title: A Transition from Localized to Strongly Correlated Electron Behavior and Mixed Valence Driven by Physical or Chemical Pressure in ACo2As2 (A = Eu and Ca).
abstract: We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. 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 EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. 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.
authors: Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306, United States.
authors: Xiaoyan
authors: X
authors: Tan
authors: Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States.|Department of Physics, Washington University , St. Louis, Missouri 63130, United States.
authors: Gilberto
authors: G
authors: Fabbris
authors: Advanced Photon Source, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors: Daniel
authors: D
authors: Haskel
authors: National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) , 115409, Moscow, Russia.|European XFEL GmbH , 22761 Hamburg, Germany.
authors: Alexander A
authors: AA
authors: Yaroslavtsev
authors: Quantum Condensed Matter Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.
authors: Huibo
authors: H
authors: Cao
authors: Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306, United States.
authors: Corey M
authors: CM
authors: Thompson
authors: Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306, United States.
authors: Kirill
authors: K
authors: Kovnir
authors: National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) , 115409, Moscow, Russia.
authors: Alexey P
authors: AP
authors: Menushenkov
authors: DESY Photon Science , 22603 Hamburg, Germany.
authors: Roman V
authors: RV
authors: Chernikov
authors: Quantum Condensed Matter Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.
authors: V Ovidiu
authors: VO
authors: Garlea
authors: Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306, United States.
authors: Michael
authors: M
authors: Shatruk