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Register a new userMagnetic properties of bilayer Sr$_3$Ir$_2$O$_7$: role of epitaxial strain and oxygen vacancies
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Using {it ab initio} methods, we investigate the modification of the magnetic properties of the $m=2$ member of the strontium iridates Ruddlesden-Popper series Sr$_{m+1}$Ir$_{m}$O$_{3m+1}$, bilayer Sr$_3$Ir$_2$O$_7$, induced by epitaxial strain and oxygen vacancies. Unlike the single layer compound Sr$_2$IrO$_4$, which exhibits a robust in-plane magnetic order, the energy difference between in-plane and out-of-plane magnetic orderings in Sr$_3$Ir$_2$O$_7$ is much smaller and it is expected that small external perturbations could induce magnetic transitions. Our results indicate that epitaxial strain yields a spin-flop transition, that is driven by the crossover between the intralayer $J_1$ and interlayer $J_2$ magnetic exchange interactions upon compressive strain. While $J_1$ is essentially insensitive to strain effects, the strength of $J_2$ changes by one order of magnitude for tensile strains $geq$ 3~%. In addition, our study clarifies that the unusual in-plane magnetic response observed in Sr$_3$Ir$_2$O$_7$ upon the application of an external magnetic field originates from the canting of the local magnetic moments due to oxygen vacancies, which tilt the octahedral networks - thereby allowing for noncollinear spin configurations.
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Rare earth pyrochlore Iridates (RE2Ir2O7) consist of two interpenetrating cation sublattices, the RE with highly-frustrated magnetic moments, and the Iridium with extended conduction orbitals significantly mixed by spin-orbit interactions. The coexistence and coupling of these two sublattices create a landscape for discovery and manipulation of quantum phenomena such as the topological Hall effect, massless conduction bands, and quantum criticality. Thin films allow extended control of the material system via symmetry-lowering effects such as strain. While bulk Pr2Ir2O7 shows a spontaneous hysteretic Hall effect below 1.5K, we observe the effect at elevated temperatures up to 15K in epitaxial thin films on (111) YSZ substrates synthesized via solid phase epitaxy. Similar to the bulk, the lack of observable long-range magnetic order in the thin films points to a topological origin. We use synchrotron-based element-specific x-ray diffraction (XRD) and x-ray magnetic circular dichroism (XMCD) to compare powders and thin films to attribute the spontaneous Hall effect in the films to localization of the Ir moments. We link the thin film Ir local moments to lattice distortions absent in the bulk-like powders. We conclude that the elevated-temperature spontaneous Hall effect is caused by the topological effect originating either from the Ir or Pr sublattice, with interaction strength enhanced by the Ir local moments. This spontaneous Hall effect with weak net moment highlights the effect of vanishingly small lattice distortions as a means to discover topological phenomena in metallic frustrated magnetic materials.
Through a neutron scattering, charge transport, and magnetization study, the correlated ground state in the bilayer iridium oxide Sr$_3$Ir$_2$O$_7$ is explored. Our combined results resolve scattering consistent with a high temperature magnetic phase that persists above 600 K, reorients at the previously defined $T_{AF}=280$ K, and coexists with an electronic ground state whose phase behavior suggests the formation of a fluctuating charge or orbital phase that freezes below $T^{*}approx70$ K. Our study provides a window into the emergence of multiple electronic order parameters near the boundary of the metal to insulator phase transition of the 5d $J_{eff}=1/2$ Mott phase.
Oxygen vacancies can be of utmost importance for improving or deteriorating physical properties of oxide materials. Here, we studied from first-principles the electronic and magnetic properties of oxygen vacancies in the double perovskite Sr$_2$FeMoO$_6$ (SFMO). We show that oxygen vacancies can increase the Curie temperature in SFMO, although the total magnetic moment is reduced at the same time. We found also that the experimentally observed valence change of the Fe ions from $3+$ to $2+$ in the x-ray magnetic circular dichroism (XMCD) measurements is better explained by oxygen vacancies than by the assumed mixed valence state. The agreement of the calculated x-ray absorption spectra and XMCD results with experimental data is considerably improved by inclusion of oxygen vacancies.
We report that in unannealed LaAlO3/SrTiO3 (LAO/STO) heterostructures the critical thickness for the appearance of the two-dimensional electron gas can be less than 4 unit cell (uc), the interface is conducting even for STO substrates with mixed terminations and the low-temperature resistance upturn in LAO/STO heterostructures with thick LAO layers strongly depends on laser fluence. Our experimental results provide fundamental insights into the different roles played by oxygen vacancies and polarization catastrophe in the two-dimensional electron gas in crystalline LAO/STO heterostructures.
The ability to manipulate oxygen anion defects rather than metal cations in complex oxides can facilitate creating new functionalities critical for emerging energy and device technologies. However, the difficulty in activating oxygen at reduced temperatures hinders the deliberate control of important defects, oxygen vacancies. Here, strontium cobaltite (SrCoOx) is used to demonstrate that epitaxial strain is a powerful tool for manipulating the oxygen vacancy concentration even under highly oxidizing environments and at annealing temperatures as low as 300 C. By applying a small biaxial tensile strain (2%), the oxygen activation energy barrier decreases by ~30%, resulting in a tunable oxygen deficient steady-state under conditions that would normally fully oxidize unstrained cobaltite. These strain-induced changes in oxygen stoichiometry drive the cobaltite from a ferromagnetic metal towards an antiferromagnetic insulator. The ability to decouple the oxygen vacancy concentration from its typical dependence on the operational environment is useful for effectively designing oxides materials with a specific oxygen stoichiometry.
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