A Mn²⁺-Based Pyrochlore Magnet With Fragile Magnetic Order and Strong Spin Fluctuations

Pyrochlore magnets, built from corner-sharing tetrahedra, serve as magnetic frameworks that attract extensive attention due to their exotic quantum spin states. Here we present K₃Mn(MoO₄)₂Cl, a chemically tailored pyrochlore magnet derived from Na₃Mn(CO₃)₂Cl, where Mn²⁺ ions form an ideal 3D frustrated network. Millimeter-sized single crystals of K₃Mn(MoO₄)₂Cl were successfully achieved via a simple high-temperature self-flux method. Thermodynamic measurements reveal a sharp transition at 258 mK, yet only ∼15% of the expected Rln6 entropy is recovered, indicating pronounced spin fluctuations in this long-range ordered state. Remarkably, the ordered state is highly sensitive to perturbations, disappearing under a magnetic field of about 0.2 T, while magnetization along [111] shows a feature consistent with a near half-saturated state. Our results establish the newly discovered pyrochlore magnets as a chemically engineered pyrochlore platform, in which geometrical frustration and fragile magnetic order interplay, thereby offering new opportunities to explore emergent magnetic phenomena in three dimensions.