Field-induced incommensurate-to-commensurate transition in Ba 2 CuGe 2 O 7

We report an observation of a commensurate-incommensurate phase transition in the Dzyaloshinskii-Moriya spiral antiferromagnet Ba${}_{2}{\mathrm{CuGe}}_{2}{\mathrm{O}}_{7}$. The transition is induced by an external magnetic field applied along the $c$ axis of the tetragonal structure, i.e., in the plane of spin rotation. Bulk magnetic measurements and neutron-diffraction experiments show that the transition occurs in a critical field ${H}_{c}\ensuremath{\approx}2.1$ T. Experimental results for the period of the magnetic structure and magnetization as functions of magnetic field are in quantitative agreement with our exact analytical solution for Dzyaloshinskii's model of commensurate-incommensurate transitions in spiral magnets.