Absence of Walker Breakdown in the Dynamics of Chiral Neel Domain Walls Driven by In-Plane Strain Gradients

The influence of mechanical strain on the static and dynamic properties of chiral domain walls (DWs) in perpendicularly magnetized strips is investigated using micromagnetic simulations and a one-dimensional model. While a uniform strain allows one to reversibly switch the domain-wall configuration at rest between Bloch and Neel patterns, strain gradients are suggested as an energy-sustainable means to drive domain-wall motion without the need for magnetic fields or electrical currents. It is shown that an in-plane strain gradient creates a force on a domain wall that drives it towards a region of higher tensile (compres-sive) strain for materials with positive (negative) magnetostriction. Moreover, due to the dependence of the domain-wall internal energy on the in-plane strain, a damping torque proportional to the local strain arises during motion that opposes the precessional torque due to the driving force, which is proportional to the strain gradient. After a transient period, where both the internal DW angle and the velocity change non -monotonically, reaching their maximum values asynchronously, the two torques balance each other. This compensation prevents the onset of turbulent domain-wall dynamics, and steady domain-wall motion with a constant velocity is asymptotically reached for an arbitrarily large strain gradient. Despite this complex dynamics, our work shows that average domain-wall velocities in the range of 500 m/s can be obtained using voltage-induced strain in piezoelectric/ferromagnetic devices under realistic conditions.