Magnetic properties of Oxygen Monolayrs on Graphite and Hexagonal Boron Nitride

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An oxygen monolayr physisorbed on a layred crystal surface is a most ideal system of the two-dimensional (2D) magnetism. In the case of graphite substrate, the high density monolayr, xi-phase, which has a triangular lattice with the molecular axis perpendicular to the surface, gives rise to an antiferromagnetic transition at 11.9 K and changes to the low-temperature epsilon-phase with a distorted triangular lattice. These features have been observed so far by the magnetic susceptibility and X-ray diffraction measurements. However, the spin structure of the low temperature phase has not determined directly. By use of the neutron diffraction measurement, we have succeeded for the first time to observe the antiferromagnetic structure of the co-linear spin configuration. The most important result is a remarkable magnetic moment reduction of an oxygen molecule ; that is, S = 0.61, being only 61% of the full moment (S=1). This reduction is not understood yet, but related plausibly to the spin fluctuation inherent in the low dimensional triangular magnet.Another important result is the investigation of oxygen monolayrs on hexagonal boron nitride (h-BN), a substance having a very similar crystal structure but a little smaller substrate potential than graphite. We have observed a new antiferromagnetic transition in the low density delta-phase. According to the SR X-ray diffraction this new phase has a 2D rectangular lattice with large anisotropic lattice constants, and it shows an anomalous kinetics of the antiferromagnetic transition, namely, the logarithmic time development of domain growth ; these facts indicate a pseudo-1D spin system. Furthermore we have observed a drastic decrease of susceptibility below the critical temperature, as chi = exp (-To/T), which represents the existence of a energy gap of spin excitation. Considering a 1D spin system with S = 1, this new phase may suggest to be the Haldene phase.