What is "multiferroics"

n. (plural of multiferroic English)

Multiferroics have been formally defined as materials that exhibit more than one primary ferroic order parameter simultaneously (i.e. in a single phase), and many researchers in the field consider materials to be multiferroics only if they exhibit coupling between primary order parameters. However, the definition of multiferroics can be expanded to include non-primary order parameters, such as antiferromagnetism or ferrimagnetism.

The four basic primary ferroic order parameters are

1. ferromagnetism
2. ferroelectricity
3. ferroelasticity
4. ferrotoroidicity

The last is a topic of some debate, as there was no evidence for switching ferrotoroidicity until recently.

Many multiferroics are transition metal oxides with perovskite crystal structure. They can be generally subdivided into two classes as introduced by D. Khomskii: Type-I and type-II multiferroics.

Type-I multiferroics which have been known for a long time are often good ferroelectrics and antiferromagnetic. They exhibit high ferroelectric and lower magnetic ordering temperatures. Examples are BiFeO (T = 1100 K, T = 643 K) and YMnO (T = 914 K, T = 76 K) and probably BiMnO and PbVO. Since ferroelectricity and magnetism develop independently from another, the magnetoelectric coupling between the two is usually weak. In a first approach, they might be regarded as ferroelectrics which happen to be (antiferro)magnetic. However, more recently there have been reports of large magnetoelectric coupling at room-temperature in type-I multiferroics such as in the "diluted" magnetic perovskite (PbZrTiO)–(PbFeTaO) (PZTFT) in certain Aurivillius phases, and in the system (BiFeCoO)-(BiKTiO) (BFC-BKT). Here, strong ME coupling has been observed on a microscopic scale using PFM under magnetic field among other techniques. The latter system, appears to be the first reported core-shell type relaxor ferroelectric multiferroic, where the magnetic structure in so-called "multiferroic clusters" is proposed to be due Fe-Co ferrimagnetism, which can be switched by an electric field.

Type-II multiferroics include rare-earth manganites such as TbMnO, HoMnO. Here, magnetism causes ferroelectricity and ordering temperatures are usually identical, which are, however, at very low temperatures (e.g. 28 K in case of TbMnO). Moreover, they exhibit low net magnetization due to antiferromagnetic spin-spiral structures and low polarization of the order of 10 μC/cm.

Other, non-perovskite multiferroic oxides include LuFeO and LiCuO, and non-oxides such as BaNiF and spinel chalcogenides, e.g. ZnCrSe. These compounds 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.