Exchange Interactions in Ferromagnetic Chromium Chalcogenide Spinels

Transition-metal spinels with magnetic ions solely on the octahedral sites have been the subject of many investigations. The properties of such spinels had always been found to be dominated by antiferromagnetic interactions. In this paper we describe the properties of a group of spinels in which ferromagnetic interactions are found to predominate. The compounds CdCr₂S₄, CdCr₂Se₄, HgCr₂S₄, and HgCr₂Se₄ have been prepared and found to be ferromagnetic with Curie temperatures of 84. 5, 129. 5, 36. 0, and 106^, respectively. With the exception of HgCr₂S₄, which becomes metamagnetic below 25^, all these materials remain ferromagnetic down to 4. 2^. These materials are also unique in that they are good electrical insulators in addition to being ferromagnets. A theoretical treatment is presented which correlates the high-temperature susceptibility with the Curie temperature and which provides a determination of the exchange interactions. The six nearest-neighbor interactions are treated by the high-temperature expansion technique of Rush-brooke and Wood, while the 30 more-distant-neighbor interactions are treated within the two-particle-cluster approximation of Callen and Callen. The analysis of the data indicates that the distant-neighbor interactions play a very important role in the determination of the type of magnetic order observed in the chalcogenide spinels.