Development of Intermetallic Compounds for Use as Magnetic Refrigerators or Regenerators

Abstract

Materials with complex magnetic order and complex phase transitions areour best hope in search for the giant magnetocaloric effect. Members ofthe RT2 family of intermetallic compounds (R=Dy, Gd, Er Ho, T=Co, Ni,Al) are known to display high magnetocaloric effects. In this study, weinvestigate, how the classical magnetic properties correlate with thepresence of high magnetocaloric effects. This knowledge provides auseful tool to identify promising materials from literature surveys andsimultaneously helps the researcher to a better understanding of thematerial's behavior and offer useful additional data to the refrigeratordesigner. We report here beyond the usual magnetic entropy changemeasurements on the compounds' response to field and temperature cyclingas well as on their time-dependent response to an instantaneous changeof the external field. It will be shown that unusually high magneticentropy changes are coupled with increased irreversibility, with respectto both field and temperature, and with a fast, asymmetrical relaxation.For the best materials, the irreversibility persists up to fields in thetesla range. Simultaneously, it is shown that slight irregularities inprocessing and consequently in structure, result in dramatic changes inthe nature of the transition and reduces the value of the observedmagnetocaloric effect as well.