Bulk GdBa2Cu3O7 Superconductors Prepared by Single Direction Melt Growth (SDMG) : Key Points to Understand their Behaviour Under a Time-Dependent Magnetic Field (invited, keynote)

Bulk, large grain GdBa2Cu3O7 superconductors prepared by the ‘Single-Direction Melt Growth’ (SDMG) process [1,2] are known to exhibit an unprecedented homogeneity of their superconducting properties [3]. Hence, they offer great ability as trapped field superconducting magnets or magnetic shields and screens [4]. In most engineering applications, however, the superconductor is subjected to several cycles of the applied field. Examples include a bulk magnet in a rotating machine or a superconducting magnetic screen subjected to a changing stray field. The goal of this communication, therefore, is to highlight key parameters required to predict and study the change of properties of an SDMG bulk superconductor when it is subjected to repeated cycles of applied field. For low-frequency magnetic fields, we study the magnetic screening properties of a disk-shaped superconducting bulk. Experimentally, the screening factor SF that we obtain after up to 30 cycles of low-frequency applied field is the same (at the maximum field) as the SF after the first ramp from zero field to its maximum. Numerically, we show that the relative decay of SF between successive occurrences of the maximum field is smaller than the decay that would be obtained if the maximal field was maintained. Hence, the key parameters are the magnetic relaxation and the penetration depth in the bulk, since a higher penetration depth leads to a smaller SF and a faster magnetic relaxation. At higher frequencies of applied field, the hysteresis losses may yield a temperature increase which, in turn, affects the magnetic properties. To predict this self-heating, we provide experimental data on the thermal properties of SDMG bulk samples (GdBa2Cu3O7 + 0.4 mol Gd2BaCuO5 + 10% Ag2O) and compare them to those of other bulk samples (YBa2Cu3O7 and Bi2Sr2CaCu2O8). Among others, we investigate the anisotropy of both the thermal conductivity and the electrical resistivity, the peaks appearing in the temperature-dependent specific heat and the temperature dependence of the irreversibility line (H||ab and H||c). Practical implications for engineering applications are discussed.

This work is partially funded by a collaboration programme between the FRS-FNRS (Belgium) and the PAN (Poland) under reference PINT-BILAT-M R.M002.24. Nicolas Rotheudt is recipient of an FRS-FNRS Research Fellow Grant.

References
1. T. Motoki et al. Appl. Phys. Express 13, 093002 (2020).
2. F. Antoncik et al. IEEE Trans. Appl. Supercond. 35, 6800205 (2025).
3. T. Motoki et al. Supercond. Sci. Technol. 35, 094003 (2022).
4. N. Rotheudt et al. Supercond. Sci. Technol. 38, 025012 (2025).