SPIN-GAP BEHAVIOR OF CUNB2O6

The magnetic properties of two polymorphs of CuNb2O6 with quasi-one-di mensional zig-zag spin (S = 1/2) chains of Cu2+ ions are studied. The magnetic excitation spectra of Cu spins measured by neutron scattering , and the T-dependence of the longitudinal spin relaxation rate of Nb- 93 nuclei by NMR indicate the existence of the spin gap in the monocli nic phase whose magnitude is consistent with the one deduced hy the de tailed analyses of uniform magnetic susceptibility at low temperatures . The behavior in the monoclinic phase is in clear contrast to that fo und in the orthorhombic phase.