High Field ESR Study of the Spin Gap System with the Field-Induced Phase

High field ESR studies of spin gap systems F 2 PNNNO and KCuCl 3 have been performed. The broadening of the linewidth in F 2 PNNNO is observed near the start of the 1/2 plateau but not near the entrance of the field induced order at 9 T. The possible origin is discussed. The reduction of the spin gap by pressure in KCuCl 3 is observed directly by high field ESR under pressure. The result is discussed in connection with the theory by Matsumoto et al.