anisotropy (PMA) in CoFeB/MgO stacks deposited on W using a Ta buffer layer is studied as a function of Ta and CoFeB layer thickness and annealing temperature. It is shown that very thin Ta “dusting” layers (thickness between 0.3 and 1 nm) enhance PMA of CoFeB layers grown on top of W. We find that Ta thickness is a crucial factor affecting magnetic anisotropy and it needs to be scaled proportionally to CoFeB thickness for obtaining PMA. Stacks without Ta have in-plane anisotropy, verifying the “PMA-enhancing” role of Ta. The maximum effective PMA energy ( 3.6×106 erg/cm3) is obtained for a stack with 1.4 nm of CoFeB and 1 nm of Ta and after annealing at 350 °C. Besides, PMA can be obtained even at the as-deposited state for certain thicknesses. This W-based CoFeB/MgO system could enable the development of low power consumption, high density, and non-volatile magnetic memories.

Characterization, electrical and magnetic properties of polyaniline/maghemite nanocomposites

Composites of polyaniline with nanocrystalline maghemite capped with oleic acid were synthesized at various maghemite loadings and their electrical and magnetic properties were studied from 5 to 300 K. Chemisorption of oleic acid by the surface iron atoms provided excellent dispersion of maghemite in organic solvents and thus enabled their homogenous dispersion in the polymer matrix. The resulting nanocomposite materials exhibit properties of both the conductive matrix and the magnetic iron oxide component. Magnetoresistance studies indicated an increase in the resistance in the presence of an external magnetic field, while the absolute conductivity values decrease due to the addition of iron oxide nanoparticles.

Microstructural modification in Co/Cu giant-magnetoresistance multilayers

Three different classes of [Co/1.1 nm/Cu/2.1 nm]30 multilayers were grown by magnetron sputtering deposition. The effect of magnetostatic interactions on the giant-magnetoresistance (GMR) and magnetic properties are examined in relation to the induced changes in the film microstructure as it is varied by: (i) the substrate surface roughness and (ii) the effect of thermal isolation of the Si(100) substrate from the cooling plate during deposition. A remarkable variation in shape and magnitude of GMR, and in the magnetic (M-H) loops, is observed for the three classes of films. It is found that there are three characteristic features in every sample that vary systematically: (i) The (ΔR/R)max ratio. (ii) The magnetic field range where a GMR loop reaches its minimum value. (iii) The (M-H) loops that vary from the characteristic antiferromagnetic to a typical ferromagnetic loop shape. Two well-separated grain size distributions below and above 12 nm were found from transmission electron microscopy. The smaller...

Effects of boron substitution on the structural and magnetic properties of melt-spun Sm(Co,Fe,Zr)7.5 and Sm(Co,Fe,Zr,Cu)7.5 magnets

In this work we examine the effect of small boron substitution (x=0, 0.005, 0.010, 0.015) on the structural and magnetic properties of Sm(Co0.86−xFe0.1Zr0.04Bx)7.5 and Sm(Co0.74−xFe0.1Cu0.12Zr0.04Bx)7.5 melt-spun samples, as a function of wheel speed and annealing conditions. Boron substituted as-spun ribbons are found to have increased coercivity, Hc>5 kOe, and small grain size of 60–100 nm. For copper containing samples, the highest coercivity (Hc=16.3 kOe) was obtained in as-spun ribbons with x=0.015. In samples without copper the coercivity increased after short annealing (Hc=12 kOe for x=0.015). The large coercivities are attributed to a fine microstructure consisting mainly of hexagonal TbCu7-type phase and a small amount of soft-phase grains.

Deposition of thin films over large areas with a simple sputtering technique for microwave applications

A simple sputtering technique is presented for the fabrication of (YBCO) films over large areas. The magnetron sputtering source combines operation in an unbalanced magnetic field configuration and use of large YBCO targets. YBCO films were deposited on substrates with sizes up to . The films are characterized by x-ray diffraction, pole figures, ac susceptibility, transport properties and thickness uniformity. The results show that these films are of good quality with perfect c-axis orientation, epitaxial growth, critical temperature of , critical current density of at 77 K and thickness uniformity better than 5% over an substrate. A two-pole bandpass filter with centre frequency of 13 GHz and 3 dB fractional bandwith of about 15% was designed to test the microwave performance of these films. A YBCO filter exhibits at 77 K an insertion loss of more than 5 dB lower than an equivalent Au filter on substrate at the same temperature.

Silver Nanoparticles and Graphitic Carbon Through Thermal Decomposition of a Silver/Acetylenedicarboxylic Salt

Spherically shaped silver nanoparticles embedded in a carbon matrix were synthesized by thermal decomposition of a Ag(I)/acetylenedicarboxylic acid salt. The silver nanoparticles, which are formed either by pyrolysis at 300 °C in an autoclave or thermolysis in xylene suspension at reflux temperature, are acting catalytically for the formation of graphite layers. Both reactions proceed through in situ reduction of the silver cations and polymerization of the central acetylene triple bonds and the exact temperature of the reaction can be monitored through DTA analysis. Interestingly, the thermal decomposition of this silver salt in xylene partly leads to a minor fraction of quasicrystalline silver, as established by HR-TEM analysis. The graphitic layers covering the silver nanoparticles are clearly seen in HR-TEM images and, furthermore, established by the presence of sp2carbon at the Raman spectrum of both samples.

Structural study, resistivity, magnetization and Raman measurements for the HTc superconducting compounds SmBa2−xSrxCu3O6+y (x = 0.0, 0.25, 0.5, 0.75, 1.0 and 1.25)

Abstract The structure and the physical properties of the oxygenated and deoxygenated compounds of the series SmBa 2− x Sr x Cu 3 O 6+ y ( x = 0.0, 0.25, 0.5, 0.75, 1.0 and 1.25) are studied. In the XRD spectra of the oxygenated compounds a structure of tetragonal symmetry is revealed beyond the value of x = 0.75 and the upper limit for substitution of Sr for Ba is x = 1.0. Resistivity and magnetization measurements of the fully oxygenated compounds show that the T c values decrease with increasing Sr content and this is discussed in relation to the observed structural changes deduced by the Rietveld analysis of the XRD spectra. The bond length changes that occur in the coordination sphere of Cu(1), Cu(2) and (Ba,Sr) atoms are consistent with the “charge redistribution” model, i.e., electrons are transferred from Cu(2) to Cu(1) atoms and the number of holes on the “Cu(2) superconducting planes” increases beyond the optimum value inducing a reduction in the T c . The Raman measurements support the charge redistribution and the stress relaxation models as probable explanations for the reduction of T c upon Sr substitution.

57Fe Mössbauer and magnetic studies of RFe4B (R = Tm, Er) alloys

Abstract Magnetic and Mossbauer measurements have been carried out in the borides (RFe4B (R = Er, Tm), which have the hexagonal Ce Co4B-type structure with an interchange of sites between Fe and B. Powder X-ray diffraction diagrams were indexed with the space group P6/mmm. Magnetization studies in the temperature range from 4.2 to 700 K reveal the presence of compensation temperatures arising from the antiferromagnetic coupling between the Fe and R sublattices. There was no indication of spin reorientation for both Mossbauer and magnetic measurements. Mossbauer spectra were analysed with a program which assumes binomial distributions of the B atoms. Independent occupancy probabilities for the i and (c, d)-sites of the CeCo4B-type structure were determined by this analysis.