Harmonic susceptibilities of a bulk superconductor MgB2 at low magnetic fields

Harmonic ac susceptibilities, chi(n) = chi(n)' + i chi(n)", and ac magnetization of a metallic granular superconductor of MgB2 have been measured as a function of temperature, ac field amplitude and additional small dc field in the temperature range between 15 and 45 K. Temperature- and amplitude-dependent measurements are similar to those obtained for a typical high-temperature superconductor. In addition, MgB2 has a very narrow transition temperature width and high critical current densities. The experimental results have been compared to the numerical solutions of a model based on Bean's critical state. Odd harmonic susceptibilities (n = 3, 5 and 7) are in good agreement with those obtained numerically from the model. In contrast, the measured even harmonic susceptibilities exhibit a peak-like behaviour below T-c, with only small magnitudes of about 10(-4) (SI). The results are interpreted in terms of flux penetration to the bulk of superconductor MgB2.