A relatively high critical temperature, T c , approaching 40 K, places the recently-discovered [1] superconductor magnesium diboride (MgB 2 ) intermediate between the families of low-and copper-oxide-based high-temperature superconductors (HTS). Supercurrent flow in MgB 2 is unhindered by grain boundaries [2, 3], unlike the HTS materials. Thus, long polycrystalline MgB 2 conductors may be easier to fabricate, and so could fill a potentially important niche of applications in the 20 to 30 K temperature range. However, one disadvantage of MgB 2 is that in bulk material the critical current density, J c , appears to drop more rapidly with increasing magnetic field than it does in the HTS phases [4]. The magnitude and field dependence of J c are related to the presence of structural defects that can "pin" the quantised magnetic vortices that permeate the material, and prevent them from moving under the action of the Lorentz force. Vortex studies [3] suggest that it is the paucity of suitable defects in MgB 2 that causes the rapid decay of J c with field. Here we show that modest levels of atomic disorder, induced by proton irradiation, enhance the pinning, and so increase J c significantly at high fields. We anticipate that chemical doping or mechanical processing should be capable of generating similar levels of disorder, and so achieve technologically-attractive performance in MgB 2 by economically-viable routes.A Type II superconductor undergoes the transition to the normal state at the upper critical field, H c2 . However, the ability to carry dissipation-free current ceases at a lower field, the irreversibility field, H irr . Above H irr the Lorentz force on the vortices is large enough for them to become detached from pinning defects, and virtually free to move. In MgB 2 powder (and also wires and tapes), H irr is approximately half of H c2 [5], so that there is an extended field domain where there might be a useful J c if only the pinning could be strengthened.Ion irradiation is a reproducible means of inducing crystalline disorder. Energetic ions displace atoms from their equilibrium lattice sites, creating a variety of defects, including vacancies and interstitials. Such defects tend to depress the superconducting order parameter locally, and thereby create pinning sites for the vortices. We chose protons for this initial study because at the maximum beam energy available to us of 2 MeV, they can penetrate 40 to 50µm into MgB 2 . A series of irradiations were performed in order to create significant defect densities. The probability of displacement per atomic site (dpa) can be simulated using commercial software [6], and we aimed to generate as uniform a profile as possible through the sample depth (Fig.1).As in our previous study [3], we selected fragments of about 100 µm size from commercial MgB 2 powder (Alfa Aesar Co., 98% purity). Several samples were prepared, each of 20 fragments embedded in silver-loaded epoxy and then polished, so that a defined geometry was obtained, with an estimated thickness of...

The recently discovered superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are 'pinned' in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the 'persistent' supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.

Bulk samples of MgB 2 were prepared with 5, 10, and 15 wt % Y 2 O 3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ϳ3-5 nm YB 4 nanoparticles embedded within MgB 2 grains of ϳ400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density J C was observed, most notably for 10% doping. At 4.2 K, the lower bound J C value was ϳ2 ϫ10 5 A cm Ϫ2 at 2 T. At 20 K, the corresponding value was ϳ8ϫ10 4 A cm Ϫ2 . Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1506184͔In slightly more than one year after the discovery of superconductivity in magnesium diboride, there is now a wide body of evidence indicating that MgB 2 does not contain intrinsic obstacles to current flow between grains, unlike the high-temperature superconducting cuprates. Evidence for strongly coupled grains has been found even in randomly aligned, porous, and impure samples, 1,2 suggesting that dense forms of MgB 2 will be attractive in high-current applications at 20-30 K and perhaps 4.2 K. So far, however, bulk samples have demonstrated modest values of the irreversibility field 0 H*(T) reaching about 4 T at 20 K and 8 T at 4.2 K.3 For comparison, established low-temperature superconductors, e.g., NbTi ͑10 T͒ and Nb 3 Sn ͑20 T͒, have significantly higher irreversibility fields at 4.2 K, while Bi 2 Sr 2 Ca 2 Cu 3 O 10 ͑3 T͒ is becoming established at 20 K. 4 MgB 2 tape results are somewhat more promising, with 0 H* values of above 5 at 20 K, 5-8 where partial orientation of crystallites parallel to the field is playing a role. Since the irreversibility field is the practical limit to magnet applications, it is desirable to make 0 H* values as high as possible.A central question is how to further increase the irreversibility field in addition to introducing crystallographic texture. Alloying additions, such as atomic substitution for Mg or B or added interstitial atoms, increase electron scattering and decrease the coherence length, producing higher upper critical and irreversibility fields.9,10 Adding nanometer-scale defects can produce similar effects. For example, proton irradiation studies showed that 0 H* increased significantly from ϳ3.5 to ϳ6 T at 20 K with only moderate damage, corresponding to atomic displacements of a few %, due to either vacancies or interstitials.11 Mechanical processing also produces structural defects, and similar increases in the irreversibility field have been reported. 6,8,12 These increases were steeper than the concomitant reductions in the critical temperature T c , suggesting it is viable to improve the accessible field range without sacrificing other superconducting properties too much.To explore more practical and scaleable routes to defect incorporation in bulk MgB 2 , the present study explores chemical and nanostructural changes via addition of nanoparticles. Coherently ordered Mg-B-O precipitates are known to ...

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Thin-film nanocalorimeter for low temperatures and high magnetic fields is described. The calorimeter is based on a commercial microchip module (thermal conductivity vacuum gauge TCG 3880 from Xensor Integration, NL). The gauge consists of submicron silicon nitride membrane with a film-thermopile and a resistive film-heater with dimensions of 50×100μm2 located at the center of the membrane. The gauge is mounted in a thermostat filled with helium exchange gas. The method of alternating current (ac) calorimetry is applied for heat capacity measurements. The noise-floor sensitivity of the calorimeter is better than 1nJ∕K below 100K and about 3nJ∕K at 300K. This allows for reliable measurements to be performed on sub-microgram samples. It is proved that the method is applicable for heat capacity measurements at temperatures in the range of 5–300K and in high magnetic fields up to 8T. We present a theoretical analysis of the thermal processes in the gauge-sample-surrounding gas system. On this basis a calibration method has been developed. We demonstrate that the technique yields correct heat capacity for test samples and that in special cases the thermal conductivity and the magnetostriction of the sample can be measured simultaneously with the heat capacity.

Results of isothermal magnetization and magnetic relaxation measurements are presented probing the nature of the magnetic-field-induced magnetostructural transition in the intermetallic compound Gd 5 Ge 4 . This transition shows the characteristics of a disorder-influenced first order transition including distinct metastable behavior. Below approximately 21 K, the transition from the magnetic-field-induced ferromagnetic state back to the antiferromagnetic state shows additional interesting features. Similarities with other classes of magnetic systems exhibiting magnetostructural transitions are pointed out.

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