Multiferroics: Materials of the Future
Materials exhibiting ferroic phase transitions are ubiquitous in nature. Ferroic materials are those which possess two or more orientation states (domains) that can be switched by an external field and show hysteresis. Typical examples include ferromagnets, ferroelectrics and ferroelastics which occur as a result of a phase transition with the onset of spontaneous magnetization (M), polarization (P) and strain (e), respectively. A material that displays two or more ferroic properties simultaneously is called a multiferroic, e.g. magnetoelectrics (simultaneous P and M). These materials find widespread applications as actuators, transducers, memory devices and shape memory elements in biomedical technology.
First I will provide a historical perspective on this technologically important class of materials and then briefly illustrate the relevant concepts through two recently discovered subclasses: one perovskite based and the other one with a hexagonal structure. I will discuss their properties, model the transitions at mesoscale and describe their microstructure. To this end I will introduce the notion of symmetry breaking: Broken rotational symmetry in a crystal leads to ferroelasticity, broken inversion symmetry leads to ferroelectricity and broken time reversal symmetry results in ferromagnetism. For the latter class and multiferroics, I will introduce the concept of color symmetry in order to describe various magnetic phases. Next, I will emphasize the role of long-range, anisotropic forces such as those arising from either the elastic compatibility constraints or the (polar and magnetic) dipolar interactions in determining the microstructure.
Much of the excitement in this field stems from the unusual optical, spin and lattice properties of these materials which renders them as truly viable candidates for future metamaterials, e.g. as negative refractive index materials (NIM).
Avadh Saxena is Deputy Group Leader in the Condensed Matter Theory group at Los Alamos National Lab. His main research interests include phase transitions, optical, electronic and vibrational properties of functional materials, device physics, soft condensed matter and nonlinear phenomena. He has published about 240 reseach papers and has organized over two dozen international conferences on these topics. He is a frequent invited speaker world-wide. He holds adjunct professor positions at the University of Barcelona, Spain and the University of Arizona, Tucson. He is Scientific Advisor to National Institute for Materials Science (NIMS), Tsukuba, Japan. He is a member of American Physical Society, the Sigma Xi Scientific Research Society and the Metals, Minerals, and Materials Society. Contact him at: firstname.lastname@example.org.
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