  The nature of the magnetic order in (La2/3Sr1/3MnO3)9/(LaNiO3)3 superlattices
is investigated using x-ray resonant magnetic reflectometry. We observe a new
c-axis magnetic helix state in the (LaNiO3)3 layers that had never been
reported in nickelates, and which mediates the ~130deg magnetic coupling
between the ferromagnetic (La2/3Sr1/3MnO3)9 layers, illustrating the power of
x-rays for discovering the magnetic state of complex oxide interfaces. Resonant
inelastic x-ray scattering and x-ray absorption spectroscopy show that Ni-O
ligand hole states from bulk LaNiO3 are mostly filled due to interfacial
electron transfer from Mn, driving the Ni orbitals closer to an atomic-like 3d8
configuration. We discuss the constraints imposed by this electronic
configuration to the microscopic origin of the observed magnetic structure. The
presence of a magnetic helix in (La2/3Sr1/3MnO3)9/(LaNiO3)3 is crucial for
modeling the potential spintronic functionality of this system and may be
important for designing emergent magnetism in novel devices in general.
