Onset of magnetic correlations in LiY1-xHoxF4 with 0.002 ≤ x ≤ 0.05 studied via μSR

Abstract

We are using μSR to probe the evolution of magnetism from single-ion to spin glass behaviour in LiY1-xHoxF4 with increasing x. For x = 0.002 the Ho spin dynamics exhibit single-ion behaviour similar to that observed in molecular magnets. Near T≈ 10 K, the energy gap between the Ho ground state electronic doublet and first excited singlet energy levels, we find that the longitudinal depolarization rate exhibits a peak that depends on the Larmor frequency. At temperatures T << 10 K we find sharp reductions in the transverse field depolarization rates in the vicinity of magnetic field induced (avoided) level crossings (ALCs) in the hyperfine-split ground state energy manifold. This is attributed to a reduction in the quasistatic magnetic disorder due to resonant Ho spin fluctuations at frequencies much larger than the muon Larmor frequency. As the Ho concentration is increased toward x = 0.05, magnetic correlations are observed to become significant, changing the 10 K peak into a plateau, and significantly broadening the rate changes observed at ALCs.