Crossed field effect measured on a GdBCO pellet at various temperatures below 77 K

In engineering applications, superconducting bulk materials might be subjected to cycles of variable magnetic fields transverse to their trapped field which can cause a significant demagnetization. Up to now, several studies have been carried out on bulk superconducting pellets in order to characterize this so-called “crossed fields” effect at 77 K only. When attempting to carry out experiments at lower temperature, the issue is that the commercial systems with rotating sample holders do not accommodate sample having a larger size than few mm³. Therefore samples have to be cut from a larger pellet to be characterized.
We designed an insertion tool for a Physical Property Measurement System (PPMS) that allows to rotate a sample of 9 mm diameter and up to 8 mm height inside the sample chamber from a parallel to a perpendicular direction with respect to the magnetic field direction. A Pt100 temperature sensor and a Hall probe were placed in the sample holder in order to measure the local temperature and the magnetic induction at the centre of the sample surface.
A GdBCO sample was characterized between 50 K and 85 K by measuring first the penetration and the remnant fields through hysteresis loops measurements. Then transverse magnetic fields of different amplitudes were applied at constant sweep rates. The effect of the transverse magnetic field on the trapped field was studied and compared to the natural relaxation due to the flux creep. Results show that the application of a transverse magnetic field of the same percentage of the axial penetration field at several temperatures causes the same relative demagnetization (corrected from the flux creep relaxation). For a relatively small number of cycles, this demagnetization is found to be a logarithmic function of the time. Advantage was also taken from the possibility to change the temperature by measuring the crossed field effect of a magnetized sample which temperature was lowered to freeze the natural relaxation.