AC losses in high pressure synthesized MgB₂ bulk rings measured by a transformer method

Recently developed manufacturing technologies use high pressure and various doping additions to prepare bulk MgB₂-based materials with a high critical current density measured by the magnetization method. We use a contactless transformer method, which is based on studying the superconductor response to an induced transport current, to measure AC losses in bulk MgB ₂ rings synthesized under high pressure. The obtained dependence of the losses on the primary current (applied magnetic field) is fitted by a power law with an exponent of ∼2.1 instead of the cubic dependence predicted by Bean's model and power law electric field-current density (E-J) characteristics with a large exponent. An unusually strong dependence of the AC losses on the frequency is also observed. It is shown that the E-J characteristic of bulk MgB₂ is well fitted by the dependence used in the extended critical state model based on account of the viscous vortex motion in the flux flow regime. Numerical simulation using this E-J characteristic gives current and frequency AC loss dependences that agree well with the experimental results.