O K-edge XAS measurements reveal a surface reconstruction in RBaCuFeO _{5+ δ} layered perovskites when δ > 0. Oxygen excess, together with some degree of A-site cation intermixing, appears to control the OER activity in these Co-free electrocatalysts.

The layered perovskite ${\mathrm{YBaCuFeO}}_5$ is a rare example of a cycloidal spiral magnet whose ordering temperature $T_{\text{spiral}}$ can be tuned far beyond room temperature by adjusting the degree of ${\mathrm{Cu}}^{2+}/{\mathrm{Fe}}^{3+}$ chemical disorder in the structure. This unusual property qualifies this material as one of the most promising spin-driven multiferroic candidates. However, very little is known about the response of the spiral to magnetic fields, crucial for magnetoelectric cross-control applications. Using bulk magnetization and neutron powder diffraction measurements under magnetic fields up to 9 T, we report here a temperature-magnetic field phase diagram of this material. Besides revealing a strong stability of the spiral state, our data uncover the presence of weak ferromagnetism coexisting with the spiral modulation. Since ferromagnets can be easily manipulated with magnetic fields, this observation opens new perspectives for the control of the spiral orientation, directly linked to the polarization direction, as well as for a possible future use of this material in technological applications.

We probe the spectrum of elementary excitations in SrIrO 3 by using heterostructured [ ( SrIrO 3 ) m / ( SrTiO 3 ) l ] samples to approach the bulk limit. Our resonant inelastic x-ray scattering (RIXS) measurements at the Ir L 3 edge reveal a robust low-lying collective magnetic mode with an antiferromagnetic (AFM) dispersion similar to the insulators Sr 2 IrO 4 and Sr 3 Ir 2 O 7 , albeit with a large gap and much larger linewidth. At higher energies we find the spin-orbit exciton, also strongly broadened, but with an inverted dispersion and doubled periodicity that are controlled by the charge hopping. These results demonstrate that the AFM paramagnon persists, somewhat counterintuitively, far into the metallic regime of the insulator-metal transition driven by the degree of confinement in the heterostructure. We conclude that these two excitations, which are contrasting but coexisting hallmarks of strong AFM pseudospin and charge fluctuations in a spin-orbit-coupled Mott-Slater material, are properties intrinsic to the ground state of semimetallic perovskite SrIrO 3 .