Defect-manipulated magnetoresistance and above-room-temperature ferromagnetism in two-dimensional BaNi2V2O8

The intricate correlation between multiple degrees of freedom and physical properties is a fascinating area in solid state chemistry and condensed matter physics. Here, we report a quantum-magnetic system BaNi2V2O8 (BNVO), in which the spin correlation was modulated by unusual oxidation state, leading to different magnetic behavior. The BNVO was modified with topochemical reduction (TR) to yield TR-BNVO with partially reduced valance state of Ni+ in the two-dimensional NiO6-honeycomb lattice. Accordingly, the antiferromagnetic order is suppressed by the introduction of locally interposed Ni+ and oxygen vacancies, resulting in a ferromagnetic ground state with the transition temperature up to 710 K. A positive magnetoresistance (7.5%) was observed in the TR-BNVO at 40 K under 7 T. These findings show that topological reduction is a powerful approach to engineer low-dimensional materials and accelerate the discovery of new quantum magnetism.