Electronically highly cubic conditions for Ru in alpha-RuCl3

We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the t2g orbitals is -12 $\pm10$ meV, i.e. negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a Jeff = 1/2 ground state. We have thus shown that as far as the Ru 4d local properties are concerned, alpha-RuCl3 is an ideal candidate for the realization of Kitaev physics

Electronic structure and magnetic exchange interactions of Cr-based van der Waals ferromagnets. A comparative study between CrBr3 and Cr2Ge2Te6

Low dimensional magnetism has been powerfully boosted as a promising candidate for numerous applications. The stability of the long-range magnetic order is directly dependent on the electronic structure and the relative strength of the competing magnetic exchange constants. Here, we report a comparative pressure-dependent theoretical and experimental study of the electronic structure and exchange interactions of two-dimensional ferromagnets CrBr3 and Cr2Ge2Te6 . While CrBr3 is found to be a Mott-Hubbard-like insulator, Cr2Ge2Te6 shows a charge-transfer character due to the broader character of the Te 5p bands at the Fermi level. This different electronic behaviour is responsible of the robust insulating state of CrBr3 , in which the magnetic exchange constants evolve monotonically with pressure, and the proximity to a metal-insulator transition predicted for Cr2Ge2Te6 , which causes a non-monotonic evolution of its magnetic ordering temperature. We provide a microscopic understanding for the pressure evolution of the magnetic properties of the two systems.This work was supported by the Spanish Govern- ment through Projects MAT2016-80762-R, PGC2018- 101334-B-C21 and PGC2018-101334-A-C22. A.O.F. also through the FPU16/02572 grant. J.S.Z was supported by National Science Foundation grant DMR-1720595.Center for Dynamics and Control of Material

Low-field switching of noncollinear spin texture at La0.7Sr0.3MnO3-SrRuO(3 )interfaces

Interfaces of ferroic oxides can show complex magnetic textures which have strong impact on spintronics devices. This has been demonstrated recently for interfaces with insulating antiferromagnets such as BiFeO3. Here, noncollinear spin textures which can be switched in very low magnetic field are reported for conducting ferromagnetic bilayers of La0.7Sr0.3MnO3-SrRuO3 (LSMO-SRO). The magnetic order and switching are fundamentally different for bilayers coherently grown in reversed stacking sequence. The SRO top layer forms a persistent exchange spring which is antiferromagnetically coupled to LSMO and drives switching in low fields of a few milliteslas. Density functional theory reveals the crucial impact of the interface termination on the strength of Mn-Ru exchange coupling across the interface. The observation of an exchange spring agrees with ultrastrong coupling for the MnO2/SrO termination. Our results demonstrate low-field switching of noncollinear spin textures at an interface between conducting oxides, opening a pathway for manipulating and utilizing electron transport phenomena in controlled spin textures at oxide interfaces

Surface states and Rashba-type spin polarization in antiferromagnetic MnBi2Te4 (0001)

The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators such as Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.We acknowledge financial support from the DFG through SFB1170 ’Tocotronics’, SFB1143 ’Correlated Magnetism’, SPP 1666 ’Topological insulators’, ERA-Chemistry Programm (RU-776/15-1), and the Wurzburg-Dresden Cluster of ¨Excellence on Complexity and Topology in Quantum Matter – ct.qmat (EXC 2147, project-id 39085490). We also acknowledge the support by Spanish Ministerio de Economia y Competitividad (MINECO Grant No. FIS2016-75862-P), Academic D.I. Mendeleev Fund Program of Tomsk State University (Project No. 8.1.01.2018), the Saint Petersburg State University grant for scientic investigations (Grant No. 15.61.202.2015), and Russian Foundation for Basic Research (Grant No. 18-52-06009). S.M. acknowledges support by the Swiss National Science Foundation (Grant No. P300P2-171221). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. We acknowledge Diamond Light Source for access to beamline I05 (proposals No. SI19278 and No. SI22468) that contributed to the results presented here. Parts of this research were carried out at PETRA III (DESY, Hamburg, Germany) under Proposal No. I-20180510. This work has been partly performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MIUR, Italy) facility. M.M.O. acknowledges support by the Diputacion Foral de Gipuzkoa ((SAREA 2018 - RED ´2018, project no. 2018-CIEN-000025-01)