Muon spin rotation studies of spin dynamics at avoided level crossings in LiY0.998Ho0.002F4

We have studied the Ho(3+) spin dynamics for LiY(0.998)Ho(0.002)F(4) via the positive muon (mu(+)) transverse field depolarization rate lambda(TF) as a function of temperature and magnetic field. We find sharp minima in lambda(TF)(H) at fields for which the Ho(3+) ion system has field-induced (avoided) level crossings. The reduction scales with calculated level repulsions, suggesting that mu(+) depolarization by slow fluctuations of nonresonant Ho(3+) spin states is partly suppressed when resonant tunneling opens new fluctuation channels at frequencies much greater than the muon precession frequency.