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Dipartimento di Fisica e Scienze della Terra

Magnetocaloric materials

Involved People: Massimo Solzi, Francesco Cugini, Nicola Sarzi Amadé, Giuseppe Allodi

in collaboration with:

F. Albertini, S. Fabbrici (IMEM-CNR Institute, Parma)

L. Righi (Dept. of Chemistry, Parma University)

E. Bruck, FAME, TU Delft (The Netherlands)

Ö. Çakir, Yıldız Teknik Üniversitesi (Turkey)

Phenomenology of the Magnetocaloric Effect and MCE materials

The magnetocaloric effect (MCE) of the materials is manifested both in the form of an isothermal entropy change and as a variation of the adiabatic temperature following the application of a magnetic field. The physical origin of MCE is the perturbation of the magnetic configuration of the material induced by the coupling between magnetic structure and applied magnetic field. An important application of MCE regards the cooling around room temperature. The construction of gas-free refrigeration systems of new generation is one of the major challenges facing research in the field of magnetism. The development of solid-state air conditioning and refrigerator systems based on MCE represents an energy-efficient and environment safer alternative to the current techniques of compression/expansion of hydrofluorocarbons (HFCs), which are greenhouse gases.

The recent research activities in this field has focused on materials that show phase transitions of the first order at the same time structural and magnetic; in the vicinity of these critical points the application of a magnetic field induces high variations of entropy of the material (giant MCE). Among these compounds, the Heusler alloys of the type Ni2MnGa show a high MCE in correspondence of the martensitic transformation, in addition to multi-functional characteristics (magnetic shape memory materials). The ability to easily control, by appropriate compositional substitutions, the temperature and the thermodynamic properties of these transitions is one of the peculiarities of these systems.

Experimental set-ups for the characterization of MCE

The development of non-standard experimental setups have allowed the complete characterization and a better understanding  of the magnetocaloric properties across first order transitions of several materials. Our last works have demonstrated the convergence, within the experimental errors, of the results from the three different and independent techniques: magnetometry (performed with a SQUID magnetometer), calorimetry (with the homemade DSC) and the direct temperature variation measurement. The attention was paid in particular on a class of Heusler alloys, with a general formula NiMnGa, characterized by several functional properties and by a good MCE across a magneto structural transformation.

NiMnGa MCE convergence