A Mn<sup>2+</sup>-Based Pyrochlore Magnet With Fragile Magnetic Order and Strong Spin Fluctuations.

Lin, Weijie; Li, Zhaoyi; Zhang, Chen; Guo, Ruixin; Xiao, Quan; An, Weiran; Wen, Bo; Wang, Zhentao et al. · Adv Mater · 2026

basic_science · Level V

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