μSR study of spin freezing and persistent spin dynamics in NaCaNi₂F₇

Abstract

A new pyrochlore compound, NaCaNiF, was recently synthesized and has a single magnetic site with spin-1 Ni . 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 Ni 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.

Frustrated Magnetism in Fluoride and Chalcogenide Pyrochlore Lattice Materials

“…In contrast, the cobalt variants agree well with this semiclassical picture where the observed 30% elastic agrees well with the expected 33% for J = 1 /2 [27]. Further evidence for the importance of quantum fluctuations in NaCaNi 2 F 7 comes from µSR measurements, which show the observed static field in the frozen state is an order of magnitude smaller than expected, perhaps signalling that the Ni moments are not fully frozen [33].…”

Structural and spin-glass properties of single crystal J eff = ½ pyrochlore antiferromagnet NaCdCo2F7: correlating T f with magnetic-bond-disorder

“…Despite the apparent spin-glass transition at 4 K, the S = 1 NaCaNi 2 F 7 shows persistent spin dynamics down below 80 mK and a continuum of excitations observed by inelastic neutron scattering suggests a large manifold of low-energy states [21,22]. A similar picture is found in both J eff = ½ NaCaCo 2 F 7 and NaSrCo 2 F 7 , where a short-range-ordered state is selected from a continuous manifold of low-energy states and frozen in at T f = 2.4 K and 3.0 K, respectively [23,24].…”