Effects of proton irradiation and ageing on the superconducting properties of single crystalline and polycrystalline MgB2

We present magnetization data on crystalline and polycrystalline MgB2 before and after irradiation with ∼1 MeV protons. In the virgin crystal the critical current density is below our noise floor of 103 A cm−2 over most of the field range. However, after irradiation a peak occurs in the current density as a function of applied magnetic field as the upper critical field Hc2 is approached. After subsequent ageing over a time period of three months, the peak effect is greatly enhanced, exhibiting much stronger pinning over a wide field range, and the upper critical field is approximately doubled, accompanied by a 2 K reduction in transition temperature. Similar studies were made on polycrystalline fragments, where irradiation leads to an increased irreversibility field (Jc is enhanced at high fields but decreased at low fields) and a suppression in transition temperature. However, after two years of ageing both parameters returned towards those of the virgin sample.