For generations humans have been engineering pillars that have more than one function. The giant stone pillars of the 9th-century Cordoba Mosque in Andalusia, Spain, for example, provide both structural stability and aesthetic appeal. Now researchers in the US have built pillar structures at the nanometre scale that combine two markedly different functions: magnetism and ferroelectricity. The technological importance of these 21st century nanopillars could be as far reaching as that of the ancient building techniques used in Andalusia.
Magnetic materials are ubiquitous, from the huge transformer cores in electrical power sub-stations to the tiny magnetic particles that are used to store data on our computer disks. The widespread applications of magnets stem from two basic properties. First, they have a spontaneous magnetic moment, which enables magnetic flux to be concentrated in transformers. Second, the orientation of the magnetism can be switched back and forth by an applied magnetic field, and can therefore be used for data storage.
Similarly, ferroelectric materials have a spontaneous electric polarization, the direction of which can be switched with an applied electric field. In fact, the “ferro” part of the name arises because their electrical properties are similar to the magnetic properties of iron-based magnetic materials; most, however, are not ferrous in the sense that they contain iron. Ferroelectrics are used to make capacitors with high dielectric constants, and also have applications in nonvolatile data storage and sonar.
Now, Haimei Zheng of the University of Maryland and colleagues in the US have adopted a different approach, which I believe is much more promising for producing useful magneto-electric multiferroics. Instead of trying to produce a single compound, they grow a closely interwoven composite material from magnetic cobalt ferrite (CoFe2O4) and ferroelectric barium titanate (BaTiO3). To do this, the team used a well established growth technique called pulsed vapour deposition, in which an oxide target containing the correct ratios of barium, titanium, cobalt and iron is bombarded with a laser. This releases atoms from the target that fortuitously self-assemble into nanometre-sized 3D pillars of cobalt ferrite within a barium titanate matrix (H Zheng et al. 2004 Science 303 661).
In the April issue of Physics World Nicola Spaldin from the University of California at Santa Barbara describes this work in more detail
