Magnetism of Europium under Extreme Pressures

Figure caption: L<sub>γ1</sub> XES spectra of Eu to pressures of 119 GPa at room temperature. No apparent change in XES spectra was observed under pressure, suggesting no change in 4ƒ magnetic moment to 119 GPa.
Figure caption: Lγ1 XES spectra of Eu to pressures of 119 GPa at room temperature. No apparent change in XES spectra was observed under pressure, suggesting no change in 4ƒ magnetic moment to 119 GPa.

The lanthanide elements show many interesting phenomena such as heavy fermion metal properties and unconventional superconductivity due to their ƒ electrons. Among all lanthanide metals, only Ce and Eu are known to become superconducting under pressure. In order to understand the interplay between magnetism and superconductivity in europium at extreme pressures, experiments using X-ray emission spectroscopy (XES) at HPCAT and nuclear forwarding scattering (NFS) at sector 3 of APS have been done recently. With pressure above 80GPa, Eu has a critical superconducting temperature around 2K. Magnetic order in Eu is found to collapse just above 80GPa in association with the superconducting state according NFS spectra; however no change in the Lγ1 line of Eu in XES is observed up to 119GPa. These results indicate that the highly localized character of the 4ƒ7 magnetic state in Eu remains intact and Eu remains nearly divalent to at least 119GPa. The proximity to a magnetically ordered phase, together with the persistence of strong local moments in the superconducting phase, points to the possibility that magnetic fluctuations may be involved in the superconducting pairing mechanism of this interesting rare-earth metal. (W. Bi, et al, Phys. Rev. B, 93, 184424, 2016)