Focus: The Best of Both Worlds

Published June 22, 2007  |  Phys. Rev. Focus 19, 20 (2007)  |  DOI: 10.1103/PhysRevFocus.19.20

Multiferroicity Induced by Dislocated Spin-Density Waves

Joseph J. Betouras, Gianluca Giovannetti, and Jeroen van den Brink

Published June 20, 2007
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New ears. Compact microphones use a material that couples vibrations with electric fields in order to efficiently change sound into electricity. Physicists hope to revolutionize computing with similar materials that combine electrical and magnetic sensitivity. A theory suggests a new class of such “multiferroic” materials.

In computers, magnetism gets the prize for storing information, but electrical signals do the actual computations. Many physicists hope that materials that have both built-in magnetic and electric fields will enable entirely new types of computing devices, but the two properties rarely coexist. Now, in the 22 June Physical Review Letters, researchers describe a new way that such a combined state might occur. The ideas widen the field of candidate materials for novel computing paradigms.

Each digital one or zero stored on a hard drive is represented by a microscopic patch of magnetic material–a piece of permanent magnet, or ferromagnet. Ferroelectric materials, which have a permanent alignment of their electrical charge, are less familiar, but are used in equipment like microphones. Physicists have dreamed of materials that are simultaneously ferromagnetic and ferroelectric. In such a “multiferroic” material, they hope, a tiny magnetic field could control an electric current. Or perhaps a tiny electric signal could reverse the magnetic field of the material, or magnetically align the spins of electrons passing through it, enabling new types of “spintronic” devices.

For these uses, however, the electric and magnetic behavior must strongly influence each other. “The interesting multiferroics are those where you have a coupling between the two,” says Jeroen van den Brink of the University of Leiden and the Radboud University in Nijmegen, both in the Netherlands. Unfortunately, the usual mechanisms that produce ferroelectricity and ferromagnetism are incompatible at an atomic level, so they rarely occur together, let alone interact. Researchers have made multiferroic materials, for example by blending components with each property. They have also found coexistence of the two in some uniform materials, but it seemed to require an unusual, helical pattern of magnetism at the atomic scale.

Van den Brink and his colleagues now propose a less exotic mechanism by which a material could become multiferroic. Their theory was motivated by recent perplexing experiments, where materials called rare-earth manganites became multiferroic at low temperatures, even though they don’t have the helical magnetic pattern. The materials form so-called spin-density waves, where the bar-magnet-like spins of the atoms organize into sheets alternately pointing in opposite directions. The researchers argue that this magnetic order alone can generate a macroscopic electric field.

The team starts with the conjecture that a magnetization that changes from place-to-place at the atomic scale–unlike the uniform magnetism in an ordinary ferromagnet–generates a small electric field. Although they don’t know in detail how this might happen, it does not violate any known laws and leads to a simple explanation of the data. Their theory then shows how a large-scale field will be generated if the period of the spin-density wave is an exact multiple of the period of the spacing between atomic layers, as well as being shifted in position with respect to the layers. These conditions appear to be met at the temperatures where the rare-earth manganites become multiferroic. The theory also suggests that researchers should look for a permanent electric field in other materials that form spin-density waves, such as certain organic molecular crystals.

Art Ramirez, of Alcatel-Lucent’s Bell Labs in Murray Hill, New Jersey, says that the model could help explain the experiments in rare-earth manganites. However, he notes that other multiferroic materials don’t seem to match the requirements of this model.

–Don Monroe