Quantitative imaging of hybrid chiral spin textures in magnetic multilayer systems by Lorentz microscopy

K. Fallon, S. McVitie, W. Legrand, F. Ajejas, D. Maccariello, S. Collin, V. Cros, and N. Reyren

Phys. Rev. B 100, 214431 – Published 24 December 2019

Abstract

Chiral magnetic textures in ultrathin perpendicularly magnetized multilayer film stacks with an interfacial Dzyaloshinskii-Moriya interaction have been the focus of much research recently. The chirality associated with the broken inversion symmetry at the interface between an ultrathin ferromagnetic layer and a heavy metal with large spin-orbit coupling supports homochiral Néel domain walls and hedgehog (Néel) skyrmions. Under spin-orbit torques these Néel-type magnetic structures are predicted, and have been measured, to move at high velocities. However recent studies have indicated that some multilayered systems may possess a more complex hybrid domain wall configuration, due to the competition between interfacial DMI and interlayer dipolar fields. These twisted textures are expected to have thickness dependent Néel and Bloch contributions to the domain or skyrmion walls. In this work, we use the methods of Lorentz microscopy to determine quantitatively experimentally both (i) the contributions of the Néel and Bloch components and (ii) their spatial spin variation at high resolution. These are compared with modeled and simulated structures that are in excellent agreement with our experimental results. Our quantitative analysis provides powerful direct evidence of the Bloch wall component which exists in these hybrid walls and will be significant when exploiting chirality in spintronic applications.

Received 1 February 2019

DOI:https://doi.org/10.1103/PhysRevB.100.214431

Physics Subject Headings (PhySH)

Dzyaloshinskii-Moriya interaction

Magnetic multilayers

Lorentz microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

K. Fallon¹, S. McVitie¹, W. Legrand², F. Ajejas², D. Maccariello², S. Collin², V. Cros², and N. Reyren²

¹Scottish Universities Physics Alliance, School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
²Unité Mixte de Physique, CNRS, Thales, Université Paris-Sud, Université Paris-Saclay, 91767, Palaiseau, France

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Vol. 100, Iss. 21 — 1 December 2019

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Images

Figure 1
(a),(b) Schematic figures to show to orientation of the electron beam in the TEM for films with (a) planar and (b) perpendicular magnetization. In both cases the films have saturation induction, $B_{s}$ , and thickness, $t$ , and are shown untilted and tilted by an angle θ. The projected thickness is indicated as $t'$ ’. Lorentz microscopy is sensitive to the magnetic induction perpendicular to the trajectory of the electron beam, this quantity is labeled as $B_{L}$ .
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Figure 2
(a)–(c) $M_{x}, M_{y}$ , and $M_{z}$ components, constructed from a simple 1D hyperbolic tangent model, of two closely spaced domain walls. The upper third, above the top red dividing line, (a)–(c) models pure Néel walls while the middle third models hybrid Bloch/Néel walls, with a Bloch to Néel ratio of 0.1 to 0.9, and the lower third models pure Bloch type walls. Calculated Fresnel images (d)–(f) of the pure Néel (upper), hybrid (middle), and Bloch (lower) wall taken at tilt angles of $+ 20^{\circ}, 0^{\circ}$ , and $- 20^{\circ}$ , respectively, about the axis indicated by arrowhead. Intensity line traces from the middle part (i.e., the hybrid wall) of images (d)–(f) are shown in (g)–(i). The dashed lines on the line traces correspond to the center positions of the domain walls.
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Figure 3
Low magnification Fresnel image of 15 × layer sample with closely spaced pairs of domain walls. The sample is in the untilted position however the film on the transparent membrane is buckled and therefore tilted with respect to the beam. The dashed red line marks a bend contour and shows where the effective tilt with respect to the electron beam changes from negative to positive and the contrast is seen to reverse.
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Figure 4
Fresnel images of 15 × layer sample with closely spaced pairs of domain walls with varying local tilt at the colored lines to be (a) $+ 5^{\circ}$ , (b) $0^{\circ}$ , and (c) $- 5^{\circ}$ . The corresponding intensity line traces from these images, averaged over 10 lines, are shown in (d)–(f). The dashed lines on the line trace are to guide the eye and indicate the central position of the domain wall with respect to the Fresnel image contrast. In Fresnel images the important feature is the contrast level relative to the background, therefore the line traces have been normalized to the background value.
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Figure 5
DPC images of the 15 × layer sample taken at two different values of sample tilt. The local tilt at the colored lines are (a) $0^{\circ}$ and (b) $+ 13 . 2^{\circ}$ . The component of induction mapped is indicated by the double headed arrow. The corresponding line traces, averaged over 20 lines, of the integrated induction from these images are shown in (c) and (d). To aid interpretation, the line traces include vertical dotted lines which indicate the position of the domain walls and horizontal dotted lines which show the salient (labeled) quantities extracted from the graphs.
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Figure 6
(a) DPC image taken of the 15 × sample tilted and in a demagnetized state close to remanence, with component of induction mapped indicated by the double headed arrow. The sample is tilted to provide strong contrast from the out-of-plane domains. (b) Integrated induction line trace taken from red line in (a) showing domain wall profile and its fit to a hyperbolic tangent function, see text for details.
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