Universal chiral-triggered magnetization switching in confined nanodots

Abstract

Spin orbit interactions are rapidly emerging as the key for enabling efficient current-controlled spintronic devices. Much work has focused on the role of spin-orbit coupling at heavy metal/ferromagnet interfaces in generating current-induced spin-orbit torques. However, the strong influence of the spin-orbit-derived Dzyaloshinskii-Moriya interaction (DMI) on spin textures in these materials is now becoming apparent. Recent reports suggest DMI-stabilized homochiral domain walls (DWs) can be driven with high efficiency by spin torque from the spin Hall effect. However, the influence of the DMI on the current-induced magnetization switching has not been explored nor is yet well-understood, due in part to the difficulty of disentangling spin torques and spin textures in nano-sized confined samples. Here we study the magnetization reversal of perpendicular magnetized ultrathin dots, and show that the switching mechanism is strongly influenced by the DMI, which promotes a universal chiral non-uniform reversal, even for small samples at the nanoscale. We show that ultrafast current-induced and field-induced magnetization switching consists on local magnetization reversal with domain wall nucleation followed by its propagation along the sample. These findings, not seen in conventional materials, provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications.

Spatially and time-resolved magnetization dynamics driven by spin–orbit torques

“…Furthermore, as we argue in the following, such a reversal scheme is unique to SOTs. Our data show that domain nucleation takes place at the edge of the sample where the DMI and B x concur to tilt the magnetization towards the current direction, thereby confirming the prediction of recent micromagnetic models 5,39 . However, the concerted action of B DL , DMI, and B x only leads to a left/right asymmetry, similar to the asymmetric nucleation induced by a perpendicular magnetic field 41 , whereas we observe that the domain nucleation site alternates between the four quadrants shown in Fig.…”

“…However, the tilt angle in Fig. 3 is opposite to that predicted by micromagnetic models of Pt/Co heterostructures 30,33,39 and observed by MOKE microscopy in Pt/Co/Ni/Co racetracks 43 . More specifically, the angle between the DW normal and the current direction is ≈ −45 • for a left-handed up-down DW (↑←↓) at positive current (see panel IV in Fig.…”

“…A second model is based on the random nucleation and isotropic expansion of magnetic bubbles, induced by a thermally-assisted DW depinning process and the DL component of the SOT 36 . Finally, micromagnetic models have been proposed, in which domain nucleation is either random and thermally-assisted 32,37 or determined by the competition between external field and DMI at the sample edge followed by DW propagation across the magnetic layer 5,39 . In such schemes, which apply to perpendicularly magnetized layers, the DL torque acts as a z-axis field B DL ∝ M x on the internal DW magnetization, favoring the expansion of a domain on the side where |M x | is larger (for example at an up-down DW of the type ↑←↓ rather than at the opposite down-up DW ↓→↑).…”

Continuous Tuning of the Magnitude and Direction of Spin‐Orbit Torque Using Bilayer Heavy Metals

“…In order to obtain the direction and amplitude of HT and HL, harmonic Hall voltage measurements have been widely used, [4,5,7,23,24] along with spin-torque ferromagnetic resonance. [2,3,27] Recently, the Dzyaloshinskii-Moriya interaction (DMI) has been found to play an important role in the current induced magnetization switching through chiral domain wall nucleation and motion by SOT, [8][9][10][28][29][30][31] while its detrimental effect on switching has been reported in some other scenarios. [26,32] DMI is also found to be responsible for the formation of magnetic skyrmions, [33,34] which serve as bits to store information in future memory and logic devices.…”

Current-driven domain wall dynamics in ferromagnetic layers synthetically exchange-coupled by a spacer: A micromagnetic study

“…On the other hand, we have also considered that the same current flowing through the HM is also flowing through the ultrathin FM layers. This is surely an exaggeration, as the electrical resistivity of the FM should be larger than the one of the FM layers 29 (2017)). In a more realistic case, the density current along the FM layer must be smaller than the one along the thicker and low-resistivity HM, and consequently the STT should play a marginal role.…”