Magnetization Measurement of NENP and NINO in High Magnetic Field

Magnetization measurements have been performed on the single crystal samples of Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ), NENP, and Ni(C 3 H 10 N 2 ) 2 NO 2 (ClO 4 ), NINO in pulsed high magnetic fields. No magnetization appears up to about 10 T reflecting the presence of the Haldane gap and the linear magnetization appears in both compounds above this field. The observed anisotropy of the transition field is explained by the Haldane gap energy E g and the crystalline field constants D and E in the lowest excited triplet. The parameters are estimated as E g =16.8 K, D =-16.1 K, and E =-1.3 K and E g =14.2 K, D =-11.5 K and E =2.1 K for NENP and NINO, respectively.