The interfacial Dzyaloshinskii-Moriya Interaction (DMI) plays a crucial role in chiral domain wall (DW) motion, favoring fast DW velocities. We explore the effect of interface disorder on DMI and DW dynamics in perpendicular magnetized Ta/CoFeB/MgO thin films. Light He + irradiation has been used to gently engineer interface intermixing on a scale of 0.1 nm. We demonstrate that a slight modification of the Ta/CoFeB interface leads to an increase of the DMI value accompanied by an enhancement of DW velocity in the flow regime. Using micromagnetic simulations based on granular structures, we show that the enhancement of DW velocity is mainly related to an increase in the distribution of magnetic parameters related to the interface. We further infer that the DMI modulation is related to the asymmetric disorder induced by irradiation leading to alloying with the Ta buffer layer. Understanding the role of disorder is therefore crucial for the design of future devices where post-growth interface alloying can be used to finely tune the DMI.
Local perturbations in the relative orientation of the magnetic moments in a continuous magnetic system can propagate in the form of waves. These so-called spin waves represent a promising candidate as an information carrier for spin-based low-power applications. A localized, energy-efficient excitation of coherent and short-wavelength spin waves is a crucial technological requirement, and alternatives to excitation via the Oersted field of an alternating current must be explored. Here, we show how a domain wall pinned at a geometrical constriction in a perpendicularly magnetized thin nanowire emits spin waves when forced to rotate by the application of a low direct current flowing along the wire. Spin waves are excited by the in-plane stray field of the rotating domain wall and propagate at an odd harmonic of the domain wall rotation frequency in the direction of the electron’s flow. The application of an external field, opposing domain wall depinning induced by the current, breaks the symmetry for spin wave propagation in the two domains, allowing emission in both directions but at different frequencies. The results presented define a new approach to manufacture tuneable high-frequency spin wave emitters of easy fabrication and low power consumption.
The domain wall depinning field represents the minimum magnetic field needed to move a domain wall, typically pinned by samples' disorder or patterned constrictions. Conventionally, such field is considered independent on the Gilbert damping since it is assumed to be the field at which the Zeeman energy equals the pinning energy barrier (both damping independent). Here, we analyse numerically the domain wall depinning field as function of the Gilbert damping in a system with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Contrary to expectations, we find that the depinning field depends on the Gilbert damping and that it strongly decreases for small damping parameters. We explain this dependence with a simple one-dimensional model and we show that the reduction of the depinning field is related to the finite size of the pinning barriers and to the domain wall internal dynamics, connected to the Dzyaloshinskii-Moriya interaction and the shape anisotropy.
We develop and analyze an advanced concept for a domain-wall-based sensing of rotations. Moving domain walls in n closed loops with n − 1 intersecting convolutions by rotating fields, we are able to sense n rotations. By combining loops with coprime numbers of rotations, we create a sensor system allowing for the total counting of millions of turns of a rotating applied magnetic field. We analyze the operation of the sensor and identify the intersecting cross structures as the critical component for reliable operation. Specifically, depending on the orientation of the applied field angle with the magnetization in the branches of the cross, a domain wall is found to propagate in an unwanted direction, yielding failures and counting errors in the device. To overcome this limiting factor, we introduce a specially designed syphon structure to the controlled pinning of the domain wall before the cross and depinning and propagation only for a selected range of applied field angles. By adjusting the syphon and the cross geometry, we find that the optimized combination of both structures prevents failures in the full sensor structure yielding robust operation. Our modeling results show that the optimized element geometry allows for the realization of the sensor with cross-shaped intersections and an operation that is tolerant to inaccuracies of the fabrication.
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