Exotic quantum phenomena are often found to occur in spin systems that exhibit low-dimensional magnetism. By combining nuclear magnetic resonance, neutron-scattering, and muon-spin spectroscopy ($\mu \mathrm{SR}$) techniques, we report a rare instance of strongly frustrated two-dimensional (2D) magnetism in a three-dimensional (3D) hexagonal perovskite. Here, ${\mathrm{Ba}}_2{\mathrm{La}}_2{\mathrm{MnTe}}_2{\mathrm{O}}_{12}$, a triangular-lattice magnet, is shown to undergo a magnetic transition at $T_{\mathrm{N}}\approx 4.4\mathrm{K}$, below which the manganese moments form a ${120}^{\circ }$ AFM order within the $ab$ plane while staying disordered along the $c$ axis. This exotic ground state, which exhibits ideal 2D magnetism, is highly consistent with the persistently strong spin fluctuations and the large internal field distributions revealed by zero-field $\mu \mathrm{SR}$. Further, the 2D magnetism also leads to a significant frustration, much larger than that of most known magnetically ordered, frustrated systems. Our work on ${\mathrm{Ba}}_2{\mathrm{La}}_2{\mathrm{MnTe}}_2{\mathrm{O}}_{12}$ not only challenges the interpretations of magnetic order in other 3D hexagonal perovskites, but it also provides insight into how the dimensionality affects the exotic magnetic states.

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• Magnetism
• Antiferromagnets
• Muon spin relaxation & rotation
• Neutron scattering
• Nuclear magnetic resonance