  Magnetic ordering phenomena have a profound influence on the macroscopic
properties of correlated-electron materials, but their realistic prediction
remains a formidable challenge. An archetypical example is the ternary nickel
oxide system RNiO3 (R = rare earth), where the period-four magnetic order with
proposals of collinear and non-collinear structures and the amplitude of
magnetic moments on different Ni sublattices have been subjects of debate for
decades. Here we introduce an elementary model system - NdNiO3 slabs embedded
in a non-magnetic NdGaO3 matrix - and use polarized resonant x-ray scattering
(RXS) to show that both collinear and non-collinear magnetic structures can be
realized, depending on the slab thickness. The crossover between both spin
structures is correctly predicted by density functional theory and can be
qualitatively understood in a low-energy spin model. We further demonstrate
that the amplitude ratio of magnetic moments in neighboring NiO6 octahedra can
be accurately determined by RXS in combination with a correlated double cluster
model. Targeted synthesis of model systems with controlled thickness and
synergistic application of polarized RXS and ab-initio theory thus provide new
perspectives for research on complex magnetism, in analogy to two-dimensional
materials created by exfoliation.
