Istituto dei materiali per l'elettronica ed il magnetismo     
Fabbrici S., Albertini F., Paoluzi A., Bolzoni F., Cabassi R., Solzi M., Giancarlo C., Cavallari C., Righi L. Magneto-structural transitions and inverse magnetocaloric effect in NiCoMnGa alloys. In: Thermag III - The 3rd International Conference of IIR on Magnetic Refrigeration at Room Temperature (Des Moines, Iowa, U.S.A., 11-15 Marzo 2009).
We studied the composition dependence of the structural and magnetic transformations of the Ni2MnGa system by introducing Co at the expenses of Ni around the Mn-rich composition Ni2Mn1.2Ga0.8, which is the composition where the structural martensitic transformation and the Curie temperature coincide. A close mapping of a wide compositional region allowed us to find a compositional range where there appears an unusual and intriguing inverse magneto-structural transition. Following ac susceptibility thermo-magnetic analysis and structural characterization, it appears that for some compositions of the quaternary alloy the Curie temperature of the martensite phase, which is usually a virtual transition being at far higher temperatures than the martensite-austenite one, is dramatically lowered by the introduction of Co, and can be measured below the martensitic transition temperature. This should be a very promising situation from the magneto-caloric point of view; in fact, in these samples the martensitic transformation here is coupled with the onset of magnetism which is still present in the austenite phase. In other words, in these samples the onset of magnetism on heating can be observed, and it is also connected to a structural transformation. A measure of the entropy change in some of these samples has been performed; we found an inverse magneto-caloric effect reaching values up to 17 J/Kg K in 5T, with dT/dH up to 10 K/T. These results are strongly dependent on composition, and still under investigation.
Subject Ni2MnGa
Martensitic phase
Structural modulation
Crystal structure

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