Mn-55 NMR spectrum in a noncollinear antiferromagnet RbMnBr3

The NMR spectrum of Mn-55 in an easy-plane noncollinear antiferromagnet RbM nBr3 was investigated in the frequency range from 380 MHz to 470 MHz under applied magnetic field H up to 85 kOe at T = 1.3 K. The NMR spectrum in the noncollinear commensurate phase can be explained by a plane magnetic struc ture with eight manganese spins per magnetic unit cell. In the collinear ph ase the NMR frequencies are strongly field dependent. In the vicinity of th e critical fields, we observed NMR signals when the radio-frequency field w as parallel to the applied magnetic field.