Ferromagnetism is the most familiar kind of magnetism, characterized by fields oriented by north and south poles. Ferroelectricity refers to materials which can be electrically polarized, having a preferred orientation of positive-negative electric charge. In ferroelastic materials "shape-memory" alloys, for example stress can cause a spontaneous change in orientation or crystal structure that can flip back again. A multiferroic material may have regions, separated by domain walls, where one or more of these different properties are differently oriented.
"One reason we are looking at oxides like bismuth ferrite is because we can control one property by changing others," says Ramesh. "These materials have a lot of personality."
Jan Seidel of the UC Department of Physics came upon the unique domain-wall properties of bismuth ferrite as he was performing scanning-probe measurements on thin films of the material created by Lane Martin of MSD; both are members of the Quantum Materials program.
"To make the films we hit targets of ceramic oxides containing bismuth and iron with a laser pulse and convert the solid to a plasma plume that is deposited on the substrate," Martin explains. "We choose the substrate structure and control the temperature and atmosphere to get the right mix and the right phase."
For the present work Martin grew films of bismuth ferrite between 50 and 200 nanometers thick on substrates of strontium titanium oxide (SrTiO3), a crystal whose similar lattice parameters and structure encouraged the growth of high-quality bismuth
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory