Magnetic measurements: A powerful tool in magnetic refrigerator design

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Abstract
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.