We have studied the magnetoelectric coupling of the frustrated triangular antiferromagnet iron jarosite using Raman spectroscopy, dielectric measurements and specific heat. Temperature dependent capacitance measurements show an anomaly in the dielectric constant at TN. Specific heat data indicate the presence of a low frequency Einstein mode at low temperature. Raman spectroscopy confirms the presence of a new mode below TN that can be attributed to folding of the Brillouin zone. This mode shifts and sharpens below TN. We evaluate the strength of the magnetoelectric coupling using the symmetry unrestricted biquadratic magnetoelectric terms in the free energy.
Sketch of two connected triangles formed by Fe3+ spins (red arrows) in the hexagonal basal plane of potassium iron jarosite. An applied magnetic field (H) below the antiferromagnetic ordering temperature induces shifts of the hydroxy ligands, giving rise to local electrical dipole moments (blue arrows). These electric displacements cancel out in pairwise fashion by symmetry. Ligand shifts are confined to the plane and shown by shadowing.