Effects of Cobalt on the Crystalline Structures of the Ni-Mn-In Giant Magnetocaloric Heusler Alloys

Abstract

The giant inverse magnetocaloric effect driven by a merged magneto-structural transformations in Ni-Mn-In-Co Heusler alloys, makes them highly promising as solid state refrigerants near room temperature. Knowledge of the crystallographic behavior of these alloys at a broad temperature range is critical to the understanding of the giant magnetocaloric effect. In this study, three Ni-Mn-In-Co alloys were investigated by neutron and synchrotron diffraction techniques. The chemical compositions of the alloys, determined by the Rutherford Backscattering Spectrometry (RBS) technique, were Ni41Mn39In12Co8, Ni48Mn34In12Co6 and Ni52Mn25In16Co7. The austenitic (A) phase of all three alloys was cubic L21 (Fm3̅m). Martensitic (M) phase of the Ni41Mn39In12Co8 alloy was a mix of 8M and 6M modulated monoclinic structures, while the other two alloys had a M composed of a mix of 7M and 5M modulated monoclinic structures. All modulated structures belong to the P 1 2/m 1 space group. Site occupancy refinements of the A phases of all three alloys, revealed that almost all the Co atoms (~97%) occupy the regular Ni (8c) sites. In the studied temperature range (50–250 K) of the M phase of the Ni41Mn39In12Co8 alloy has very low magnetization. Also, no antiferromagnetic ordring was observed in the neutron diffraction refinement of the M phase. Therefore by eliminating the possibilities of ferromagnetism and antiferromagnetism, it is concluded that the M phase of the Ni41Mn39In12Co8 alloy is spin glass.