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

Abstract

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 Néel 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 (compressive) 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 nonmonotonically, 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.

Generation of imprinted strain gradients for spintronics

“…In analogy with the conventional field-driven case, the magnetoelastic field can be considered as a force that pushes the DW along the direction of decreasing energy, i.e., increasing compressive strain if λ s > 0 for the in-plane-strain-gradient case. This force is proportional to the local gradient of the spatially variable quantity 18,38,39 , and its effect is essentially that of an effective (magnetoelastic) field…”

“…One of the key challenges with these devices is the control of DWs 14 , typically realized using geometric constraints (notches) [15][16][17] or the local manipulation of the magnetic anisotropy through strain 18,19 using magnetostrictive/piezoelectric systems [20][21][22][23] . However, these approaches are not attractive for most sensor manufacturers due to high cost and complexity.…”

The Combined Effect of Spin-Transfer Torque and Voltage-Controlled Strain Gradient on Magnetic Domain-Wall Dynamics: Toward Tunable Spintronic Neuron

“…Consequently, many external stimuli have been studied that serve as driving forces, such as currents, [15,16] magnetic fields, [17][18][19] electric fields, [20] spin waves, [21] and other physical fields. [22][23][24] Among these, the current-driven DW motion has shown promising features for emulating various biological synaptic and neural characteristics. [25][26][27] Spintronic devices based on magnetic solitons (e.g., DW, skyrmion) with the advantages of high speed and low power consumption have shown great potential for developing artificial neuromorphic computing systems.…”

“…To elucidate the micromagnetic simulation results, the expressions of 𝐷𝑥𝑥 and 𝐹 ME 𝑥 need to be explicitly, then we choose the following ansatz for the Néel type DW: [23]…”

“…This also simply explains the origin of the DW leaky directions which is consistent with the recent work that the tensile strain favors Néel DW for materials with negative magnetoelastic coupling constant. [23] 0 -2 -4 -6 -8 -10 -12 -14 DW Integrated Dynamics with the Combined Effect of STT and VCSG. After proving the DW leaky dynamics solely induced by VCSG, now we focus on the integration process.…”