ABSTRACT

The recent discovery of the spin supersolid candidate Na_2⁢BaCo⁢(PO₄)₂ has stimulated a great deal of research on triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic, and magnetocaloric properties of polycrystalline Na_2⁢𝐴⁢𝑇⁢(PO₄)₂ (𝐴 = Ba,Sr; 𝑇 = Co,Ni,Mn). X-ray and neutron diffraction measurements confirm that Na₂⁢Ba⁢𝑇⁢(PO₄)₂ (NB⁢𝑇⁢P) crystallizes in a trigonal structure, while Na₂⁢Sr⁢𝑇⁢(PO₄)₂ (NS⁢𝑇⁢P) forms a monoclinic structure with a slight distortion of the triangular network of 𝑇²⁺ ions. The dc magnetization data show that all six compounds order antiferromagnetically below 2 K, and the Néel temperatures of NS⁢𝑇⁢P are consistently higher than those of NB⁢𝑇⁢P for 𝑇 = Co, Ni, and Mn, due to the release of geometrical frustration by monoclinical distortions. Furthermore, magnetocaloric measurements show that trigonal NB⁢𝑇⁢P can reach a lower temperature in the quasiadiabatic demagnetization process and thus it demonstrates a better performance in the magnetic refrigeration, compared with monoclinic NS⁢𝑇⁢P. Our findings highlight the outstanding magnetocaloric performances of the trigonal transition-metal phosphates and disclose two necessary ingredients for a superior magnetic coolant that can reach an ultralow temperature, including a perfect geometrically frustrated lattice and a small effective spin number associated with the magnetic ions.