High-field magnetization of a quasi-one-dimensional S=1 antiferromagnet Ni(C2H8N2)2NO2(ClO4: Observation of the Haldane gap

Abstract

High-field magnetization measurements were perform- ed for the quasi-one-dimensional S=1 antiferromagnet Ni(${\mathrm{C}}_2$ ${\mathrm{H}}_8$ ${\mathrm{N}}_2$ ${)}_2$ ${\mathrm{NO}}_2$(${\mathrm{ClO}}_4$) up to 39 T. Applied magnetic fields parallel and perpendicular to the chain axis induce a transition from a nonmagnetic to a magnetic state at a critical field, ${\mathit{H}}_{\mathit{c}}^{\mathrm{\parallel }}$=9.8 T, and ${\mathit{H}}_{\mathit{c}}^{\mathrm{\perp }}$=13.1 T, indicating clear evidence for the existence of the Haldane gap. The observed gaps are markedly different from those in neutron-scattering experiments.