Biaxial magnetic alignment and orthorhombic twin microstructure in layered nickelates La₃Ni₂O₇

Abstract The synthesis of large bulk nickelate single crystals remains challenging. We demonstrated room-temperature biaxial magnetic alignment of polycrystalline La 3 Ni 2 O 7+ δ ( δ = 0) using a modulated rotating field (MRF). Phase-pure powders (∼5 μm) were aligned under 7 T-MRF during epoxy curing, exploiting triaxial susceptibility ( χ a > χ b > χ c ). X-ray diffraction (XRD) and pole-figures confirm: Δ ϕ = 4° in-plane orientation degrees and orthorhombic twin domains via ( h 00)/(0 k 0) peaks and fourfold symmetry. These twins, analogous to cuprates but distinct in NiO 6 origin, provide the first crystallographic evidence of domain microstructure in nickelates, enabling direction-resolved studies of anisotropy of density-order phase and superconductivity obscured by heterogeneity.