Nuclear Spin Driven Quantum Relaxation inLiY0.998Ho0.002F4

Staircaselike hysteresis loops of the magnetization of a ${\mathrm{LiY}}_{0.998}{\mathrm{Ho}}_{0.002}{\mathrm{F}}_4$ single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single ${\mathrm{Ho}}^{3+}$ ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.