Transverse staggered moment in the Haldane-gap antiferromagnet [Ni(C2H8N2)2NO2(ClO4)] observed by proton nuclear magnetic resonance

A quasi-one-dimensional S=1 antiferromagnet NENP [Ni(${\mathrm{C}}_{2}$${\mathrm{H}}_{8}$${\mathrm{N}}_{2}$${)}_{2}$${\mathrm{NO}}_{2}$(${\mathrm{ClO}}_{4}$)] has been studied through the measurement of proton NMR under an applied magnetic field, ${\mathit{B}}_{0}$, up to 12 T. At 4.2 K with the magnetic field parallel to the chain, a large transverse moment perpendicular to the magnetic field has been observed even below the critical field, ${\mathit{B}}_{\mathit{c}}$=9.8 T. The moment increases with increasing applied field, showing no singularity at ${\mathit{B}}_{\mathit{c}}$. It is not caused by the three-dimensional order. It is due to the field-induced staggered moment caused by the combined action of the staggered inclination of the principal axes of the g tensor of ${\mathrm{Ni}}^{2+}$ ions from the crystal b axis (chain axis) and the large transverse staggered susceptibility. The induced transverse moment is estimated to be 1.5${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ at ${\mathit{B}}_{0}$=11.7 T.