Superconducting fluctuations in neutron irradiated MgB2 in relation to precursor diamagnetism in Al-doped MgB2

The fluctuation-related diamagnetism above the superconducting transition temperature Tc in neutron irradiated and in Al-doped MgB2 is studied by means of high-resolution isothermal measurements of the diamagnetic contribution to the magnetization, Mdia=Mdia(H,T=const). In both the neutron irradiated and the Aldoped compounds, Tc decreases on increasing the fluence and the Al amount, respectively. The magnetic field dependence of Mdia is apparently similar in both types of compounds: in the limit H→0, −Mdia goes as H^n (with n in between 1/2 and 1), while by increasing the field above a given value Hup, an upturn in the field dependence occurs and |Mdia| decreases. From the temperature behaviors of Hup, it is proved that the origin of the precursor diamagnetism is quite different in neutron irradiated and in Al-doped MgB2. In the latter, the magnetization curves reflect the precursor diamagnetism typical of heterogeneous systems and unrelated to superconducting fluctuations, due to site dependence of the transition temperature. At variance, neutron irradiated MgB2 displays different properties. The fluence-dependent transition temperature is practically site independent, the superconducting fluctuations and the related diamagnetism basically retaining the features of the pure (unirradiated) MgB2. Upon irradiation, the anisotropy parameter involved in the fluctuation spectrum decreases. Correspondingly, also the upturn field Hup decreases, consistent with a less anisotropic coherence length in strongly irradiated compounds. The implications of these experimental findings on the disorders induced by heterovalent substitutions and by neutron irradiation in MgB2 are discussed.