PHYSICAL REVIEW B 93, 014403 (2016) Domain wall dynamics in ultrathin Pt/Co/AlOx microstrips under large combined magnetic fields E. Ju´ e, 1, 2, 3, * A. Thiaville, 4 , † S. Pizzini, 5, 1 J. Miltat, 4 J. Sampaio, 4 L. D. Buda-Prejbeanu, 1, 2, 3 S. Rohart, 4 J. Vogel, 5, 1 M. Bonfim, 6 O. Boulle, 1, 2, 3 S. Auffret, 1, 2, 3 I. M. Miron, 1, 2, 3 and G. Gaudin 1, 2, 3 1 Universit´ e Grenoble Alpes, 38000 Grenoble, France 2 CEA, INAC, SPINTEC, 38000 Grenoble, France 3 CNRS, SPINTEC, 38000 Grenoble, France 4 Laboratoire de Physique des Solides, Universit´ e Paris–Sud, CNRS, UMR 8502, 91405 Orsay, France 5 CNRS, Institut N´ eel, 38042 Grenoble, France 6 Departamento de Engenharia El´ etrica, Universidade Federal do Paran´ a, Curitiba, Brazil (Received 29 October 2014; revised manuscript received 15 October 2015; published 4 January 2016) The dynamics of magnetic domain walls in ultrathin strip-patterned Pt/Co/AlOx samples with perpendicular easy axis has been studied experimentally under an easy-axis field, superposed to a hard-axis field oriented along the strip. The easy-axis field is large so that the domain walls move well beyond the creep regime. A chiral effect is observed where the domain wall velocity shows a monotonous and surprisingly large variation with an in-plane field. A micromagnetic analysis, combining analytic, one-dimensional, and two-dimensional simulations with structural disorder, shows that this behavior can be reproduced with a Dzyaloshinskii-Moriya interaction of the interfacial type, with due consideration of the dynamics of the tilt degree of freedom of the domain wall. The estimated effective value of this interaction (D ≈−2.2 mJ/m 2 for a 0.6 nm Co thickness) is consistent with values obtained by other techniques. It is also shown, by micromagnetic analysis, that several modes and characteristic times occur in the dynamics of the tilt of such domain walls. DOI: 10.1103/PhysRevB.93.014403 I. INTRODUCTION Recent studies have revealed that the magnetism of ultrathin films is richer than what is conceived from the mere reduction of film thickness. Indeed, beyond the expected change in the Curie temperature and in thermal fluctuation effects, and beyond the appearance of surface/interface anisotropy and of interfacial exchange coupling, etc., chiral effects have been discovered in such samples. As chirality cannot be defined in a two-dimensional world, and as also shown by experiments, these effects appear when upward- and downward-oriented normals to the film are structurally different (structural inversion asymmetry). The emblematic sample in this respect [1] is an ultrathin cobalt layer with a perpendicular easy axis, capped by aluminum oxide and grown on platinum, a 5d metal with large spin-orbit coupling, whereas the capping insulator is made of light elements. The first observed chiral effect appears to be the switching of the sample’s perpendicular magnetization for a unique combination of domain magnetization (±z), in-plane current (±x ), and in-plane field (±x )[2,3], where z denotes the film normal and x is an in-plane direction. The second is a chiral change of the velocity of domain walls (DWs) when driven by an in-plane current flowing transverse to the DW, under an in-plane field applied parallel to the current [4,5], wherein for successive DWs, opposite in-plane fields are required to increase the velocity. A closely related effect is a chiral variation of DW energy under an in-plane field applied transverse to the DW, which manifests itself by a chiral change of field-driven velocity in the creep regime * Present address: NIST, Boulder, CO 80305, USA; emilie.jue. spintec@gmail.com † andre.thiaville@u-psud.fr [6,7], and by an asymmetric nucleation of reversed domains at the edges of the sample that are normal to the in-plane field [8]. In the first case, the observations have been interpreted by a spin-orbit torque exerted by the in-plane current (in the form of a Rashba effective field [9], or of a spin Hall effect [2,3]). The DW experiments of the second case have been accounted for by the micromagnetic exploration of the dynamics of so-called Dzyaloshinskii domain walls [10], chiral DWs with a (partly or fully) N´ eel structure, whose stability derives from an interfacial Dzyaloshinskii-Moriya interaction (DMI) [11–14], following the observation of such structures in monolayer and bilayer epitaxial films [15]. These two types of chiral effects are interconnected, because the same forms of spin-orbit torque are invoked when current is applied, and also because magnetization switching in samples extending over more than tens of nanometers occurs via DW nucleation and motion [16]. Thus, the structure and dynamics of DWs in such ultrathin films with perpendicular magnetization and structural inversion asymmetry along the film normal constitute an interesting topic in itself for physics as well as for applications, and also as the basic process of magnetization reversal in these samples. The simplest and best understood means for acting on a DW is to apply a magnetic field. The field value and time scale fix the regime of DW motion. On the one hand, applying low fields for long times gives access to the creep regime [17], which has been studied in particular in ultrathin films with perpendicular magnetization [17–19] and recently in asymmetric samples [6,7]. In this regime, the DW energy (surface tension) plays a key role, and the intrinsic magnetization dynamics (damped precession around the effective field) is not important. On the other hand, at large fields the flow regime of motion is reached, with the two well-known steady-state and preces- sional regimes according to the dynamics of the DW moment [20]. For ultrathin films with perpendicular magnetization, 2469-9950/2016/93(1)/014403(11) 014403-1 ©2016 American Physical Society