Successive magnetic transitions in the spin-5 2 easy-axis triangular-lattice antiferromagnet Na2BaMn(PO4)2: A neutron diffraction study

Motivated by the recent observations of various exotic quantum states in the equilateral triangular-lattice phosphates Na2BaCo(PO4)2 with J eff = 1/2 and Na2BaNi(PO4)2 with S = 1, the magnetic properties of spin-5/2 antiferromagnet Na2BaMn(PO4)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 approximate to 1.13 K and T N2 approximate to 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 Mn2+ 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 Na2BaMn(PO4)2 are in line with the expectation for a triangle-lattice antiferromagnet with an easy-axis magnetic anisotropy.