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Register a new userMagneto-optical studies of the uniform critical state in bulk MgB2
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We present a detailed magneto-optical investigation of the magnetic flux penetration in polycrystalline MgB2 slabs made by direct reaction of B and Mg. Our results unambiguously indicate a uniform, Bean critical state magnetization behavior with almost no electromagnetic granularity. From the measured magnetic flux profiles we were able to extract the temperature dependence of the critical current density Jc(T). The Jc(T) value reaches 1.8x10^5 A/cm2 at 10K and 0.12T, in good agreement with global magnetization measurements.
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An extended magneto-optical (MO) analysis of samples cut from high-density pellets of MgB2 is reported. For sake of comparison some magnetic measurements and critical values are also shown. Notwithstanding the fact that the optical and SEM images exhibit grains with different shape and size, all the samples investigated by MO analysis (three specimens of different shape and size) enter as a whole into a critical state. The current at different temperatures and fields, in a phase zone accessible to MO are calculated by means of a quantitative analysis. The analysis, based on the inversion of the Biot-Savart law, corresponds to three regimes: under-critical, critical and over-critical, respectively. A possible qualitative interpretation of the absence of magnetic granularity is given in the framework of a critical state likely reached by a network of strongly coupled Josephson Junctions.
Temperature dependent optical conductivities and DC resistivity of c-axis oriented superconducting (Tc = 39.6 K) MgB2 films (~ 450 nm) have been measured. The normal state ab-plane optical conductivities can be described by the Drude model with a temperature independent Drude plasma frequency of omega_{p,D}=13,600 +/- 100 cm-1 or 1.68 +/- 0.01 eV. The normal state resistivity is fitted by the Bloch-Gruneisen formula with an electron-phonon coupling constant lambda_{tr} = 0.13 +/- 0.02. The optical conductivity spectra below T_c of these films suggest that MgB2 is a multi-gap superconductor.
We have developed disk-shaped MgB2 bulk superconducting magnets (20, 30 mm in diameter, 10 mm in thickness) using the in-situ process from Mg and B powders and evaluated the temperature dependence of trapped magnetic field. A pair of two disc-shaped bulks of 30 mm in diameter and 10 mm in thickness magnetized by field-cooling condition showed trapped fields of 1.2, 2.8 and 3.1 T at 30, 20 and 17.5 K, respectively. High trapped field over 3 T was recorded for the first time.
Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystalline MgB_2 pellets over the whole binding energy range with a view to having an idea of the charge state of Magnesium (Mg). We observe 3 distinct peaks in Mg 2p spectra at 49.3 eV (trace), 51.3 eV (major) and 54.0 eV (trace), corresponding to metallic Mg, MgB_2 and MgCO_3 or, divalent Mg species respectively. Similar trend has been noticed in Mg 2s spectra. The binding energy of Mg in MgB_2 is lower than that corresponding to Mg(2+), indicative of the fact that the charge state of Mg in MgB2 is less than (2+). Lowering of the formal charge of Mg promotes the sigma to pi electron transfer in Boron (B) giving rise to holes on the top of the sigma-band which are involved in coupling with B E_2g phonons for superconductivity. Through this charge transfer, Mg plays a positive role in hole superconductivity. B 1s spectra consist of 3 peaks corresponding to MgB_2, boron and B_2O_3. There is also evidence of MgO due to surface oxidation as seen from O 1s spectra.
Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,a)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.
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