Magnetic cooling towards millikelvin temperatures using rare earth triangular magnets

• Thesis title in Czech: Magnetické chlazení směrem k milikelvinovým teplotám pomocí trojúhelníkových magnetů ze vzácných zemin
• Thesis title in English: Magnetic cooling towards millikelvin temperatures using rare earth triangular magnets
• Key words: fyzika pevných látek|frustrovaný magnetizmus|kvantový magnetizmus|magnetická kritikalita|termodynamika|chlazení
• English key words: solid state physics|frustrated magnetism|quantum magnetism|magnetic criticality|thermodynamics|refrigeration
• Academic year of topic announcement: 2023/2024
• Thesis type: diploma thesis
• Thesis language: angličtina
• Department: Department of Condensed Matter Physics (32-KFKL)
• Supervisor: Ross Harvey Colman, Dr.
• Author: hidden - assigned and confirmed by the Study Dept.
• Date of registration: 02.11.2023
• Date of assignment: 18.12.2023
• Confirmed by Study dept. on: 18.12.2023
• Date and time of defence: 03.09.2025 08:30
• Date of electronic submission: 17.07.2025
• Date of submission of printed version: 17.07.2025
• Date of proceeded defence: 03.09.2025
• Opponents: RNDr. Jiří Kaštil, Ph.D.

Guidelines

The work will consist in the characterization of the low temperature magnetic properties of hexaaluminate single crystals: LnMgAl11O19 (Ln=Ce, Gd) and EuAl12O19, in order to evaluate the potential interest of these compounds for magnetic cooling down to millikelvin temperatures. While CeMgAl11O19 and EuAl12O19 are already available, the growth of GdMgAl11O19 will be part of the project. The specific heat and the magnetization of these compounds will be measured down to 0.5K or 0.05K using a dilution refrigerator. The cooling power both upon switching the magnetic field off and upon rotating the single crystal with respect to magnetic field will be evaluated based on these data. Then a test of magnetic cooling will be performed by the end of the project on the most promising of the compounds.

References

[1] Y. Tokiwa et al., Frustrated magnet for adiabatic demagnetization cooling to milli-Kelvin temperatures, Communication materials 2, 42 (2021)
[2] E. C. Koskelo et al., Comparative study of magnetocaloric properties for Gd3+ compounds with different frustrated lattice geometries, PRX Energy 2, 033005 (2023)
[3] M. Ashtar et al., REZnAl11O19 (RE = Pr, Nd, Sm–Tb): a new family of ideal 2D triangular lattice frustrated magnets, J. Mater. Chem. C, 7, 10073 (2019)
[4] M. Balli; S. Jandl; P. Fournier; M. M. Gospodinov, Anisotropy-enhanced giant reversible rotating magnetocaloric effect in HoMn2O5 single crystals, Appl. Phys. Lett. 104, 232402 (2014)
[5] R. Bag et al., Realization of quantum dipoles in triangular lattice crystal Ba3Yb(BO3)3, Phys. Rev. B 104, L220403 (2021)
[6] J. Xiang et al., Dipolar spin liquid ending with quantum critical point in a Gd-based triangular magnet, arXiv preprint arXiv:2301.03571 (2023)

Preliminary scope of work in English

Magnetic materials with weak magnetic interactions can efficiently be used for cooling down to millikelvin temperature based on the principle of adiabatic demagnetization [1, 2]. The sample is cooled under magnetic field to reach its magnetic saturation, then the magnetic field is released implying an increased magnetic entropy and the crystal cools further down. The triangular lattice antiferromagnets LnMgAl11O19 (Ln=Ce, Gd) and EuAl12O19 are promising materials for such applications thanks to their weak magnetic interactions and their magnetic frustration [3]. Moreover CeMgAl11O19 and EuAl12O19 harbor large magnetic anisotropy, thus the magnetic cooling can also be achieved by rotating the crystal with respect to magnetic field. Such rotational magnetocaloric effect are of great interest for energy efficient and compact devices [4]. This master project aims at the evaluation of the interests of these materials for magnetic cooling by evaluating both the cooling power and the lowest achievable temperature. This work has also interest in terms of fundamental research since we expect the possible arise of new quantum magnetic phases due to the combination of exchange and dipolar magnetic interactions with similar magnitudes [5,6].