Tunable Quantum Tunneling of Magnetic Domain Walls

Perhaps the most anticipated, yet experimentally elusive, macroscopic quantum phenomenon 1 has been spin tunneling in a ferromagnet 2 , which may be formulated in terms of domain wall tunneling 3,4. One approach is to focus on mesoscopic systems where the number of domain walls is finite and the motion of a single wall has measurable consequences. Research of this type includes magnetotransport measurements on thin ferromagnetic wires 5 and magnetization experiments on single particles 6,7 , nanomagnet ensembles 8-10 , and rare earth multilayers 11. A second method is to investigate macroscopic disordered ferromagnets 12-15 , whose dynamics are dominated by domain wall motion, and search the associated relaxation time distribution functions for quantum effects. Both approaches have revealed clear deviations from thermal relaxation in the form of finite timescales that persist as temperature T approaches zero. But while the classical, thermal processes in these experiments are easily regulated via T, the quantum processes have not been tunable, making definitive interpretation in terms of tunneling difficult. Here we report on a disordered magnetic system for which it is possible to adjust the quantum tunneling probabilities with a knob in the laboratory. We are able to model both the classical, thermally activated response at high 2 temperatures and the athermal, tunneling behavior at low temperatures within a simple, unified framework. Fig. 1a depicts domain wall motion in the classical and quantum limits against the background of a fixed potential landscape, which pins the walls. In the classical case,