Abstract Body

Due to their exceptional ability to trap magnetic flux lines, large bulk, melt-textured (RE)Ba₂Cu₃O_7-x superconductors (RE = rare earth element) can be used as powerful permanent magnets. Such permanent magnets are desired in a range of large-scale applications including e.g. rotating machines, magnetic levitation or undulators. The present work focuses on using such magnets for generating large magnetic field gradients and forces over a centimeter scale, a typical size for designing magnetic drug delivery systems [1,2]. Previous works have focused on the field gradients and forces generated by one bulk superconductor magnetized permanently or several bulk superconductors magnetized with parallel magnetization directions [3,4]. Such configurations can be achieved using stationary bulks placed in a time-varying magnetic field having a fixed direction. In the present work we focus on the magnetic field gradient between two bulk superconductors with anti-parallel magnetization directions, as shown schematically in figure 1(a). The goal of our work is to explore experimentally the amplitude of the magnetic field gradient that can be reached in practice in such anti-parallel configuration. The work includes (i) designing an experimental system for magnetizing simultaneously the two bulks at high field and then rotating one of them, (ii) designing the system to be able to measure the flux density distribution between the bulks and (iii) determining experimentally the improvement of the gradient compared to that of a single bulk.

The experimental system relies on a bespoke insertion tool designed [5] to be accommodated in the sample chamber of a commercial Quantum Design (QD) Physical Property Measurement System (PPMS). Unlike commercially available insertion tools (e.g. the QD ‘horizontal rotator’) the system developed in-house allows the rotation of relatively large size samples, e.g. a cube of 6 mm side. The insertion tool combines a driving shaft operating in translation with a rack and pinion system to convert the translation motion into a rotational one. The final mechanism is shown in figure 1(b) and allows for a rotation of 208°, hence enabling the direction of magnetization to be easily reversed by 180°. The mechanical motion is achieved in the vacuum-sealed chamber of the PPMS with low helium gas pressure. Experiments can therefore be carried out at various cryogenic temperatures set by the PPMS.

In order to measure the flux density distribution, a home-made gradient measuring probe was designed, developed and calibrated. The probe contains 13 aligned miniature Hall sensors and is shown in figure 1(b). The connecting wires required for reading the Hall sensor signals are connected to the existing 12-pin connector placed at the bottom of the PPMS sample chamber and an external instrumentation feedthrough. The exact distance between the active areas of the Hall sensors was determined by a calibration experiment carried out prior to placing the measuring probe in the insertion tool. The calibration experiment involved moving a permanent magnet along a straight line and recording the positions of the different sensors.

Experiments were carried out on cubic (RE)Ba₂Cu₃O_7-x bulk superconductors (6 mm side), permanently magnetized using a zero-field cooled (ZFC) procedure [5]. The measured flux density B_z(z) along the line z joining the centers of the faces when the magnetizations are anti-parallel is shown in figure 1(c) at 65 K. Also plotted is the flux density measured when only one bulk sample (either 1 or 2) is used. The anti-parallel configuration is found to result in an increased magnetic flux density gradient ∂B_z(z)/∂z in the region between the bulks. Other experiments were carried out with various DC background fields and at other temperatures.

In summary, this work shows that it is possible to characterize experimentally the magnetic field gradient between two bulk superconductors with anti-parallel magnetization directions, and to investigate the resulting increase in the magnetic flux density gradient. Perspectives using bulk superconductors of larger size will be given. The results of this study are helpful to understand how the rotation of magnetized bulk superconductors can be used to achieve larger gradients and forces for practical applications.

References

[1] A. Arsenault et al. IEEE Trans. Appl. Supercond. 33 4401409 (2023)
[2] Senapati et al. Signal Transduct Target Ther. 3 7 (2018)
[3] F. Mishima et al. IEEE Trans. Appl. Supercond. 17 2303 (2007)
[4] S. Nishijima et al. IEEE Trans. Appl. Supercond. 18 874 (2008)
[5] M. Houbart et al. Supercond. Sci. Technol. 37 095009 (2024)

Acknowledgment

This work was supported by the Fonds de la Recherche Scientifique − FNRS under grant CDR n° J.0218.20 (35325237). Michel Houbart was recipient of a FRS-FNRS Research Fellow grant.

Figure 1. (a) Schematic illustration of magnetized bulk superconductors with anti-parallel magnetization directions. (b) Bespoke insertion tool designed to be inserted in a PPMS system in order to allow the rotation of large magnetized superconducting samples. The magnetic field gradient measuring probe is placed in the gap between the holders for Sample 1 and Sample 2 (c) Distribution of the magnetic flux density B_z at 65 K using either only one bulk sample (Sample 1 or Sample 2) or two bulk superconductors with anti-parallel magnetization directions (Samples 1+2)

Keywords: bulk superconductor, trapped-field magnet, rotating superconductor.