Spin-degree manipulation for one-dimensional room-temperature ferromagnetism in a haldane system

The intricate correlation between lattice geometry, topological behavior and charge degrees of freedom plays a key role in determining the physical and chemical properties of a quantum-magnetic system. Herein, we investigate the introduction of the unusual oxidation state as an alternative pathway to modulate the magnetic ground state in the well-known S = 1 Haldane system nickelate Y₂BaNiO₅ (YBNO). YBNO is topologically reduced to incorporate d⁹-Ni⁺ (S = 1/2) in the one-dimensional Haldane chain system. The random distribution of Ni⁺ for the first time results in the emergence of a one-dimensional ferromagnetic phase with a transition temperature far above room temperature. Theoretical calculations reveal that the antiferromagnetic interplay can evolve into ferromagnetic interactions with the presence of oxygen vacancies, which promotes the formation of ferromagnetic order within one-dimensional nickel chains. The unusual electronic instabilities in the nickel-based Haldane system may offer new possibilities towards unconventional physical and chemical properties from quantum interactions.