Chirality-Induced Asymmetric Magnetic Nucleation inPt/Co/AlOxUltrathin Microstructures

Abstract

The nucleation of reversed magnetic domains in Pt/Co/AlOx microstructures with perpendicular anisotropy was studied experimentally in the presence of an in-plane magnetic field. For large enough in-plane field, nucleation was observed preferentially at an edge of the sample normal to this field. The position at which nucleation takes place was observed to depend in a chiral way on the initial magnetization and applied field directions. A quantitative explanation of these results is proposed, based on the existence of a sizable Dzyaloshinskii-Moriya interaction in this sample. Another consequence of this interaction is that the energy of domain walls can become negative for in-plane fields smaller than the effective anisotropy field. [5]. In the latter case, the transcription of the chirality from the atomic scale to the macroscopic scale of textures may be impeded by the existence of an anisotropy. This is exemplified in liquid crystals, where under a DC magnetic or electric field that induces anisotropy, the cholestericnematic transition takes place [4]. Thus, the detection and quantification of a chiral interaction when it is too weak to give rise to a global chiral texture is difficult.Magnetism is another prominent field where chiral textures are considered. A chiral magnetic interaction indeed exists, namely an anti-symmetric exchange called Dzyaloshinskii-Moriya interaction (DMI) [5, 6], that is allowed when central symmetry is broken. In many compounds having this property, especially the cubic B20 structures with depressed magnetic anisotropy, chiral textures like homochiral spin spirals or 2D skyrmion lattices have been observed, both in reciprocal [7] and in real space [8]. Another class of chiral magnetic systems has recently appeared, namely the few atomic layer thick samples grown on an underlayer with large spinorbit interaction, showing structural inversion asymmetry and perpendicular magnetic anisotropy (PMA) [9][10][11][12]. In these systems, PMA is very strong so that chiral spin spirals are not stable, and as a result DMI has remained unnoticed for about 20 years. However, at magnetic edges like a domain wall (DW) separating two uniformly magnetized domains or at physical edges in a microstructure, the competition of chiral interaction with anisotropy is modified. Indeed, the peculiarities of field and current-induced dynamics of domain walls in such samples [13][14][15] have been found to be consistent with a chiral texture localized on the DW, deriving from the presence of interface-induced DMI [16]. These local chiral magnetization textures, that appear as Néel walls of a fixed chirality, have also been observed recently by lowenergy electron microscopy [17,18], on samples with wide domain walls.In this Letter we show that chiral interactions can also be detected at the edges of a microstructure: in the presence of an additional in-plane field, nucleation of reversed domains takes place preferentially at one edge of the sample, oriented perpendicular to this field. The side at which nucle...

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

“…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. 3b, with an additional top/bottom asymmetry.…”

Very large domain wall velocities in Pt/Co/GdOx and Pt/Co/Gd trilayers with Dzyaloshinskii-Moriya interaction

“…A first demonstration was given by Miron et al [2] who showed that in Pt/Co/AlOx trilayers DWs move with a larger velocity than in Pt/Co/Pt. The presence of a large DMI in Pt/Co/AlOx was confirmed experimentally using chiral nucleation experiments [18] and Brillouin Light Scattering spectroscopy (BLS) [19].…”

Breathing modes of confined skyrmions in ultrathin magnetic dots

“…[21][22][23][24] Some examples of such material systems include Pt/Co (0.6)/Al 2 O 3 , 25 Pt (2.5)/Co (0.3)/Pt (1.5), 26 Pt (3)/CoFe (0.6)/MgO (1.8) and Ta (5)/CoFe (0.6)/MgO (1.8), 27 Pt (1.5)/Co (0.3)/Ni (0.7)/Co (0.15), 28 , and Pt/[Ni/Co] n and Ir/[Ni/Co] n , 29 where the figures in parentheses represent film thicknesses in nm and are presented here to highlight the importance of the interfacial origin of the effect. At present, the existence of a sizeable DMI has been inferred from experiments involving imaging of static domain wall structures, 29 domain wall nucleation, 30 and from field 26 and current-driven magnetic domain wall dynamics 27,28 in which a clear dependence on the wall chirality has been observed. In these cases, the experimental results suggest that the domain wall structure involves a Néel profile, rather than a Bloch form, which is possible in these strong ferromagnets if a sufficiently large DMI is present.…”