Yanwei Ma

X-ray diffraction study of the structural phase transition of Ni2MnGa alloys in high magnetic fields

Abstract The Ni 2 MnGa alloys have been systematically studied in a wide temperature range at different fields, by means of a low temperature X-ray diffraction apparatus in high magnetic fields. Using the width of the (220) diffraction peak as a measure of the degree of transformation to the low-temperature tetragonal phase, two kink anomalies in the temperature dependence of (220) peak width were detected, both on cooling and warming, between the parent and martensitic phases, corresponding to the martensitic transition-temperature ( T m ) and premartensitic transition-temperature ( T p ), respectively. Furthermore, effects of the magnetic field were also clearly observed.

Effect of high magnetic field on the two-step martensitic-phase transition in Ni2MnGa

Using a low-temperature x-ray diffraction apparatus in a high field, a systematic study was performed of the stoichiometric Ni2MnGa alloys in a wide temperature range from 10 K to room temperature and in fields up to 5 T. X-ray diffraction data at different fields clearly show the martensitic-phase transition is a two-step process. It is found that the martensitic-transition temperature Tm increases and the premartensitic-transition temperature Tp decreases with increasing magnetic fields up to 2 T, and then both remain unchanged in fields up to 5 T.

Jc enhancement of YBa2Cu3O7 films on polycrystalline silver substrates by metalorganic chemical vapor deposition in high magnetic fields

High-quality YBa2Cu3O7 (YBCO) films were grown on polycrystalline silver substrates by a metalorganic chemical vapor deposition process in high magnetic fields. Property characterizations revealed that the YBCO films deposited in high magnetic fields have excellent structural and superconducting properties. At 77 K, the transport critical current densities of such films were over 10 000 A/cm2 in the field of 1 T (H∥c axis). The improvement of the critical currents in YBCO films deposited in high magnetic fields is correlated with the better intergrain connections, as observed by scanning electron microscopy.

Magnetic field effects on synthesis process of high-Tc superconductors

Abstract Materials synthesis processes in high magnetic fields are investigated for high- T c superconductors at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. On a melt growth process of YBa 2 Cu 3 O 7 bulk in magnetic fields, it was found that the crystallinity is improved. These results are due to not only the magnetic field orientation but also the decrease of the crystal growth rate by the magnetic field, which is suggested from a differential temperature analysis measurement in magnetic fields. On a chemical vapor deposition process of YBa 2 Cu 3 O 7 films deposited on the polycrystalline Ag substrates in high magnetic fields, the microstructure changes drastically from the rectangular large grains with 10 μm in size to the small complex shaped grains. The spiral growth mode is observed for the samples prepared at 0 T but it changes into the island growth mode for that prepared in the presence of a magnetic filed. On a partial-melt process of Bi 2 Sr 2 CaCu 2 O 8 tapes in magnetic fields, the homogeneous texture is obtained and it results in the increase of the critical current density.

Surface morphology and growth mechanism of YBa2Cu3O7 films by chemical vapor deposition in a magnetic field

We have investigated the influence of a magnetic field on YBa2Cu3O7 (YBCO) films grown directly on polycrystalline silver substrates by metalorganic chemical vapor deposition (CVD). Clearly, application of a magnetic field during the CVD process not only changed the surface morphology of YBCO films, but also improved their crystallinity and grain connectivity. Furthermore, it is found that the growth mechanism of YBCO films was changed from the spiral growth mode at zero field to the 2D or 3D island growth mode under an external magnetic field. A discussion on the mechanism of this field-induced growth mode change is also presented.

New Metalorganic Chemical Vapor Deposition Process in a High Magnetic Field for YBa2Cu3O7.

A hot-wall-type metalorganic chemical vapor deposition (MOCVD) apparatus combined with a cryocooled superconducting magnet was recently developed. This system allows the application of magnetic fields up to 11 T for the MOCVD process. After making various improvements, we succeeded in fabricating YBa2Cu3O7 (YBCO) films under optimal deposition conditions. It is found that the crystallinity of YBCO films produced on polycrystalline silver substrates by the magnetic-field-assisted CVD process is improved as compared with those without the magnetic field.

Evaporation of Silver during Chemical Vapor Deposition Process for YBa2Cu3O7 and its Effect on Microstructure

YBa2Cu3O7 (YBCO) films were simultaneously grown on both silver and MgO substrates by a chemical vapor deposition (CVD) technique. It is found that the transportation of silver occurs through the evaporation of silver from the silver substrate at the processing temperature of 850°C. The effect of the evaporation of silver during deposition was investigated. The silver vapor travels a distance of a few millimeters over the surface of a neighboring MgO substrate. It is also demonstrated for the first time that the presence of silver vapor causes some microstructural changes in the film grown on the MgO substrate.

Growth and superconducting properties of YBa2Cu3O7 films on silver substrates by MOCVD in magnetic fields

Abstract YBa 2 Cu 3 O 7 (YBCO) films were grown on polycrystalline silver substrates by metalorganic chemical vapor deposition (MOCVD) in high magnetic fields. It is demonstrated that a magnetic field affected the microstructure of the films on polycrystalline silver substrates. Large square-shaped grains were observed in the 0 T films. The grain size was drastically reduced by application of a few tesla fields, then changed to small grains with an irregular shape above 4 T. Furthermore, it is found that application of a large magnetic field during the deposition improved the crystallinity and grain connectivity of YBCO films, leading to the improvement of the critical current density J c .

Investigation of phase transformations in Ni2MnGa using high magnetic field low-temperature X-ray diffraction system

Abstract We present results on the magnetic field-induced martensitic phase transformation in Ni2MnGa. The experiments were carried out in a wide temperature range from 10 to 300 K and in magnetic fields up to 5 T. Two successive anomalies in the temperature dependence of main (2 2 0) peak width, corresponding to martensitic transition temperature (Tm) and premartensitic transition temperature (Tp), respectively, are detected. It is found that Tm increases and Tp decreases with applying magnetic fields up to 2 T, and then the transition temperatures do not change in fields up to 5 T.

On the role of silver evaporation during YBa2Cu3O7−x film processing

Abstract YBa 2 Cu 3 O 7− x (YBCO) films were simultaneously grown on both silver and MgO substrates by metalorganic chemical vapor deposition. Studies show that the transportation of silver occurs via the evaporation from the silver substrate at high deposition temperatures. The silver vapor can migrate over a distance of a few millimeters on the surface of MgO substrate. We argue that the neighboring silver substrate could act as the source of the silver vapor. The presence of silver vapor may play a role as a catalyst for the microstructure change of YBCO film on the MgO substrate.