Kavli Affiliate: Gang Su
| First 5 Authors: Chuandi Zhang, Junsen Xiang, Quanliang Zhu, Longfei Wu, Shanfeng Zhang
| Summary:
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 N'{e}el 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.
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