Magnetic fluctuations and spin freezing in nonsuperconducting LiFeAs derivatives

Abstract

We present detailed magnetometry and muon-spin rotation data on polycrystalline samples of overdoped, nonsuperconducting LiFe${}_{1\ensuremath{-}x}$Ni${}_{x}$As ($x=0.1,0.2$) and Li${}_{1\ensuremath{-}y}$Fe${}_{1+y}$As ($0\ensuremath{\le}y\ensuremath{\le}0.04$) as well as superconducting LiFeAs. While LiFe${}_{1\ensuremath{-}x}$Ni${}_{x}$As exhibits weak antiferromagnetic fluctuations down to $1.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, Li${}_{1\ensuremath{-}y}$Fe${}_{1+y}$As samples, which have a much smaller deviation from the $1:1:1$ stoichiometry, show a crossover from ferromagnetic to antiferromagnetic fluctuations on cooling and a freezing of dynamically fluctuating moments at low temperatures. We do not find any signatures of time-reversal symmetry breaking in stoichiometric LiFeAs that would support recent predictions of triplet pairing.