In most ferromagnets the magnetization rotates from one domain to the next with no preferred handedness. However, broken inversion symmetry can lift the chiral degeneracy, leading to topologically-rich spin textures such as spin-spirals 1,2 and skyrmions 3-5 via the Dzyaloshinskii-Moriya interaction (DMI)6 . Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy metal and an oxide, the DMI stabilizes chiral domain walls (DWs) 2,7 whose spin texture enables extremely efficient current-driven motion [8][9][10][11] . We show that spin torque from the spin Hall effect [12][13][14][15] The Rashba field lacks the correct symmetry to drive DWs directly 16,26,27 , and the spin Hall effect (SHE) in the adjacent heavy metal has emerged as a possible alternative mechanism [12][13][14][15][16]27 . SHE-driven spin accumulation at the heavy-metal/ferromagnet interface generates a Slonczeswki-like torque 16,26,27 strong enough to switch uniformly-magnetized films [12][13][14][15]18 . However, the Bloch DWs expected in typical nanowire geometries [8][9][10][11]28 have their plane oriented perpendicular to the nanowire axis, in which case the Slonczewski-like torque vanishes 16 . This behavior was recently confirmed in asymmetric Pt/Co/Pt stacks in which the SHE-induced torques from the Pt layers did not cancel completely 15 . In that case, currentassisted DW depinning was observed when an applied field rotated the DW plane towards the current axis, but up-down and down-up DWs were driven in opposite directions and the current had no effect in the absence of the bias field. The SHE alone is therefore incapable of uniformly 3 driving trains of DWs in devices, and is insufficient to explain the high spin-torque efficiencies and DW velocities observed in Pt/Co/oxide 8-11 without applied fields.Here we characterize current-induced torques and DW dynamics in out-of-plane magnetized Pt/CoFe/MgO and Ta/CoFe/MgO stacks that are nominally identical except for the heavy-metal underlayers, whose spin Hall angles are large and of opposite sign [12][13][14] . By considering the symmetry of the measured current-induced torque along with the DW dynamics driven by this torque, we uniquely identify the DW configuration as Néel with a fixed chirality.Magnetostatics alone makes this configuration unstable and does not favor one chirality over the other, but the DMI has been theoretically shown to promote chiral Néel DWs 2,7 . By applying inplane magnetic fields, we verify that the DW magnetization aligns rigidly along the nanowire axis, and that the DW spin spiral exhibits a global chirality common to both Pt/CoFe/MgO and Ta/CoFe/MgO. Current-driven DW motion in heavy-metal/ferromagnet/oxide structures is naturally explained by the combination of the SHE, which produces the sole current-induced torque, and the DMI, which stabilizes chiral DWs whose symmetry permits uniform motion with very high efficiency.DW motion was characterized in 500-nm wide, 40-μm long nanowires overlaid with an orthogonal DW nucleation li...

Magnetic storage based on racetrack memory is very promising for the design of ultra-dense, low-cost and low-power storage technology. Information can be coded in a magnetic region between two domain walls or, as predicted recently, in topological magnetic objects known as skyrmions. Here, we show the technological advantages and limitations of using Bloch and Néel skyrmions manipulated by spin current generated within the ferromagnet or via the spin-Hall effect arising from a non-magnetic heavy metal underlayer. We found that the Néel skyrmion moved by the spin-Hall effect is a very promising strategy for technological implementation of the next generation of skyrmion racetrack memories (zero field, high thermal stability, and ultra-dense storage). We employed micromagnetics reinforced with an analytical formulation of skyrmion dynamics that we developed from the Thiele equation. We identified that the excitation, at high currents, of a breathing mode of the skyrmion limits the maximal velocity of the memory.

Current-induced domain wall motion in the presence of the Dzyaloshinskii-Moriya interaction (DMI) is experimentally and theoretically investigated in heavy-metal/ferromagnet bilayers. The angular dependence of the current-induced torque and the magnetization structure of Dzyaloshinskii domain walls are described and quantified simultaneously in the presence of in-plane fields. We show that the DMI strength depends strongly on the heavy metal, varying by a factor of 20 between Ta and Pa, and that strong DMI leads to wall distortions not seen in conventional materials. These findings provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications.

Current-induced domain wall motion along high perpendicular magnetocrystalline anisotropy multilayers is studied by means of full micromagnetic simulations and a one-dimensional model in the presence of in-plane fields. We consider domain wall motion driven by the spin Hall effect in the presence of the Dzyaloshinskii-Moriya interaction (DMI). In the case of relatively weak DMI, the wall propagates without significant tilting of the wall plane, and the full micromagnetic results are quantitatively reproduced by a simple rigid one-dimensional model. By contrast, significant wall-plane tilting is observed in the case of strong DMI, and a one-dimensional description including the wall tilting is required to qualitatively describe the micromagnetic results. However, in this strong-DMI case, the one-dimensional model exhibits significant quantitative discrepancies from the full micromagnetic results, in particular, when high longitudinal fields are applied in the direction of the internal domain wall magnetization. It is also shown that, even under thermal fluctuations and edge roughness, the domain wall develops a net tilting angle during its current-induced motion along samples with strong DMI. V

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