Dielectric ordering and colossal magnetodielectricity in the antiferromagnetic insulating state of λ-(BEDT-TSF) ₂FeCl ₄

We report on the temperature and magnetic-field dependence of dielectric constant ε ₁ ^c and conductivity σ ₁ ^calong the c axis in the antiferromagnetic insulating (AFM-I) state of λ-(BEDT-TSF) ₂FeCl ₄, which indicate some dielectric ordering in coexistence with AFM-I. There appears a sharp upturn of saturated electric polarizations and a colossal magnetodielectricity (CMD) in magnetic fields. On a basis of the π-d superexchange interaction we propose a charge ordering-induced polarization model leading to two candidates of ferroelectric or antiferroelectric orderings, and discuss the prominent magnetoelectric effects associated with CMD in terms of the field-induced unlocking or melting charge ordering.