  We thoroughly study the zero temperature properties of monoaxial chiral spin
systems under a tilted magnetic field. The magnetic phase diagram includes two
kinds of continuous phase transitions and one discontinuous phase transition.
We clarify the properties of the phase transition in terms of the helical wave
picture and the particle (soliton) picture and also in terms of the interaction
between solitons. The interacting-soliton picture properly describes most of
the discontinuous phase transition. In addition, we investigate several
instabilities of the modulated structures such as an isolated soliton and the
surface modulation, since their instabilities should be important to the
changes of the magnetic properties. For this purpose, we perform the analyses
based on the energy landscape as well as the excitation spectrum; the former
approach gives an intuitive interpretation. We clarify the mechanisms of the
instabilities through these analyses and draw the stability lines of solitons
in the magnetic phase diagram.
