Structural, magnetic, and magnetocaloric properties of triangular-lattice transition-metal phosphates
Chuandi Zhang, Junsen Xiang, Quanliang Zhu, Long‐Fei Wu, Shanfeng Zhang, Juping Xu, Wen Yin, Peijie Sun, Wei Li, Gang Su, Wentao Jin
Abstract
The recent discovery of the spin supersolid candidate ${\mathrm{Na}}_{2}\mathrm{BaCo}({\mathrm{PO}}_{4}{)}_{2}$ 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 ${\mathrm{Na}}_{2}AT({\mathrm{PO}}_{4}{)}_{2}$ ($A$ = Ba,Sr; $T$ = Co,Ni,Mn). X-ray and neutron diffraction measurements confirm that ${\mathrm{Na}}_{2}\mathrm{Ba}T({\mathrm{PO}}_{4}{)}_{2}$ ($\mathrm{NB}T\mathrm{P}$) crystallizes in a trigonal structure, while ${\mathrm{Na}}_{2}\mathrm{Sr}T({\mathrm{PO}}_{4}{)}_{2}$ ($\mathrm{NS}T\mathrm{P}$) forms a monoclinic structure with a slight distortion of the triangular network of ${T}^{2+}$ ions. The dc magnetization data show that all six compounds order antiferromagnetically below 2 K, and the N\'eel temperatures of $\mathrm{NS}T\mathrm{P}$ are consistently higher than those of $\mathrm{NB}T\mathrm{P}$ for $T$ = Co, Ni, and Mn, due to the release of geometrical frustration by monoclinical distortions. Furthermore, magnetocaloric measurements show that trigonal $\mathrm{NB}T\mathrm{P}$ can reach a lower temperature in the quasiadiabatic demagnetization process and thus it demonstrates a better performance in the magnetic refrigeration, compared with monoclinic $\mathrm{NS}T\mathrm{P}$. 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.