Magnetic structure of antiferromagnetic high-pressure phases of dysprosium and holmium*

Dysprosium (Dy) and holmium (Ho) have been studied using neutron diffraction under high pressures and low temperatures at a spallation neutron source by employing a large-volume diamond anvil cell. Companion measurements at different central wavelengths allow the collection over extended reciprocal space with momentum transfer Q covering the range from 0.5 Å -1 to 5.5 Å -1 . Upon cooling to 15 K, magnetic ordering was observed in the hexagonal close-packed ( hcp ), alpha-samarium ( α-Sm ), and double hexagonal close packed ( dhcp ) phases of Dy to 22 GPa as well as Ho to 31 GPa. We report on previously undetected magnetic superlattice reflections signaling antiferromagnetic transition for both the α-Sm and dhcp phases of Dy and Ho. Magnetic structure refinements for the α-Sm phase of Dy shows a complex phase comprising two magnetic propagation vectors k = (1/2, 1/2, 1/2) and k = (1/2, 0, 0). Magnetic structure refinements for the dhcp phase of Dy yield a single magnetic propagation vector k = (1/2, 0, 1/3) and a possible magnetic space group P_bnma. Similar magnetic structures were observed in Ho in the α-Sm phase. Very weak magnetic reflections were observed in the dhcp phase of Ho, however the quality is not sufficient for magnetic structure refinement. The observed magnetic structures are presented to the highest pressure of 22 GPa for Dy and 31 GPa for Ho.