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Disorder-induced chirality in superconductor-ferromagnet heterostructures revealed by neutron scattering and multiscale modeling

Annika Stellhorn, Juan G. C. Palma, Alicia Backs, Anders Bergman, Angela B. Klautau, Emmanuel Kentzinger, Connie Bednarski-Meinke, Steffen Tober, Elizabeth Blackburn, Juri Barthel, Nina-Juliane Steinke, Helena M. Petrilli, Ivan P. Miranda

Abstract

Chirality in superconductor-ferromagnet hybrids strongly influences phenomena such as the observable signatures of long-range triplet superconductivity, but its microscopic origin in nominally centrosymmetric ferromagnets is still unclear. Here, we combine structural characterization, polarization-analyzed grazing-incidence small-angle neutron scattering (PA-GISANS), first-principles calculations, and deep-learning-assisted multiscale modeling to study FePd and Nb/FePd heterostructures. Experimentally, we observe partial L1$_0$ order, atomic intermixing, anti-phase boundaries, and a depth-dependent defect gradient across the FePd layer, together with a finite net magnetic chirality at room temperature. The GISANS asymmetry indicates that the main chiral contribution lies in-plane, with an additional out-of-plane component associated with depth-dependent magnetic inhomogeneity. Theoretically, we show that chemical disorder in FePd, especially when combined with a compositional gradient, produces finite Dzyaloshinskii-Moriya interactions and stabilizes chiral finite-$\mathbf{q}$ magnetic modulations with mixed Bloch-Néel character. In the mesoscopic model, the resulting in-plane modulation length approaches the experimentally observed range. These results identify disorder and compositional gradients as intrinsic microscopic sources of net chirality in FePd-based films, showing that the observed chirality does not arise only from interface effects.

Disorder-induced chirality in superconductor-ferromagnet heterostructures revealed by neutron scattering and multiscale modeling

Abstract

Chirality in superconductor-ferromagnet hybrids strongly influences phenomena such as the observable signatures of long-range triplet superconductivity, but its microscopic origin in nominally centrosymmetric ferromagnets is still unclear. Here, we combine structural characterization, polarization-analyzed grazing-incidence small-angle neutron scattering (PA-GISANS), first-principles calculations, and deep-learning-assisted multiscale modeling to study FePd and Nb/FePd heterostructures. Experimentally, we observe partial L1 order, atomic intermixing, anti-phase boundaries, and a depth-dependent defect gradient across the FePd layer, together with a finite net magnetic chirality at room temperature. The GISANS asymmetry indicates that the main chiral contribution lies in-plane, with an additional out-of-plane component associated with depth-dependent magnetic inhomogeneity. Theoretically, we show that chemical disorder in FePd, especially when combined with a compositional gradient, produces finite Dzyaloshinskii-Moriya interactions and stabilizes chiral finite- magnetic modulations with mixed Bloch-Néel character. In the mesoscopic model, the resulting in-plane modulation length approaches the experimentally observed range. These results identify disorder and compositional gradients as intrinsic microscopic sources of net chirality in FePd-based films, showing that the observed chirality does not arise only from interface effects.

Paper Structure

This paper contains 27 sections, 24 equations, 11 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Experimental Results
  3. Structural and magnetic properties of the L$1_0$ FePd films
  4. Intermixing and compositional gradient distribution
  5. Anti-phase boundaries
  6. Determination of chirality via PA-GISANS
  7. Theoretical Results
  8. Ordered L$1_0$ FePd: electronic structure and magnetic properties
  9. Intermixing states: proof-of-concept in a more localized perspective
  10. Interdiffusion with ab initio accuracy: disorder- and gradient-induced net global chirality
  11. Modeling via deep learning
  12. Construction and analysis of the mesoscopic FePd model
  13. Nature of the disorder-induced DMI: insights from the DFT dataset
  14. Summary and Outlook
  15. Acknowledgments
  16. ...and 12 more sections

Figures (11)

  • Figure 1: Magnetic structure of i.Nb/FePd and ii.FePd. (a,b) Hysteresis loops obtained by MPMS, and surface magnetic domain structure obtained by MFM of i.Nb/FePd (top) and ii.FePd (bottom). (c) XRD measurements of the FePd(002) and FePd(001) Bragg peaks, with the peak ratio indicating the quality of the L$1_0$ phase of FePd. All measurements are obtained at 300 K.
  • Figure 2: Structural characterization obtained at 300 K. (a) HAADF-STEM measurements of the Nb/FePd interface in i.Nb/FePd (top) and the FePd/Pd interface (bottom). (b) HAADF-STEM EDX measurements of the Nb/FePd interface in i.Nb/FePd (top), and the respective schematic monolayer structure of the L$1_0$ phase of FePd with alternating Fe (red) and Pd (yellow) layers (bottom).
  • Figure 3: Anti-phase boundaries in i.Nb/FePd. (a) Cross section of the sample obtained from HAADF-STEM measurements, (b) AFM image showing the top surface morphology, and (c) schematic cross-section of anti-phase boundaries along $[111]$. All measurements were performed at 300 K. Images are adapted from PhDStellhorn.
  • Figure 4: PA-GISANS measurements and extracted chirality.$I(Q_y)$ plots of (a) i.Nb/FePd and (b) ii.FePd, in comparison with (c) the respective chirality plots $(I^{+-}-I^{-+})(Q_y)$. Displayed for a polarization along the surface normal $<$001$>$ (left), and within the surface plane (100) (right). All measurements were made at 300 K.
  • Figure 5: Structural and theoretical magnetic interactions of pristine L$1_0$ FePd. (a) Unit cell of FePd crystal (blue spheres: Pd; orange sphere: Fe), (b) application of the Moriya's rule in the Fe-Pd bond Moriya (c) Exchange and (d) Dzyaloshinskii-Moriya coupling parameters for Fe-Fe (red dashed-lines), Fe-Pd (green dashed lines) and Pd-Pd (blue-dashed lines) interactions as a function of the normalized distance between the atomic pairs (in units of the lattice parameter). In (c), the solid black curve shows the RKKY fit.
  • ...and 6 more figures