Inhomogeneous dynamic state in the double trillium lattice antiferromagnet KBaFe$_2$(PO$_4$)$_3$

S. J. Sebastian; S. S. Islam; R. Kolay; S. Mohanty; Q. P. Ding; Y. Skourski; J. Sichelschmidt; M. Baenitz; Jonas A. Krieger; T. J. Hicken; H. Luetkens; A. A. Tsirlin; Y. Furukawa; R. Nath

arXiv · 2511.07844

Inhomogeneous dynamic state in the double trillium lattice antiferromagnet KBaFe$_2$(PO$_4$)$_3$

S. J. Sebastian·S. S. Islam·R. Kolay·S. Mohanty·Q. P. Ding·Y. Skourski·J. Sichelschmidt·M. Baenitz·Jonas A. Krieger·T. J. Hicken·H. Luetkens·A. A. Tsirlin·Y. Furukawa·R. Nath

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Abstract

The three-dimensional (3D) magnet KBaFe$_2$(PO$_4$)$_3$ hosts a double-trillium lattice of Fe$^{3+}$ (spin, $S=5/2$) ions offering a prototypical platform to study the frustration induced effects in 3D. Through magnetization, specific heat, $^{31}$P nuclear magnetic resonance (NMR), and muon spin relaxation ($\mu$SR) experiments, supported by first principles calculations, we uncover an unconventional ground state. Despite strong antiferromagnetic interactions with a large Curie-Weiss temperature $\theta_{\rm CW} = -70(2)$ K, no magnetic long-range order is observed down to 30 mK. Below $T^{\ast}\simeq 3.5$ K, the NMR linewidth becomes nearly field-independent and the spin-spin relaxation rate $1/T_2$ saturates, accompanied by an inhomogeneous distribution of transverse nuclear magnetization $M_{xy}$. The latter indicates the emergence of short-range dynamical correlations, which was further corroborated by a robust and field-insensitive broad maximum in specific heat. In $\mu$SR, we detect neither a static internal field nor spin freezing; instead the relaxation remains dynamic and is best described by two coexisting dynamic relaxation channels: a dominant fast (sporadic) channel and a slower Markovian component. Their differing weights and fluctuation rates suggest microscopic inhomogeneity in spin dynamics. Altogether, KBaFe$_2$(PO$_4$)$_3$ exemplifies a rare high-spin stochiometric 3D antiferromagnet that evades ordering and instead fosters a mosaic of spin dynamics driven by strong geometric frustration intrinsic to the trillium lattice.

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S. J. Sebastian, S. S. Islam, R. Kolay, S. Mohanty, Q. P. Ding, Y. Skourski, J. Sichelschmidt, M. Baenitz, Jonas A. Krieger, T. J. Hicken, H. Luetkens, A. A. Tsirlin, Y. Furukawa, R. Nath. 2025-11-11. Inhomogeneous dynamic state in the double trillium lattice antiferromagnet KBaFe$_2$(PO$_4$)$_3$. https://arxiv.org/abs/2511.07844

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