Magnetic phase diagram of a 2-dimensional triangular lattice antiferromagnet Na₂BaMn(PO₄)₂

We report the magnetic phase transitions of a spin-5/2, 2-dimensional triangular lattice antiferromagnet (AFM) Na₂BaMn(PO₄)₂. From specific heat measurements, we observe two magnetic transitions at temperatures 1.15 and 1.30 K at zero magnetic field. Detailed AC magnetic susceptibility measurements reveal multiple phases including the↑↑↓(up-up-down)-phase between 1.9 and 2.9 T at 47 mK when magnetic field is applied along thecaxis, implying that Na₂BaMn(PO₄)₂is a classical 2dTL Heisenberg AFM with easy-axis anisotropy. However, it deviates from an ideal model as evidenced by a hump region with hysteresis between the↑↑↓andV-phases and weak phase transitions. Our work provides another experimental example to study frustrated magnetism in 2dTL AFM which also serves as a reference to understand the possible quantum spin liquid behavior and anomalous phase diagrams observed in sibling systems Na₂BaM(PO₄)₂(M= Co, Ni).