Successive magnetic transitions in the spin- 5 2 easy-axis triangular-lattice antiferromagnet Na 2 BaMn ( PO 4 ) 2 : A neutron diffraction study

Motivated by the recent observations of various exotic quantum states in the equilateral triangular-lattice phosphates ${\mathrm{Na}}₂\mathrm{BaCo}{({\mathrm{PO}}₄)}₂$ with ${J}_{\mathrm{eff}}=1/2$ and ${\mathrm{Na}}₂\mathrm{BaNi}{({\mathrm{PO}}₄)}₂$ with $S=1$, the magnetic properties of spin-5/2 antiferromagnet ${\mathrm{Na}}₂\mathrm{BaMn}{({\mathrm{PO}}₄)}₂$, their classical counterpart, are comprehensively investigated experimentally. DC magnetization and specific heat measurements on polycrystalline samples indicate two successive magnetic transitions at ${T}_{\mathrm{N}1}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}1.13$ K and ${T}_{\mathrm{N}2}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}1.28$ 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 ${\mathrm{Mn}}²⁺$ moments long-range ordered. The incommensurate magnetic propagation vector $k$ shows a dramatic change for the intermediate phase between ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$, 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 ${\mathrm{Na}}₂\mathrm{BaMn}{({\mathrm{PO}}₄)}₂$ are in line with the expectation for a triangle-lattice antiferromagnet with an easy-axis magnetic anisotropy.