  Strong hyperfine coupling in a 0.2% Holmium doped LiYF_4 single crystal
induces staircaselike hysteresis loops of the magnetization at very low
temperatures. The field sweep rate dependence of hysteresis loops allows the
study of two different regimes in the magnetic relaxation of these weakly
coupled magnetic moments. At slow field sweep rates, quantum tunneling of the
magnetization occurs at avoided level crossings in the low-energy scheme of a
single ion Ho^3+. At faster sweep rates, nonequilibrated spin-phonon and
spin-spin transitions, mediated by weak dipolar interactions, lead to
magnetization oscillations and additional steps.
