Spiral plane flops in frustrated helimagnets in external magnetic field
We discuss theoretically frustrated Heisenberg spiral magnets in magnetic field H. We demonstrate that small
anisotropic spin interactions (single-ion biaxial anisotropy or dipolar forces) select the plane in which spins rotate
(spiral plane) and can lead to the spiral plane flop upon in-plane field increasing. Expressions for the critical
fields Hflop are derived. It is shown that measuring of Hflop is an efficient and simple method of quantifying the
anisotropy in the system (as the measurement of spin-flop fields in collinear magnets with axial anisotropy). Corresponding recent experiments are considered in spiral magnets some of which are multiferroics of spin origin.