This invention relates to a process for producing a single crystal of garnet ferrites, and more specifically to a process for producing a single crystal of garnet ferrite suitable for use as a Faraday rotation material in an optical isolator adapted to prevent light from returning to a semiconductor laser employed for optical communication or in an optical magnetic field sensor adapted to control or protect electric power system in transformer substations.
As techniques for producing a single crystal of garnet ferrite useful as magnetooptical materials, the flux technique and LPE technique have been usually employed to date.
Of these techniques, the flux technique comprises melting raw materials for such respective components that form a ferrite having the garnet structure together with a flux component such as PbO, PbF.sub.2 and/or B.sub.2 O.sub.3 in a crucible, for example, made of platinum into a homogeneous melt and cooling the melt gradually to cause a single crystal of the garnet ferrite to grow.
Further, the LPE technique also comprises similarly to the flux technique, melting raw materials for such respective components that form a ferrite having the garnet structure into a homogeneous melt as described above and dipping a substrate having the garnet structure in the resulting melt while maintaining it at a temperature slightly lower than the saturation temperature to cause a single crystal of the garnet ferrite to grow on the substrate as a thin film.
However, the flux technique is accompanied by such problems that it requires long time, i.e., from a week to a month, for the growth of a single crystal and it cannot avoid the inclusion of impurities from the flux and crucible used.
On the other hand, the LPE technique requires shorter time for the growth of each crystal. However, in this technique, a larger single crystal having a three-dimensional structure cannot be obtained and mixing of impurities is also unavoidable because a flux and a crusible have to be used similarly to in the former technique.
Among these impurities, mixing of impurity ions other than trivalent ions, especially, Pb.sup.2+, Pt.sup.4+ and the like in a single crystal of garnet ferrite leads to a modification to the ionic valence of ferric ions (trivalent ions), thereby adversely affecting primarily the light absorption characteristic of the single crystal.
In addition, the above-employed crucible is usually made of a noble metal such as platinum. In order to cause a large single crystal to grow, it is indispensable to enlarge the crucible, resulting in a problem that the production cost jumps up.
Moreover, although it has been known that the degree of Faraday rotation .theta..sub.F (deg/cm) of the garnet ferrite can be remarkably increased by substituting the c-site component in the garnet structure partially with Bi.sup.3+, and increase in Faraday rotation is useful for miniaturing an element, it has been difficult to prepare single crystals of the garnet ferrite having such a large Faraday rotation.