Helical magnetic state in the distorted triangular lattice of α-CaCr2O4

The magnetic properties of the high temperature a form of the CaCr2O4 compound have been investigated for the first time by magnetic susceptibility, specific heat, and powder neutron diffraction. The system undergoes a unique magnetic phase transition at 43 K to a long-range ordered incommensurate helical phase with magnetic propagation vector k = (0,0.3317(2),0). The magnetic model proposed from neutron diffraction data shows that the plane of rotation of the spins is perpendicular to the wave vector and that the magnetic modulation is consistent with two modes belonging to distinct irreducible representations of the group. The magnetic point group 2221 ' is not compatible with ferroelectricity unlike the CuCrO2 delafossite [Kimura et al., Phys. Rev. B, 78 140401 (2008)], but predicts the existence of quadratic magnetoelectric effects, discussed based on a Landau analysis.