Visualization of Co-3d high- and low-spin states in an Ising spin chain magnet

Abstract

Properties of trivalent cobalt oxide compounds are largely influenced by the spin state of the six 3d electrons at each Co site. High-spin Co3+ ions, where orbital angular momentum is only partially quenched, often exhibit significant anisotropy, providing playgrounds for Ising spin systems. However, real-space observations of their orbital states have remained limited. Here, we determine the Co-3d spin and orbital states in an Ising spin chain magnet Ca3Co2O6, where high- and low-spin states alternate along the chain. Synchrotron x-ray diffraction and valence electron density (VED) analysis, utilizing the core differential Fourier synthesis (CDFS) method, reveal distinct anisotropic VED distributions around the two inequivalent Co sites. The VED distribution around the octahedral Co site corresponds to a low-spin state, while the trigonal-prismatic Co site exhibits anisotropic VED that cannot be explained by the crystal electric field (CEF) alone. This anisotropy is quantitatively captured by a model incorporating CEF, spin-orbit coupling, and on-site 3d-4p orbital hybridization, consistent with a high-spin state exhibiting Ising magnetism.