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

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 Ni₄₁Mn₃₉In₁₂Co_8, 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.