Successive magnetic transitions in the spin-5/2 easy-axis triangular-lattice antiferromagnet Na${}_2$BaMn(PO${}_4$)${}_2$: A neutron diffraction study

Abstract

Motivated by the recent observations of various exotic quantum states in the equilateral triangular-lattice phosphates Na${}_2$BaCo(PO${}_4$)${}_2$ with $J{}_{\mathrm{eff}}$ = 1/2 and Na${}_2$BaNi(PO${}_4$)${}_2$ with $S$ = 1, the magnetic properties of spin-5/2 antiferromagnet Na${}_2$BaMn(PO${}_4$)${}_2$, their classical counterpart, are comprehensively investigated experimentally. DC magnetization and specific heat measurements on polycrystalline samples indicate two successive magnetic transitions at $T{}_{N1}$ $\approx$ 1.13 K and $T{}_{N2}$ $\approx$ 1.28 K, respectively. Zero-field neutron powder diffraction measurement at 67 mK reveals a Y-like spin configuration as its ground-state magnetic structure, with both the $ab$-plane and $c$-axis components of the Mn${}^{2+}$ moments long-range ordered. The incommensurate magnetic propagation vector $k$ shows a dramatic change for the intermediate phase between $T{}_{N1}$ and $T{}_{N2}$, in which the spin state is speculated to change into a collinear structure with only the $c$-axis moments ordered, as stabilized by thermal fluctuations. The successive magnetic transitions observed in Na${}_2$BaMn(PO${}_4$)${}_2$ are in line with the expectation for a triangle-lattice antiferromagnet with an easy-axis magnetic anisotropy.

8 Pages, 6 figures