Structural, magnetic and magnetocaloric properties of triangular-lattice transition-metal phosphates

Abstract

The recent discovery of the spin supersolid candidate Na${}_2$BaCo(PO${}_4$)${}_2$ stimulates numerous research interest on the triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic and magnetocaloric properties of polycrystalline Na${}_2$$A$$T$(PO${}_4$)${}_2$ ($A$ = Ba, Sr; $T$ = Co, Ni, Mn). X-ray and neutron diffraction measurements confirm that Na${}_2$Ba$T$(PO${}_4$)${}_2$ (NB$T$P) crystallizes in a trigonal structure, while Na${}_2$Sr$T$(PO${}_4$)${}_2$ (NS$T$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 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 monoclinic distortions. Further magnetocaloric measurements show that trigonal NB$T$P can reach a lower temperature in the quasi-adiabatic demagnetization process and thus shows 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 ultra-low temperature, including a perfect geometrically frustrated lattice and a small effective spin number associated with the magnetic ions.

10 Pages, 6 figures, accepted for publication in Physical Review Materials

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Cited by in corpus (7)

• Double Magnon-Roton Excitations in the Triangular-Lattice Spin Supersolid
• Zeeman split Kramers doublets in spin-supersolid candidate Na${}_2$BaCo(PO${}_4$)${}_2$
• NMR study of supersolid phases in the triangular-lattice antiferromagnet Na2BaCo(PO4)2
• Successive magnetic transitions in the spin-5/2 easy-axis triangular-lattice antiferromagnet Na${}_2$BaMn(PO${}_4$)${}_2$: A neutron diffraction study
• Antiferromagnetic ordering and critical behavior induced giant magnetocaloric effect in distorted kagome lattice Gd${}_3$BWO${}_9$
• Giant Magnetocaloric Effect in a Honeycomb Spiral Spin-Liquid Candidate
• Spin structures and phase diagrams of the spin-$\frac{5}{2}$ triangular-lattice antiferromagnet Na${}_2$BaMn(PO${}_4$)${}_2$ under magnetic field