Biquadratic antisymmetric exchange and the magnetic phase diagram of magnetoelectric CuFeO2

Biquadratic antisymmetric exchange terms of the form -[C(ij)e(ij)(alpha)(s(i)xs(j))(z)](2), where e(ij) is the unit vector connecting sites i and j and alpha=x,y, due partially to magnetoelectric coupling effects, are shown to be responsible for the spin-flop helical phase in CuFeO2 at low magnetic field and temperature. Usual biquadratic symmetric exchange, likely due to magnetoelastic coupling, is found to support the stability of axial magnetic states at higher fields in this nearly-Heisenberg-like stacked triangular antiferromagnet. A model Hamiltonian which also includes substantial interplane and higher-neighbor intraplane exchange interactions reproduces the unique series of observed commensurate and incommensurate periodicity phases with increasing applied magnetic field in this highly frustrated system. The magnetic-field-temperature phase diagram is discussed in terms of a Landau-type free energy.