First- and second-order phase transitions in RE6Co2Ga (RE = Ho, Dy or Gd) cryogenic magnetocaloric materials

Abstract Rare-earth (RE) rich intermetallics crystallizing in orthorhombic Ho 6 Co 2 Ga-type crystal structure exhibit peculiar magnetic properties that are not widely reported for their magnetic ordering, order of magnetic phase transition, and related magnetocaloric behavior. By tuning the type of RE element in RE 6 Co 2 Ga (RE = Ho, Dy or Gd) compounds, metamagnetic anti-to-paramagnetic (AF to PM) phase transitions could be tuned to ferro-to-paramagnetic (FM to PM) phase transitions. Furthermore, the FM ground state for Gd 6 Co 2 Ga is confirmed by density functional theory calculations in addition to experimental observations. The field dependence magnetocaloric and Banerjee’s criteria demonstrate that Ho 6 Co 2 Ga and Dy 6 Co 2 Ga undergo a first-order phase transition in addition to a second-order phase transition, whereas only the latter is observed for Gd 6 Co 2 Ga. The two extreme alloys of the series, Ho 6 Co 2 Ga and Gd 6 Co 2 Ga, show maximum isothermal entropy change (∣Δ S iso max (5 T)∣) of 10.1 and 9.1 J kg −1 K −1 at 26 and 75 K, close to H 2 and N 2 liquefaction, respectively. This outstanding magnetocaloric effect performance makes the RE 6 Co 2 Ga series of potential for cryogenic magnetic refrigeration applications.