Magnetization process of anS=1linear-chain Heisenberg antiferromagnet

Magnetization measurements in strong pulsed magnetic fields on single-crystal samples of the S=1 linear-chain Heisenberg antiferromagnet Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{H}}_2$ ${)}_2$ ${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$) are reported. The magnetization is very small at the low-field region and begins to increase sharply at a finite field for all of the three principal axes. This is evidence for the existence of the Haldane gap in this compound.