Strongly correlated oxide nanosheets: A novel platform for 2D magnetism

Abstract

The 2017 discovery of intrinsic 2D magnetism in CrI3 and Cr2Ge2Te6 demonstrated that magnetic order can persist in atomically thin layers, challenging the Hohenberg-Mermin-Wagner theorem. While most 2D magnets are van der Waals materials, this thesis explores oxide nanosheets as a novel platform. In 4d and 5d layered transition metal oxides, the interplay of electronic correlations, spin-orbit coupling (SOC), and kinetic energy gives rise to exotic physics. The thesis investigates Kitaev spin-liquid behavior in Na2IrO3 nanosheets and room-temperature antiferromagnetism in SrRu2O6 flakes. In ultrathin Na2IrO3, first-principles calculations reveal a stabilized zigzag antiferromagnetic state due to enhanced Heisenberg and off-diagonal interactions. Self-doping in unpassivated flakes induces transitions from Mott insulating to metallic and from AFM to FM phases, showcasing the promise of non-van der Waals oxides for unconventional 2D magnetism. SrRu2O6 nanosheets maintain AFM order above 430 K despite 2D spin fluctuations. Surface-induced charge doping modulates Néel temperatures and electronic phases, with distinct effects for electron and hole doping. The study also examines magnetic anisotropy in d3 compounds with honeycomb and triangular lattices. SOC and orbital configurations near the Fermi level govern spin alignment, with doping, strain, and confinement inducing easy-plane anisotropy. The role of pz and dz2 orbitals in materials like CrI3, CrTe2, and SrRu2O6 is highlighted. Overall, the thesis advances understanding of 2D correlated oxide magnetism, emphasizing the critical roles of SOC, electron correlations, and structural distortions in designing quantum and spintronic devices.