Multiferroic Materials

Multiferroic materials constitute a class of multifunctional materials presenting in the same time coupled properties in terms of ferromagnetic-ferroelectric and ferroelastic order. On a fundamental scale, the nature of the interactions and in particular the magnetoelastic coupling mechanism is not fully understood despite the amount of experiments. On a technological level, this class of material is interesting in the field of information storage or for microelectromechanical systems development. Among multiferroic materials, BiFeO₃ is one is one of the few which presents in the same time both ferroelectric and magnetic order above 300K. However despite this unique property, BiFeO₃ also presents a magnetic helicoïdal structure of Fe³⁺ magnetic moments, this magnetic structure leads to weak magnetoelectric coupling. A drastic increase of magnetoelectric coupling can be obtained through the breakdown of helicoïdal spirale and the formation of an antiferromagnetic linear structure of type G leading to a non-zero mean magnetization.

Multiples ways can be followed to breakdown this helicoidal structure among them:

1. Reducing the size of the material dimension to a value lower than the cycloid period (~64 nm). This can be obtained by a polyol method for synthetizing BiFeO₃ nanoparticles.
2. Substituting Bi³⁺ and Fe³⁺ by Ti⁴⁺ and Zr⁴⁺ using a ceramic synthesis in order to partially break helicoïdal structure of Fe³⁺ magnetic moments

MEB picture of BiFeO3 particles obtained with polyol processModification of helicoïdal magnetic order and apparition of ferromagnetic order