Pulsed Laser Deposition as a Novel Growth Technique of Multiferroic LuFe 2 O 4 Thin Films
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Pulsed Laser Deposition as a Novel Growth Technique of Multiferroic LuFe2O4 Thin Films J. Rejman, T. Dhakal, P. Mukherjee, S. Hariharan, S. Witanachchi. Laboratory for Materials Science and Technology, University of South Florida, 4202 E. Fowler Ave., Tampa, FL, 33620 ABSTRACT Growth of polycrystalline Lutetium Iron Oxide via pulsed laser deposition in thin film form is reported for the first time herein, and the multiferroic LuFe2O4 phase is stabilized. Fluence and pressure dependent phase growth is demonstrated, along with crystalline structure in vacuum (~10-5 torr) conditions. Thermodynamic considerations at the laser-target interaction were investigated, as well as at the plume-substrate interface, which reveal that the necessary Gibbs free energy is available in the optimized growth environment to allow formation of the LuFe2O4 polycrystalline phase. The resulting growth rate is found to be related to the Gibbs free energy and concentration of nucleation sites on the substrate. Characterization of the multiferroic aspect of LuFe2O4 entailed direct measurement of the ferroelectricity in the thin film, as well as magnetic behavior, both at various temperatures. In particular, the ferroelectric polarization vs. voltage data yield values of 0.61 µC/cm2 at 300 K to 3.29 µC/cm2 at 183 K; moreover, these data are in agreement with those reported in the literature. Magnetization vs. applied field data shows the magnetization at 300 K to be 180 emu/cm3 and increasing to 200 emu/cm3 at 10 K. INTRODUCTION Pulsed laser deposition (PLD) was used as a novel approach to deposit thin films of the target material on heated substrates in order to realize the hexagonal LuFe2O4 phase. Thin films of LFO have not heretofore been reported or characterized, nor does the literature contain information regarding the use of PLD for LFO thin film deposition. For the purposes of this work thin films will be defined as in [1], i.e. the thin film regime begins at tfilm < 500 nm. PLD is superior to other conventional thin film deposition techniques such as ion beam sputtering, magnetron sputtering, and molecular beam epitaxy due to having several orders of magnitude [2] larger affinity towards producing stable nucleation sites. EXPERIMENT §1. Laser-target interaction A LuFeO3 target was used in the PLD process to make thin films of LuFe2O4. Scanning electron microscopy (SEM) images of the target show the surface features at the Excimer lasertarget surface interaction regions (figure 1). The number of pulses per track is on the order of 20,000 shots, the repetition rate = 10 Hz, the rate of rotation is 23 rpm, and the difference between the outer and inner diameter of each track is 3 mm. Utilizing multiple tracks on the target allows for a uniform surface deformation on the target, which ultimately admits higher
quality thin films. Figure 1a reveals the surface features and grain sizes for the non-ablated central region of the target, and 1b an ablated region with an inset at 1000 times magnification to show the target surface
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