Ji-g Chen

Interface controlled electrical and magnetic properties in Fe–Fe3O4–silica gel nanocomposites

Iron nanoparticles with a shell of Fe3O4 phase with a total diameter of 5.3 nm have been grown within a silica gel matrix in the percolative configuration by suitable reduction followed by oxidation treatments. dc electrical resistivity measurements were carried out in the temperature range 80–300 K. The resistivity of the nanocomposites was found to be about 7 orders of magnitude lower than that of the reference gel. The electrical conduction has been explained on the basis of a small polaron hopping mechanism. The activation energy in the case of the composites was calculated from experimental data to be about one-fifth that for the reference sample. An interfacial amorphous phase is believed to cause such reduction in resistivity. The effective dielectric constant of this phase was estimated to be about four times that of the reference glass. Magnetization measurements on these specimens were carried out in the temperature range 5–300 K both in zero field cooled and field cooled states. A peak in the m...

Coexistence of antiferromagnetic order and superconductivity with TN higher than Tc in DyNi2B2C

Abstract A superconducting transition temperature T c of 3.8 K with an onset as high as 5.2–5.5 K was reported for quaternary borocarbide DyNi 2 B 2 C with a high antiferromagnetic Neel temperature T N around 10.1–10.8 K. No reentrant behavior was observed down to 0.3 K demonstrating a perfect coexistence between superconductivity and antiferromagnetic order. A superconducting upper critical field H c2 (0) of 3.0 kG, lower critical field H c1 (0) of 200 G and Ginzburg-Landau parameter κ of 3.7 were derived. A broad T c transition points to a possible small T c varying homogeneity range of DyNi 2 − α B 2 − β C 1 − δ in the phase diagram. The magnetic entropy S m of 10.3 J/mol K or 90% R ln 4 indicates antiferromagnetic ordering of Dy 3+ with a quadruplet ground state in the tetragonal crystal field. Two field-induced weak-ferromagnetic-like transitions with a field of around 1 T were observed at 2 K.

Influence of Mn substitution on microstructure and magnetic properties of Cu1−xMnxO nanoparticles

Mn-doped CuO nanoparticles were synthesized using a coprecipitation method. Powder x-ray analysis reveals that all the samples crystallize into a monoclinic-type structure. Significant changes occur in both the microstructure and particle size with calcination temperature TA. The average particle size d increases from 11to85nm as the TA increases from 250to850°C. For particles of size d⩽22nm, energy dispersive spectroscopy indicates the existence of a crystalline Cu1−xMnxO core surrounded by an amorphous CuO shell. M(T) curves reveal a spin-glass behavior for the 11 and 22nm particles and a ferromagnetic behavior for particles of size d⩾35nm. The observed ferromagnetic behavior is attributed to the coexistence of Mn2+ and Mn3+ ions.

High-Tc superconducting surface coils for improving the image quality on a 3 T imager

A surface coil composed of a miniature spiral YBa2Cu3Oy (YBCO) thin film resonator and a Bi2Sr2Ca2Cu2O3 (BSCCO) tape resonator were designed. The spiral YBCO resonator was fabricated onto one side of a 2 × 2 cm2 LaAlO3 substrate, which was then coupled to the receiver via a tank circuit and a fine-tuning copper coil to image small samples. Additionally, a BSCCO tape resonator of 20 cm in diameter was coupled to a receiver coil and a fine-tuning coil to image large samples. An improvement of the signal-to-noise ratio for high-Tc resonators over a conventional copper resonator is demonstrated.

Integrated high-TC radio frequency superconducting quantum interference device using SrTiO3 bicrystal substrate resonators

In contrast to the previous high-TC radio frequency (rf) superconducting quantum interference device (SQUID) magnetometer, in which the rf SQUID is placed face to face with the substrate resonator, a simple integrated washer-type rf SQUID magnetometer based on a single SrTiO3 bicrystal substrate resonator is designed and fabricated in this work. The magnetometer combines the rf SQUID, flux focusing, and resonator into a single chip. This integrated washer-type rf SQUID magnetometer offers a superior advantage in integrating the device to a scanning SQUID detection system in which the distance between the sample and SQUID can be minimized.

Superconducting-quantum-interference-device array magnetometers with directly coupled pickup loop and serial flux dams

In this work, we studied the engineering of high-transition-temperature superconductor Josephson junctions and superconducting quantum interference device (SQUID) by using step-edge or the bicrystal grain-boundary technologies. Serial Josephson junctions and bare SQUID array reveal high quality device characteristics. A high-Tc SQUID magnetometer exhibiting magnetic field sensitivity of 33fT∕Hz1∕2 in the white regime and 80fT∕Hz1∕2 at 1Hz was demonstrated by incorporating the flux dams and serial SQUID into the pickup loop of magnetometer. Furthermore, we demonstrate the opening of the flux dams by applying an external magnetic field to induce a current higher than the critical current of the serial flux dams. We show that the serial flux dams effectively suppress the low frequency 1∕f-like noises.

Spin accumulation and magnetotransport in NiFe∕Al∕NiFe single-electron transistors

Spin accumulation effect and anomalous magnetoresistance resulting in spin dependent transport in NiFe∕Al∕NiFe single-electron transistors have been studied. The magnetic tunneling process is strongly influenced by the charging energy and a superconducting gap. The magnetoresistance is enhanced with decreasing bias voltage. In addition, a suppressed superconducting gap caused by spin accumulation was experimentally demonstrated. For higher bias voltage, the spin accumulation induced by the spin-valve effect is seen as periodic oscillation in a Coulomb blockade region. Below the critical voltage, which is the sum of the superconducting gap and charging energy, spin accumulation can suppress the superconductivity and induce anomalous magnetoresistance.

Superparamagnetic behaviour of antiferromagnetic CuO nanoparticles

The magnetic properties of CuO nanoparticles prepared by calcining are investigated. Above the so-called blocking temperature, it is observed that the thermal energy became larger than the anisotropy energy of Cu/sup 2+/ moments indicating a superparamagnetic behaviour on the magnetization of CuO nanoparticles. Furthermore, the M-H data exhibit features of superparamagnetic particles, such as symmetrical hysteresis loops with a coercive field of 200 Oe and a remanence 0.00385 emu/g at 5 K.

High-T/sub c/ electronic planar gradiometer constructed from magnetometers on a chip

We have fabricated a high-T/sub c/ electronic planar gradiometer composed of dc SQUIDs on a 10 mm /spl times/ 10 mm SrTiO/sub 3/ bicrystal substrate. The SQUIDs were patterned in a single-layer YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film which is prepared by pulsed laser deposition. The outputs of the magnetometers are added and subtracted by analog electronic circuits to form the gradiometer. These magnetometers could be used to construct two sets electronic planar first-order gradiometers or an electronic planar second-order gradiometer, which could measure the elements /spl part/B/sub z///spl part/x and /spl part/B/sub z///spl part/y or /spl part//sup 2/B/sub z///spl part/x/spl part/y, respectively. The noise spectra of each magnetometer and a first-order electronic planar gradiometer were measured.