The phase evolution inside Fe-sheathed wires containing precursor powders consisting of a mixture of Mg and B has been studied in situ by means of x-ray diffraction with hard synchrotron radiation (90 keV). Mg was found to disappear progressively during the heating stage. At 500• C, the intensity of the Mg diffraction lines is reduced by about 20%. This effect is partly attributable to MgO formation. The MgB 2 phase was detected from 575• C. Fe 2 B was forming at the interface between the sheath and the ceramic core at sintering temperatures of 780 and 700• C, but not at 650• C. The formation rate of this phase is strongly dependent on the heat treatment temperature. Its presence can be readily detected as soon as the average interface reaction thickness exceeds 150-200 nm.

We present synchrotron x-ray diffraction studies revealing that the lattice of thulium borocarbide is distorted below T Q Ӎ 13.5 K at zero field. T Q increases and the amplitude of the displacements is drastically enhanced by a factor of 10 at 60 kOe when a magnetic field is applied along ͓100͔. The distortion occurs at the same wave vector as the antiferromagnetic ordering induced by the a-axis field. A model is presented that accounts for the properties of the quadrupolar phase and explains the peculiar behavior of the antiferromagnetic ordering previously observed in this compound. DOI: 10.1103/PhysRevB.73.020504 PACS number͑s͒: 74.70.Dd, 75.25.ϩz, 75.80.ϩq More than ten years ago it was discovered that four of the rare-earth borocarbides ͑Tm, Ho, Er, and Dy͒ show a coexistence of superconductivity and antiferromagnetic ordering with comparable transition temperatures.1,2 Superconductivity occurs below T c = 11 K and the Néel temperature is T N = 1.52 K in TmNi 2 B 2 C.3 The characterization and the understanding of these intermetallic compounds have made much progress during the previous decade, as may be seen in a recent review. 4 Although unusual phenomena have been detected, the type-II superconductivity of these materials seems to be described by the BCS theory. The electronic system strongly influences the acoustic and optical ⌬ 4 phonon branches close to ͑0.5, 0, 0͒ and ͑0, 0.5, 0͒ in the nonmagnetic Lu and Y versions of these compounds. [5][6][7] The two vectors connect parallel areas of the Fermi surface, 8 so nesting explains why the electron-phonon interaction is particularly large at these wave vectors. This enhancement of the electron-phonon interaction is probably the primary reason for the relatively high T c values of the borocarbides. Nesting is generally also assumed to be important for the magnetic susceptibility of the band electrons by causing a maximum in the Ruderman-Kittel-Kasuya-Yoshida ͑RKKY͒ coupling of the rare-earth moments at the nesting vectors. 8 The magnetic properties of TmNi 2 B 2 C single crystals have been studied by magnetization measurements 9 and by neutron diffraction. [10][11][12] The ordering vector at zero field is Q F = ͑0.094, 0.094, 0͒ with the magnetic moments transversely polarized along the easy c axis. The shift of the magnetic ordering vector away from zero is probably a consequence of the Anderson-Suhl screening of the longwavelength susceptibility of the superconducting electrons. 11Applying a field in excess of 10-15 kOe along ͓100͔, the magnetic ordering changes into another antiferromagnetic one at the wave vector Q A = ͑0.483,0,0͒. In the original experiment, only fields of up to 18 kOe were applied, 11 and the new magnetic phase was considered to be the stable one at zero field in case the electrons were normal, but because the superconducting electrons were more strongly affected by the superzone energy gaps at Q A than at Q F , the longwavelength ordering became the favorable one of the combined system at small fields. The extension of the exper...

No abstract

The phase transformations occurring in the ceramic core of Fe-sheathed MgB 2 wires and tapes prepared by in-situ reaction of Mg and B precursor powders, have been studied by means of high-energy x-ray diffraction. In particular, the time evolution of the Fe 2 B phase, forming at the interface between the sheath and the ceramic, was studied at different sintering temperatures. The reactivity of the sheath towards Fe 2 B formation is strongly dependent on powder pre-treatment. In wires produced with commercial Mg and B powders without additional mechanical activation, the Fe 2 B phase starts forming around 650°C. In contrast, in tapes produced from a mixture of Mg and B powders subjected to high-energy ball milling, the interfacial Fe 2 B layer forms readily at 600°C. The increase of Fe 2 B volume fraction is linear to first approximation, showing that the interfacial layer does not act as a diffusion barrier against further reaction between the sheath and the ceramic core. If the ceramic core is converted to MgB 2 at a temperature, which is low enough to avoid Fe 2 B formation, the interface is stable during further annealing at temperatures up to 700°C at least. However, too high annealing temperatures (T > 800°C), would result in formation of Fe 2 B, probably following the partial decomposition of MgB 2 .

MgB 2 wires, tapes and bulk samples produced within the EU-funded HIPERMAG project have been studied by a combination of X-ray diffraction and electron microscopy. The reaction layers forming at the interface between the ceramic core and Fe or Ni sheaths can be studied with both methods. The complementary techniques enable to study both the microstructure and the formation kinetics of the interface layers. Grain sizes can be determined either by direct observation or by analysis of the shape of X-ray diffraction peaks. Electron microscopy can detect B-rich secondary phases and phases present in small fractions that are not accessible by x-ray diffraction. On the other hand, synchrotron diffraction provides a fast and non-destructive method for the study of the main phases and their development during insitu, high-temperature investigations. The combination of the two techniques is a very valuable tool for the optimisation of MgB 2-based superconducting material.