Magnetocaloric Mn(Co1-xNix)Ge - structural and magnetic transitions

The structural and magnetic properties of MnCoGe-based alloys have been studied extensively in recent years due to their potential application as magnetic cooling materials based on the magnetocaloric effect (MCE). The Mn(Co1-xNix)Ge series is of particular interest as magnetic transitions in the range 275 K to 345 K generally coincide with a martensitic structural transition TM, with such an overlap then allowing scope for the formation of magneto-structural transitions (ferromagnetic orthorhombic to paramagnetic hexagonal) and hence an associated large MCE [e.g. 1]. Neutron diffraction, magnetisation and x-ray experiments on Mn(Co1-xNix)Ge compounds (x = 0.12 to 1.00) have demonstrated magnetic structures ranging from ferromagnetic for x < 0.50 to non-collinear spiral antiferromagnetic for x > 0.55 at low temperature (e.g. 5 K). TM is found to decrease initially with increasing Ni content and then increase. First-order magneto-structural transitions are observed in Mn(Co1-xNix)Ge samples for ~0.20 < x < ~0.65 with the presence of ferro-/antiferro-magnetic structures in Mn(Co1-xNix)Ge allowing investigation of both direct and inverse magnetocaloric effects. Our results (including the magnetic phase diagram for Mn(Co1-xNix)Ge) are discussed in terms of the increase of valence electron concentration on substitution of Ni (3d84s2) for Co (3d74s2) in the orthorhombic phase, leading to expansion of the unit cell and redistribution of the valence electrons [2].