National Institute Of Materials Physics - Romania

The crystal structure, electronic and magnetic properties of predicted new full-Heusler compounds Zr(2)MnZ (Z=Al, Ga, In) were studied within the density functional theory (DFT) framework. These materials exhibit unique properties that connect the spin gapless semiconducting character with the completely compensated ferrimagnetism. Magnetically ordered Zr(2)MnZ (Z=Al, Ga, In) compounds crystallize in inverse Heusler structure are stable against decomposition and have zero magnetic moment per formula unit, in agreement with Slater-Pauling rule. The Zr2MnAl compound presents semiconducting properties with an energy band gap of 0.41 eV in the majority spin channel and a zero band gap in the minority spin channel. By substituting completely the Al in Zr2MnAl via Ga and In elements, semiconducting pseudo band gaps are formed in the majority spin channels due to different neighborhoods around the manganese atoms, which decreases the energy of Mn triple degenerated anti-bonding states. (C) 2016 Elsevier B.V. All rights reserved.

Dense (95-98.6%) bulk boron carbide prepared by Spark Plasma Sintering (SPS) in Ar or N-2 atmospheres were subject to three-point flexural tests at room and at 1600 degrees C. Eight different consolidation conditions were used via SPS of commercially available B4C powder. Resulting specimens had similar grain size not exceeding 4 mu m and room-temperature bending strength (sigma(25) (degrees C)) of 300-600 MPa, suggesting that difference in sigma(25) (degrees C) m is due to development of secondary phases in monolithic boron carbide ceramics during SPS processing. To explain such difference the composition of boron carbide and secondary phases observed by XRD and Raman spectroscopy. The variation in intensity of the Raman peak at 490 cm(-1) of boron carbide suggests modification of the boron carbide composition and a higher intensity correlates with a higher room temperature bending strength (sigma(25) (degrees C)) and Vickers hardness (HV). Secondary phases can modify the level of mechanical characteristics within some general trends that are not dependent on additives (with some exceptions) or technologies. Namely, HV increases, sigma(25) (degrees C) decreases, and the ratio sigma(1600 degrees C)/sigma(25 degrees C) (sigma(1600 degrees C) - bending strength at 1600 degrees C) is lower when fracture toughness (K-IC) is higher. The ratio sigma(1600 degrees C)/sigma(25 degrees C) shows two regions of low and high K-IC delimited by K-IC=4.1 MPa m(0.5): in the low K-IC region, boron carbide specimens are produced in nitrogen. (c) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

High density monolithic boron carbide specimens with grain size of 3-5 mu m were consolidated by spark plasma sintering using identical heating conditions and Ar or N-2 atmospheres. The effect of impurities from two different raw powders on the flexural strength was revealed. The increase in flexural strength was observed up to 1600 degrees C. Specimens consolidated in nitrogen had a higher strength than that consolidated in argon. Samples had the room temperature strength ranging from 350 to 550 MPa. The high temperature strength of our samples exceeding 400 MPa is higher than that previously reported for polycrystalline monolithic B4C. The supporting cracking/strengthening mechanism was discussed and proposed. (C) 2016 The Ceramic Society of Japan. All rights reserved.

Graphene nanopowder (G) with average thickness particle size of about 6-8 nm was added to MgB2 commercial powder. Starting composition was (MgB2)((1-x))(G)(x),x = 0.0125, 0.025, 0.05. Processing was performed by Spark Plasma Sintering (SPS) technique. All added samples have high density (above 95%). The critical temperature (T-c) and the lattice parameter a (c-axis lattice parameter is constant) show a small variation suggesting that carbon substitution for boron is low. TEM observations show the presence of un-reacted graphene plates supporting the T-c and structural results. It also indicates that G-addition does not modify the MgB2 microstructure. Despite this, there is an optimum doped sample (MgB2)(0.9875)(G)(0.0125) for which the critical current density at temperatures below 25 K is slightly higher at high magnetic fields than for the pristine sample. The addition of G is found as one of the least effective C-source additions enhancing J(c). We discuss results as being strongly related to variation of the residual stress. (C) 2016 Elsevier Ltd. All rights reserved.

In melt-spun FePtB-based ribbons, the addition of Ag has been proven to decrease the temperature of phase transformation from the A1 fcc FePt phase to the hard magnetic tetragonal L1(0) phase. Alloys with 6 and 9 at.% Ag added to the initial FePtB have been synthesized by rapid solidification from the melt. The samples have been laser irradiated and submitted to nitriding procedure. This procedure has been proven beneficial for inducing complete transformation of A1 to L1(0) phase and a strong (001) texturing. Ag segregation combined to mechanisms of creation of vacancies and diffusion of N give rise to the formation of an intergranular disordered region and due to an improved interfacial coupling between FePt grains, enhanced coercivity and two-phase magnetic behavior is obtained.

Thin films of Bi2Sr2CaCu2O8 (Bi-2212) with the non-c-axis (117) orientation were grown by MOCVD on (110) LaAlO3 single crystal substrate. XRD theta - 2 theta scans show that films contain also (119) and (011) Bi-2212 impurity grains. We propose and report characterization of the non-c-axis films by XRD phi-psi scans. By this approach, the assumed theoretical film-substrate relationship of the (117) Bi-2212 grains is demonstrated experimentally. The result also confirms twinning in the span rooflike (117) Bi-2212 grains. The impurity (119) and (011) Bi-2212 grains are also rooflike and in-plane aligned according to film-substrate relation and AFM images so that the film can be considered in-plane epitaxial.

Dense bulk samples, with relative density of 88-99%, of MgB2 added with Ge2C6H10O7 were obtained by ex situ Spark Plasma Sintering. The critical current density of the added samples is improved at high magnetic fields when compared to a pristine reference sample. The optimum composition was found for MgB2(Ge2C6H10O7)(0.0014), where J(c)(20 K) = 10(2) A/cm(2) is obtained at 5.8 T, versus 3.9 T for the reference sample. This sample has also the highest product J(c0) center dot H-irr among the added samples, and values are similar to those for the pristine MgB2 sample. There is no significant Ge substitution in the crystal lattice of MgB2, while C substitutes for B. Pinningmechanism shifts from point to grain boundary type with increasing of addition amount. The pinning shift tendency is higher at lower temperatures (5 K).

FePt system attracts currently a great deal of interest for applications as future RE free permanent magnets. Among the key issues to be solved one may count the decreasing of the ordering temperature and improvement of magnetic behavior. For that purpose we have studied the effect of a nitriding post-synthesis procedure on the FePtAgB melt spun ribbons, aimed at refining the microstructure and enhancing the magnetic performances. Deep structural characterization by transmission electron microscopy, electron diffraction, energy dispersive X-ray spectroscopy and X-ray diffraction allowed us to observe the morphology and to correctly assign and identify the nature of the main granular phases observed. Nitriding procedure is shown to strongly enhance the (001) texturing and the degree of ordering of the L1(0) FePt phase, as well as largely increase of coercivity, compared to the as-cast state. These changes are interpreted in terms of Ag segregation towards intergranular region associated to N diffusion and creation of vacancies that favor consistently the process of ordering the FePt grains into the L1(0) tetragonal phase. (C) 2015 Elsevier B.V. All rights reserved.

FePt-based alloys are currently under scrutiny for their possible use as materials for perpendicular magnetic recording. Another possible application is in the field of permanent magnets without rare-earths, magnets that may operate at higher temperatures than the classic Nd-Fe-B magnets. Within this study, FeCoPt alloys prepared by rapid solidification from the melt are structurally and magnetically characterized. In the as-cast FeCoPt ribbons, a three-phase structure comprising well-ordered CoFePt and CoPt L1(0) phases embedded in a disordered fcc FePt matrix was evidenced by XRD, HREM and SAED. Extended transmission electron microscopy analysis demonstrates the incipient formation of ordered L1(0) phases. X-ray diffraction was used to characterize the phase structure and to obtain the structural parameters of interest for L1(0) ordering. In the as-cast state, the co-existence of hard magnetic CoFePt and CoPt L1(0) tetragonal phases with the soft fcc FePt phase is obtained within a refined microstructure made of alternatively disposed grains (grain sizes from 1 to 7 nm). Following a thermal treatment of 1 h at 670 degrees C, the soft magnetic fcc matrix phase transforms to tetragonal L1(0) phases (disorder-order transition). The resulting CoPt and CoFePt L1(0) phases have grains of around 5-20 nm in size. In the as-cast state, magnetic measurements show a quite large remanence (0.75 T), close to the value of the parent L1(0) FePt phase. Coercive fields of about 200 kA/m at 5 K were obtained, comparable with those reported for some FePt-based bulk alloys. Upon annealing both remanence and coercivity are increased and values of up to 254 kA/m at 300 K are obtained. The polycrystalline structure of the annealed FeCoPt samples, as well as the formation of multiple c-axis domains in different CoPt and CoFePt regions (which leads to a reduction of the magneto-crystalline anisotropy) may account for the observed coercive fields that are lower than in the case of very thin FeCoPt films. A Curie temperature of about 820 K (close to 550 degrees C) is reported for the Fe35Co15Pt50 alloy which opens wide possibilities for the use of such magnets in high operating temperature industrial applications. The present results indicate that ternary FeCoPt alloys hold a great potential as a novel class of rare earth free exchange-spring coupled nanocomposite magnets. (C) 2015 Elsevier B.V. All rights reserved.

Aluminophosphate glasses from the Li2O-BaO-Al2O3-P2O5 system with the addition of nonmagnetic and paramagnetic rare earth ions, were prepared using a wet nonconventional method to process the raw materials, followed by a melting-quenching procedure. The glasses obtained were characterized with respect to their magnetic and magneto-optical properties using superconducting quantum interference device magnetometry and spectroscopic ellipsometry. The assumption of a linear dependence of the Verdet constant on the magnetic susceptibility, with a proportionality constant dependent on the type of vitreous matrix and doping ion, is critically discussed. The diamagnetic and paramagnetic contributions to the Faraday rotation were separately analyzed and specific designs for optimal active and passive elements are proposed.