Helical-to-Fan Transitions under Magnetic Fields in the Noncentrosymmetric Tetragonal Magnet EuRhGe 3

The magnetic structure of EuRhGe 3 , a noncentrosymmetric body-centered tetragonal magnet with the space group I4mm, has been investigated by resonant X-ray diffraction. Below T N = 12 K, EuRhGe 3 undergoes a helical magnetic ordering with an incommensurate propagation vector [Formula: see text], in which the magnetic moments lie in the ab plane and rotate by a constant turn angle of 145.8° between adjacent layers. When a magnetic field is applied along the a axis at 2 K, a second-harmonic 2q peak develops, indicating that the circular helix is gradually distorted into a helimagnetic soliton-lattice state, which eventually undergoes a lock-in transition to the commensurate structure with q = 0.8 at 3.8 T. Above the subsequent phase boundary at 5 T, the helicity is lost, and a spin-flop xy[Formula: see text]-fan (elliptic conical) state is realized, in which the moments oscillate predominantly along the b axis but are accompanied by a small c-axis component. At higher fields, the system enters a conventional planar xy-fan phase without a c-axis component. EuRhGe 3 provides a prototypical example of a helimagnet that exhibits a full sequence of field-induced structures, evolving from a circular helix to a spin-flop xy[Formula: see text]-fan (elliptic conical), and finally to a planar xy-fan structure, which has been theoretically predicted.