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Magnetic refrigeration, an emerging new technology for cooling and gas liquefaction, needs magnetic materials with specific thermomagnetic behavior. Depending on the thermodynamic cycle selected, the isothermal magnetic entropy change or the adiabatic temperature change upon field application needs to be a preselected function of temperature. To obtain these properties, most designers rely on calorimetry, an expensive and time consuming technique. The present article describes that, classical magnetic measurements, when evaluated within the framework of the Landau theory for the second order phase transition or the thermodynamics in magnetic fields, are able to provide the preliminary information needed for the design of magnetic refrigerators. After reviewing the theory, experimental results on ferromagnetic gadolinium (Gd) and helimagnetic dysprosium (Dy) are analyzed and compared to direct experimental results as well as to those obtained using the molecular field model. The results demonstrate the reproducibility of entropy calculations and the good agreement between the experimental and the calculated specific heat anomalies. While the molecular field theory which assumes simple ferromagnetic order clearly fails for helimagnetic dysprosium, an analysis of the experimental data based on the Landau theory gives reliable results. Besides, the field and temperature dependencies of the isothermal magnetic entropy change allows one to characterize the magnetic structure (nature of the magnetic order) of the sample. Furthermore, magnetic measurements define the useful field range and provide information on transitions that influence the thermal behavior and magnetic losses.

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The magnetization MH(T) and the specific heat capacity cP,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 cP,H(T) measurements both the magnetic entropy change and the adiabatic temperature change were calculated and compared to values obtained from MH(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.

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.