8.1 Ferroic Material

• Ferroics are materials like ferroelectrics, ferromagnetic or ferroelastics which exhibit a large change in the properties of the materials across a critical temperature and show a characteristic hysteresis loop with two equivalent response states at zero value of stimuli.

• The critical temperature, in general, is also accompanied with a symmetry breaking.

• Typically known orderings are ferroelectric (coupling of charge polarization and electric field), ferromagnetic (coupling of magnetic moment and magnetic field) and ferroelastic (coupling of stress and strain) ordering. Another proposed ordering mechanism is ferrotordoicity which exhibit arrangement of magnetic vortices in an ordered manner, called tordoization.

      Figure 8.1 explains the various possible scenarios. While there are a large number of magnetically       and electrically polarizable materials, there are only a few materials which show ferroelectric and       ferromagnetic ordering.  Magnetoelectric materials are those materials which are simultaneously       electrically and magnetically polarizable, while Multiferroics are strictly those materials which show       ferroelectric and ferromagnetic ordering.  

While, strictly speaking multiferroism means only for those materials in which there is coupling of more than one order parameter, now a days, researchers have also started including antiferromagnetism as well as ferrimagnetism also with multiferroic materials.

The multiferroic materials are either rare earth manganites or ferrites or transition metal perovskite oxides. The examples are TbMnO₃, TbMn₂O₅, HoMn₂O₅, LuFe₂O₄, BiFeO₃, BiMnO₃ and YMnO₃. Some non-oxides are also multiferroics such as BaNiF₄ and spinel chalcogenides, e.g. ZnCr₂Se₄.

Given that the multiferroic materials show more than one ferroic ordering, the envisaged applications are numerous. Some of these applications can be future memory devices with multiple degree of control, sensors and actuators that be controlled by more than one type of stimuli, spintronic devices where spin of electron can be controlled electrically.

Recent reports also classify the multiferroics into Type I and Type II multiferroics. Type I multiferroics are those materials in which the source of ferroelectricity and magnetism is different and the effects are fairly independent of each other, albeit with a small degree of coupling. In contrast, type II materials are those where magnetism causes the existence of ferroelectricity attributed to the strong coupling between two states. However, the magnitude of polarization is these materials remains very small, typically less than 10^-2 μC/cm².

There are no text books yet on Multiferroics, however there are a few good reviews^1,2,3,4 in the international journals which can be referred for an elaborate reading. These reviews have also been source of much of the basic information in this module.