[Paper Review] Magnetism in artificial Ruddlesden-Popper iridates leveraged by structural distortions

Meyers, D., Cao, Yue+19 more|arXiv (Cornell University)|Jul 27, 2017

Advanced Condensed Matter Physics37 citations

TL;DR

This study demonstrates that artificial Ruddlesden-Popper iridate superlattices (nSIO/1STO) exhibit c-axis collinear antiferromagnetic order in bilayer systems, confirmed by azimuthal resonant elastic x-ray scattering and resonant inelastic x-ray scattering (RIXS). The magnon gap in the bilayer (55 meV) is reduced to nearly half that of bulk Sr3Ir2O7 due to structural distortions—specifically, bending of the c-axis Ir-O-Ir bonds and altered local environments—highlighting how spin-orbit coupling amplifies sensitivity to subtle lattice changes in iridates.

ABSTRACT

We report on the tuning of magnetic interactions in superlattices composed of single and bilayer SrIrO$_3$ inter-spaced with SrTiO$_3$. Magnetic scattering shows predominately $c$-axis antiferromagnetic orientation of the magnetic moments for the bilayer justifying these systems as viable artificial analogues of the bulk Ruddlesden-Popper series iridates. Magnon gaps are observed in both superlattices, with the magnitude of the gap in the bilayer being reduced to nearly half that in its bulk structural analogue, Sr$_3$Ir$_2$O$_7$. We assign this to modifications in the anisotropic exchange driven by bending of the $c$-axis Ir-O-Ir bond and subsequent local environment changes, as detected by x-ray diffraction and modeled using spin wave theory. These findings explain how even subtle structural modulations driven by heterostructuring in iridates are leveraged by spin orbit coupling to drive large changes in the magnetic interactions.

Motivation & Objective

• To resolve the controversy over the magnetic ground state in artificial bilayer SrIrO3/SrTiO3 superlattices (2SIO/1STO), which was previously reported as canted in-plane antiferromagnetism despite structural analogies to bulk Sr3Ir2O7.
• To directly probe the magnetic ordering direction and exchange coupling in nSIO/1STO superlattices using azimuthal resonant elastic x-ray scattering (REXS).
• To quantify the magnetic excitation spectrum and magnon gap using resonant inelastic x-ray scattering (RIXS) to assess the impact of heterostructuring on magnetic interactions.
• To link observed changes in magnetic anisotropy and magnon gaps to specific structural distortions, particularly Ir-O-Ir bond bending and local environment modifications.
• To establish whether artificial superlattices can serve as true analogues to bulk Ruddlesden-Popper iridates, especially in terms of magnetic ground state tuning via structural control.

Proposed method

• Growth of high-quality [nSIO/1STO]×m superlattices (n=1,2; m=60,30) via pulsed laser deposition on single-crystal substrates.
• Use of resonant elastic x-ray scattering (REXS) with azimuthal angle-dependent measurements to determine the magnetic moment orientation (c-axis vs. in-plane).
• Acquisition of resonant inelastic x-ray scattering (RIXS) spectra at the Ir L3 edge with 35 meV energy resolution to probe magnon excitation dispersions and gaps.
• X-ray diffraction and structural refinement to quantify octahedral distortions, bond angles, and Ir-O-Ir bond bending in the superlattices.
• Spin wave theory modeling of the magnetic dispersion to extract exchange coupling parameters and compare with bulk Sr3Ir2O7 and Sr2IrO4.
• Comparison of experimental RIXS data with theoretical predictions to validate the c-axis magnetic order and assess the role of oxygen vacancies and strain.

Experimental results

Research questions

• RQ1 Does the bilayer SrIrO3/SrTiO3 superlattice (2SIO/1STO) exhibit c-axis collinear antiferromagnetic order, as in bulk Sr3Ir2O7, or canted in-plane order as previously reported?
• RQ2 How do structural distortions—particularly c-axis Ir-O-Ir bond bending and local environment changes—affect the anisotropic exchange coupling and magnon gap in artificial iridate superlattices?
• RQ3 To what extent does spin-orbit coupling amplify the sensitivity of magnetic interactions to subtle lattice distortions in these heterostructures?
• RQ4 Can the observed reduction in magnon gap (from ~100 meV in bulk to 55 meV in 2SIO/1STO) be quantitatively explained by changes in tetragonal distortion and Ir-O-Ir bond angles?
• RQ5 What is the role of epitaxial strain and oxygen vacancies in stabilizing or destabilizing the c-axis magnetic order in these artificial systems?

Key findings

• The magnetic ground state in 2SIO/1STO is confirmed as c-axis collinear antiferromagnetic order, resolving prior controversy and aligning with bulk Sr3Ir2O7.
• Azimuthal REXS scans show a clear maximum intensity at 90° azimuth, matching the expected response for c-axis moments and ruling out in-plane order.
• RIXS measurements reveal a magnon gap of 55 meV in 2SIO/1STO, reduced to nearly half the 92–100 meV gap observed in bulk Sr3Ir2O7.
• The reduction in magnon gap is attributed to a significant decrease in tetragonal distortion and Ir-O-Ir bond bending, as confirmed by x-ray diffraction and structural refinement.
• Spin wave theory modeling confirms that the modified anisotropic exchange coupling, driven by structural distortions, is responsible for the reduced gap and c-axis stabilization.
• The findings demonstrate that even subtle structural modulations in heterostructured iridates are leveraged by strong spin-orbit coupling to induce large changes in magnetic interactions, pushing the system toward a quantum critical point between ab-plane and c-axis antiferromagnets.