High-pressure oxygen: a non-conventional magnet studied by means of neutron diffraction

While being among the most casually treated elements on our planet, elementary oxygen shows many unusual features at low temperatures and high pressures. At low pressure, oxygen is the only elementary molecular magnet. Under high pressures solid oxygen undergoes several structural transformations and finally becomes a metal and superconductor under the very high pressure of 96 GPa. For the first time, we studied magnetic ordering in solid oxygen under very high pressures by a direct method, namely by means of neutron diffraction. A new type of magnetic order with ferromagnetic stacking of the antiferromagnetic 02 planes was discovered in delta-O-2 at P = 6.2 GPa. The structural transitions from alpha-O-2 to beta-O-2 and delta-O-2 correlate with the magnetic transitions; therefore in the pressure range P < 7 GPa, oxygen should be considered as a unique,spin-controlled crystal'.