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.

Measurements of the adiabatic temperature change (ΔT) and the specific heat transfer (ΔQ) of Ni_2.18Mn_0.82Ga Heusler alloy were taken in order to quantify the direct giant magnetocaloric effect of the alloy when it is in the vicinity of magneto-structural phase transition (PT) from paramagnetic austenite to ferromagnetic martensite, and their results are presented. A new vacuum calorimeter was used to simultaneously measure ΔT and ΔQ of magnetocaloric materials with a Bitter coil magnet in fields of up to H = 140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in H = 140 kOe were found to be ΔT = 8.4 K at initial temperature 340 K and ΔQ = 4900 J/kg at 343 K. Using this direct method, we show that the alloy indeed demonstrates the largest value of ΔQ as compared with previously published results for direct measurements of magnetocaloric materials, even though at 140 kOe the magnetic field-induced magnetostructural PT is still not complete.

High cooling power of magnetocaloric refrigeration can be achieved only at large amounts of heat, which can be transferred in one cycle from cold end hot end at quasi-isothermal conditions. The simple and robust experimental method of direct measuring of the transferred heat of materials with magnetocaloric effect (MCE) in thermal contact with massive copper block with definite heat capacity in quasi-isothermal regime was proposed. The vacuum calorimeter for the specific transferred heat ΔQ and adiabatic temperature change ΔT measurements of MCE materials in the fields of Bitter coil magnet up to H = 140 kOe was designed and tested on samples of Ni43Mn37.9In12.1Co7 Heusler alloy with inverse MCE in the vicinity of meta-magnetostructural phase transition (PT). It was found, that the magnetic field H = 80 kOe produces complete PT from martensite to austenite with ΔQ = - 1600 J/kg at initial temperature 273 K.

Magnetocaloric effect (MCE) in pulsed magnetic fields up to 50 T was directly studied in alloy Fe₄₉Rh₅₁. An inverse MCE ΔT ≈ –8 K is observed at different initial temperatures around the metamagnetic phase transition upon field rising to 20 T; further growth of the field to 50 T leads to a decrease in the absolute adiabatic temperature change by nearly 1 K, which is due to the direct MCE and proves that the whole sample undergoes a transition into the ferromagnetic phase. Upon the field decrease, the maximal absolute value of the adiabatic temperature change of |ΔT| = 9.8 K was revealed at 6 T when the initial temperature is 310 K.