Tunable quantum tunnelling of magnetic domain walls

Perhaps the most anticipated, yet experimentally elusive, macroscopic quant um phenomenon(1) is spin tunnelling in a ferromagnet(2), which may be formu lated in terms of domain wall tunnelling(3,4). One approach to identifying such a process is to focus on mesoscopic systems where the number of domain walls is finite and the motion of a single wall has measurable consequence s. Research of this type includes magnetotransport measurements on thin fer romagnetic wires(5), and magnetization experiments on single particles(6,7) , nanomagnet ensembles(8-10) and rare-earth multilayers(11). A second metho d is to investigate macroscopic disordered ferromagnets(12-15), whose dynam ics are dominated by domain wall motion, and search the associated relaxati on-time distribution functions for the signature of quantum effects. But wh ereas the classical, thermal processes that operate in these experiments ar e easily regulated via temperature, the quantum processes have so far not b een tunable, making difficult a definitive interpretation of the results in terms of tunnelling. Here we describe a disordered magnetic system for whi ch it is possible to adjust the quantum tunnelling probabilities. For this material, we can model both the classical, thermally activated response at high temperatures and the athermal, tunnelling behaviour at low temperature s within a unified framework, where the domain wall is described as a parti cle with a fixed mass. We show that it is possible to tune the quantum tunn elling processes by adjusting the 'mass' of this particle with an external magnetic field.