  Single-molecule magnets (SMMs) are promising elements for quantum
informatics. In the presence of strong magnetic anisotropy, they exhibit
magnetization blocking - a magnetic memory effect at the level of a single
molecule. Recent studies have shown that the SMM performance scales with the
height of magnetization blocking barrier. By employing molecular engineering
this can be significantly modified, remaining independent from other external
factors such as magnetic field. Taking advantage of hyperfine coupling of
electronic and nuclear spins further enhances their functionality, however, a
poor understanding of relaxation mechanisms in such SMMs limits the
exploitation of nuclear-spin molecular qubits. Here we report the opening
discovery of field-dependent oscillation of the magnetization blocking barrier
in a new holmium metallacrown magnet driven by the switch of relaxation
mechanisms involving hyperfine interaction. Single-crystal magnetic hysteresis
measurements combined with first-principles calculations reveal an activated
temperature dependence of magnetic relaxation dominated either by incoherent
quantum tunneling of magnetization at anti-crossing points of
exchange-hyperfine states or by Orbach-like processes at crossing points. We
demonstrate that these relaxation mechanisms can be consecutively switched on
and off by increasing the external field, which paves a way for manipulating
the magnetization dynamics of SMMs using hyperfine interaction.
