High pressure studies of the T−P phase diagrams of erbium and thulium up to 30 GPa by using ac magnetization experiments

The pressure dependence of the magnetic ordering temperatures for lanthanide ferromagnets Er and Tm has been investigated in the pressure region of up to 30 GPa through ac magnetization measurements by employing a superconducting quantum interference device. Six $4f$ lanthanide ferromagnets, from Gd to Tm, commonly have successive structural transitions starting from a hexagonal close-packed (hcp) structure under pressure. The experiments reported here have been performed with high sensitivity to allow for a more comprehensive discussion of the magnetostructural correlation when compared to four other $4f$ ferromagnets, namely Gd, Tb, Dy, and Ho. Our results conclude that only Er exhibits the increase in ferromagnetic ordering temperature against initial compression among the six lanthanide ferromagnets. The ferromagnetic magnetization anomaly for Er reduces below the detection limit before the structural transformation from Sm-type to the dhcp structures, and that for Tm exhibits similar behavior before the structure transformation from the hcp to Sm-type structures. However, the helimagnetic magnetization anomalies of both Er and Tm remain stable at least within the Sm-type structure below 30 GPa.