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Disorder-broadened topological Hall phase and anomalous Hall scaling in FeGe

Chaman Gupta, Chris Matsumura, Hongbin Yang, Sarah Edwards, Rebeca M. Gurrola, Jiun-Haw Chu, Hanjong Paik, Yongqiang Wang, David A. Muller, Robert Streubel, Tzu-Ming Lu, Serena Eley

TL;DR

The study demonstrates that controlled disorder introduced by Ne$^{+}$ irradiation in epitaxial FeGe films markedly enlarges the temperature window and strength of the topological Hall effect, indicating broader skyrmion stability under defect-rich conditions. Concurrently, disorder shifts the anomalous Hall effect from Berry-curvature–driven quadratic scaling to skew-scattering–dominated linear scaling, with the skew coefficient $\alpha$ increasing ~3×, revealing that defects act as both nucleation/pinning centers for topological textures and enhancers of extrinsic electronic scattering. These findings establish defect engineering as a viable route to tailor both real-space magnetic textures and momentum-space transport in chiral magnets, with direct implications for robust skyrmion-based devices. The work also highlights the need for direct imaging of magnetic textures in disordered regions to confirm the nature of the low-temperature THE and to explore current-driven dynamics in defect-rich regimes.

Abstract

Magnetic skyrmions are topologically protected spin textures that are promising candidates for low-power spintronic memory and logic devices. Realizing skyrmion-based devices requires an understanding of how structural disorder affects their stability and transport properties. This study uses Ne$^{+}$ ion irradiation at fluences from $10^{11}$ to $10^{14}$ ions-cm$^{-2}$ to systematically vary defect densities in 80 nm epitaxial FeGe films and quantify the resulting modifications to magnetic phase boundaries and electronic scattering. Temperature- and field-dependent Hall measurements reveal that increasing disorder progressively extends the topological Hall signal from a narrow window near 200K in pristine films down to 4K at the highest fluence, with peak amplitude more than doubling. Simultaneously, the anomalous Hall effect transitions from quadratic Berry curvature scaling to linear skew scattering behavior, with the skew coefficient increasing threefold. These results establish quantitative correlations between defect concentration, skyrmion phase space, and transport mechanisms in a chiral magnet. It demonstrates that ion-beam modification provides systematic control over both topological texture stability and electrical detectability.

Disorder-broadened topological Hall phase and anomalous Hall scaling in FeGe

TL;DR

The study demonstrates that controlled disorder introduced by Ne irradiation in epitaxial FeGe films markedly enlarges the temperature window and strength of the topological Hall effect, indicating broader skyrmion stability under defect-rich conditions. Concurrently, disorder shifts the anomalous Hall effect from Berry-curvature–driven quadratic scaling to skew-scattering–dominated linear scaling, with the skew coefficient increasing ~3×, revealing that defects act as both nucleation/pinning centers for topological textures and enhancers of extrinsic electronic scattering. These findings establish defect engineering as a viable route to tailor both real-space magnetic textures and momentum-space transport in chiral magnets, with direct implications for robust skyrmion-based devices. The work also highlights the need for direct imaging of magnetic textures in disordered regions to confirm the nature of the low-temperature THE and to explore current-driven dynamics in defect-rich regimes.

Abstract

Magnetic skyrmions are topologically protected spin textures that are promising candidates for low-power spintronic memory and logic devices. Realizing skyrmion-based devices requires an understanding of how structural disorder affects their stability and transport properties. This study uses Ne ion irradiation at fluences from to ions-cm to systematically vary defect densities in 80 nm epitaxial FeGe films and quantify the resulting modifications to magnetic phase boundaries and electronic scattering. Temperature- and field-dependent Hall measurements reveal that increasing disorder progressively extends the topological Hall signal from a narrow window near 200K in pristine films down to 4K at the highest fluence, with peak amplitude more than doubling. Simultaneously, the anomalous Hall effect transitions from quadratic Berry curvature scaling to linear skew scattering behavior, with the skew coefficient increasing threefold. These results establish quantitative correlations between defect concentration, skyrmion phase space, and transport mechanisms in a chiral magnet. It demonstrates that ion-beam modification provides systematic control over both topological texture stability and electrical detectability.

Paper Structure

This paper contains 12 sections, 4 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Results and Discussion
  3. Sample preparation and disorder characterization
  4. Topological Hall Effect (THE)
  5. Anomalous Hall Effect (AHE)
  6. Conclusion
  7. Experimental Section
  8. Sample Fabrication
  9. Ion-beam modification
  10. Transport Measurements and Analysis
  11. Magnetometry
  12. STEM imaging and Multislice Electron Ptychography (MEP)

Figures (5)

  • Figure 1: Identification of defects.\ref{['fig:image:SRIM']} SRIM full-cascade calculations for 400 Ne$^{+}$ irradiation (fluences $10^{11}–10^{14}$ ions-cm$^{-2}$; samples S1--S5) give depth-resolved displacements-per-atom (dpa). The FeGe film (0--80, pink) experiences a nearly uniform damage level. \ref{['fig:image:MAADFS2']} Cross-sectional medium angle annular dark-field (MAADF) STEM of a lightly irradiated film (S2, $5\times10^{11}$ ions-cm$^{-2}$), and, \ref{['fig:image:MAADFS5']} of the highest-dose fillm (S5, $10^{14}$ ions-cm$^{-2}$) reveal an increasing density of nanoscale contrast variations attributed to point-defect clusters within the FeGe layer. Atomic-resolution multislice electron ptychography images taken from the red boxed area in \ref{['fig:image:MAADFS5']} for depths of \ref{['fig:image:MEPD1']}1113, \ref{['fig:image:MEPD2']}1416, and \ref{['fig:image:MEPD3']}1719
  • Figure 2: Topological Hall extraction and temperature evolution in pristine and irradiated FeGe. \ref{['fig:the:extraction:S0']} Decomposition of the Hall signal at $T = 200~\mathrm{K}$ for pristine S0 over the field range $-2 \le \mu_0H \le 2~\mathrm{T}$. Total Hall resistivity after ordinary Hall removal, $\rho_{xy} - R_0H$ (red, left $y$-axis), anomalous Hall contribution $\rho_{\mathrm{AHE}}$ (black dashed, left $y$-axis), and residual topological Hall resistivity $\rho_{\mathrm{THE}}$ (blue, right $y$-axis). Black curved arrows indicate field-sweep direction; colored arrows mark corresponding $y$-axes. \ref{['fig:the:extraction:S5']} Same decomposition for highly disordered S5 ($10^{14}~\mathrm{ions\,cm}^{-2}$), showing enhanced THE amplitude compared to pristine film. \ref{['fig:the:temperature:S0']} Temperature evolution of $\rho_{\mathrm{THE}}(H)$ from $4~\mathrm{K}$ to $80~\mathrm{K}$ in pristine S0 (curves vertically offset for clarity), revealing diminishing THE signals at low temperatures. \ref{['fig:the:temperature:S5']} Temperature evolution of $\rho_{\mathrm{THE}}(H)$ in irradiated S5, showing persistent THE signals across all temperatures, demonstrating disorder-stabilized skyrmions at low temperatures.
  • Figure 3: Disorder evolution of the topological Hall effect.\ref{['fig:the:all']} Field dependence of the topological Hall resistivity $\rho_{\mathrm{THE}}(H)$ at $T=120$ K for S0 (pristine), S3, and S5 over the range $-1\le \mu_0H \le 1$ T, showing a progressive increase in amplitude and width with irradiation fluence. Temperature–field maps of $\rho_{\mathrm{THE}}(T,H)$ for \ref{['fig:the:S0']} S0, \ref{['fig:the:S1']} S1 ($10^{11}$ ions/cm$^{2}$), \ref{['fig:the:S3']} S3 ($10^{12}$ ions/cm$^{2}$), \ref{['fig:the:S4']} S4 ($10^{13}$ ions/cm$^{2}$), and \ref{['fig:the:S5']} S5 ($10^{14}$ ions/cm$^{2}$) are plotted with identical axes and a common color scale (right; red/blue denote positive/negative $\rho_{\mathrm{THE}}$). The vertical dashed lines in \ref{['fig:the:S0']}, \ref{['fig:the:S3']}, and \ref{['fig:the:S5']} mark $T=120K$, corresponding to the cut shown in \ref{['fig:the:all']}. Together, the maps reveal a systematic broadening and strengthening of the low-temperature $\rho_{\mathrm{THE}}$ lobe with increasing defect density.
  • Figure 4: Anomalous Hall effect across disorder.\ref{['fig:ahe:temperature']} Saturated anomalous Hall resistivity $\rho_{\mathrm{AHE,sat}}(T)$ for S0, S3, and S5, extracted from the positive-field saturation window ($\mu_0H>1.5T$). Disorder enhances the low-$T$ AHE and slightly shifts the peak near 150200K. \ref{['fig:ahe:sigmaxx']}$\sigma_{\mathrm{AHE}}$ vs. $\sigma_{xx}$ with temperature as a parametric color (10280K), mapping the conductivity regimes: trajectories move from the high-$\sigma_{xx}$ intrinsic plateau at high $T$ toward extrinsic, disorder-dominated behavior at lower $\sigma_{xx}$; irradiation shifts the curves to smaller $\sigma_{xx}$.
  • Figure 5: \ref{['fig:ahemech:cartoon']} Schematic showing different AHE mechanisms. \ref{['fig:ahemech:intrinsic']} Intrinsic window: $R_s\equiv\rho_{\mathrm{AHE}}/M$ scales nearly quadratically with $\rho_{xx}$; $R_s\propto\rho_{xx}^{n}$ with $n\approx2$ (fit exponents printed on panel). \ref{['fig:ahemech:bad']} Bad-metal/mixed window: sub-quadratic power law $R_s=A\rho_{xx}^{\gamma}$ with $\gamma\approx1.5$–$1.7$, signaling deviation from the intrinsic limit at highest resistivities. \ref{['fig:ahemech:skew']} Skew-scattering window: linear relation $R_s=\alpha\rho_{xx}$; extracted $\alpha$ increases systematically with disorder (S0 $<$ S3 $<$ S5; values shown). \ref{['fig:ahemech:map']} Conductivity–temperature map: $\sigma_{xx}(T)$ for S0, S3, and S5, indicating the temperature ranges used for the three scaling analyses (shaded bands labeled to match panels). Symbols: S0 (orange circles), S3 (purple diamonds), S5 (blue crosses). Fits are performed over the highlighted $\rho_{xx}$ windows.