Preparation and Characterization of Francisite Solid Solutions Cu₃Bi(Se_1−xTe_xO₃)₂O₂Br (x = 0−1): Possibility for Francisites as Starting Materials for Oxide van der Waals Ferromagnets

We synthesized the solid solutions Cu₃Bi-(Se_1−xTe_xO₃)₂O₂Br (x = 0, 0.25, 0.5, 0.75, 1) for the first time and characterized their structures by X-ray diffraction (XRD) measurements and their magnetic properties by electron spin resonance (ESR), magnetic susceptibility, magnetization, and specific heat measurements as well as by density functional theory (DFT) calculations. The Neél temperature T_N and the critical field μ₀H_C needed for the metamagnetic transition increase with x, while the magnetization at a given magnetic field and the Curie− Weiss temperature Θ decrease with x. We show that the tendency for the interlayer antiferromagnetic (AFM) coupling in francisites is not explained by interlayer spin exchange but by the interlayer high-spin orbital interaction that occur across the van der Waals (vdW) gaps, hence indicating that francisites are vdW ferromagnets. This is surprising because, so far, well-established vdW ferromagnets are either layered tellurides or layered iodides. The trends in T_N, μ₀H_C, and the magnetization of Cu₃Bi(Se_1−xTe_xO₃)₂O₂Br as a function of x are well explained by the interlayer AFM interactions, and that in the Θ is explained by the intralayer spin exchanges. We proposed how one might modify francisites to make their interlayer interactions ferromagnetic (FM) hence leading to oxide vdW ferromagnets (Figure Presented).