Current‐Induced Skyrmion Generation through Morphological Thermal Transitions in Chiral Ferromagnetic Heterostructures

Abstract

anisotropy (PMA). In ultrathin films, skyrmions can exhibit sub-nanometer scale size [8][9][10][11] and move in response to an applied current with velocities exceeding 100 m s -1 [5] in a controllable [12,13] and reliable [13] way. Therefore, they promise great technological utility for racetracktype memories, [14] logic gates, [15] probabilistic computing, [16] and neuromorphic devices, [17] for which they have to be readily created and manipulated. Homochiral skyrmions can be stabilized by the Dzyaloshinkii-Moriya interaction (DMI) [18,19] in materials with strong spin-orbit coupling and broken inversion symmetry. Since asymmetric multilayer stacks of a ferromagnet and a heavy metal [5][6][7] possess DMI and can also exhibit large current-induced spin-orbit torques that can provide an efficient means to create and manipulate skyrmions, [20][21][22] these systems are now a central focus of current research. Magnetic skyrmions can exist as isolated topological excitations, [23,24] or as ordered arrays (hexagonal lattice) comprising the magnetic ground state, [2,5] depending on material and Magnetic skyrmions promise breakthroughs in future memory and computing devices due to their inherent stability and small size. Their creation and current driven motion have been recently observed at room temperature, but the key mechanisms of their formation are not yet well-understood. Here it is shown that in heavy metal/ferromagnet heterostructures, pulsed currents can drive morphological transitions between labyrinth-like, stripe-like, and skyrmionic states. Using high-resolution X-ray microscopy, the spin texture evolution with temperature and magnetic field is imaged and it is demonstrated that with transient Joule heating, topological charges can be injected into the system, driving it across the stripe-skyrmion boundary. The observations are explained through atomistic spin dynamic and micromagnetic simulations that reveal a crossover to a global skyrmionic ground state above a threshold magnetic field, which is found to decrease with increasing temperature. It is demonstrated how by tuning the phase stability, one can reliably generate skyrmions by short current pulses and stabilize them at zero field, providing new means to create and manipulate spin textures in engineered chiral ferromagnets. Magnetic SkyrmionsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Individual skyrmion manipulation by local magnetic field gradients

“…On the other hand, lower estimations of D s in these systems have been extracted through Brillouin Light Scattering (BLS) measurements 37,38 . Regarding the Pt/CoFeB/MgO multilayer our measured D s is lower than what observed in similar multilayers through measurements of equilibrium domain patterns 18,25,27 , but in good agreement with BLS experiments performed on individual layers [38][39][40] . In any case, only for W/CoFeB/MgO and Pt/CoFeB/MgO multilayers the DMI magnitude is significantly larger than the critical amount required to stabilise Néel domain walls 29 , with |D c | = 2ln(2)µ 0 M 2 S t/π 2 being 0.18 mJ/m 2 , 0.15 mJ/m 2 , and 0.33 mJ/m 2 for Ta, W and Pt-based stacks, respectively.…”

“…While MFM is well established as a tool to measure DMI and investigate nanoscale magnetic skyrmions in Co-based multilayer stacks 17,42,[44][45][46] , similar studies for CoFeB-based systems have involved using less widely available techniques, such as Scanning Transmission X-ray Microscopy (STXM) 16,18,26,27 , Magnetic Transmission soft X-ray Microscopy (MTXM) 16,19 or X-ray holography 25 . MFM imaging of soft magnetic materials like CoFeB can actually be very challenging due to image deformations caused by the probe-sample magnetic interaction.…”

Thermal nucleation and high-resolution imaging of submicrometer magnetic bubbles in thin thulium iron garnet films with perpendicular anisotropy

“…(Note that the field was applied via rotating permanent magnets with > 200 mT saturation field, possibly resulting in in-plane and out-of-plane field offsets of a few mT, which may have been responsible for the preferred in-plane orientation of the stripe domains). Bubble domains are not observed here, which is not surprising, since at zero field, the parallel stripe phase is lower in energy than the bubble domain phase [23,24]. Transformation to a bubble phase would require overcoming sizable energy barriers, which is not expected during a quasistatic (adiabatic) increase in the applied field [23,24].…”

“…This contrasts with the high-remanence out-of-plane loops for thinner TmIG films [8,9], pointing to stronger stray field interactions in these thicker films that promote a demagnetized state. The out-of-plane loop exhibits hysteresis near saturation, suggesting the presence of metastable states which can be transformed into a bubble ground state [22][23][24]. Finally, the small out-of-plane remanence and coercivity <1 mT suggest that the films have very low pinning.…”

Stripe domains reorientation in ferromagnetic films with perpendicular magnetic anisotropy

“…logic circuits, racetrack memories, sensors and rf devices [58][59][60][61]. Different methods have been pursued for the generation and controlled propagation, including magnetic field, electric current, electric field or mechanical stress [56,59,[62][63][64][65][66]. In some cases, the geometry and local asymmetries of stripe domain patterns have been used to guide the propagation of skyrmions or bubbles [66,67].…”