Phenomenological theory of magnetic 90^° helical state in RbEuFe₄As₄

Helical state with 90^° turn angle between magnetic moments in the neighboring Eu layers is formed in the superconducting iron arsenide RbEuFe₄As₄. Such spin state is not realized within the standard Heisenberg model with bilinear exchange interactions and requires the biquadratic interaction term. In addition, the helix orientation may be fixed by the in-plane four-fold anisotropy term. We explored a rich phase diagram for such a state in the magnetic field applied perpendicular to the helix direction. The magnetic field induces the metamagnetic transition to the double-periodic state with the moment angles (α, α, - α, - α) with respect to the field for the four subsequent spins at the transition field determined by the strength of biquadratic interaction. The transition is second order for small biquadratic coupling and becomes first order when this coupling exceeds the critical value. On the other hand, at high magnetic fields, the aligned state becomes unstable with respect to formation of an incommensurate fan state which transforms into the double-periodic state with decreasing magnetic field. In addition, when the magnetic field is applied along one of four the equilibrium moment directions, the deformed helix state experiences the first-order rotation transition at the field determined by the four-fold anisotropy. We will discuss possible experiments to detect these metamagnetic transitions.