We report on ferromagnetic characteristics of Zn 1Ϫx Mn x O ͑xϭ0.1 and 0.3͒ thin films grown on Al 2 O 3 (00•1) substrates using laser molecular-beam epitaxy. By increasing the Mn content, the films exhibited increases in both the c-axis lattice constant and fundamental band gap energy. The Curie temperature obtained from temperature-dependent magnetization curves was 45 K for the film with xϭ0.3, depending on the Mn composition in the films. The remanent magnetization and coercive field of Zn 0.9 Mn 0.1 O at 5 K were 0.9 emu/g and 300 Oe, respectively. For Zn 0.7 Mn 0.3 O, the remanent magnetization at 5 K increased to 3.4 emu/g.

We report the growth of high-quality c-axis-oriented epitaxial MgB 2 thin films by using a pulsed laser deposition technique. The thin films grown on (1    1 0 2) Al 2 O 3 substrates show a T c of 39 K. The critical current density in zero field is ∼ ∼ ∼ ∼ 6 x 10 6 A/cm 2 at 5 K and ∼ ∼ ∼ ∼ 3 x 10 5 A/cm 2 at 35 K, suggesting that this compound has great potential for electronic device applications, such as microwave devices and superconducting quantum interference devices (SQUIDs). For the films deposited on Al 2 O 3 , X-ray diffraction patterns indicate a highly c-axis-oriented crystal structure perpendicular to the substrate surface.The recent discovery of the binary metallic MgB 2 superconductor [1] having a remarkably high transition temperature (T c ) of 39 K has attracted great scientific interest [2][3][4][5][6][7][8]. With its metallic charge carrier density [2] and the strongly linked nature of the inter-grains in a polycrystalline form [9,10], this material is expected to be a very promising candidate for superconducting device [11] as well as large-scale applications. Furthermore, since the single crystal growth of MgB 2 seems very difficult, the fabrication of epitaxial thin film should be an important development for future basic research studies. However, the fabrication of thin films of this material has not been reported yet.We used a two-step method to fabricate MgB 2 thin films. First, we deposited amorphous B thin films; we sintered then at high temperature in Mg vapor, which is very similar to the growth techniques of cuprate Hg-based superconducting thin films [12,13]. We pressed commercial Boron (99.99%) powder into a disk shape with a diameter of 12.7 mm and a height of 5 mm under a pressure of 6 tons. Precursor thin films of B were deposited on Al 2 O 3 (AO) and SrTiO 3 (STO) substrates at room temperature by using pulsed laser deposition. The laser energy density was 20 -30 J/cm 2 at a laser flux of 600 mJ/pulse and a pulse frequency of 8 Hz. After a precursor thin film had been fabricated, it was put into a Ta tube together with a high purity Mg metal (99.9%) and sealed in an Ar atmosphere. The heat treatment was carried out in an evacuated quartz ampoule to prevent oxidation of the Ta tube. The typical sintering procedure was fast heating to 900 °C in 5 minutes; this temperature was held for 10 -30 minutes, and then quenched to room temperature. The typical film thickness used in this study was 0.4 µm, which was measured by a scanning electron microscope. This simple technique can be applied to other physical deposition methods, such as sputtering and electron-beam evaporation, and is highly reproducible, so mass production should be possible. The resistivity measurements were carried out using the dc four-probe method. The dc magnetic properties were measured with a Quantum Design MPMS superconducting quantum interference device magnetometer. The structures were analyzed using a x-ray diffractometer (XRD).The typical temperature dependence of the resistivity of MgB 2 grown on AO ...

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We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)10.1103/PhysRevB.73.014512] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

We report that the ͑Ba, K͒Fe 2 As 2 crystal with T c = 32 K shows a pinning potential, U 0 , as high as 10 4 K, with U 0 showing very little field dependence. The ͑Ba, K͒Fe 2 As 2 single crystals become isotropic at low temperatures and high magnetic fields, resulting in a very rigid vortex lattice, even in fields very close to H c2 . The isotropic rigid vortices observed in the two-dimensional ͑2D͒ ͑Ba, K͒Fe 2 As 2 distinguish this compound from 2D high-T c cuprate superconductors with 2D vortices. The vortex avalanches were also observed at low temperatures in the ͑Ba, K͒Fe 2 As 2 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, K͒Fe 2 As 2 crystal.A high critical current density, J c , upper critical field, B c2 , and irreversibility field, B irr , a high superconducting transition temperature, T c , strong magnetic-flux pinning, good grain connectivity, and isotropic superconductivity are the major physical requirements for superconducting materials used in practical applications operating at low and, in particular, high magnetic fields. The conventional low-T c superconductors, where H c2 is also small, can only carry large J c at very low temperatures. The cuprate high-T c superconductors suffer from poor grain connectivity and easy melting of the vortex lattice, leading to small J c in high magnetic fields at relatively high temperatures. For MgB 2 superconductor with T c of 39 K, B irr is far below H c2 , and J c drops quickly with both field and temperature, preventing its use above 20 K. The newly discovered Fe-based superconductors 1-7 show T c as high as 55 K and B c2 above 200 T, in combination with a small anisotropy for REFeAsO 1−x F x ͑RE-1111 phase, with RE a rare-earth element͒ 8 and an almost isotropic superconductivity for ͑Ba, K͒Fe 2 As 2 ͑122 phase͒. 9 These properties make the Fe-based superconductors extremely promising candidates for high magnetic field applications at relatively high temperatures. The current carrying ability of these superconductors at high fields and temperatures is largely determined by the flux-pinning strength and the behavior of the vortex matter. Therefore, the determination of their intrinsic vortex pinning strength is a central issue from both an applied and a fundamental perspective. Both 1111 and 122 phase compounds have typical two-dimensional ͑2D͒ crystal structures. In RE-1111 phase, where RE is a rare-earth element, the FeAs superconducting layers are separated by insulating LaO layers 10 while in Ba͑K͒-122 phase, the FeAs layer is sandwiched between conductive Ba layers. 5 It is expected that the 122 phase containing two FeAs layers would have small anisotropy and thus higher intrinsic pinning compared to the single layer 1111 phase. Co-doped BaFe 2 As 2 single crystal shows an anisotropy of 1-3 and upper criticalfield values of B c2 ͑B ʈ ab͒ = 20 T and B c2 ͑B ʈ c͒ = 10 T at 20 K, with dB c2 / dT Ϸ 5 T/ K. 11 For single crystals of the optimally do...