$\mu$SR study of spin freezing and persistent spin dynamics in NaCaNi$_2$F$_7$

TL;DR

This study investigates the magnetic properties of the newly synthesized pyrochlore NaCaNi$_2$F$_7$ using muon spin rotation, revealing spin freezing below 4 K and persistent spin dynamics down to 75 mK, indicative of geometrical frustration.

Contribution

The paper provides the first muon spin rotation analysis of NaCaNi$_2$F$_7$, demonstrating spin freezing and persistent dynamics in a spin-1 pyrochlore with potential for frustrated magnetism.

Findings

Spin freezing occurs below 4 K with a 140 G internal field.

Persistent spin dynamics are observed down to 75 mK.

Muon site is likely between fluorine ions, off-centre.

Abstract

A new pyrochlore compound, NaCaNi$_2$F$_7$, was recently synthesized and has a single magnetic site with spin-1 Ni$^{2+}$. We present zero field (ZF) and longitudinal field (LF) muon spin rotation ($\mu$SR) measurements on this pyrochlore. Density functional theory (DFT) calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-$\mu$-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni$^{2+}$ 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.