Chirality-Induced Asymmetric Magnetic Nucleation in $\mathrm{Pt}/\mathrm{Co}/{\mathrm{AlO}}_x$ Ultrathin Microstructures

The nucleation of reversed magnetic domains in $\mathrm{Pt}/\mathrm{Co}/{\mathrm{AlO}}_x$ 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.

Figure 1

Kerr images showing the chiral nucleation of domains at one edge of the pad of the $\mathrm{Pt}/\mathrm{Co}/{\mathrm{AlO}}_x$ microstructure, by application of an out-of-plane field pulse. (a)–(d) Magnetization is initially saturated $\uparrow$ and $B_x=0$, $+160$, $+215$, and $+260\mathrm{mT}$, (e)—(f) magnetization is initially saturated $\uparrow$ and $B_x=-160$ and $-260\mathrm{mT}$, (g)–(h) magnetization is initially saturated $\downarrow$ and $B_x$ is $+160$ and $+260\mathrm{mT}$. The width of the pad is $70\mu \mathrm{m}$. The dotted lines highlight the left and right edges of the pad and the arrows show the side of the sample where nucleation takes place.

Figure 2

Nucleation fields measured as a function of in-plane field $H_x$ for the reversed domain in the middle (squares) and at the left edge of the sample (circles). Note the difference of scales between the two axes.

Figure 3

(a) Sketch of the micromagnetic configuration within a microstructure with the DMI in zero applied field (i), under an $x$ field (ii), under an additional negative $z$ field (iii), and after reversal, with a domain wall of magnetization parallel to the $x$ field (iv). (b) results of a 1D calculation showing the reversal field for $D/D_{c0}=0$ (dashes) and 0.5 (lines). For $D\ne 0$ an easy and a hard branch develop, corresponding to the reversal at the two edges of the microstructure. Inset: complete astroids.

Figure 4

Nucleation field vs $H_x$ for a domain at the edge and in the film calculated using the rigid droplet model (full and dotted line) and the relaxed model (dots and square symbols). Insets: calculated droplet shapes for $H_x/H_{K_0}=0$ (lines) and 0.21 (dots).