Magnetic Exchange Interactions of Elemental Europium at Extreme Pressures

Elemental lanthanides are known to show strong variations in their magnetic ordering temperatures under pressure. Eu is unique that ordering disappears in the 75-80 GPa range, giving way to superconductivity at 3-4K. To clarify how the magnetism interplays with pressure, we use density functional theory in combination with linear response and the magnetic force theorem to compute the exchange constants between 4f local moments. We confirm that Eu retains divalency with strong 4f 7 (J = 7 2) local moments despite structural, Hubbard U, or pressure variations. The crystal structures are found to be crucial to the AFM-PM phase transition. High symmetry structures such as bcc and hcp tend to enhance AFM ordering, while in Pnma phase the exchange constants decrease to zero rapidly with increasing pressure. Increasing U is observed to enhance AFM exchange constants in all structures, making AFM order more robust. By choosing U about 4∼5 eV at high
pressure, the critical volume for the AFM-PM phase transition is in good agreement with experiment.