A Transition from Localized to Strongly Correlated Electron Behavior and Mixed Valence Driven by Physical or Chemical Pressure in ACo_2As_2 (A = Eu and Ca)

摘要

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo_2As_2 (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferro- magnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (＜0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo_2As_2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo_2As_2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo_2As_2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca_(0.9)Eu_(0.1)Co_(1.91)As_2 or direct electron doping in Ca_(0.85)La_(0.15)Co_(1.89)As_2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.