Magnetic Structure of the High-Pressure Phases of Holmium*

The magnetic phases in rare earth metals are well established under ambient pressure conditions, however, little is known about the magnetic ordering in their corresponding high pressure crystalline modifications. Holmium (Ho) was studied in a large-volume diamond anvil cell at the Spallation Neutron Source to 20 GPa and 10 K. The ambient pressure hexagonal close packed phase (hcp) of holmium shows two magnetic transitions below 10 GPa one to Antiferromagnetic (AFM) incommensurate phase and another to a Ferromagnetic (FM) phase. At pressures above 10 GPa, Ho transforms to the alpha-Samarium (α-Sm) phase which shows only one transition marked by appearance of a magnetic peak at 3 Å below 20 K that is more intense than the nuclear peaks. This magnetic order remains even when pressure is increased past 19 GPa where the sample again transforms, this time to the double hexagonal close packed (dhcp) phase. The magnetic ordering temperature increases with pressure from below 20 K at 15 GPa to around 25 K at 20 GPa. The magnetic phase diagram of Ho is presented to 20 GPa and 10 K.