BERKELEY, CA -- Combinatorial synthesis -- a strategy developed at Ernest Orlando Lawrence Berkeley National Laboratory for making and testing many complex metal materials in parallel -- has paid quick dividends for its inventors.
Scientists at the Berkeley Lab's Molecular Design Institute have used the technique to discover new magnetoresistive (MR) compounds, materials whose electrical conductivity changes in a magnetic field.
Berkeley Lab researchers Xiao-Dong Xiang and Peter Schultz published their findings in the Oct. 13 issue of the journal Science. Schultz is also a professor of chemistry at UC Berkeley.
Xiang and Schultz first described their combinatorial technique only four months earlier, in the June 23 issue of Science.
"The technique is a powerful way to examine a broad base of advanced materials," Xiang said. "Magnetoresistant materials are just the first."
Combinatorial synthesis enables materials scientists to create thousands of complex materials in the time it normally takes to create one. The technique lays down the materials in a checkerboard arrangement, as thin squares arranged in a grid.
Different metal ingredients are stenciled onto the grid through cut-out "masks." Because of the different cut-outs used for each ingredient, each square in the grid receives a different combination of metals.
Heat treatment mixes the ingredients and creates a grid of stable compounds--a "combinatorial library." The materials can be scanned for interesting electrical properties with a matching set of contact pins. Xiang and Schultz have shown they can put as many as 10,000 different hi-tech materials into a single square-inch library.
The MR compounds in the study are members of a materials class known as perovskite oxides, conductive metal crystals that are made of complex ratios of four to six metal atoms. Their many-atom structure makes them particularly suited for combinatorial study. Interesting materials in such classes are usually discovered by substituting atoms in an already known compound or changing the ratios of the existing atoms. With a combinatorial strategy, scientists can look at many atom substitutions and ratio adjustments in a single combinatorial library.
To look for new materials with magnetoresistance, the researchers began with a well-studied class of MR materials based on manganese oxide. They substituted similar elements from the periodic table -- iron, vanadium and cobalt -- for manganese, and made a separate combinatorial library for each one.
The researchers found success with the cobalt oxide library, which yielded 26 new MR materials. The materials showed resistance changes as high as 72 percent, making the cobalt oxides similar to the so-called "colossal" class of MR materials. Colossal MR materials lose a great deal of their resistance in magnetic fields, some as much as 99.99 percent.
Although the phenomenon is still not well understood, scientists suspect magnetoresistance has to do with a physical characteristic known as spin polarization. Spin polarization affects how electrons can be passed back and forth between neighboring atoms. Atoms with similar spin polarizations are much more likely to transfer electrons between one another than those with different spin polarizations.
It is thought that magnetism aligns the spin polarization in MR materials. This lets electrons jump more freely between atoms, causing resistance in a material to decrease.
In the past, combinatorial approaches have been used successfully by researchers in the life sciences, who have searched for potential drugs by screening vast libraries of protein combinations. Xiang and Schultz are the first to apply the philosophy to solid-state materials.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, Calif. It conducts unclassified scientific research and is managed by the University of California.
An additional article provides further details on how the combinatorial synthesis technique can be used to create and test whole libraries of materials.