Sung-Ik Lee

Very strong intrinsic flux pinning and vortex avalanches in (Ba,K)Fe2As2 superconducting single crystals

We report that the (Ba,K)Fe2As2 crystal with Tc =3 2u2002K shows a pinning potential, U0, as high as 104u2002K, with U0 showing very little field dependence. The (Ba,K)Fe2As2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to Hc2. The isotropic rigid vortices observed in the two-dimensional (2D) (Ba,K)Fe2As2 distinguish this compound from 2D high-Tc cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the (Ba,K)Fe2As2 crystal. It is proposed that it is the K substitution that induces both almost isotropic superconductivity and the very strong intrinsic pinning in the (Ba,K)Fe2As2 crystal.

Coexistence of the δl and δTc flux pinning mechanisms in nano-Si-doped MgB2

The flux pinning mechanisms of nano-Si-doped MgB2 are reported in this work. The field dependence of the critical current density, Jc(B), was analyzed within the collective pinning model. We found that the mechanisms for both δl pinning, i.e., pinning associated with charge-carrier mean free path fluctuations, and δTc pinning, which is associated with spatial fluctuations of the transition temperature, coexist in the nano-Si-doped MgB2 samples, while Hc2 increases greatly with increasing nano-Si doping level. However, their contributions are strongly temperature dependent. The δl pinning is dominant at low temperatures, decreases with increasing temperature, and is suppressed completely at temperatures close to the critical temperature, Tc. However, the δTc pinning mechanism shows opposite trends.

Fluctuation conductivity of single-crystalline BaFe1.8Co0.2As2 in the critical region

The magnetofluctuation conductivity, called excess conductivity, originated from the forming of the superconducting droplet near to the mean-field transition temperature, was measured for the optimally doped BaFe1.8Co0.2As2 single crystals with a critical temperature, Tc, of 24.6 K. This measurement of the excess conductivity for magnetic fields up to 9 T was compared with the thermodynamic scaling theory in the critical region, in which not only the Gaussian fluctuation but also fourth order terms of the order parameter are included. An analysis of the excess conductivity showed that the superconductivity followed three-dimensional scaling rather than two-dimensional scaling even though the sample had a layered structure.

Fabrication of FeSe1−x superconducting films with bulk properties

Abstract We have fabricated high-quality FeSe 1− x superconducting films with a bulk T c of 11–12xa0K on different substrates, Al 2 O 3 (0xa00xa00xa01), SrTiO 3 (1xa00xa00), MgO(1xa00xa00), and LaAlO 3 (1xa00xa00), by using a pulsed laser deposition technique. All the films were grown at a high substrate temperature of 610xa0°C, and were preferentially oriented along the (1xa00xa01) direction, the latter being to be a key to fabricating of FeSe 1− x superconducting thin films with high T c . According to the energy dispersive spectroscopy data, the Fe:Se composition ratio was 1:0.90xa0±xa00.02. The FeSe 1− x film grown on a SrTiO 3 substrate showed the best quality with a high upper critical magnetic field [ H c 2 (0)] of 56xa0T.

Growth and characterization of 144 μm thick barium ferrite single crystalline film for microwave device application

Liquid phase epitaxy technique was used to grow 144u2002μm thick barium ferrite (BaFe12O19; BaM) single crystalline films on (111) Gd3Ga5O12 substrate. The growth rate of 72u2002μm/h was achieved with a flux system of Fe2O3–BaCO3–Na2CO3. The grown BaM films show single crystalline (000l) orientation that was confirmed by x-ray diffraction and magnetic torque curves. The saturation magnetization (4πMs) and the anisotropy field (Hk) were found to be 4.2 kG and 16.0 kOe, respectively. The ferrimagnetic resonance linewidth (ΔH) at 35 GHz was measured to be 0.1 kOe.

Suppression of dendritic flux jumps in MgB2 films coated with a gold rim

An effective method for suppressing dendritic flux avalanches in MgB2 films is demonstrated by coating the films with a metal rim along the edges, which is where the avalanches nucleate. The effect of the partial coating has been investigated by means of magneto-optical imaging on a series of samples with golden rims of different width. Measurements of the onset field of instability reveal that such rims can substantially improve the thermo-magnetic stability of the superconducting films.

Magnetic and Microwave Properties of Sm-Doped SrFe

Samarium (Sm) doped M-type strontium ferrite single crystals have been successfully grown from melts using a flux system of SrCO3 and Na2CO3. The crystals exhibited the typical hexagonal ferrite growth habit with a plate-like geometry, and the largest crystals obtained are of 10 mm in length and 3-4 mm in thickness, respectively. It was observed that the saturation magnetization and anisotropy field are greatly affected by the doping concentration of Sm. A saturation magnetization (sigmas) of 69.8 emu/g and an anisotropy field (H k) of 25 kOe was achieved. Room temperature FMR measurements show 85 Oe of linewidth (DeltaH) at 53 GHz.

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We have successfully fabricated c-axis-oriented MgB2 thin films by using a laser-assisted chemical vapor deposition (LACVD) technique. Mg and B sources were supplied by pulsed laser ablation and a mixture of B2H6 gas in H2, respectively. The MgB2 thin films grown on Al2O3(0001) substrates at 400u2009°C showed a superconducting transition temperature of 40.2xa0K with a sharp transition width of 0.2xa0K and a high critical current density of about 107xa0Axa0cm−2 at 20xa0K and zero field, suggesting that this low-temperature process would be a promising technique for electronic devices operating at high temperatures above 20xa0K.

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The magnetic penetration depth

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