Spin stripe order in a square planar trilayer nickelate*

Highly orbitally polarized trilayer nickelates represent a potential platform for searching for high-T_C superconductivity. La₄Ni₃O₈ undergoes a semiconductor-insulator transition on cooling at T_SI ~ 105 K, which is accompanied by formation of charge stripes. Here we report polarized and unpolarized neutron diffraction measurements which show that the magnetic ground state consists of antiferromagnetic spin stripes that are commensurate with the charge stripes. The magnetic stripes are modeled as occurring within uncorrelated trilayers. Comparison of the charge and spin stripe order parameters reveals that, in contrast to single-layer nickelates and most transition metal oxides, the orders both appear at T_SI, suggesting a strong coupling between the two.