Jaehong Jeong, Benjamin Lenz, Arsen Gukasov, Xavier Fabrèges, Andrew Sazonov, Vladimir Hutanu, Alex Louat, Dalila Bounoua, Cyril Martins, Silke Biermann, Véronique Brouet, Yvan Sidis, and Philippe Bourges, Phys. Rev. Lett. 125 (2020) 097202.

5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr₂IrO₄ is particularly in the spotlight because of the so-called j_eff = 1/2 state consisting of a quantum superposition of the three local t_2g orbitals with, in its simplest version, nearly equal populations, which stabilizes an unconventional Mott insulating state.

Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local j_eff = 1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows four cross-shaped positive lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.

https://doi.org/10.1103/PhysRevLett.125.097202.

Premakumar Yanda, I. V. Golosovsky, I. Mirebeau, N. V. Ter-Oganessian, Juan Rodríguez-Carvajal and A. Sundaresan

In most of the spin-induced multiferroics, the ferroelectricity is caused by inversion symmetry breaking by complex spin structures of the transition-metal ions. Here, we report the importance of interplay of 4 f -3d magnetic interactions in inducing ferroelectricity in the centrosymmetric (Pnma) green phase compound Gd2BaCuO5. With decreasing temperature, a long-range incommensurate ordering of both Gd3+ and Cu2+ spins at TN = 11.8 K occurs with the modulation vector k = (0, 0, g) and a lock-in transition to a strongly noncollinear structure with kc = (0, 0, 1/2) at Tloc ∼ 6 K. Both spin structures induce electric polarization consistent with the polar magnetic space groups Pm1 (α, 0, g)ss and Paca21, respectively. Based on the symmetry analysis of magnetoelectric interactions, we suggest that the ferroelectricity in both commensurate and incommensurate phases is driven by a complex interplay of two-spins and single-spin contributions from magnetic ions located in noncentrosymmetric environments. Our study demonstrates that the green phase family of compounds may serve as a playground for studying the multiferroic phenomena, where the interplay of 4 f -3d interactions demonstrates an alternative route to find magnetoelectric materials.

https://www.doi.org/10.1103/PhysRevResearch.2.023271