Effects of Co and Mn doping on the structure and superconductivity of MgB2

A series of Co and Mn doped MgB2 polycrystalline samples have been synthesized and studied by x-ray diffraction and Raman spectrum. It is found that for Co-doping the a-axis of the lattice remains unchanged as the Co content increases, while the c-axis shows a small increase. In contrast, for Mn-doping, the c-axis shows an evident decrease as the Mn content increases, besides a small decrease of the a-axis. The T-c of Mn doped samples decreases quickly with increasing Mn content while that of Co-doped samples decreases rather slowly. The Raman spectra show a smaller change on Co content. For Mn-doped samples, however, the linewidth is obviously broadened, although the frequency is almost unchanged on an increase of x. The variations of the lattice parameters and Raman spectra for Co doping reveal that the Co ion is divalent when substituted at the Mg site. It is suggested that additional structural disorder may be induced by a localized magnetic moment, which will further give an effect on T, and Raman linewidth, besides the effect of the magnetic moment. (C) 2008 Elsevier Ltd. All rights reserved.