Magnetic phase diagram of a 2-dimensional triangular lattice antiferromagnet Na 2 BaMn(PO 4 ) 2 .

We report the magnetic phase transitions of a spin-5/2, 2-dimensional triangular lattice antiferromagnet (AFM) Na 2 BaMn(PO 4 ) 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 the c axis, implying that Na 2 BaMn(PO 4 ) 2 is a classical 2 d TL Heisenberg AFM with easy-axis anisotropy. However, it deviates from an ideal model as evidenced by a hump region with hysteresis between the↑↑↓and V -phases and weak phase transitions. Our work provides another experimental example to study frustrated magnetism in 2 d TL AFM which also serves as a reference to understand the possible quantum spin liquid behavior and anomalous phase diagrams observed in sibling systems Na 2 Ba M (PO 4 ) 2 ( M = Co, Ni).