Charge ordering and multiferroicity in Fe3BO5 and Fe2MnBO5 oxyborates

The comparison of Fe 3 BO 5 and Fe 2 MnBO 5 reveals that the 2Fe 2+ : Fe 3+ charge ordering of the former is suppressed in the latter. Spin dynamics probed by ac susceptibility are strongly affected by the substitution, inducing superparamagnetism at low temperature in Fe 2 MnBO 5 . Interestingly, for both oxyborates, glassiness is observed in the dielectric properties at low temperature, but only Fe 3 BO 5 shows a magnetodielectric effect close to its lower magnetic transition. A change in the electrical polarization, measured by pyroelectric current integration, is observed in Fe 3 BO 5 and is even more pronounced in Fe 2 MnBO 5 . Such results suggest that these oxyborates behave like antiferromagnetic relaxor ferroelectrics. These features are proposed to be related to the distribution of the species (Fe 3+ , Fe 2+ and Mn 2+ ) over the four transition metal sites forming the ludwigite structure.