Physicists in the US have developed a new technique for making nanostructures that have both ferroelectric and ferromagnetic properties. So-called ferroelectromagnetic materials could be used to help convert electric energy into magnetic energy, and vice versa, in devices such as transducers, sensors and actuators (H Zheng et al. 2004 Science 303 661).
Only a few single-phase materials exhibit both strong electric and magnetic properties. Composites that contain both electric and magnetic materials can be made, but this is difficult because the two materials must have compatible lattice structures and must also interact effectively with each other.
Now Ramamoorthy Ramesh at the University of California at Berkeley and co-workers at the University of Maryland, Rowan University, Virginia Tech and Pennsylvania State University have used laser deposition on a ceramic substrate to make a composite ferroelectromagnet from barium titanate and cobalt ferrite. This former is ferroelectric, while the latter is ferromagnetic, and both have similar lattice dimensions.
When they looked at the composite with atomic force and transmission electron microscopes, Ramesh and co-workers found that the two compounds had self-assembled into hexagonal arrays with nanopillars made of cobalt ferrite embedded in a barium titanate matrix. The ferrite nanopillars were evenly sized and spaced about 20 to 30 nanometres apart (see figure).
The composite nanostructure shows three-dimensional heteroepitaxy, which means that the two phases are epitaxial with respect to the substrate and also with respect to each other. This leads to strong mechanical coupling between the two phases, which the team demonstrated by measuring how the magnetic properties of the material changed with temperature. They found a distinct drop in the magnetization around the Curie temperature, which indicated that the ferroelectric and ferromagnetic phases had indeed coupled.
“We now need to get long-range order among the nanopillars, meaning that they arrange themselves into a lattice with a periodicity of say 50 to 100 nanometres,” Ramesh told PhysicsWeb. “We could then have electrically and magnetically tuneable photonic structures.” The researchers now plan to build nanostructures on technologically important substrates such as silicon, and to apply the technique to other materials.