Growth of Single Crystal Yig Fibers by the Laser Heated Pedestal Growth Method
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ABSTRACT Yttrium iron garnet (YIG) single crystal fibers of nominal composition Y3Fe5 O1 2 were grown by the laser heated pedestal growth (LHPG) technique, a miniaturized floated-zone process. YIG which melts incongruently, was grown at a temperature below the peritectic decomposition temperature under self-adjusting conditions even though it has very narrow solidification region according to the Y 20 3-Fe 20 3 phase diagram. YIG fibers in diameter ranges from 100 to 750 gtm were grown at various growth rates and conditions, and analyzed by x-ray diffraction, electron microprobe, and IR-VIS spectroscopy. Infrared transparent YIG fibers were grown at rates below 12 mm/h in air. At these growth rates, yttrium orthoferrite and iron-oxide inclusions within the YIG fiber, which act as IR scattering centers, were significantly reduced. The transparency of the fibers was more dependent on the growth rate than the stability of the molten zone. Surface ridges containing an Fe-rich composition were observed at all growth rates. These were associated with molten zone instability. INTRODUCTION Y3Fe5 Ol 2 (YIG) has been widely studied because of its excellent microwave properties. These include relatively low magnetization, extremely narrow line width and low dielectric loss behavior [1,2]. These properties make it suitable for microwave devices such as circulators, isolators, and phase shifters. YIG also has unique magneto-optical properties in the near infrared (1-5 gim) [3,4], and is also useful for infrared isolators, optical switches, and sensor applications etc. Single crystals of bulk YIG have been grown principally by the flux growth [5-7] and traveling solvent zone methods [8-12]. In this fiber growth study the laser-heated pedestal growth technique [13] was used. Since it is actually a miniaturized float zone process, the conclusions reached in Kimura's paper on float zone growth of YIG [9-11 ] are directly relevant. The phase relationships in the system Fe 20 3-Y20 3 were first presented by Van Hook [14,15]. YIG was found to be an incongruently melting compound as can be seen from the phase diagram given in Fig. 1. This makes crystal growth by many commonly used melt growth methods difficult. The float zone process [16], however, is one of the best methods for growing uniform single crystals of this type of compound. For the preparation of single crystal fibers, the laser-heated pedestal growth method (LHPG) was chosen because laser heating permits formation of small molten zones. EXPERIMENTAL PROCEDURE Starting with a YIG seed and source rod, steady state growth of YIG can be achieved from an Fe 20 3-rich solution whose composition gives a liquid phase below the peritectic decomposition temperature as shown in Fig. 1. While the growth rates will be lower than for 83 Mat. Res. Soc. Symp. Proc. Vol. 481 0 1998 Materials Research Society
growth from a pure melt, due to partitioning effects, the high temperature gradients in the LHPG system allow much greater rates than those encountered in normal flux growth.
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