Kavli Affiliate: Gang Su
| First 5 Authors: Chuandi Zhang, Junsen Xiang, Cheng Su, Denis Sheptyakov, Xinyang Liu
| Summary:
Motivated by the recent observations of various exotic quantum states in the
equilateral triangular-lattice phosphates Na$_2$BaCo(PO$_4$)$_2$ with
$Jrm_{eff}$ = 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 $Trm_{N1}$ $approx$ 1.13 K and $Trm_{N2}$
$approx$ 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
$Trm_{N1}$ and $Trm_{N2}$, 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.
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