A relatively inexpensive technique for making thin films with novel magnetic properties out of the same class of ceramic materials that exhibit high-temperature superconductivity has been developed by scientists in the Materials Sciences Division.
Kannan Krishnan and Anjaneya Modak are using a chemical process to produce magnetic thin films composed of lanthanum, strontium, manganese, and oxygen (LSMO). This technique costs a fraction of the current method of choice for fabricating these films -- pulsed laser ablation. Plus, it holds potential for important commercial as well as scientific applications, whereas laser ablation has been mostly limited to laboratory use.
Thin films, layers of material only a couple thousand angstroms in thickness, have become a staple of the electronics industry. Interest in thin films made from magnetic ceramics is especially keen because these materials lose electrical resistance (become conductors) in the presence of a magnetic field -- a property known as magnetoresistance.
"LSMO thin films exhibit giant and even colossal magnetoresistance (GMR and CMR)," says Krishnan. "In the presence of a sufficiently strong magnetic field (about 5 to 10 tesla), the electrical resistance of our LSMO thin films will decrease more than a thousand percent compared to the less than 20 percent change observed in current thin films."
At low enough temperatures (about 77 degrees Kelvin), the order of change in resistance for the LSMO thin films made by Krishnan and Modak is even greater than for the ceramic "high temperature" superconductors.
"LSMO thin films and similar materials synthesized by our process could find applications in the next generation of magnetic field sensors, with potential applications in computer information storage and the automobile industry," Krishnan says.
Characterizing their LSMO thin films using the powerful transmission electron microscopes at the National Center for Electron Microscopy (NCEM), Krishnan and Modak found they could easily vary the composition and structure of the films in order to make a variety of products, including multilayer films with alternating magnetic layers.
A key to this versatility is in the preparation. Solutions of each of the film's constituents, in this case, lanthanum, strontium, and manganite, are prepared separately from cheap, readily available chemicals. These separate solutions are then subjected to further processing steps, including hydrolysis, spin coating, and heat treatment that transform the solution into a gelatinous ceramic. Depositing the solution on one type of substrate, such as lanthanum aluminum oxide, will produce an epitaxial film (one whose crystal orientation is the same as that of the substrate), while a different substrate, such as silicon, yields a polycrystalline thin film (a film composed of aggregates of crystals).
"This is the first demonstration of a reproducible sol-gel process that produces polycrystalline as well as epitaxial LSMO thin films," Krishnan says. "Our thin films demonstrate magnetic properties comparable to the best of existing technologies, and we offer a significant economic advantage."
The total cost of equipment and chemicals for the sol-gel polymeric process of Krishnan and Modak is less than $5,000, compared to the other techniques, which typically require investments in excess of $100,000. Krishnan and Modak will use the capabilities at NCEM and the Advanced Light Source to better understand the GMR and CMR characteristics of their thin films. One goal will be to substantially reduce the strength of the magnetic field needed to trigger this phenomenon.