Frustrated Spin-Hexamer Lattices with Enhanced Sub-Kelvin Magnetic Cooling

Significant efforts are being devoted to the exploration of spin–orbit frustrated magnets due to their potential to host novel quantum ground states. This work reports the synthesis, structure, magnetic, and magnetocaloric properties of LiSrRE2(BO3)3 (RE = Nd, Gd–Yb) series crystallizing in a trigonal (P̅3m1) structure, where face-sharing of REO9 polyhedra form spatially confined magnetic RE6 hexamers. The RE6 hexamer connections in the ab-plane constitute a triangular spin-hexamer lattice, interleaved by Sr2+/B3+ nonmagnetic spacers along the c-axis. All LiSrRE2(BO3)3 compounds remain paramagnetic down to 1.8 K and exhibit significant magnetocaloric effects (MCE) in the sub-Kelvin regime. The cooling curves of the Gd and Nd compounds show signatures of cooperative magnetism, whereas the Ho and Yb counterparts exhibit continuous cooling to zero field, with the latter reaching a minimum temperature of 88 mK, demonstrating an excellent magnetic cooling performance. Specific heat measurements down to 0.1 K reveal an antiferromagnetic (AFM) transition at TN = 0.8 K in LiSrGd2(BO3)3, driven by the S = 7/2 spin-only moment. Meanwhile, LiSrNd2(BO3)3 with Jeff = 1/2 moment shows only short-range correlations, with no sign of long-range order or spin freezing, indicating its potential as a quantum spin liquid (QSL) derived from the spin-hexamer lattice.