Development of Intermetallic Compounds for Use as Magnetic Refrigerators or Regenerators

Abstract

Materials with complex magnetic order and complex phase transitions are our best hope in search for the giant magnetocaloric effect. Members of the RT₂ family of intermetallic compounds (R=Dy, Gd, Er Ho, T=Co, Ni, Al) are known to display high magnetocaloric effects. In this study, we investigate, how the classical magnetic properties correlate with the presence of high magnetocaloric effects. This knowledge provides a useful tool to identify promising materials from literature surveys and simultaneously helps the researcher to a better understanding of the material’s behavior and offer useful additional data to the refrigerator designer. We report here beyond the usual magnetic entropy change measurements on the compounds’ response to field and temperature cycling as well as on their time-dependent response to an instantaneous change of the external field. It will be shown that unusually high magnetic entropy changes are coupled with increased irreversibility, with respect to both field and temperature, and with a fast, asymmetrical relaxation. For the best materials, the irreversibility persists up to fields in the tesla range. Simultaneously, it is shown that slight irregularities in processing and consequently in structure, result in dramatic changes in the nature of the transition and reduces the value of the observed magnetocaloric effect as well.