Evidence for a Magnetic Collapse in the Epsilon Phase of Solid Oxygen

Solid oxygen is the only elementary molecular magnet. Under the very high pressure of 96 GPa oxygen transforms into a metal and a superconductor. Theory predicts a nonmagnetic state occurring before the transition into the superconducting ξ phase. Nevertheless, until now there was no direct evidence of a magnetic collapse in high-pressure oxygen. For the first time direct information is provided on magnetic properties of the ε phase, which is sandwiched between the antiferromagnetic δ phase and the superconducting ξ phase. We used magnetic neutron diffraction. The data show that the long-range magnetic order disappears at the δ-ε transition. The magnetic collapse occurs at P≈8  GPa, far below the pressure of the insulator-metal (superconductor) transition. The collapse is preceded by a decrease in temperature of transition towards the long-range magnetically ordered state (T_LRO) in the δ phase, at P=7.6  GPa.