Optimization of the Magnetocaloric Effect in Low Applied Magnetic Fields in LnOHCO₃ Frameworks

This study probes the structure and magnetocaloric effect of the LnOHCO₃ (Ln = Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, and Er³⁺) frameworks. The combination of single crystal X-ray and neutron powder diffraction indicates that these materials solely adopt the P2₁2₁2₁ structure under these synthetic conditions and magnetic susceptibility measurements indicate they remain paramagnetic down to 2 K. We show that the magnetocaloric effects of TbOHCO₃ and DyOHCO₃ have peak entropy changes of 30.99 and 33.34 J kg^-1 K^-1 for a 2-0 T field change, respectively, which are higher than that of the promising GdOHCO₃ framework above 4 K in moderate magnetic fields. The magnetic entropy changes of TbOHCO₃ and DyOHCO₃ above 4 K for smaller than 2 T field changes also exceed those of Gd₃Ga₅O₁₂ and Dy₃Ga₅O₁₂, making them suitable magnetic cooling materials for use at liquid helium temperatures using the low applied magnetic fields accessible using permanent magnets, advantageous for efficient practical cooling devices.