Effects of high pressure on magnetic transitions in heavy rare earth metals.

摘要

Rare earth metals bear close resemblance to the 5f actinide metals that are critical to nuclear stockpile stewardship research. To better understand these materials, we have studied erbium, holmium, and terbium under high pressure and low temperatures in order to elucidate the magnetic transitions as these materials are compressed to different crystallographic phases. In erbium, we have undertaken four-probe electrical resistance measurements in a diamond anvil cell to temperatures as low as 10 K and pressures up to 20.3 GPa. We have also conducted x-ray diffraction measurements on erbium to 10 K and 20.4 GPa. In the electrical resistance measurements we saw a distinct change in the resistance slope at the antiferromagnetic Neel temperature, signaling a transition to its c-axis modulated antiferromagnetic phase. At pressures of 10.6 GPa and above the change in slope signaling antiferromagnetic order was suppressed. The x-ray diffraction measurements showed that this loss of transition occurred at the same pressure as the transition from the hexagonal close packed crystal structure (hcp) to the &agr;-Sm structure. In holmium, we made electrical resistance measurements to 10 K and 30.1 GPa which also showed a change in slope at the Neel temperature. The transition was suppressed at pressures above 9 GPa. Additionally, we collected neutron diffraction spectra of holmium at 89 K, 110 K, and 300 K at pressures as high as 6.6 GPa. These spectra confirmed the paramagnetic-to-antiferromagnetic transition at temperatures and pressures similar to those determined from the electrical resistance measurements. In terbium we made both electrical resistance and neutron diffraction measurements as well. The electrical resistance measurements showed a transition to the ferromagnetic phase at the Curie temperature at pressures up to 3.6 GPa, where the transition was suppressed. The neutron diffraction measurements showed that the ferromagnetic transition was indeed suppressed at this pressure and above and also confirmed that above 3.6 GPa the crystal structure undergoes a transition from hcp to &agr;-Sm. It is postulated that a change in the Fermi surface as the structure shifts from hcp to &agr;-Sm is responsible for the loss of ferromagnetism in terbium and antiferromagnetism in erbium and holmium.