Multiferroic Properties of Sr_1-xBa_xMn_1-yTi_yO₃ Perovskites

Multiferroics are the subject of intense interest with elaborate investigations designed to understand how such two disparate order parameters could interact, coexist and compete within the same host structure ultimately allowing the remote control of magnetic properties via the application of external electric fields or vice versa. I will discuss the achievement of robust ferroelectricity with T_F > 300 K and magnetism (T_N ~ 200 K) in Sr_1-xBa_xMnO₃ perovskites only if enough of the large Ba²⁺ ions were forced into the structure at the Sr²⁺ site with contents extending well beyond the normal solubility limit. Neutron and x-ray diffraction were successfully used to demonstrate the strong and tunable coupling of the two order parameters that originate exclusively at the same Mn⁴⁺ sublattice. The ferroelectric phase transition exhibits a crossover signature from displacive to order-disorder type as revealed by spectroscopic phonon measurements. Ti-substituted Sr_1-xBa_xMn_1-yTi_yO₃ have also been synthesized with small amounts of Ti that maintain the integrity of the long range ordered magnetic Mn sublattice. The doubly substituted materials exhibit ferroelectric properties above 400 K with spontaneous polarization properties significantly exceeding those of the classical titanates.