Neutron diffraction study of magnetic ordering in high pressure phases of rare earth metal holmium

The magnetic ordering in rare earth metals is well established for ambient pressure crystal structures, 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 high-pressure up to 20 GPa and to low-temperature to 10 K. We have conducted two independent high-pressure low-temperature experiments under non-hydrostatic and quasi-hydrostatic pressure conditions respectively. The ambient pressure hexagonal close packed (hcp) phase of holmium shows two magnetic transitions below 10 GPa one to an incommensurate Antiferromagnetic (AFM) phase and another to a conical-Ferromagnetic (c-FM) phase. In contrast, alpha-Samarium-type (a-Sm) phase above 10 GPa and the double hexagonal close packed (dhcp) phase above 19 GPa show only one FM transition marked by the appearance of a magnetic peak at 3 angstrom and the concurrent enhancement of nuclear peaks below 30 K. These new transitions observed by neutron diffraction can be accounted by a commensurate superlattice formation along c-axis in both the a-Sm-type phase and the dhcp phase.