The Origin of Ising Magnetism in Ca₃Co₂O₆ Unveiled by Orbital Imaging

One-dimensional CoO₆ chains in Ca₃Co₂O₆ arranged in a triangular lattice give rise to an Ising-like magnetism with an intriguing quantum tunneling staircase structure in its magnetization. To resolve the underlying local electronic configuration of the Co ions we applied s-core-level non-resonant inelastic x-ray scattering (s-NIXS), a new technique that is capable of imaging the shape of the 3d orbitals in real space. The orbital shapes that we found established unequivocally that both Co sites (octahedral and trigonal prismatic) are in a 3+ valence state (i.e. 3d⁶); the trigonal Co site has a high-spin configuration while the octahedral Co site is low spin. Interestingly, we directly ‘see’ that it is the complex d₂ orbital that is stabilized by the prismatic trigonal coordination, which naturally explains the Ising magnetism in the system. Utilizing this ability to image electron orbitals, and thus directly relating the orbital occupation with the local crystal structure—without the need for theoretical modeling—is essential for modeling magnetic properties. This is especially true in situations where one would like to make use of the delicate balance of competing interactions to stabilize a particular orbital state for a desired or optimized physical property.