The results of a study of linear and nonlinear properties of Alfvén-cyclotron waves propagating along a background magnetic field in collisionless plasmas are presented. Kinetic theory and one-dimensional hybrid simulations are used to identify the complete family of growing and damped wave modes and the connection with the spontaneous fluctuations in plasmas. The self-consistent reshaping of initially cold ion velocity distributions, the simultaneous generation of beam protons, and the preferential heating and acceleration of alpha particles are shown to occur by the same mechanism: ion trapping and induced pitch-angle scattering by parametrically driven ion acoustic waves.