Magnetic fluctuations and spin freezing in nonsuperconducting LiFeAs derivatives

We present detailed magnetometry and muon-spin rotation data on polycrystalline samples of overdoped, nonsuperconducting LiFe1-xNixAs (x = 0.1,0.2) and Li1-yFe1+yAs (0 <= y <= 0.04) as well as superconducting LiFeAs. While LiFe1-xNixAs exhibits weak antiferromagnetic fluctuations down to 1.5 K, Li1-yFe1+yAs 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.