Electromagnetic evolution in proton-containing spinel NiCo2O4 thin films via ionic liquid gating

Proton incorporation through ionic liquid gating (ILG) has become an efficient strategy to manipulate the novel ground-state properties in complex oxides. However, the mechanisms of proton evolution and their correlations with other degrees of freedom in quantum material systems are still unclear. Herein, we provide insights into the correlated interactions among different lattice-spin-orbital-spin degrees of freedom in spinel ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ (NCO) films with a complex local electronic configuration and coordination environments. Our results reveal that the Ni ions near the Fermi level show the prior ${\mathrm{Ni}}^{3+}\ensuremath{\rightarrow}{\mathrm{Ni}}^{2+}$ response, whereas the Co ions at different octahedral and tetrahedral sites exhibit a stepwise ${\mathrm{Co}}^{3+}\ensuremath{\rightarrow}{\mathrm{Co}}^{2+}$ evolution. The gradual evolutions reflected in both structure and electronic configuration aspects determine the different electronic conducting mechanisms as well as the variation of the magnetic ground states in the protonated NCO films. Our systematic structure-property investigations shed light on the fundamental understanding of the strongly correlated nature in quantum materials and the manipulation of quantum materials with desired functionalities through the ILG approach.