1. The James Franck Institute and Department of Physics, The University of Chicago, Chicago, Illinois 60637, USA
2. NEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540, USA

Correspondence to: T. F. Rosenbaum¹ Correspondence and requests for materials should be addressed to T.F.R. (e-mail: Email: t-rosenbaum@uchicago.edu).

Abstract

Perhaps the most anticipated, yet experimentally elusive, macroscopic quantum phenomenon¹ is spin tunnelling in a ferromagnet², which may be formulated 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 consequences. Research of this type includes magnetotransport measurements on thin ferromagnetic wires⁵, and magnetization experiments on single particles^{6, 7}, nanomagnet ensembles^{8, 9, 10} and rare-earth multilayers¹¹. A second method is to investigate macroscopic disordered ferromagnets^{12, 13, 14, 15}, whose dynamics are dominated by domain wall motion, and search the associated relaxation-time distribution functions for the signature of quantum effects. But whereas the classical, thermal processes that operate in these experiments are easily regulated via temperature, the quantum processes have so far not been tunable, making difficult a definitive interpretation of the results in terms of tunnelling. Here we describe a disordered magnetic system for which 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 temperatures within a unified framework, where the domain wall is described as a particle with a fixed mass. We show that it is possible to tune the quantum tunnelling processes by adjusting the 'mass' of this particle with an external magnetic field.