MAGNETIC DYNAMICS IN CONDENSED OXYGEN - RECENT EXPERIMENTAL RESULTS

The spin dynamics in the condensed phases of ambient-pressure molecula r oxygen (liquid, plastic-crystalline (gamma-), magnetically disordere d (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 fas t paramagnetic fluctuations whose spectrum is determined from the S(Q, omega) dynamic structure factors accessible from inelastic neutron sc attering. Such information is shown to provide a key to understand the non-trivial temperature dependence of longitudinal relaxation rates f ound in muon spin relaxation measurements. Recent neutron scattering m easurements performed within the magnetically ordered alpha-phase unde r high-resolution conditions reveal the presence of a low-energy excit ation of magnetic origin, unnoticed in a previous polarized-neutron ex periment, which corresponds to the continuation to longer wavevectors of the spin-wave mode detected in antiferromagnetic resonance (AFMR) e xperiments carried out by optical means.