  We report an observation of a commensurate-incommensurate phase transition in
a Dzyaloshinskii-Moriya spiral magnet Ba_2CuGe_2O_7. The transition is induced
by applying a magnetic field in the plane of spin rotation. In this experiment
we have direct control over the strength of the commensurate potential, while
the preferred incommensurate period of the spin system remains unchanged.
Experimental results for the period of the soliton lattice and bulk
magnetization as a function of external magnetic field are in quantitative
agreement with theory.
