The Upper Critical Magnetic Field of Superconducting YBa 2 (Cu 1-x Ni x ) 3 O 7
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THE UPPER CRITICAL MAGNETIC FIELD OF SUPERCONDUCTING YBa 2 (Cul-xNix) 307 YOUWEN XU, A. R. MOODENBAUGH, AND M. SUENAGA Materials Science Division, Brookhaven National Lab., Upton, NY 11973 ABSTRACT The upper critical magnetic field of a series samples of YBa 2 (CuI xNix)30 7 is measured. Other related parameters, such as the lower critical magnetic 'ield cl, the Ginsberg-Landau parameter K, the jump in the specific heat AC/Tc, the superconducting coherence length t, and the penetration depth A, are calculated from Tc, dHc 2 /dT and dM/dH. The results show that He (0) and consequently t(0), are essentially constant, but Hc and AC/Tc decrease wits increasing Ni content. INTRODUCTION Knowledge of the upper critical magnetic field Hc2 is important for understanding the mechanism of superconductivity, since its value is directly related to the superconducting coherence length and anisotropy in the superconducting state. The measurements of Hc2 and the field dependence of the magnetization give information about the Ginsberg-Landau parameters K and the jump in specific heat at T AC/Tc, which is related to the electronic density of state at the Fermi surface N(0). The upper critical magnetic field H 2 of a YBa 2 Cu 30 7 single crystal y as been studied both resistively and magnetically %y Welp et al.' and Crabtree et al. . Previously we also reported a study on Hc2 on "c"-axis aligned polycrystalline powder YBa 2 (Cu 9 8 M 0 2 ) 3 0 7 where M=Cu, Ni, and Al. Our result on the "pure" 1:2:3 sample is consistent with that of Welp et al. from a single crystal. In this work, we carried out a systemetic study of Hc2 determined magnetically on a "c"-axis aligned powder of YBa 2(Cul1 xNix) 3 0 7 , where x=0.00-0.06. Related parameters, such as the thermodynamic and lower critical magnetic fields, Hc and Hcil the GinsbergLandau parameter K, the jump in the specific heat AC at Tc, the superconducting coherence length t, and the penetration depth A, were also calculated. EXPERIMENTAL METHOD The well crystallized samples 4 , 5 with grain size typically >10-20pm were ground and sieved, mixed with epoxy, and aligned in a magnetic field of 8T during the solidification of the epoxy. Transmission Laue diffraction showed the sample to be oriented along the "c"-axis with a high degree of alignment. The magnetic measurement was performed in a Quantum Design SQUID magnetometer with a DC magnetic fields 4 5 tesla. Values of Hc 2 (T) are obtained with the crystallographic "c"-axis of the specimen parallel to the magnetic field. RESULTS AND DISCUSSION The upper critical magnetic field was determined by measuring the temperature dependence of magnetization M(T) in various fields. In a temperature interval below Tc, the values of the magnetization are the same for zero-field-cooled and field-cooled measurement. This "reversible" range depends on the magnetic field and the sample. Its span varies from 1-2K (for H=0.5T) to about 14K (for H=5T). In this "reversible" range M is linear with temperature. Tc(H) is determined by extrapolating the linear p
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