Metamagnetic transition and magnetocaloric effect in ErCo₂

The magnetization M_H(T) and the specific heat capacity c_P,H(T) of the ErCo₂ intermetallic compound were measured in the temperature range 5 - 100 K and in 0, 7 or 14 T applied field, respectively. A clear first-order phase transition is found at the magnetic ordering of the Er sublattice. While for order-disorder transitions in simple ferromagnets there is a good agreement between magnetocaloric performance predicted on the basis of magnetization measurements compared to calorimetric measurements, it is necessary to investigate whether the agreement is still present for materials with more complex transitions (e.g. order - order, metamagnetic, first order etc). From the magnetization data the magnetic entropy change at the transition was calculated using the Maxwell relations. From the c_P,H(T) measurements both the magnetic entropy change and the adiabatic temperature change were calculated and compared to values obtained from M_H(T) and to the values calculated by the usual approximative expressions. The agreement is less good than in the case of second-order phase transitions. The discrepancy is interpreted in terms of the theory of first-order/metamagnetic transitions showing that the boundary conditions used in the derivation of the approximative formulae for simple ferromagnetic materials are not appropriate for more complex transitions as in ErCo₂.