Tayebeh Mousavi

Study of the structural, electric and magnetic properties of Mn-doped Bi2Te3 single crystals

Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi2Te3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi2Te3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with TC varying between 9 and 13 K (bulk values) and a saturation moment that reaches 4.4(5) μB per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab-plane. The electrical transport data show an anomaly around TC that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n-doped with carrier concentrations of ~ 0.5–3.0 × 1020 cm−3. X-ray magnetic circular dichroism (XMCD) at the Mn L2,3 edge at 1.8 K reveals a large spin magnetic moment of 4.3(3) μB/Mn, and a small orbital magnetic moment of 0.18(2) μB/Mn. The results also indicate a ground state of mixed d4–d5–d6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga1−xMnxAs. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c-axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.

Persistent current joints between technological superconductors

Persistent current joints are crucial components of superconducting magnets—enabling the production of the high and ultra-stable magnetic fields required, for instance, for magnetic resonance measurements. At this critical juncture when persistent mode magnets containing commercial high temperature superconductors may soon become a reality, it is of value to take stock and evaluate current challenges faced in the field of jointing. This paper provides a review of progress made to date on the production and characterization of joints between the five major technological superconductors—NbTi, Nb3Sn, MgB2, BiSCCO and REBCO, including the materials that are used to make these joints.

Microstructure and superconducting properties of Sn–In and Sn–In–Bi alloys as Pb-free superconducting solders

© 2016 IOP Publishing Ltd. In this work the microstructure and superconducting properties of various compositions of binary Sn-In and ternary Sn-In-Bi alloys have been studied as potential replacements for the Pb-based superconducting solders currently used for superconducting joints between technological low temperature superconducting wires. The influence of chemistry and microstructure on the superconducting properties are investigated. Both the Sn-In β and γ phases are stable over a fairly wide range of chemical composition, with the superconducting properties of each phase improving with increasing solute concentration, but the results show that the In-rich β phase has better superconducting properties than the Sn-rich γ phase. For this reason, in both the binary Sn-In and the ternary Sn-In-Bi systems, the best superconducting properties are achieved in alloys with a high volume fraction of β-phase. The ternary alloys generally show better superconducting properties than the binary alloys, with the highest values of 6.9 K, 0.18 T and 1.3 ×108 A m-2 measured for T C, B C2 (at 4.2 K) and J C (at 4.2 K, 0.01 T) respectively in the Sn35In50Bi15 alloy that consists of a matrix of β-phase with fine fibres of γ and BiIn2. The dependence of the pinning force on the reduced magnetic field exhibits a scaling law behaviour with the maximum at B/B C2 ∼ 0.2 indicating that the dominant mechanism is surface normal pinning in these solders. Rapid solidification in liquid nitrogen results in manipulation of the phase chemistry by both increasing the solute content in the majority phase, changing the fraction of each phase and reducing the scale of the microstructure, leading to significant improvements in superconducting performance. However, the quenched alloys age considerably, even at room temperature, resulting in a deterioration in B C2 and J C values over a few days.

Phase Evolution of Superconducting Sn–In–Bi Solder Alloys

Development of Pb-free superconducting solders is of great importance for the superconducting magnet manufacturers who often use Pb-Bi alloys as superconducting solders. Here, we study the phase evolution, microstructure, and superconducting properties of a number of alloys in the Pb-free Sn-In-Bi system using analytical scanning electron microscopy, differential thermal analysis, X-ray diffraction, and SQUID magnetometry. All the alloys exhibit melting points below 200°C and superconducting transition temperatures above 4.2 K. The choice of initial composition and cooling rate controls the microstructure (chemistry, volume fraction, morphology, and scale) of these alloys and strongly influences the final superconducting properties. The In-rich β-phase shows better superconducting properties compared with the other phases in the Sn-In-Bi system. As a result, by increasing the indium content in the alloy, volume fraction of the In-rich β-phase increases, leading to lower melting point, higher wettability, and better superconducting properties.

Effects of Processing Condition on the Properties of Fe(Se,Te) Thin Films Grown by Sputtering

We studied the effects of processing conditions on the properties and microstructure of Fey(Se1-xTex) thin films grown on MgO substrates by RF sputtering. The thin films were grown in two ways; in situ deposition onto heated substrates and deposition onto cold substrates followed by an ex situ annealing process. Various properties of the deposited Fe(Se,Te) thin films including the phase, texture, surface morphology, composition, and superconducting properties were investigated. The results showed that in situ sputtering leads to higher quality films, e.g., smoother surface, pure phase, more uniform composition, and better crystallographic alignment. For the ex situ grown films, depending on the annealing conditions, phase separation, appearance of the impurity phases such as hexagonal Fe(Se,Te), poor texture, higher Fe-content, nonuniform composition, and rough surfaces developed in the films.

Lead-Free Solders for Superconducting Applications

In this paper, the low-temperature properties of some commercial Pb-free solders developed for electronics and aerospace applications were investigated for possible use as superconducting solder materials. Their properties are also compared with in-house Sn₃₅In₅₀Bi₁₅ solders being developed to replace the widely used Pb-Bi alloys. Of these materials, the Sn₃₅In₅₀Bi₁₅ solder is the best potential candidate for superconducting applications.

Analytical microscopy of iron-based superconducting materials

Abstract Inhomogeneity and phase separation in unconventional superconducting materials is a topic of increasing interest, as the competition between phases with different types of ordering is thought to play an important role in the emergence of the high temperature superconducting state. A wide range of experimental techniques are available for investigating the crystallography of these complex materials, however the majority are bulk probes. Here we briefly review some spatially resolved techniques that are being used to investigate phase separation in ironbased superconductors, with a focus on analytic Scanning Electron Microscopy techniques and synchrotron-based Photoelectron Microscopy.