We report experimental results of second-harmonic (SH) response at microwave frequency in several ceramic MgB 2 samples, prepared by different methods. The SH signal has been investigated as a function of the temperature and DC magnetic field. The investigation has been carried out at low magnetic fields, where nonlinear processes arising from motion of Abrikosov fluxons are ineffective. We show that the low-temperature SH emission is ascribable to processes involving weak links. Comparison among the peculiarities of the SH signal radiated by the different samples shows that the presence of weak links strongly depends on the sample preparation method, as well as the purity and morphology of the components used for the preparation.

The AC susceptibility, χ, at zero DC magnetic field of a polycrystalline sample of LaFeAsO 0.94 F 0.06 (T c ≈ 24 K) has been investigated as a function of the temperature, the amplitude of the AC magnetic field (in the range H ac = 0.003 Oe ÷ 4 Oe) and the frequency (in the range f = 10 kHz ÷ 100 kHz). The χ(T ) curve exhibits the typical two-step transition arising from the combined response of superconducting grains and intergranular weak-coupled medium. The intergranular part of χ strongly depends on both the amplitude and the frequency of the AC driving field, from few Kelvin below T c down to T = 4.2 K. Our results show that, in the investigated sample, the intergrain critical current is not determined by pinning of Josephson vortices but by Josephson critical current across neighboring grains.

We report results of microwave second‐harmonic generation in ceramic samples of MgB2, prepared by different methods. The SH signal has been investigated as a function of the temperature and the static magnetic field. The results are discussed in the framework of models reported in the literature. We show that the peculiarities of the SH signal are related to the specific properties of the sample. A comparison with the results obtained in ceramic and crystalline YBa2Cu3O7 shows that the second‐harmonic emission in MgB2 is weaker than that observed in ceramic YBa2Cu3O7. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2482–2486, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22001