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Next: Neutron scattering measurements [#!RenardEPL87!#,#!RenardJAP88!#,#!MaPRB92!#,#!RegnaultPRB94!#,#!MaPRB95!#] Up: 5.1 Introduction Previous: Susceptibility [#!RenardEPL87!#,#!RenardJAP88!#,#!GadetPRB91!#,#!AvenelPRB92!#]

High-field magnetization [#!KatsumataPRL89!#,#!AjiroPRL89!#]

The magnetization of NENP at 1.3 K was small and almost field independent up to a critical field $H_c\sim9.5$ T, and above Hc, linearly increased. This phenomenon was attributed to the crossover in the field between the singlet ground state and one of the triplet excited states. The Haldane gap ($E_{\rm g}\approx17$ K) and the single ion anisotropy ($D\approx16$ K) were obtained from the critical field and its crystalline axis dependence.