Magnetic Order Induced by the Cooperation of Valence and Structural Instabilities in Eu₁−xCaxCo₂P₂

Magnetism of layered compound Eu1−xCaxCo2P2 was investigated by means of magnetization, specific heat, and electrical resistivity measurements. It is found that Eu undergoes a phase transition from a divalent state to a mixed-valence state of Eu2+δ and Eu(3−δ′)+ simultaneously with a structural transition from the uncollapsed tetragonal (ucT) phase to the collapsed tetragonal (cT) phase near x = 0.5. Whereas the compounds with the ucT structure exhibit antiferromagnetic ordering of Eu2+ moments, those with the cT structure exhibit additional magnetic ordering of Co moments. The ordering temperature of Co moments is much higher than those in other ACo2P2 systems with a divalent A cation. These results suggest that an electron-doping effect on Co 3d bands owing to the Eu valence transition stabilizes the magnetic ordering of Co moments.