Magnetic Domain Dynamics in an Insulating Ferromagnet

Drag effects in domain-wall dynamics in metallic ferromagnets have been observed in a wide range of materials, and traditionally have been attributed to eddy current back-action. Recently, drag effects have been observed in the electrically-insulating disordered Ising ferromagnet, LiHo_xY_1-xF_4, which cannot sustain eddy-currents. It was hypothesized that these drag effects were instead driven by disorder-induced random-field pinning. We test this hypothesis by measuring the statistics and form of magnetic avalanches due to domain-wall motion in the pure compound, LiHoF_4, free from random-field pinning. Following the approach derived from the ABBM model, we examine the average scaled shape of Barkhausen events and test for drag effects by characterizing the skewness. We find that shorter Barkhausen events approach a symmetric lineshape, indicating a drag-free response, whereas longer avalanches do retain a weak drag signature. Furthermore, the dynamics of the system can be tuned by enhancing the quantum tunneling rate via an applied magnetic field transverse to the Ising axis. 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.