Charge ordering and multiferroicity in Fe3BO5 and Fe2MnBO5 oxyborates
Résumé
The comparison of Fe3BO5 and Fe2MnBO5 reveals that the 2Fe(2+) Fe3+ 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 Fe2MnBO5. Interestingly, for both oxyborates, glassiness is observed in the dielectric properties at low temperature, but only Fe3BO5 shows a magnetodielectric effect close to its lower magnetic transition. A change in the electrical polarization, measured by pyroelectric current integration, is observed in Fe3BO5 and is even more pronounced in Fe2MnBO5. Such results suggest that these oxyborates behave like antiferromagnetic relaxor ferroelectrics. These features are proposed to be related to the distribution of the species (Fe3+, Fe+ and Mn+) over the four transition metal sites forming the ludwigite structure.