On a microscopic scale, atoms and molecules produce electric and magnetic fields. On our own scale, the electrical and magnetic properties of various atoms offset and cancel each other out in most crystals. Sometimes, however, this does not happen and for some compounds, known as ferromagnetics, magnetic properties remain on a macroscopic scale and can therefore be used as magnets. Less commonly, an electric order exists on a macroscopic scale. This can be observed in ferroelectric compounds. And even more rarely, electric and magnetic orders co-exist. This is the case of multiferroics. Furthermore, electric and magnetic orders interact in these materials. This interaction makes it possible to control the spin (or magnetic moment of the atoms) through an electric field, which opens up many new horizons, especially for information storage.

Researchers from the Laboratoire de physique des solides (CNRS/University Paris-Sud 11), the Institut rayonnement-matière de Saclay (CEA IRAMIS) and the Institut Néel (CNRS) synthesised and studied a multiferroic compound, BiFeO3. They revealed the interaction between the electric and magnetic orders, then manufactured a material composed of a BiFeO3 layer and a ferromagnetic film. They showed that they could apply an electric field to alter the preferred direction of magnetisation of the ferromagnetic film. These groundbreaking results validate the concept of storing and writing magnetic data using an electric field.

In today's hard discs, data - or bits - are written using a magnetic field which directs magnetisation, thus imposing the value of the bit. There are two possible magnetisation states and thus two possible bit values (0 or 1). With a multiferroic material, each memory element could be set to four separate states rather than two (two electrical polarisation states and two magnetisation states). It is also possible to imagine magnetic memories that have two states (like today's), but can be modified by applying an electric field. Being able to write and erase data using an electric field represents a genuine step forward for mobile electronic devices (mobile phones, laptop computers, GPS, etc.) in two ways. First, applying an electric field requires less energy than applying a magnetic field, which would increase battery life. Second, the electric field would be more localised, meaning that more memory elements could be placed on a surface and components could be even more highly miniaturised.

Bibliography

Electric field switching of the magnetic anisotropy of a ferromagnetic layer exchange coupled to the multiferroic compound BiFeO3. D. Lebeugle, A. Mougin, M. Viret, D. Colson, L. Ranno. Physical Review Letters, 18 November 2009.

* Laboratoire de physique des solides (CNRS/University Paris-Sud 11 Solid-State Physics Laboratory) and Institut Néel (CNRS)
** Saclay Institute of Matter and Radiation (CEA IRAMIS)