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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(Co$_{1-x}$Ni$_x$)Ge series is of particular interest as magnetic transitions in the range 275 K to 345 K generally coincide with a martensitic structural transition $T$$_{M}$, with such an overlap then allowing scope for the formation of a magneto-structural transition (ferromagnetic orthorhombic to paramagnetic hexagonal) and hence an associated large MCE [e.g. 1].
Neutron diffraction, magnetisation and x-ray experiments on Mn(Co$_{1-x}$Ni$_x$)Ge compounds (x = 0.12 to 1.00) have demonstrated magnetic structures ranging from ferromagnetic for x < 0.50 to non-collinear spiral antiferromagnetic for x > 0.55 at low temperature (e.g. 5 K). $T$$_M$ is found to decrease initially with increasing Ni content and then increase. First-order magneto-structural transitions are observed in Mn(Co$_{1-x}$Ni$_x$)Ge samples for ~0.20 < x < ~0.65 with the presence of ferromagnetic and antiferromagnetic structures in Mn(Co$_{1-x}$Ni$_x$)Ge allowing investigation of both direct and inverse magnetocaloric effects. Our results (including the magnetic phase diagram for Mn(Co$_{1-x}$Ni$_x$)Ge) are discussed in terms of the increase of valence electron concentration on substitution of Ni (3$d$$^8$4s$^2$) for Co (3$d$$^7$4$s$$^2$) in the orthorhombic phase, leading to expansion of the unit cell and redistribution of the valence electrons [2].
[1] T. Trung, L. Zhang, L. Caron, K. H. J. Buschow, E. Brück, Appl. Phys. Lett. 2010, 96, 172504.
[2] Q. Ren, W. D. Hutchison, J. Wang, A. J. Studer and S. J. Campbell, Chem. Mater. 2018, 30, 1324.
| Topic | Advanced Materials |
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