Multiferroics

Multiferroics have been formally defined as materials that exhibit more than one primary ferroic order parameter simultaneously (i.e. in a single phase).

The four basic primary ferroic order parameters are

1. ferromagnetism
2. ferroelectricity
3. ferroelasticity
4. ferrotoroidicity (?)

the latter still being under debate as there has been no proof for switching ferrotoroidicity till today (Mar. 2012). Many researchers in the field consider materials as multiferroics only if they exhibit coupling between the order parameters. On the other hand, the definition of multiferroics can be expanded as to include non-primary order parameters, such as antiferromagnetism or ferrimagnetism.

Typical multiferroics belong to the group of the perovskite transition metal oxides, and include rare-earth manganites and -ferrites (e.g. TbMnO₃, HoMn₂O₅, LuFe₂O₄). Other examples are the bismuth compounds BiFeO₃ and BiMnO₃, and non-oxides such as BaNiF₄ and spinel chalcogenides, e.g. ZnCr₂Se₄. These alloys show rich phase diagrams combining different ferroic orders in separate phases. Apart from single phase multiferroics, composites and heterostructures exhibiting more than one ferroic order parameter are studied extensively. Some examples include magnetic thin films on piezoelectric PMN-PT substrates and Metglass/PVDF/Metglass trilayer structures. Besides scientific interest in their physical properties, multiferroics have potential for applications as actuators, switches, magnetic field sensors or new types of electronic memory devices.