Hong-Wei Jiang

Magnetic properties of Si-substituted Y2Fe17

Abstract Magnetic properties of the Si-substituted Y 2 Fe 17 have been investigated. The Curie temperature increases with increasing Si content although the unit cell volume decreases. The average Fe magnetic moment decreases with Si substitution, and the easy magnetization direction does not change. Neutron-diffraction showed that the Si atoms do not preferentially occupy the dumbbell site. The occupancy of Si atoms is very small for the 6g site, and almost statistical for the other three sites.

A superconductive shielding can for high T/sub c/ SQUID

The authors have fabricated yttrium-barium-copper-oxygen ceramic superconductive shielding cans and measured the magnetic shielding properties at liquid-nitrogen temperature by using a Hall magnetic sensor and high-T/sub c/ SQUID (superconducting quantum interference device). They report the fabrication processing of the shielding cans and the measurement results. The magnetic properties of the bulk samples are very different from those of conventional superconductors. There are lots of grain boundaries and defects inside the ceramic samples. The magnetic field can penetrate through those defects into the sample easily. The physical density of the samples is of great importance for their superconductive magnetic shielding ability. The isostatic pressing technique, which has been used in this work to prepare a dense and uniform cylinder, is a key processing method. >

Josephson effects observed in constricted YBaCuO films at liquid nitrogen temperature

Abstract The DC and AC Josephson effects have been observed in constricted superconducting Y-Ba-Cu-O films at liquid nitrogen temperature. The temperature dependence of the critical current was studied. It was found that I c ∝ (T c -T) 3 2 . The induced steps were seen in the I-V characteristics when the sample was irradiated by a microwave field. Separations between steps agrees with the Josephson relation.

DC SQUIDs fabricated with YBaCuO thin films

DC superconducting quantum interference devices (SQUIDs) have been fabricated from thin films of YBaCuO. The devices were made by the photolithographic method with wet etching. The oscillatory output voltage of the devices in a sweeping magnetic field is shown to have a good periodicity. Triangular wave patterns of the DC SQUID output were observed. >

Observations of the ac Josephson effect in Y-Ba-Cu-O superconductors at 77 K

Abstract The observations of the ac Josephson effect in Y-Ba-Cu-O superconducting weak link were reported. The Y-Ba-Cu-O sample had a Tc =90 K . The I–V characteristics of the weak link junction were mearsured at 77 K. The junction had a Ic =0.43 mA and the normal resistance Rn =0.38 ohm. 10.15 GHz microwave was applied to the junction and the rf-induced steps up to n =10 were observed. The dependence of the step height on the rf level was in agreement with ordinary Josephson tunneling behavior.

Thermal diffusion of vortices and self-organized criticality

Abstract A logarithmic barrier, U(J) ∝ ln ( Jc J ) , is often observed in transport and magnetic relaxation experiments. We show that this kind of current dependency is a natural result of a unified materials equation for type-II superconductors. With finite barrier and T → 0, this equation coincides with the nonlinear diffusion equation describing one-dimensional sandpile with self-organized criticality.

INVESTIGATION OF PHOTOLITHOGRAPHY FOR FABRICATING OF YBCO SUPERCONDUCTING THIN FILMS DEVICES WITH CHEMICAL ETCHING

We have investigated the photolithography on the superconducting YBCO thin films with chemical etching. It was found that the films can be etched by HCl—H3PO4 mixing acid and the side-etching can be suppressed down to one micrometer. The influence on the critical temperature can be minimized to two degrees under well-controlled conditions.

Anomalous thermal expansion of Sm/sub 2/Fe/sub 15/Si/sub 2/

Crystallographic and magnetic properties of Sm/sub 2/Fe/sub 17-x/Si/sub x/ with x=0, 0.5, 1.0, 1.5, 2.0, and 2.5 have been investigated. X-ray diffraction and thermomagnetic analyses showed that all samples studied are single-phase with Th/sub 2/Zn/sub 17/ structure. The Curie temperature increases with increasing Si content, although the lattice parameters decrease. Anomalous thermal expansion of Sm/sub 2/Fe/sub 15/Si/sub 2/ was observed below Curie temperature by high-temperature X-ray diffraction experiments. >

THE NEUTRON DIFFRACTION STUDY ON Dy2Fe17C0.5

A sample of Dy2Fe17C0.5 was prepared and identified to be of a single Th2Ni17 type structure by X-ray phase analysis. It is shown that a small amount of carbon raises the Curie temperature in Dy2Fe17C0.5 from 396 K to 483 K, while the magnetic moment does not increase. In order to understand the effect of carbon on the magnetic properties, a neutron diffraction study was carried out. A preliminary discussion is given.