Experimental evidence of a haldane gap in an S=2 quasi-linear-chain antiferromagnet

The magnetic susceptibility of the S = 2 quasi-linear-chain Heisenberg antiferromagnet (2,2'-bipyridine)trichloromanganese(III) has been measured from 1.8 to 300 K with the magnetic field H parallel and perpendicular to the chains. The analyzed data yield g approximate to 2 and J approximate to 35 K. The magnetization M has been studied at 30 mK and 1.4 K in H up to 16 T. No evidence of long-range order is observed. Depending on crystal orientation, M approximate to 0 at 30 mK until a critical field is achieved (H-c parallel to = 1.2 +/- 0.2 T and H-c perpendicular to = 1.8 +/- 0.2 T), where M increases continuously as H is increased. These results are interpreted as evidence of a Haldane gap.