A Mn ²⁺ ‐Based Pyrochlore Magnet With Fragile Magnetic Order and Strong Spin Fluctuations

Abstract

ABSTRACT 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 K 3 Mn(MoO 4 ) 2 Cl, a chemically tailored pyrochlore magnet derived from Na 3 Mn(CO 3 ) 2 Cl, where Mn 2+ ions form an ideal 3D frustrated network. Millimeter‐sized single crystals of K 3 Mn(MoO 4 ) 2 Cl 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.