178 Journal of Magnetism and Magnetic Materials 50 (1985) 178-188 North-Holland, Amsterdam A THERMAL NEUTRON INVESTIGATION OF MAGNETIC CORRELATIONS IN LIQUID OXYGEN M. DERAMAN, J.C. DORE Physics Laboratory, University of Kent, Canterbury, UK and J. SCHWEIZER Institut Laue-Langevin, Grenoble, DN/RFG, CENG, Grenoble, France Received 3 January 1985 The neutron scattering cross section for liquid and gaseous oxygen is reported. Polarisation techniques are used to separate nuclear and magnetic contributions for the liquid phase. The profile for magnetic scattering indicates significant magnetic correlations in the liquid which are consistent with antiferromagnetic ordering in the condensed phase. The results are related to the magnetic structures of the various crystalline phases of oxygen. I. Introduction The use of thermal neutrons for the investiga- tion of magnetic structure in crystalline [1] or disordered materials [2] has been particularly in- fluenced by the devlopment of new techniques for the production and analysis of polarised neutron beams. Most of the research activity has naturally centered on the magnetic properties of the transi- tion metal and rare-earth ions and there has been surprisingly little interest in the aspects of molecu- lar magnetism. In this context, oxygen is of par- ticular interest as it exhibits three distinct crystal- line forms below the freezing point. The mono- clinic a phase (T < 23.9 K) and the rhombohedral /3 phase (23.9 K < T < 43.8 K), are orientationally ordered, but the cubic 3' phase (43.8 K < T < 54.4 K) is orientationally disordered. From susceptibility measurements [3], the a and /3 phase behave antiferromagnetically while the 3' and the liquid phases are paramagnetic. Neutron diffraction has already been used to in- vestigate oxygen magnetic properties. Collins [4], Alikhanov et al. [5] and Meier et al. [6], have studied polycrystalline samples with unpolarised neutrons, and shown that the monoclinic a phase is antiferromagnetic with the two moments of the C 2/m cell opposed to each other. They did not find any long range order in the fl phase, but only indications of short range order. The structure of the a and fl phases have been the subject of recent theoretical studies by Etters, Helmy and Kobayashi [7] who also review the available experimental data. Cox et al. [8] have studied the molecular magnetic form factor of the 3' phase, using polarised neutrons and paramagnetic single crystals polarised by high magnetic fields. These crystallographic methods are char- acterised by the fact that information of the mag- netic form factor is obtained at Q-values corre- sponding to Bragg diffraction conditions. In con- trast, neutron diffraction studies of liquid oxygen reported by Dore, Walford and Page [9] show clearly a continuous magnetic scattering contribu- tion in the low Q-value region of the differential scattering cross section. The measurements were made primarily to study the liquid structure but the data indicated that the shape of the magnetic 0304-8853/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)