Crystalline structure and magnetic behavior of the Ni₄₁Mn₃₉In₁₂Co₈ alloy demonstrating giant magnetocaloric effect

Magnetic cooling is a green cooling technology, which is more energy efficient than existing fluid-compression cooling machines. Ni₄₁Mn₃₉In₁₂Co₈ alloy, which demonstrates promising magnetocaloric performances, was investigated using neutron diffraction and thermomagnetic measurements. The austenite structure is cubic L2₁ $\left(Fm\overline{3}m\right),$ while that of the martensite is a mix of 8 and 6 M modulated monoclinic structures $\left(P12/m1\right).$ The austenitic site occupancy refinements reveal that all substituting Co atoms occupy Ni-sites. Most Mn atoms (65%) are in the Mn-sites and the rest go to In-sites (about 35%) and Ni-sites (less than 5%). This disorder of the magnetic atoms (Mn, Ni and Co) in the austenitic phase remains unchanged during the martensitic transition. The distortions of the interatomic distances due to the modulation of the martensitic structures further enhance the disorder in the magnetic interactions. Thermomagnetic measurements indicate that the austenitic phase is ferromagnetic. Cooling to below 250 K, where the alloy loses its ferromagnetic nature, and down to 50 K, the lack of any antiferromagnetic Bragg peaks suggests no antiferromagnetic ordering in the martensitic phase. At very low temperatures in the martensitic phase, spin glass magnetic nature is identified by magnetic measurements, and the spin-glass transition temperature is ∼19 K. (paper)