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## Formation of spiral ordering by magnetic field in frustrated anisotropic antiferromagnets

We discuss theoretically phase transitions in frustrated antiferromagnets with biaxial anisotropy or dipolar

forces in magnetic field applied along the easy axis at T = 0. There are well-known sequences of phase

transitions on the field increasing: the conventional spin-flop transition and the flop of the spiral plane at strong

and weak easy-axis anisotropy, respectively. We argue that much less studied scenarios can appear at moderate

anisotropy in which the magnetic field induces transitions of the first order from the collinear state to phases with

spiral orderings. Critical fields of these transitions are derived in the mean-field approximation and the necessary

conditions are found for the realization of these scenarios. We show that one of the considered sequences of

phase transitions was found in multiferroic MnWO4 both experimentally and numerically (in a relevant model)

and our theory reproduces quantitatively the numerical findings.