Influence of carbon doping in the vortex matter properties of MgB₂

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We have studied the vortex-matter phase diagram of low C doped MgB₂ superconductor. The significant finding is the appearance of a peak effect, which, as both our global magnetization and local ac susceptibility measurements revealed, is placed well below the Hc₂-line. The absence of significant bulk pinning below the onset line Hon(T) of the peak effect implies that the Bragg glass phase is present for H < Hon(T). Interestingly, the unexpected absence of bulk pinning above the end point line Hep(T) of the peak effect implies the presence of a slightly pinned vortex phase in the regime Hep(T) < H < Hc₂(T). In addition, the observed increase of the Hc₂ᶜ(0) since the carbon doped MgB₂ becomes more dirty and the reduced anisotropy, in comparison to the pure MgB₂, makes the C-doped MgB₂ more favorable for practical applications.