The magnetocaloric effect (MCE) in an Fe₄₈Rh₅₂ alloy and Sm_0.6Sr_0.4MnO₃ manganite was studied in cyclic magnetic fields. The adiabatic temperature change in the Fe₄₈Rh₅₂ alloy for a magnetic field change (ΔB) of 8 T and a frequency (f) of 0.13 Hz reaches the highest value of (ΔT_ad) of −20.2 K at 298 K. The magnitude of the MCE in Sm_0.6Sr_0.4MnO₃ reaches ΔT_ad = 6.1 K at the same magnetic field change at 143 K. The temperature regions, where a strong MCE is exhibited in an alternating magnetic field, are bounded in both compounds. In the case of the Fe₄₈Rh₅₂ alloy, the temperature range for this phenomenon is bounded above by the ferromagnetic to antiferromagnetic transition temperature in the zero field condition during cooling. In the case of the Sm_0.6Sr_0.4MnO₃ manganite, the temperature range for the MCE is bounded below by the ferromagnetic-paramagnetic transition temperature in zero field during heating. The presence of these phase boundaries is a consequence of the existence of areas of irreversible magnetic-field-induced phase transitions. It is found that the effect of long-term action of thousands of cycles of magnetization/demagnetization degrades the magnetocaloric properties of the Fe₄₈Rh₅₂ alloy. This can be explained by the gradual decrease in the size of the ferromagnetic domains and increasing role of the domain walls due to giant magnetostriction at the ferromagnetic to antiferromagnetic transition temperature. The initial magnetocaloric properties can be restored by heating of the material above their Curie temperature.