Experimental Evidence of a Haldane Gap in anS=2Quasi-Linear-Chain Antiferromagnet

The magnetic susceptibility of the $S=2\mathrm{}$ 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 $g\approx 2$ and $J\approx 35\mathrm{K}$. 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, $M\approx 0$ at 30 mK until a critical field is achieved ( $H_{c\Vert }=1.2\mathrm{}\pm 0.2\mathrm{T}$ and $H_{c\perp }=1.8\mathrm{}\pm 0.2\mathrm{T}$), where $M$ increases continuously as $H$ is increased. These results are interpreted as evidence of a Haldane gap.