ANTIFERROMAGNETISM IN SOLID OXYGEN

Solid oxygen is known to have spontaneous antiferromagnetic order in i ts lowest temperature phase. Muons implanted in alpha-O-2 in zero appl ied magnetic field take up positions where the local held is non-zero and precess at a frequency proportional to the sublattice magnetisatio n. The reduction of the magnetisation as the temperature increases tow ard T-N is similar to that expected for the case of an anisotropic AFM coupling among spins of O-2 molecules. A fit to a simple non-interact ing, harmonic spin wave model yields an anisotropy-induced gap in the spin wave density of states E-g/k(B) = 80.5(6) K, implying a large ani sotropy constant K approximate to 16J.