Unusual spin frozen state in a frustrated pyrochlore system NaCaCo₂F₇ as observed by NMR

We present ²³Na -and ¹⁹F NMR results on the magnetically frustrated pyrochlore NaCaCo₂F₇ with a frustration index of f = θ_CW/T_f ∝ 56. Recent neutron scattering experiments proposed XY like antiferromagnetic spin clusters at low energies in NaCaCo₂F₇. ²³Na NMR -spectra reveal the presence of two magnetically non equivalent Na sites in conjunction with the local Co²⁺ spin structure. Below 3.6 K both the ²³Na -and ¹⁹F spectra broaden due to the formation of static spin correlations. A huge reduction of the ¹⁹F -and ²³Na NMR signal intensity hints at a quasi-static field distribution in NaCaCo₂F₇ in this regime. The ¹⁹F spin-lattice relaxation rate ¹⁹(1/T₁) exhibits a peak at around 2.9 K, at the same temperature range where ac and dc susceptibility data show a broad maximum. The character of the spin fluctuation appears to be isotropic. The overall temperature dependence of ¹⁹(1/T₁) can be described by the BPP theory considering a fluctuating hyperfine field with an autocorrelation function. The correlation time of the autocorrelation function exhibits an activation behavior further indicating the spin-frozen state. While the present NMR studies suggest the spin frozen state at low temperatures, μSR investigations however reveal the presence of so called persistent spin dynamics down to 20 mK implying an exotic ground state in NaCaCo₂F₇.