Chirality in Magnetic Multilayers Probed by the Symmetry and the Amplitude of Dichroism in X-Ray Resonant Magnetic Scattering

Chirality in condensed matter has recently become a topic of the utmost importance because of its significant role in the understanding and mastering of a large variety of new fundamental physical mechanisms. Versatile experimental approaches, capable to reveal easily the exact winding of order parameters, are therefore essential. Here we report x-ray resonant magnetic scattering as a straightforward tool to reveal directly the properties of chiral magnetic systems. We show that it can straightforwardly and unambiguously determine the main characteristics of chiral magnetic distributions: i.e., its chiral nature, the quantitative winding sense (clockwise or counterclockwise), and its type, i.e., Néel [cycloidal] or Bloch [helical]. This method is model independent, does not require a priori knowledge of the magnetic parameters, and can be applied to any system with magnetic domains ranging from a few nanometers (wavelength limited) to several microns. By using prototypical multilayers with tailored magnetic chiralities driven by spin-orbit-related effects at $\mathrm{Co}|\mathrm{Pt}$ interfaces, we illustrate the strength of this method.

Figure 1

(a) XRMS measurement principle and experimental configuration. (b) $6\times 6\mu {\mathrm{m}}^2$ MFM phase image showing the out-of-plane magnetic contrast of the $[\mathrm{Ir}(1)|\mathrm{Co}(0.6)|\mathrm{Pt}(1)]\times 5$ multilayer with its corresponding FFT pattern (inset) evidencing a $180\pm 30\mathrm{nm}$ period disordered magnetic stripe pattern. Corresponding diffracted patterns for a CL (c) and CR (d) polarized incident x-ray beam. (e) Resulting sum image of the diffracted pattern ($\mathrm{CL}+\mathrm{CR}$), confirming that the diffraction corresponds to the magnetic domains observed by MFM. (f) Normalized difference image ($\mathrm{CL}\text{−}\mathrm{CR}/\mathrm{CL}+\mathrm{CR}$) evidencing a pronounced magnetic asymmetry. Note that the straight black area crossing the diffraction ring is the shadow of a beamstop used to block the specular reflection.

Figure 2

(a) Diagram of a Bloch (helicoidal) winding and of a (b) Néel (cycloidal) winding. (c) Calculated orthoradial profile of the circular dichroism of different magnetic textures: CW (orange) and CCW (green) Bloch (dashed) and Néel (solid) windings for an incident angle $\theta =17.5°$. (d) Comparison with the experimental magnetic asymmetry orthoradial profile of a $[\mathrm{Ir}(1)|\mathrm{Co}(0.8)|\mathrm{Pt}(1)]\times 5$ multilayer for the same incident angle $\theta =17.5°$ (dots). Note that the red rectangles indicate the position of the beamstop.

Figure 3

Comparison of two samples with inverted stacking $\Vert [\mathrm{Ir}(1)|\mathrm{Co}(0.8)|\mathrm{Pt}(1)]\times 5$ (a)–(c) and $\Vert [\mathrm{Pt}(1)|\mathrm{Co}(0.8)|\mathrm{Ir}(1)]\times 5$ (d),(e). The corresponding magnetization textures are obtained by micromagnetic simulations based on material parameters (a),(d). (b),(e) MFM images with labyrinth-shape domains and their Fourier transform in the inset. (c),(f) Normalized magnetic asymmetry signal $(I_{\mathrm{CL}}-I_{\mathrm{CR}})/(I_{\mathrm{CL}}+I_{\mathrm{CR}})$ measured on sample a (e) and sample b (f). The diffraction sum images are shown in the inset, confirming again that the diffraction corresponds to the magnetic domains.