What is the importance of the study of the properties of multiferroic materials?

Answer 1

To understand what a multiferroic material is and why it is important, it is first necessary to understand what a ferroic material is. A ferroic material is one in which some property can be switched between a number of equivalent states by the application of some external field. The best known example of a ferroic material is a ferroamagnet. In these materials the magnetic polarisation can be switched between a number of equivalent stable directions by the application of an external magnetic field. It is easy to show this with a steel nail and a permanent magnet. The steel is ferromagnetic. Take the north pole of the magnet and stroke it in one direction down the nail towards the point of a 6 inch nail. After doing this, the nail will be magnetised with the south pole at the point. You can demonstrate this by holding it close to a compass needle. The north pole of the compass needle will be attracted towards the point of the nail. If you stroke the north pole of the permanent magnet along the nail in the opposite direction, the point of the nail will now become the north pole, and will attract the south pole of the compass needle. In this way you can demonstrate switching the polarisation of the nail between two opposite, but equivalent states. The same thing can be done by winding a coil of wire around the nail and passing a current through the coil. Passing the current one way will generate a magnetic field in the nail and polarise it in one direction, so that when the current is removed the nail will stay magnetised. Reversing the direction of current flow will reverse the nail's direction of magnetisation. This switching between two magnetically poled states is called ferromagnetic hysteresis, and it involves lining up the unpaired electron spins in the material. Ferromagnetic materials are used all over the place - in Transformers, sensors, relay switches, loudspeakers etc. One of the more important modern uses of ferromagnetic materials is in information storage, both as the storage layer in a hard disk and in the read head to read-out the information. One sort of possible information storage mechanism is the spin-valve device, which uses two polarised ferromagnetic conducing layers separated by a very thin insulating layer of oxide (e.g. MgO). Electrons can cross the insulating oxide by quantum mechanical tunnelling, but they can do this much easier if the two magnetic layers are polarised in the same direction. It is much harder to cross the barrier if the conducting layers are oppositely polarised. This is a possible new form of solid state memory. It could be very small. Spin valve devices can also be made with ferromagnetic insulating oxides as the barrier. In this case the direction of polarisation of the magnetic oxide determines how easily the electrons can tunnel through it to magnetic conductors, depending on the relative directions of polarisation. Let's consider now what other sorts of ferroics there are. Another, less famous but equally widely used, type of ferroic is the ferroelectric effect. In ferroelectrics, there is a permanent electrical polarisation which can be switched between different stable directions by applying an electric field - giving ferroelectric hysteresis. Ferroelectrics can be strongly piezoelectric and are used in devices like spark generators for gas ignition, microphones, tuning lasers and driving the fuel injectors of modern diesel engines. They are also strongly pyroelectric and are used as infra-red sensors in PIR burglar alarms and light switches. Another major application is in ceramic capacitors. A third type of ferroic is the ferroelastic material, in which we can switch a mechanical strain between equivalent states by applying a mechanical stress. Memory metals are a form of ferroelastic. OK. So far so good, so what is a multiferroic? It is a material in which there are two or more ferroic properties, which are coupled together. The most sought-after forms of multiferroic materials at the moment are those in which the ferroelectric and ferromagnetic states are coupled together. In these (and there are a few), the magnetic polarisation can be switched by the application of an electric field, or the electric polarisation by the application of a magnetic field. One example of such a material is bismuth ferrite, BiFeO3, which is both ferroelectric and antiferromagnetic. Other exmples exist within a complex group of compounds called the boracites, which have the general chemical formula M3B7O13X, where M is a divalent metal and X is a halogen. Why are multiferroics potentially important (or why is it important to study their properties)? In the example given above of the spin valve device, there is the potential to make the barrier layer out of a ferroelectric/ferromagnetic insulator. In this case there would be the potential to switch the barrier layer magnetic polarisation by applying a simple eletrical pulse to switch the ferroelectric polarisation direction. Such a device would be very small and low power, but no-one has yet succeeded in making one.