Magnetism in iridate heterostructures leveraged by structural distortions

Abstract

Fundamental control of magnetic coupling through heterostructure morphology is a prerequisite for rational engineering of magnetic ground states. We report the tuning of magnetic interactions in superlattices composed of single and bilayers of srIro 3 inter-spaced with srtio 3 in analogy to the Ruddlesden-popper series iridates. Magnetic scattering shows predominately c-axis antiferromagnetic orientation of the magnetic moments for the bilayer, as in sr 3 Ir 2 o 7. However, the magnetic excitation gap, measured by resonant inelastic x-ray scattering, is quite different between the two structures, evidencing a significant change in the stability of the competing magnetic phases. In contrast, the single layer iridate hosts a more bulk-like gap. We find these changes are driven by bending of the caxis Ir-o-Ir bond, which is much weaker in the single layer, and subsequent local environment changes, evidenced through x-ray diffraction and magnetic excitation modeling. Our findings demonstrate how large changes in the magnetic interactions can be tailored and probed in spin-orbit coupled heterostructures by engineering subtle structural modulations. Atomic scale layering of disparate materials to form an artificial heterostructure is a promising method for the intelligent design of emergent properties 1-3. Systems incorporating strong spin-orbit coupling are particularly intriguing as candidates for designer magnetic phases due to the coupling of the magnetic moments to structural distortions. Towards this goal, iridates, compounds composed of active Ir 5d orbitals in oxygen octahedra, have emerged as an important new class of correlated material 4-13. The combination of crystal field interactions and strong spin-orbit coupling, λ~0.5 eV, generates narrow bands often leading to Mott insulating antiferromagnetic ground states despite the modest values of the Coulomb repulsion U~1 eV 5,14,15. The resulting = J eff 1 2 pseudospin states are then intimately coupled with the Ir orbitals and thus are much more sensitive to structural changes than pure spin = S 1 2 moments. This is exemplified by the different magnetic ground states appearing in iridates composed of similar Ir-O octahedra when they are subtly distorted or interspaced with different atoms. For instance, Sr 2 IrO 4 , hosting isolated IrO 2 layers, forms an ab-plane canted antiferromagnetic state 6,16 , while Sr 3 Ir 2 O 7 , hosting isolated IrO 2 bi-layers, has robust c-axis collinear antiferromagnetic ordering stabilized by a 90 meV magnon gap 17,18. These competing magnetic ground states establish the iridates as ideal candidates for utilizing controlled structural perturbation to rationally engineer the magnetic behavior. Towards this goal, layered iridate heterostructures, or superlattices (SL), were recently realized via alternating layers of nSrIrO 3 and SrTiO 3 (nSIO/1STO) opening up routes to tailor magnetic phenomena within artificial Ruddlesden-Popper analogues 14,19. How magnetic couplings change within such heterostructures as compared to their bulk...

Probing surfaces and interfaces in complex oxide films via in situ X-ray photoelectron spectroscopy

“…(111)-oriented SrIrO 3 /SrTiO 3 heterostructures have also been synthesized and proposed as topological materials that exhibit the desirable honeycomb lattice for topological quantum information systems [44]. Similar (100)-oriented SrIrO 3 /SrTiO 3 superlattices have also been shown to behave like layered Ruddelsden-Popper iridates by reducing the iridate layer to 2 unit cells (UCs) [45]. The SrMnO 3 /SrIrO 3 interface has also been studied through superlattice synthesis and shown to exhibit ferromagnetism due to electron transfer from Ir to Mn across the interface, leading to the anomalous Hall effect in these structures [46,47].…”

Low-field magnetic anisotropy of Sr2IrO4

“…There are two distinguishable domains: one with isospins along the b axis and the other with isospins along the a axis. A correlation exists between the isospin direction and the stacking pattern, which implies the presence of anisotropic interlayer coupling [30]. The regions can be divided into a low field (<0.3 T) with a mixed-domain state and a high field (>0.3 T) with the WFM state.…”

Anomalous magnetoresistance due to longitudinal spin fluctuations in a Jeff = 1/2 Mott semiconductor

“…The structure of our SL is designed to minimize such competition as the octahedral network is rotated in the same way among all IrO 6 layers (Fig. 1d) 49 . For comparison, the pseudospin-half iridate Sr 2 IrO 4 has a much more complicated layered structure and a magnetic unit cell that contains four IrO 6 layers with a substantial and nontrivial interlayer interaction that favors cancellation of the canted moments 33 .…”