Antiferromagnetic Dichroism in a Complex Multisublattice Magnetoelectric CuB(2)O(4).

Magnetic control of the crystal chirality was announced by Saito et al. [Phys. Rev. Lett. 101, 117402 (2008)] on the ground of experiments in CuB(2)O(4). This claim has raised a sharp dispute in the literature because it seemed to contradict the fundamental symmetry principles. We settle this dispute on the basis of a high-resolution optical spectroscopy study of excitonic transitions in CuB(2)O(4). We find that a large sublattice-sensitive antiferromagnetic linear dichroism (LD) emerges at the Néel temperature T(N)=21  K and show how it could simulate a "magnetic-field control of the crystal chirality." We prove that the discovered LD is related microscopically to the magnetic Davydov splitting. This LD is highly sensitive to subtle changes in the spin subsystems, which allowed us to observe a splitting of the phase transition into an incommensurate magnetic phase into two transitions (T(1)(*)=8.5 and T(2)(*)=7.9  K) and to suggest elliptical spiral structures below T(1)(*), instead of a simple circular helix proposed earlier.