Chirality-induced asymmetric magnetic nucleation in Pt/Co/AlOx ultrathin microstructures

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 these samples.

In recent years, we have extensively studied field- and current-induced domain wall motion in Pt/Co/AlOx trilayers with perpendicular magnetic anisotropy, together with the Spintec laboratory [1,2,3]. It was proposed recently [4] that the high efficiency of current-induced domain wall motion in this system could be explained if the domain walls were chiral Néel walls driven by the spin Hall current from the Pt underlayer. A chiral magnetic interaction that induces such chiral Néel walls indeed exists, namely an anti-symmetric exchange called Dzyaloshinskii-Moriya interaction (DMI) [5, 6], that is allowed when central symmetry is broken like in Pt/Co/AlOx.

Figure 1 : Kerr images showing the chiral nucleation of domains at one edge of the pad of the Pt/Co/AlOx microstructure, by application of an out-of-plane field pulse. (a)–(d) Magnetization is initially saturated ↑ and Bx = 0, +160, +215, and +260 mT, (e)—(f) magnetization is initially saturated ↑ and Bx = −160 and −260 mT, (g)–(h) magnetization is initially saturated ↓ and Bx is +160 and +260 mT. The width of the pad is 70 μm. 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.

In these systems, the Perpendicular Magnetic Anisotropy (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 [7-10] have been found to be consistent with a chiral texture localized on the DW, deriving from the presence of interface-induced DMI [4]. In a recent paper in Physical Review Letters we have shown 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 nucleation takes place depends on the direction of both the additional field and the initial magnetization. This asymmetry is thus chiral, and we propose a quantitative explanation of these results, based on the existence of a sizable Dzyaloshinskii-Moriya interaction in this sample. This work was performed together with the Laboratoire de Physique des Solides and Spintec.

Our related publication

    Chirality-Induced Asymmetric Magnetic Nucleation in Pt/Co/AlOx Ultrathin Microstructures, S. Pizzini, J. Vogel, S. Rohart, L. D. Buda-Prejbeanu, E. Jué, O. Boulle, I. M. Miron, C. K. Safeer, S. Auffret, G. Gaudin, and A. Thiaville, Physical Review Letters 113, 047203 (2014)

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