The field-induced first-order phase transition in $\left({\text{Er}}_{1-x}{\text{Y}}_x\right){\text{Co}}_2$ 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 $\gamma$ of the $T$-linear specific heat decreases by $\sim 48\%$ under application and removal of a magnetic field up to 20 kOe. This behavior is attributed to the itinerant electron metamagnetism of $\text{Co}3d$ electrons. The isothermal magnetic entropy change $\Delta S_m$ in ${\text{Er}}_{0.55}{\text{Y}}_{0.45}{\text{Co}}_2$ includes a large contribution associated with spin fluctuations induced by the $f\text{−}d$ exchange interaction in the hybridized $3d\text{−}5d$-electron subsystem. These spin fluctuations are suggested to contribute substantially to the magnetocaloric effect of the $R{\text{Co}}_2$ type compounds. The maximal $\Delta S_m$ value observed for ${\text{ErCo}}_2$ just above the Curie temperature is ascribed to the closeness of the $T_{\text{C}}$ value to the spin-fluctuation temperature $T_{\text{sf}}$ of itinerant $\text{Co}3d$ electrons. The nonmonotonous change in $\Delta S_m$ with the Curie temperature of $R_{1-x}{R}_x^{\prime }{\text{Co}}_2$ compounds is explained by the temperature variation in the spin-fluctuation contribution to the magnetocaloric effect.

• Received 31 October 2008

DOI:https://doi.org/10.1103/PhysRevB.79.184420