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Ferromagnetism and ferroelectricity are examples of ferroic properties, which are distinguished by spontaneous order formation and phase transitions that are history-dependent. Multiferroics blend multiple ferroic properties, with these combinations possibly leading to new properties and capabilities for materials.
In general, the term 'multiferroics' refers to the combination of magnetism and ferroelectricity because of the major on-going pursuit to combine these properties in search of technological applications.
Magnets exhibit spontaneous magnetization as a result of magnetic dipoles within a material. If these dipoles all line up in the same direction, it results in ferromagnetic ordering. The orientation of the dipoles can be altered through the use of an external magnetic field.
Analogous behaviour can be observed in ferroelectrics, such as certain kinds of non-conducting crystals exhibiting spontaneous electric polarization, which involves one side having a positive charge and the reverse side having a negative charge. The electric polarization of ferroelectrics can be alternated via an electric field.
Magnetism and ferroelectricity are crucial to modern technology, and while they are almost entirely mutually exclusive, a recent major discovery has indicated that a weak magnetoelectric interaction can result in impressive cross-coupling effects when triggering electric polarization in a magnetically-ordered system. Magnetic ferroelectricity has been identified in 'frustrated magnets' -- materials with contending interactions between spins and complicated magnetic orders.
Modern computing is based on the ability to alternately switch magnetic states. In theory, multiferroics would permit the transitioning of magnetic state by electric field, as opposed to magnetic, which needs significantly less energy. However, there are a small quantity of materials with multiferroic behaviours that are useful at room temperature.
A major step toward making viable magnetoelectric technology would be the ability to alternate ferroelectric states with relatively minor voltages. The discovery of more magnetoelectric materials would help in the potential development of such devices.
One possible approach to developing multiferroic devices with new functionality is to design novel topological structures, like the domain walls that separate individual ferroelectric domains. These structures can be engineered to carry data or function as charge-carrying elements in unique device designs.
While multiferroicity can be appear in a single material, it can also appear in composites produced by merging ferroelectric and magnetic materials, which might be useful for applications like sensor or transmission technology. Focusing on particular magnetoelectric qualities in thin films is even more difficult than fabrication of other semiconductor-based devices. While the development of multiferroics technology is crucial to pushing the technology to the market, educating end users about such technology is essential to making it commercially viable.
One major obstacle to commercial viability is a dependence on extreme conditions. For instance, some ferroelectrics exhibit quantum critical behaviour, in which a phase transition can take place at 0 degrees Kelvin.
Recent Developments
There has been significant progress in multiferroics research over the past decade, including developments in the fundamental understanding of ferroics. For instance, a new order has been identified, ferrotoroidicity, which involves the spontaneous ordering of magnetic vortices.
Researchers have also discovered that domain walls can have distinct electrical conductivity, a finding that appears crucial to memory applications. There have also been major advancements in the processing of multiferroics, with developments in film-deposition methods that enable the creation of composite materials with customized qualities, like the manipulation of magnetism at room temperature with an electric field.
Even the most basic study of multiferroics has led to significant scientific discoveries. However, there has been a relative scarcity in the general adoption of multiferroic technology and the rate of commercial progress. For multiferroics to have an effect beyond the academic world, more novel applications should be investigated.
References and Further Reading
https://www.nature.com/collections/badcahbjie
https://www.nature.com/articles/s41563-019-0310-y
https://www.britannica.com/science/ferroelectricity
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