New Superconducting State of MgB2Induced by Mechanical Milling

Abstract

Since the discovery of superconductivity in MgB2 by Nagamatsu et al., many researches for this material have been made. However, there are only some reports on the relations between the imperfection of the structure and the superconductivity, though such study is necessary and important to its applications. Recently Okuma et al. measured the resistivity of amorphous MgxB1 x (x 1⁄4 0:3{ 0:4), which were prepared by coevapolation of pure Mg and B from the electron beam crucibles onto the glass substrate. They indicated that the superconducting transition temperature Tc was dependent on x and the maximum Tc was 6.1K for x 0:33. We performed the mechanical milling of MgB2 to introduce randomness into the structure, and found that Tc did not change by mechanical milling and the absolute value of the diamagnetization at the same temperature below Tc decreased with milling time. In this preliminary research, the quality of the starting material was not good, and we observed positive magnetization below Tc for 384 h of milling, which suggested mixing of some magnetic contamination to the sample. In this note, we have used mechanical milling with little mixing of contamination to the samples even in longer periods of milling and investigated the superconductivity by magnetization measurements. We find a new superconducting phase induced by long periods of milling. Starting MgB2 was a commercial powder (Furuuchi Chemical, 99%). Mechanical milling was carried out in a hardened steel vial with one steel ball under a purified helium atmosphere. We improved a vibrating frame (Fritsch, Pulverisette 0) and realized the method in which mixing of contamination to the sample was hardly recognized in longer periods of milling. Six samples milled for 0, 60, 120, 240, 480 and 720 h were prepared. The structure of all samples was examined from X-ray diffraction (XRD) patterns obtained using Cu K radiation with graphite monochromator (Rigaku Co., RAD-2X). SQUID magnetometers (Quantum Design, MPMS2 and MPMS XL) were used to measure the temperature dependence of the magnetization (M–T) in 100Oe and the field dependence of the magnetization (M–H) at 2.5K, 20K and 30K for all samples. We also obtained the M–T curves of the samples unmilled and milled for 240 h in field of 30Oe. In Fig. 1, the XRD patterns of MgB2 milled for several periods up to 720 h are plotted together with the pattern of the starting MgB2 powder (0 h of milling). The present starting powder contains a little amount of MgO as an impurity. Mechanical milling effects on XRD patterns, which are the decrease of the intensity of Bragg peaks and the increase of the full width at half-maximum with milling time, are confirmed. We evaluated quantitatively the crystallite size D and the lattice strain using the Wilson’s method. We obtain the results that D decreases monotonously with milling time and tends to become constant after long periods of milling, while increases with milling time and tends to reach a constant value (D 12 nm and 0:6% for 720 h of milling). Even after 720 h of milling, the original structure is very clearly recognized from the XRD pattern and any other Bragg peak cannot be detected except for the peaks of the starting material. The integrated intensities of the peaks decreases gradually with milling time, though any halo pattern suggesting amorphous phase formation is not found after longer periods of milling. Figure 2 shows the zero-field-cooled magnetization (ZFCM) over a temperature range of 2 to 50K under an applied field of 100Oe. The inset in Fig. 2 shows ZFCM in 10 20 30 40 50 60 70 80 in te ns ity ( ar b. un its )