μsR study of spin freezing and persistent spin dynamics in NaCaNi₂F₇

A new pyrochlore compound, NaCaNi₂F₇, was recently synthesized and has a single magnetic site with spin-1 Ni²⁺. We present zero field and longitudinal field muon spin rotation (μSR) measurements on this pyrochlore. Density functional theory calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-μ-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni²⁺ spins undergo spin freezing into a disordered ground state below 4 K, with a characteristic internal field strength of 140 G. Persistent Ni spin dynamics are present to our lowest temperatures (75 mK), a feature characteristic of many geometrically frustrated magnetic systems.