Trapped magnetic field in a (NdFeB)–(MgB2) pair-type bulk magnet

• Dense MgB2 discs were obtained by ex-situ Spark Plasma Sintering.
• A NdFeB–MgB2 pair-type bulk magnet was tested for different working conditions.
• The polarity of the NdFeB permanent magnet influences macro flux jumps of MgB2.
• Trapped field of the pair was 2.45 T (20 K) and 3.3 T (12 K).

Superconducting bulk discs, S, of 20 mm in diameter and 3.5 or 3.3 mm thickness of MgB2 (pristine or added with cubic BN, respectively) with density above 97% were prepared by Spark Plasma Sintering. Discs were combined in a pair-type sandwich-like arrangement with a permanent NdFeB axially magnetised magnet, PM (∼0.5 T). Measurement of the trapped field, Btr, with temperature, time, and the reduction rate of the applied magnetic field was performed using a Hall sensor positioned at the centre between the superconductor and the permanent magnet. It is shown that the permanent magnet with certain polarity favors higher trapped field of the superconductor owing to suppression of flux jumps specific for high density MgB2 samples. The Btr of the PM–S pair was 2.45 T (20 K) and 3.3 T (12 K).