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Magnetic and Magnetooptical Films made by Pulsed Laser DepositionProject Staff:C.A. Ross, V. Sivakumar, A. Taussig, M. Bolduc, in collaboration with G. Dionne, Y. Shao-Horn and G. Ceder Sponsors:MicroPhotonics Center Consortium Funding, Institute for Soldier Nanotechnology In Pulsed Laser Deposition (PLD) a high energy excimer laser is used to ablate a target, releasing a plume of material which deposits on a substrate to form a thin film. PLD is particularly useful for making complex materials such as oxides because it preserves the stoichiometry of the target material. We have used PLD to deposit a variety of oxide films for magnetooptical isolators. These materials include iron oxide, which can adopt one of four different ferri- or antiferromagnetic structures depending on deposition conditions, bismuth iron garnet (BIG, Bi3Fe5O12), and Fe-doped BaTiO3. The ideal material for an isolator combines high Faraday rotation with high optical transparency. Garnets have excellent properties, but do not grow well on silicon substrates making it difficult to integrate these materials. In contrast, iron oxide (maghemite) grows very well on MgO or Si, with high Faraday rotation, but its optical absorption is too high to be useful. A perovskite, Fe-doped barium titanate, however, has a moderate Faraday rotation. We are exploring the doping of perovskites with other elements in order to improve the magnetooptical figure of merit. In particular, we observe Faraday rotation in CeFeO3 and YCeFeCoO6 films. A second project involves the use of electrochemical methods to control the magnetization of nanoscale transition metal oxides, making a chemically switchable material. The accessibility of multiple electronic configurations and coordination of cations in these materials enables the control of magnetism by external stimuli, such as by electrochemical lithiation and delithiation, as occurs in a lithium ion battery. We have investigated changes in magnetization and structure of pulsed laser deposition (PLD)-grown Fe3O4 (magnetite) thin films, Fe3O4 nanoparticles, and CrO2 nanoparticles upon electrochemical lithiation. We observed a substantial decrease in saturation magnetization Ms (up to 30%) in thin films of magnetite grown by PLD. Significantly larger reduction in moment (up to 75%) was observed in commercially available nanoparticles upon addition of 2 moles of Li per formula unit, along with changes in remanence and coercivity. The smaller drop in Ms observed in thin films is attributed to a kinetic effect due to high density and greater diffusion lengths in PLD-grown films. This process has also been applied to needle-shaped particles of chromium dioxide. The effects of cycling, discharge-charge rate, temperature of cycling and particle size are also being studied. It has been shown that the process may be partially reversible for low Li contents. These changes in magnetic moment may be rendered useful in magnetomechanical or magnetoelectronic applications.
Figure 1. Faraday rotation vs. applied field for 500-nm-thick CeFeO3 and YCeCoFeO6 films grown in a vacuum or under 6-mTorr O2 pressure on MgO substrates, with the field perpendicular to the film.
Figure 2. Magnetic Hysteresis loops of the iron oxide films grown on Cu before and after lithiation at 0.393 µA/cm2. Note the drop in magnetization after lithiation, which recovers as the Li diffuses out of the material.
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