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The magnetic properties and field-dependent specific heat of melt-spun amorphous RE70TM30 (RE=Gd, Tb, Dy, Ho and Er; TM=Fe and Ni) and Gd65Co35 alloys were investigated as potential magnetic refrigerants. Essentially zero magnetic hysteresis was observed in all the Gd–TM alloys at temperatures from 5 K up to the ordering temperatures. The coercive force of the RE70TM30 alloys depended mainly on the RE species and increased according to the order of RE=Gd<Ho<Er<Dy<Tb. The magnetic susceptibility of most of the alloys showed apparently normal Curie–Weiss behavior above the ordering temperatures. The heat capacity measurements in zero field and applied fields of 4 and 8 T indicated that the magnetic transition in these alloys are significantly broadened. The maximum adiabatic temperature changes for Er70Fe30, Gd70Ni30 and Gd65Co35 amorphous alloys in a field change of 8 T are 4.0, 3.4, and 3.0 K, respectively. Mössbauer spectroscopy revealed that Fe atoms in the amorphous RE70Fe30 alloys carry a small magnetic moment that may complicate the magnetic ordering in the alloys. A simple model assuming a Gaussian distribution of ordering temperatures around the apparent Curie temperature was constructed to attempt to reconcile the differences in the observed magnetic properties of these amorphous alloys. The broad magnetic transition is attributed to the fluctuation of the exchange integral caused by the structural disorder in amorphous alloys. The calculated susceptibility, magnetization, and heat capacity agreed reasonably well with the experimental data and show that the magnetic susceptibility and magnetization are only weakly affected by the distribution of ordering temperatures, but the heat capacity is much more sensitive to such a distribution. To effectively screen out magnetic refrigerants with sharp magnetic transitions and correspondingly large adiabatic temperature changes from those with broadened transitions and small adiabatic temperature changes, the field-dependent heat capacity measurement technique is a powerful tool to use.

Certain applications of magnetic materials require the knowledge of the magnetocaloric effects, i.e., the adiabatic change of temperature or the isothermal change of the entropy upon application or removal of magnetic field. While the isothermal magnetic entropy change can be calculated from magnetic data only, the calculation of the adiabatic temperature change requires the knowledge of the field-dependent specific heat as well. To compare magnetocaloric effect results obtained from magnetic-only, thermal-only, or combined methods requires the evaluation of the mutual reliability of all methods. In crystalline materials it usually does not present a problem to perform thermal and magnetic measurements on the same sample or at least on samples of identical state. For amorphous materials, unfortunately, thermal measurements are extremely difficult to perform on the ribbons themselves so usually pellets pressed from powdered ribbons, with silver as a binder, are applied. After the magnetic entropy change calculated from thermal measurements performed on such a composite did not agree with the data obtained from magnetic measurements performed on the original ribbon, magnetic measurements were carried out on the pellet and its predecessor powders. Comparison of the results revealed that the pellet’s properties do not reflect the properties of the original ribbon. However, comparison of magnetic and thermal measurements carried out on the pellet itself show good agreement if proper numerical methods are used for evaluation. The study concludes that amorphous ribbons are far more competitive for certain applications than the thermal measurements, performed on the pellet only, would indicate.

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