  Trilayer nickelates, which exhibit a high degree of orbital polarization
combined with an electron count (d8.67) corresponding to overdoped cuprates,
have been identified as a promising candidate platform for achieving high-Tc
superconductivity. One such material, La4Ni3O8, undergoes a
semiconductor-insulator transition at ~105 K, which was recently shown to arise
from the formation of charge stripes. However, an outstanding issue has been
the origin of an anomaly in the magnetic susceptibility at the transition and
whether it signifies formation of spin stripes akin to single layer nickelates.
Here we report single crystal neutron diffraction measurements (both polarized
and unpolarized) that establish that the ground state is indeed magnetic. The
ordering is modeled as antiferromagnetic spin stripes that are commensurate
with the charge stripes, the magnetic ordering occurring in individual
trilayers that are essentially uncorrelated along the crystallographic c-axis.
Comparison of the charge and spin stripe order parameters reveals that, in
contrast to single-layer nickelates such as La2-xSrxNiO4 as well as related
quasi-2D oxides including manganites, cobaltates, and cuprates, these orders
uniquely appear simultaneously, thus demonstrating a stronger coupling between
spin and charge than in these related low-dimensional correlated oxides.
