Large Magnetoelectric Effect Research Articles

Crystal growth of ferromagnetic semiconductors

Ferromagnetic semiconductors are groups of transition metal-and rare-earth compounds which show large magneto-electric effects at their Curie temperatures as a result of the interaction between charge carriers and localized magnetic moments. A survey of the nature and occurrence of these compounds is presented, including a classification according to crystal structure. The most important compounds are the Eu 2+ chalcogenides with NaCl structure, the cubic ternary Cr 3+ sulpho-and seleno-spinels, and the ternary chalcogenides based on the hexagonal PbCr 2S 4 structure. A review of thermodynamic properties is used to establish, where possible, basic data for the growth of bulk crystals of these materials. The Eu 2+ chalcogenides melt congruently at temperatures above 1950° C and may be prepared by either high temperature melt growth or high temperature vapour growth techniques in sealed refractory metal crucibles. The incongruent nature of melting and evaporation of the Cr 3+ spinels, on the other hand, have resulted in the application of multicomponent crystal growth techniques such as flux growth and chemical vapour transport in quartz crucibles at temperatures below 1200°C. Compounds with the PbCr 2S 4 structure are prepared by techniques similar to the Cr 3+ spinels. A review of the growth and characterization of each compound is presented with emphasis on crystal quality. There is little known about lattice strain and dislocations in any of these compounds. Investigations to date have concentrated on the major problems of chemical purity and stoichiometry. New compounds with high Curie temperatures are chemically complex, and control of purity and stoichiometry will remain a major problem.