Angular dependence of the peak effect in MgB2−xCx

Evolution of the peak effect with the angle between the c-axis and the external magnetic field, using pure and carbon substituted MgB2 single crystals field is presented. For given angle Θ(c,H) the peak effect appears only for H>H∗(Θ), where H∗(Θ) increases with the angle for the crystals x=0 and x=0.04, while practically remains constant for x=0.1 The peak critical current diminishes with angle and disappears when the magnetic field is parallel to the c-axis. Both the irreversibility and the second peak lines, for each temperature, can be reproduced by the equation H(Θ)=H(0)/cos2Θ+(1/γ)2sin2Θ, for at least Θ<80°, with the same effective mass anisotropy parameter, γ. This type of scaling implies that the peak effect and the irreversibility lines are driven by thermal fluctuations. The relation of the experimental results with the type of pinning centers producing the peak effect will be discussed.