A comprehensive study of the pinning mechanisms of MgB2 wires treated with malic acid and their relationships with lattice defects
The effects of sintering temperature on the lattice parameters, structural strain, critical temperature (Tc), critical current density (Jc) irreversibility field, upper critical field (Bc2), and resistivity (p) of MgB2 wires treated with 10 wt alic acid (C4H6O5) are investigated in this paper. The a-lattice parameter of the sample treated with malic acid was drastically reduced, to 3.0745, as compared to those for the undoped samples. Reduction in the a-lattice parameter is related to crystalline imperfections arising from carbon substitution - as confirmed by x-ray diffraction and Raman spectra - which play a vital role in enhancing Jc, Bc2 and Birr. We have also analyzed the pinning mechanisms, and concluded that flux pinning is dominated by point and correlated pinning at lower and higher magnetic fields, respectively, for the carbon-doped samples sintered at both 700 and 900 degrees celcius. The degree of flux pinning enhancement and the ratio RHH (Bc2/Birr) have been estimated to guide us towards further enhancement of Jc. We argue that oe and otc pinning mechanisms, based on variation of the mean free path (e) and the critical temperature, respectively, coexist in the MgB 2 wires treated with malic acid, regardless of the sintering temperature. The oe pinning is predominant at lower operating temperatures, and otc pinning starts close to Tc, which means that spatial variation in the charge carrier mean free path is the main mechanism responsible for the flux pinning in the MgB2 wires treated with malic acid that were sintered at 700 and 900 degrees celcius.