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), [Formula presented], 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≈2 and J≈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≈0 at 30 mK until a critical field is achieved [Formula presented], where M increases continuously as H is increased. These results are interpreted as evidence of a Haldane gap.