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.

Revisiting Bond Breaking and Making in EuCo2P2: Where are the Electrons?

“…We and others have previously demonstrated that above the critical pressure of 3.1 GPa the LP‐EuCo 2 P 2 phase transforms into the HP‐EuCo 2 P 2 phase, with the clear change in the oxidation state of Eu, which was also confirmed in the present experiments by the dramatic change in the XAS spectra observed at the Eu L 3 ‐edge (Figure a). Based on these spectral changes, Eu undergoes a nearly complete conversion from the +2 to +3 formal oxidation state.…”

“…Finally, we should point out that EuCo 2 As 2 also undergoes a pressure‐induced structural collapse with the critical pressure of ≈4.5 GPa at room temperature . In this compound, however, the change in the Eu oxidation state is not as drastic as seen in the case of EuCo 2 P 2 . We carried out XAS measurements on the L 3 ‐edge of Eu and K‐edges of Co and As and observed the same spectral changes as seen in EuCo 2 P 2 , albeit to a lesser extent (Figure S3).…”

Hydrostatic pressure effects on the static magnetism in Eu(Fe0.925Co0.075)2As2

“…As revealed by high-pressure XAS, the average valence state of Eu in both EuFe 2 As 2 and EuCo 2 As 2 increases with the applied pressure due to a part transition from Eu 2+ to Eu 3+ . However, the mean valence in them gets stabilized around +2.3 at 10 GPa and +2.25 at 12.6 GPa, for EuFe 2 As 2 and EuCo 2 As 2 , respectively 43, 44 . Therefore, the dip in T Eu ( P ) around 11 GPa in Fig.…”

Strain-Induced Isostructural and Magnetic Phase Transitions in Monolayer MoN2

“…However, such instances are still rare and the transition usually requires a very high pressure that is not easily accessible. [4][5][6] Starting from the discovery of graphene, 7 the field of two-dimensional (2D) materials has undergone rapid development in the past decade. Many new kinds of 2D materials have been proposed and fabricated, which exhibit a vast range of novel material properties, not seen in the usual 3D bulk materials.…”