New Magnetic and Ferroelectric Cubic Phase of Thin-Film Fe-Doped BaTiO 3
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R. MAIER,' J. L. COHN, J. J. NEUMEIER, AND L. A. BENDERSKY3 SDepartment of Physics, University of Miami, Coral Gables, FL 33124 2 Department of Physics, Florida Atlantic University, Boca Raton, FL 33431 3 Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899
ABSTRACT The properties of a new cubic perovskite phase of thin-film BaFexTilxO 3 (0.5 _x_ 500 °C) and ferroelectric (Tc - 200-300 °C). INTRODUCTION Compounds in which magnetic and electric polarization exist simultaneously [1] are quite rare in nature, particularly at room temperature and above. Such materials offer the prospect of devices in which, e.g., the magneto- or electrostriction may be used to manipulate the polarization or transition temperature. The ferroelectric barium titanate (BaTiO 3) is currently employed in positive-temperaturecoefficient-resistance (PTCR) thermistors and has been widely explored for its potential use in applications such as memories, electro-optic switches and modulators. Iron substitution for titanium in bulk BaTiO 3 stabilizes the hexagonal structure [2] rather than the cubic/tetragonal perovskite. Solid solutions of bulk hexagonal BaTij,5 FexO 3 with 0.06•x•0.84 have been reported [3]. Recently we reported structural characterization of thin-film BaTil-,FeO
3
(x-0.5)
grown by pulsed laser deposition and having the cubic/tetragonal perovskite structure [4]. Here we discuss preliminary measurements of physical properties, establishing the coexistence of magnetic and electric order well above room temperature. EXPERIMENT
Thin films were grown by pulsed laser deposition, using a 248 nm KrF excimer laser (Lambda Physik, Compex 205), from targets of BaFeo.5 Tio.50 3, BaFe0 75 Ti0 .250 3, and Ba 4Fe 4Ti 3O06 (BFTO-E), prepared by standard solid state reaction. The energy density at the target was -1.5-2.0 J/cm 2, pulse repetition rate 10 Hz, and target-substrate distance 4 cm. The films were deposited on MgO substrates of (100), (110), and (11) orientation, as well as SrTiO 3 (100), with substrate temperature 880 °C and oxygen pressure 100 mTorr. Following the depositions, the chamber was filled to 500 Torr oxygen, held at 500 °C for 30 min., and subsequently cooled to room temperature. X-ray diffraction (XRD) was performed using a Philips X'Pert diffractometer with parallel-beam optics. Magnetization was measured with a Quantum Design SQUID magnetometer. Capacitance measurements (LCR meter, HP 4363B) were performed on tri-layer structures composed of films deposited on metallic, Nb-doped SrTiO 3 (100) substrates and vapor-deposited gold overlayers. The excitation voltage was 20 mV at 20 kHz. Two-probe dc electrical resistance was investigated by measuring current (Keithley, 6512 electrometer) at various applied dc voltages (Keithley, 230 voltage source). 29 Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
RESULTS All films have a similar microstructure; a detailed investigation of the films grown from the BFTO-E target on MgO is present
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