Magnetic phase diagrams and large magnetocaloric effects of the two-dimensional antiferromagnetic triangular lattice of Gd3+ions in KBaGd(BO3)2

We report a detailed study of the magnetic properties of KBaGd(BO3)2 in which magnetic Gd3+ (S = 7/2) ions form into two-dimensional triangular layers. Magnetization, specific heat, and magnetocaloric effect (MCE) measurements were performed on KBaGd(BO3)2 single crystals. The results show that a long-range antiferro-magnetic state is established below TN = 0.24 K. In zero fields, only about half of the full entropy is released at TN, indicating that not all the magnetic moments are frozen below the ordering temperature, as expected from the geometrical frustration of the triangular spin lattice. Further studies under external fields were performed down to 50 mK and the magnetic phase diagrams are established with magnetic fields applied both within and perpendicular to the triangular plane. KBaGd(BO3)2 serves as an example of a two-dimensional triangular lattice with large spin values (S = 7/2) and can be directly compared with the isostructure KBaR(BO3)2 (R = Dy-Yb) family of doublet ground states, which exhibit effective spins of S = 1/2.