Butterflylike specific heat, magnetocaloric effect, and itinerant metamagnetism in (Er,Y) Co2 compounds

The field-induced first-order phase transition in (Er1-x Yx) Co2 with the yttrium concentration x=0.45 is observed to be accompanied by a butterflylike behavior and significant irreversibility of the specific heat. The coefficient γ of the T -linear specific heat decreases by ∼48% under application and removal of a magnetic field up to 20 kOe. This behavior is attributed to the itinerant electron metamagnetism of Co3d electrons. The isothermal magnetic entropy change Δ Sm in Er0.55 Y0.45 Co2 includes a large contribution associated with spin fluctuations induced by the f-d exchange interaction in the hybridized 3d-5d -electron subsystem. These spin fluctuations are suggested to contribute substantially to the magnetocaloric effect of the R Co2 type compounds. The maximal Δ Sm value observed for ErCo2 just above the Curie temperature is ascribed to the closeness of the TC value to the spin-fluctuation temperature Tsf of itinerant Co3d electrons. The nonmonotonous change in Δ Sm with the Curie temperature of R1-x Rx′ Co2 compounds is explained by the temperature variation in the spin-fluctuation contribution to the magnetocaloric effect.