Successive magnetic transitions in the spin-52 easy-axis triangular-lattice antiferromagnet Na2BaMn(PO4)2: A neutron diffraction study

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