Short-range Spin Freezing State in the Double Trillium Lattice Spin-Liquid Candidate KSrFe$_2$(PO$_4$)$_3$ Revealed via $^{31}$P NMR

TL;DR

This study reveals that KSrFe$_2$(PO$_4$)$_3$ exhibits a complex magnetic ground state with spin freezing and persistent fluctuations, challenging previous notions of its magnetic order and highlighting its potential for exploring 3D frustrated magnetism.

Contribution

The paper provides the first detailed NMR and thermodynamic analysis of KSrFe$_2$(PO$_4$)$_3$, demonstrating its unconventional ground state with coexisting spin freezing and fluctuations, distinct from prior assumptions.

Findings

No conventional 3D long-range order observed.

Presence of spin freezing below 3.5 K.

Persistent magnetic fluctuations despite freezing.

Abstract

A comprehensive $^{31}$P nuclear magnetic resonance (NMR) study, combined with thermodynamic measurements and first-principle band-structure calculations, has been conducted to explore the ground state of the $S = 5/2$ double trillium lattice antiferromagnet KSrFe$_2$(PO$_4$)$_3$. Our experimental results indicate that the magnetic ground state is neither a conventional three-dimensional (3D) long-range order (LRO) nor a pure gapless spin-liquid state, as conjectured previously [Boya et al., APL Mater. 10, 101103 (2022)]. Specifically, the observation of a nearly field-independent NMR linewidth below $T^{*}$ = (3.5 $\pm$ 0.4) K, and a significant enhancement of spin-spin relaxation rate $1/T_2$ below $2T^{*}$ (where $T^{*}$ is the characteristic temperature identified from the magnetic susceptibility), indicate a complex magnetic ground state where spin freezing coexists with persistent dynamics. Furthermore, we argue that the lack of magnetic LRO and the persistence of strong magnetic fluctuations in KSrFe$_2$(PO$_4$)$_3$ are unlikely to originate from intersite K/Sr disorder, rather arise due to intrinsic magnetic frustration. Our findings position KSrFe$_2$(PO$_4$)$_3$ into a broader family of geometrically frustrated magnets characterized by coexisting spin freezing and pronounced antiferromagnetic fluctuations, marking it as a promising platform for investigating exotic phenomena in 3D frustrated magnets.