Magnetic dynamics in condensed oxygen: Recent experimental results

The spin dynamics in the condensed phases of ambient-pressure molecular oxygen (liquid, plastic-crystalline (gamma-), magnetically disordered (beta-) and fully ordered alpha-O-2) is investigated by means of the concurrent use of neutron scattering and muon-spin relaxation. Above the alpha --> beta transition the magnetic dynamics is governed by fast paramagnetic fluctuations whose spectrum is determined from the S(Q, omega) dynamic structure factors accessible from inelastic neutron scattering. Such information is shown to provide a key to understand the non-trivial temperature dependence of longitudinal relaxation rates found in muon spin relaxation measurements. Recent neutron scattering measurements performed within the magnetically ordered alpha-phase under high-resolution conditions reveal the presence of a low-energy excitation of magnetic origin, unnoticed in a previous polarized-neutron experiment, which corresponds to the continuation to longer wavevectors of the spin-wave mode detected in antiferromagnetic resonance (AFMR) experiments carried out by optical means.