Dynamical magnetism in the disordered cubic lattice material $\gamma$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$

TL;DR

This study investigates a disordered cubic lattice material exhibiting persistent spin dynamics and short-range correlations without static order, highlighting the role of quantum fluctuations and dilution near the percolation threshold.

Contribution

It provides experimental and theoretical evidence for a disorder-driven dynamical magnetic state in a three-dimensional quantum spin system near the percolation threshold.

Findings

No static magnetic order or spin-glass freezing observed.

Broad magnetic specific-heat anomaly consistent with a distribution of spin environments.

Monte Carlo and exact diagonalization support coexistence of weakly and strongly correlated spins.

Abstract

$\gamma$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$ realizes a disordered simple-cubic spin-$1/2$ lattice in which Co$^{2+}$ ions randomly occupy one third of the sites, placing the system close to the site-percolation threshold for magnetic order. Specific-heat, susceptibility, neutron spin-echo, and muon spin-rotation measurements reveal a broad thermodynamic crossover, short-range magnetic correlations, and persistent fast spin dynamics down to at least 0.1~K, with no evidence for static order or conventional spin-glass freezing. Monte Carlo simulations yield a broad distribution of orphan spins, finite clusters, and an infinite network. The calculated orphan-spin fraction ($\approx 8.8\%$) agrees well with the weakly correlated spin fraction inferred from magnetization ($\approx 8.2\%$). Exact diagonalization of a diluted $S = 1/2$ Heisenberg model captures the broad magnetic specific-heat anomaly and supports the coexistence of weakly and strongly correlated spin environments. These results support a picture in which spin-$1/2$ quantum fluctuations, together with dilution and proximity to the percolation threshold, can support a disorder-driven dynamical state with short-range correlations in three dimensions, distinct from both classical spin glasses and geometrically frustrated quantum spin liquids.