eTheses Repository

Crystal growth of functional oxides using an image furnace

Koohpayeh, Seyed Mojtaba (2007)
Ph.D. thesis, University of Birmingham.

The present study is concerned with the single crystal growth of functional oxides by the floating zone technique using a four mirror image furnace. This thesis attempts to extend our knowledge of the subject by firstly studying the optical floating zone method as a technique in terms of the experimental growth parameters and then by performing crystal growth and characterization on a number of functional oxides such as rutile (TiO$$_2$$), Co-doped rutile and rare earth orthoferrite crystals (RFeO$$_3$$, R=Er and Y). When growing high quality, large and homogeneous crystals using an image furnace, finding the optimum growth parameters (such as growth speed, rotation rate, gas pressure and type of atmosphere) is a crucial first step since it has been found that these parameters can all affect the eventual crystal quality and properties. Comprehensive characterization of an image furnace revealed that the growth behaviour, and consequently, the quality of the as-grown crystal, can be explained by the effects that varying these growth parameters have upon the temperature profiles within growing crystals. Characterization of as-grown crystals has been performed using a combination of different techniques such as scanning electron microscopy, energy dispersive x-ray analysis, x-ray diffractometry, back reflection x-ray Laue, polarised optical microscopy and vibrating sample magnetometry. The best quality TiO$$_2$$ crystals were prepared when neither the growing crystal nor the feed rod were rotated during crystal growth. When rotation was employed, this was found to introduce sub-grain boundaries in crystals when the linear growth rate was low, or bubble inclusions when the linear rate was high. The use of lower molten zone temperatures was found to give pale-yellow crystals, while crystals grown using higher molten zone temperatures tended to have the blue colour characteristic of oxygen deficient material. A study of Co-doped TiO$$_2$$ single crystals, prepared from the melt using the FZ technique, revealed that Co does not dissolve into the rutile matrix and that the type of atmosphere used during FZ affects the type of cobalt-based second phases formed. Magnetization measurements of Co-doped rutile bulk samples (both single crystals prepared using the FZ technique and polycrystalline powders prepared using a solid state reaction) also indicated that the magnetic properties depend upon the material preparation conditions. An oxygen deficient environment during the preparation of Co-doped TiO$$_2$$ powders and single crystals was crucial for the observation of room temperature ferromagnetism, since preparation in oxygen rich conditions led to the formation of paramagnetic material that included the second phase CoTiO$$_3$$. Finally, growth of ErFeO$$_3$$ and YFeO$$_3$$ single crystals by the FZ technique revealed that the best quality crystals were prepared when the growth rate was 6 mm/h or less, since the use of higher growth rates (12, 18 and 24 mm/h) was found to result in the formation of second phase inclusions within the crystals. The magnetic properties of these crystals, as represented by the hysteresis loops, showed that crystals of both materials grown at higher rates have significantly lower coercivities; this was attributed to the effect of the second phase in nucleating reverse magnetic domains and so aiding the demagnetization process.