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Surface-Localized Magnetic Order in RuO2 Thin Films Revealed by Low-Energy Muon Probes

Akashdeep Akashdeep, Sachin Krishnia, Jae-Hyun Ha, Siyeon An, Maik Gaerner, Thomas Prokscha, Andreas Suter, Gianluca Janka, Günter Reiss, Timo Kuschel, Dong-Soo Han, Angelo Di Bernardo, Zaher Salman, Gerhard Jakob, Mathias Kläui

TL;DR

The paper addresses the unclear magnetic ground state of RuO2, particularly in thin films, and whether magnetism is a bulk property or confined to surfaces. It employs depth-resolved low-energy muon spin rotation (LE-muSR) to probe near-surface magnetic order in RuO2 films grown on two substrates and by two deposition methods. The main finding is that magnetism is surface-confined to the top approximately 10 nm with a maximum near-surface magnetic volume fraction of about 8.5%, and no evidence of bulk long-range order is observed. This work reconciles conflicting reports by highlighting the importance of reduced dimensionality and surface defects, with implications for interpreting altermagnetism and guiding future spintronic applications in oxide thin films.

Abstract

Ruthenium dioxide (RuO2) has recently emerged as a candidate altermagnet, yet its intrinsic magnetic ground state, particularly in thin films, remains debated. This study aims to clarify the nature and spatial extent of the magnetic order in RuO2 thin films grown under different conditions. Thin films of RuO2 with thicknesses of 30 nm and 33 nm are fabricated by pulsed laser deposition and sputtering onto TiO2(110) and Al2O3(1-102) substrates, respectively. Low-energy muon spin rotation/relaxation (LE-muSR) with depth-resolved sensitivity measurements is performed in transverse magnetic fields (TF) from 4 K to 290 K. The muSR data collected with a muon implantation energy of 1 keV reveal that magnetic signals originate from the near-surface region of the film (<10 nm), and the affected volume fraction is at most about 8.5%. The localized magnetic response is consistent across different substrates, growth techniques, and parameter sets, suggesting a common origin related to surface defects and dimensionality effects. The combined use of TF-muSR and the study of depth-dependent implantation with low-energy muons provides direct evidence for surface-confined, inhomogeneous static magnetic order in RuO2 thin films, helping reconcile discrepancies. These findings underscore the importance of considering reduced-dimensional contributions and motivate further investigation into the role of defects, strain, and stoichiometry on the magnetic properties of RuO2, especially at the surface.

Surface-Localized Magnetic Order in RuO2 Thin Films Revealed by Low-Energy Muon Probes

TL;DR

The paper addresses the unclear magnetic ground state of RuO2, particularly in thin films, and whether magnetism is a bulk property or confined to surfaces. It employs depth-resolved low-energy muon spin rotation (LE-muSR) to probe near-surface magnetic order in RuO2 films grown on two substrates and by two deposition methods. The main finding is that magnetism is surface-confined to the top approximately 10 nm with a maximum near-surface magnetic volume fraction of about 8.5%, and no evidence of bulk long-range order is observed. This work reconciles conflicting reports by highlighting the importance of reduced dimensionality and surface defects, with implications for interpreting altermagnetism and guiding future spintronic applications in oxide thin films.

Abstract

Ruthenium dioxide (RuO2) has recently emerged as a candidate altermagnet, yet its intrinsic magnetic ground state, particularly in thin films, remains debated. This study aims to clarify the nature and spatial extent of the magnetic order in RuO2 thin films grown under different conditions. Thin films of RuO2 with thicknesses of 30 nm and 33 nm are fabricated by pulsed laser deposition and sputtering onto TiO2(110) and Al2O3(1-102) substrates, respectively. Low-energy muon spin rotation/relaxation (LE-muSR) with depth-resolved sensitivity measurements is performed in transverse magnetic fields (TF) from 4 K to 290 K. The muSR data collected with a muon implantation energy of 1 keV reveal that magnetic signals originate from the near-surface region of the film (<10 nm), and the affected volume fraction is at most about 8.5%. The localized magnetic response is consistent across different substrates, growth techniques, and parameter sets, suggesting a common origin related to surface defects and dimensionality effects. The combined use of TF-muSR and the study of depth-dependent implantation with low-energy muons provides direct evidence for surface-confined, inhomogeneous static magnetic order in RuO2 thin films, helping reconcile discrepancies. These findings underscore the importance of considering reduced-dimensional contributions and motivate further investigation into the role of defects, strain, and stoichiometry on the magnetic properties of RuO2, especially at the surface.

Paper Structure

This paper contains 5 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Results
  4. Discussion
  5. Conclusions

Figures (3)

  • Figure 1: XRD patterns measured with the scattering vector oriented normal to the (110)-oriented rutile TiO$_2$ and the (1̄102)-oriented Al$_2$O$_3$ substrate.
  • Figure 2: Dependence of muon spin damping rate $\sigma$, initial asymmetry $A_0$, and fraction of muons stopping in the thin film and substrate on implantation energy at different temperatures for (a) RuO$_2$(110)/TiO$_2$(110) films (b) RuO$_2$(101)/Al$_2$O$_3$(1̄102) films at applied magnetic field of 10mT and (c) The green and orange shaded areas represent the fraction of muons in the RuO$_2$ and substrate layers, respectively.
  • Figure 3: Temperature dependence of normalized initial asymmetry $A_0^{N}$ and muon spin damping rate $\sigma$ films at different implantation energies for (a) RuO$_2$(110)/TiO$_2$(110) films and (b) RuO$_2$(101)/Al$_2$O$_3$(1̄102) films at applied magnetic field of 10mT.