Enhanced magnetic ordering in the heavy lanthanide Er metal under megabar pressures

Four-point electrical resistivity measurements were carried out on erbium (Er) metal and dilute magnetic alloy containing 0.5 at.$%$ Er in superconducting Y for temperatures 3.5--295 K under pressures to 183 GPa in a diamond anvil cell. The magnetic ordering temperature ${T}_{o}$ of Er appears to rise rapidly above 80 GPa. Y(Er) alloys display giant superconducting pair breaking $\mathrm{\ensuremath{\Delta}}{T}_{c}$ per atomic percent as large as 19 K/at.$%$ Er. The present results give evidence that, for pressures above 80 GPa, the exchange coupling $J$ between Er ions and conduction electrons becomes negative, thus activating Kondo physics in this highly correlated electron system. We suggest that these anomalously high magnetic ordering temperatures are an unrecognized feature of the Kondo lattice state, providing another perspective for designing magnetic materials with high ordering temperature.