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(C2H8N2)2NO2(ClO4)] has been studied through the measurement of proton NMR under an applied magnetic field, B0, 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, B(c) = 9.8 T. The moment increases with increasing applied field, showing no singularity at B(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 Ni2+ ions from the crystal b axis (chain axis) and the large transverse staggered susceptibility. The induced transverse moment is estimated to be 1.5-mu-B at B0 = 11.7 T.