A Ferroelectric ferromagnet

We found the simultaneous occurrence of the technologically important properties ferroelectricity and ferromagnetism in CdCr2S4, a material with spinel crystal structure. Ferromagnetic materials are known to mankind since ancient times. Nowadays they find numerous applications in modern technology, e.g. in electric motors or computer hard discs. The ferroelectrics, discovered about 80 years ago, are the electrical analogues to ferromagnets. They are used, e.g., in electronic components and non-volatile memory devices. Ferroelectric ferromagnets, combining both properties in one material, could pave the way for a new generation of electronic devices, e.g. for the development of new types of electronic storage media. Despite an intense search for ferroelectric ferromagnets, until now only few such materials are known, their properties seeming not very promising for possible technological applications.CdCr2S4 belongs to the group of spinel crystals, which are important constituents of the earth mantle and also used in many applications, e.g. in high-frequency electronics. However, spinels are best known as the great impostors in gemstone history, often mistaken for rubies, even in crown jewels. Among the ferroelectric ferromagnets discovered so far, CdCr2S4 stands out due to its reasonable ordering temperatures (order 100 K) and sizable values of magnetization and polarization. Especially the occurrence of very strong ("colossal") magneto-electric effects, i.e. the strong coupling of electrical and magnetic properties (see Fig.) is amazing and gives rise to the hope for further advances in the understanding and application of these unusual materials.

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Temperature-dependent dielectric constant for two frequencies measured without and with magnetic field. A strong magneto-electric effect is revealed by a relative variation of the dielectric constant reaching nearly 500% (inset).


To learn more, see:
J. Hemberger, P. Lunkenheimer, R. Fichtl, H.-A. Krug von Nidda, V. Tsurkan and A. Loidl, Relaxor ferroelectricity and colossal magneto-capacitive coupling in ferromagnetic CdCr2S4, Nature 434, 364 (2005).