Improvements in or relating to methods of making single crystals of ferrite

摘要

Ferrites, containing in addition to iron one or more other metals are formed by crystallizing the ferrite from a solution thereof in lead monoxide by gradually reducing the temperature at a rate less than 30 DEG C. per hour. According to an example a nickel ferrite was made by adding to lead monoxide powder a mixture of nickel and ferric oxides in stoichiometric proportions and heating the mixture in a platinum crucible to 1300 DEG C. for one hour. The temperature was then lowered at about 10 DEG C. per hour and when the nickel ferrite became supersaturated in the lead monoxide solution small crystals of nickel ferrite were formed of a somewhat irregular structure as shown in Fig. 1. The crystals so obtained were then employed as seed crystals in a lead monoxide melt maintained at 1300 DEG C. and containing 30 per cent by weight of nickel ferrite whereby a crystal as shown in Fig. 2 was obtained, where 12 indicates the new growth. The upper portion of the crystal of Fig. 2 may be employed in a further crystallization whereby a uniform crystal as shown in Fig. 3 may be obtained. Crystallization may suitably be effected in a platinum crucible as shown in Fig. 5 where a lid 16 supports a wire 18 to the end of which a seed crystal 20 is attached. The lid with the attached crystal is lowered into the saturated solution of nickel ferrite in lead monoxide, 17. The temperature is then reduced and the resulting nickel ferrite crystal may be withdrawn by removing the lid or alternatively the lead monoxide melt may be poured away before it solidifies or the ferrite crystal may be allowed to cool in the solidified lead monoxide, the latter then being dissolved with a solution of nitric acid. The outer surface of the crystals tends to be strained and is suitably removed by machining or etching. Other ferrites which may be similarly obtained are those of cobalt, zinc magnesium and cadmium, manganese and copper, all of which have a spinel structure, and also ferrites of calcium, strontium and barium which have a hexagonal crystal structure. Complex ferrites such as Ni5Zn5Fe2O4 may also be prepared.