Magnetic domain dynamics in an insulating quantum ferromagnet

The structure of free-energy manifolds with many local minima are of great interest in the context of quantum computation and optimization. In the disordered ferromagnet, LiHo_xY_1-xF_4, quenched disorder and frustration, driven by the combination of chemical substitution and the anisotropic magnetic dipolar coupling, give rise to one such example of a complex, traversable free-energy landscape. We probe the structure of this landscape and the dynamics of the driven system by measuring the statistics and form of magnetic avalanches due to domain-wall motion. For large avalanches at temperatures approaching the Curie point, we find a response free of the signatures of drag effects. By contrast, in the low temperature limit, drag effects contribute to the dynamics, which we attribute to enhanced pinning from local random fields due to the disorder. Furthermore, as an applied transverse magnetic field is increased, the dynamics of the system can be tuned by progressively strengthening the random-field pinning and then increasing the quantum tunneling rate. Tuning from a classical regime to a quantum regime dominated by tunneling reveals the structure of the free-energy surface and barriers to domain reorientation, with applications to quantum annealing.