Triangular-lattice systems host a variety of ground states, ranging from quantum spin liquids to magnetically ordered phases, the latter of which can exhibit a sequence of magnetic phase transitions under applied magnetic fields. Here, we report magnetic and thermodynamic measurements, combined with powder and single-crystal neutron diffraction, on a high-spin, nearly isotropic ${\mathrm{Mn}}^{2+}$ triangular-lattice system ${\mathrm{K}}_2\mathrm{Mn}{\left({\mathrm{SeO}}_3\right)}_2$. The compound undergoes long-range magnetic ordering below $T_{\mathrm{N}}\sim 4\mathrm{K}$ in zero field. Contrary to expectations for an ideal Heisenberg system, an up-down-zero (UD0) magnetic structure is favored down to the lowest temperature (0.05 K), rather than the commonly expected Y-type structure. This UD0 state is, however, highly sensitive to external magnetic fields. For fields applied along the $c$ axis, it is readily destabilized and replaced by the Y-type structure, followed by an up-up-down (UUD) phase corresponding to the 1/3 magnetization plateau. In contrast, when the field is applied within the triangular plane, the system evolves into a canted Y state at a higher critical field. These results reveal that weak anisotropy, though small in magnitude, exerts a strongly orientation-dependent influence, playing a key role in selecting the field-induced phases in this frustrated magnet.

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• Antiferromagnetism
• Frustrated magnetism
• Magnetic phase transitions
• Magnetic susceptibility
• Phase diagrams
• Quantum phase transitions
• Specific heat
• 2-dimensional systems
• Antiferromagnets
• Triangular lattice
• Crystal growth
• Neutron diffraction
• Specific heat measurements
• Susceptibility measurements
• X-ray diffraction