Electron spin resonance spectroscopy on complex iridium oxides

• Applicant: Dr. Vladislav Kataev
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2014 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 260000137

• Final Report Year: 2020

Final Report Abstract

The primary goal of the project was to apply high-frequency high-magnetic field electron spin resonance spectroscopy (HF–ESR) for a systematic study of static and dynamic magnetic properties of complex iridium oxides which in the recent past have emerged as a novel kind of quantum magnets called spinorbital (SO) Mott insulators and attracted strong interdisciplinary interest worldwide. During the implementation of the project, a number of significant results were obtained, giving new interesting insights into the rich physics of this class of materials: (i) The HF–ESR studies of the first prototype SO Mott insulator, the layered Sr2 IrO4 , revealed a surprising inversion of the spectroscopic g -factor implying an inversion of the ordering of the t2g orbital states of the Ir4+ (5d 5 ) ions. This effect was confirmed and understood by ab initio quantum chemistry methods as being due to a specific distribution of the ionic charges in the layered structure of Sr2 IrO4 which opens a promising route to tailoring the electronic properties in artificial multilayered oxide heterostructures. (ii) A combined HF–ESR and nuclear magnetic resonance (NMR) investigation of three representatives of the important class of iridates, the double perovskites of the La2 BIrO6 family with magnetic 3d ions Co2+ and Cu2+ , as well as with non-magnetic Zn2+ at the B-site revealed a complex cross-coupling between 5d and 3d spin systems prone to magnetic frustration and exhibiting a remarkable gradual temperature evolution of the spin dynamics across the magnetic ordering temperature. The obtained results point at the important role of the multiple, competing exchange pathways involving spatially extended, strongly anisotropic 5d magnetic orbitals in the Ir-based double perovskites and stimulate further development of theoretical concepts behind the nontrivial magnetism of this type of strongly SO coupled materials. (iii) The double-perovskite iridates Sr2 YIrO6, Ba2 YIrO6, and their solid solutions containing usually nonmagnetic Ir5+ (5d 4) ions have received a great deal of interest due to controversial reports on the observation of either strongly antiferromagnetic behavior with static magnetic order at a low temperature or only a weak paramagnetism. Our multi-frequency HF–ESR experiments have ultimately resolved this controversy. Our data evidence that the magnetism of the studied material is solely due to a few percent of Ir4+ (5d 5) and Ir6+ (5d 3) magnetic defects while the regular Ir5+ (5d 4) sites remain nonmagnetic. Our findings highlight the relevance of the long-range magnetic interactions in 5d double perovskites proposed in recent theoretical models which might be even responsible for the magnetic order of defect Ir-based spin centers in Ba2 YIrO6 if their concentration exceeds a certain threshold value. (iv) Realization of the celebrated Kitaev model with compass, bond-dependent Ising interactions on the honeycomb lattice, yielding an exotic spin liquid ground state whose fractionalized excitations are dispersive Majorana fermions and static Ising gauge fluxes (visons) was searched originally in the honeycomb 5d 5 iridates. However, recently the 4d 5 compound αRuCl3 was proposed as a promising candidate to host Kitaev physics. Using the HF–ESR technique we studied the non-resonant low-energy microwave absorption (MWA) in αRuCl3 . With MWA we observed a continuum of magnetic excitations at the Brillouin-zone center q→ = 0 which gets progressively gapped above the critical field Hc for the suppression of the magnetic order. The continuum suggests a natural explanation in terms of fractionalized excitations and we consider it likely that the continuum probed by MWA may represent genuine spin-liquid physics.

Publications

• Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers, Nature Comm 6, 7306 (2015)
  N. A. Bogdanov, V. M. Katukuri, J. Romhanyi, V. Yushankhai, V. Kataev, B. Büchner, J. van den Brink, L. Hozoi
  
• Noncollinear antiferromagnetism of coupled spins and pseudospins in the double perovskite La2 CuIrO6 , Phys. Rev. B 94, 144437 (2016)
  K. Manna, R. Sarkar, S. Fuchs, Y. A. Onykiienko, A. K. Bera, G. A. Cansever, S. Kamusella, A. Maljuk, C. G. F. Blum, L. T. Corredor, A. U. B. Wolter, S. M. Yusuf, M. Frontzek, L. Keller, M. Iakovleva, E. Vavilova, H. J. Grafe, V. Kataev, H. H. Klauss, D. S. Inosov, S. Wurmehl, B. B. Büchner
  
• Elektronenspinresonanz an Iridaten in Doppelperowskitstrukturen, Dissertation zur Erlangung des Hochschulgrades Doctor rerum naturalium, Technische Universität Dresden, Dresden (2017)
  S. Fuchs
  
• Signatures of low-energy fractionalized excitations in ¸-RuCl3 from field-dependent microwave absorption, Phys. Rev. B 98, 184408 (2018)
  C. Wellm, J. Zeisner, A. Alfonsov, A. U. B. Wolter, M. Roslova, A. Isaeva, T. Doert, M. Vojta, B. Büchner, V. Kataev
  
• Static and dynamic magnetism of the Ir-based double perovskites La2 BIrO6 (B = Co, Zn) probed by magnetic resonance spectroscopies, Phys. Rev. B 98, 174401 (2018)
  M. Iakovleva, S. Fuchs, A. Alfonsov, H. J. Grafe, M. Vogl, S. Aswartham, S. Wurmehl, T. Dey, B. Büchner, E. Vavilova, V. Kataev
  
• Unraveling the Nature of Magnetism of the 5d4 Double Perovskite Ba2 YIrO6 , Phys. Rev. Lett. 120, 237204 (2018)
  S. Fuchs, T. Dey, G. Aslan-Cansever, A. Maljuk, S. Wurmehl, B. Büchner, V. Kataev
  
• Magnetic resonance spectroscopy on low-dimensional and frustrated magnets, Dissertation zur Erlangung des Hochschulgrades Doctor rerum naturalium, Technische Universität Dresden, Dresden (2019)
  M. Iakovleva