A Spin-5/2 Triangular-Lattice Antiferromagnet Exhibiting Field-Driven Competing Magnetic Phases

Although magnetic systems with large spin quantum numbers are typically associated with classical magnetism, geometrical frustration can stabilize competing spin configurations, resulting in degenerate low-energy states and complex magnetic phases that emerge from the interplay between classical thermal fluctuations and residual quantum fluctuations. Here, a previously unreported S = 5/2 triangular lattice (TL) antiferromagnet, BaMnBe₂(BO₃)₂F₂, has been successfully designed based on a structural template and synthesized via the high-temperature flux method. The structure features magnetic layers built from equilateral TLs, where nearest-neighbor Mn²⁺ ions are separated by 4.63 Å within the ab plane and 7.80 Å along the c-axis. Zero-field specific heat measurements reveal two successive magnetic transitions at 0.57 and 0.72 K. Combined thermodynamic and magnetic susceptibility measurements, along with theoretical calculations, enable us to establish a magnetic phase diagram comprising multiple field-induced competing magnetic phases. Despite the large spin value of Mn²⁺, signatures of residual quantum fluctuations remain evident and influence the stabilization of these competing magnetic states. These results highlight BaMnBe₂(BO₃)₂F₂ as a large spin TL antiferromagnet for investigating frustrated magnetism and emergent quantum spin states.