Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-7 2 triangular antiferromagnet KBaGd(BO 3 ) 2 are reported. With the average nearest-neighbor exchange coupling of 44 mK, this compound shows magnetic order below T N = 263 mK in zero field. The ADR tests reach the temperature of T min = 122 mK, more than twice lower than T N , along with the entropy storage capacity of 192 mJ K −1 cm −3 and the hold time of more than 8 h in the PPMS setup, both significantly improved compared to the spin-1 2 Yb 3+ analog. We argue that KBaGd(BO 3 ) 2 shows a balanced interplay of exchange and dipolar couplings that together with structural randomness and geometrical frustration shift T min to well below the ordering temperature T N , therefore facilitating the cooling.

We study single crystals of the magnetic superconductor EuRbFe 4 As 4 by magnetization, electron spin resonance (ESR), angle-resolved photoemission spectroscopy, and electrical resistance in pulsed magnetic fields up to 63 T. The superconducting state below 36.5 K is almost isotropic and is only weakly affected by the development of Eu 2+ magnetic order at 15 K. On the other hand, for the external magnetic field applied along the c axis the temperature dependence of the ESR linewidth reveals a Berezinskii-Kosterlitz-Thouless topological transition below 15 K. This indicates that Eu 2+ planes are a good realization of a two-dimensional XY magnet, which reflects the decoupling of the Eu 2+ magnetic moments from superconducting FeAs layers.

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