Magnetic and structural transitions in magnetocaloric Mn(Co1-xNix)Ge alloys

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Australian Institute of Physics
The magnetocaloric effect (MCE) - a significant temperature change due to the entropy change around magnetic transitions in materials driven by magnetisation or demagnetisation - has emerged as an increasingly important topic in condensed matter physics in the past two decades. A direct (positive) MCE occurs around a magnetic transition from ferromagnetism (FM) to paramagnetism (PM), while an inverse (negative) MCE is obtained around a magnetic transition from antiferromagnetism (AFM) to FM. If such magnetic transitions couple with a structural transition, a first-order magneto-structural transition can form and hence strengthen the MCE. In this work, the magnetic and structural transitions have been tuned by substitution of Ni for Co in MnCoGe. The Mn(Co1-xNix)Ge samples (x = 0.14 - 1.00) were studied by magnetisation, x-ray and neutron powder diffraction measurements over the temperature range 5 - 450 K. Mn(Co1-xNix)Ge alloys have an orthorhombic (Orth) TiNiSi-type structure (Pnma) at low temperature with transformation to a hexagonal (Hex) Ni2In-type structure (P63/mmc) at the martensitic transformation temperature TM. The increase of the Ni content changes the orthorhombic phase from FM (x < 0.55) to spiral-AFM (x ≥ 0.55). In addition, the transformation temperature TM for the reverse martensitic transformation - from orthorhombic to hexagonal - decreases with Ni content x when x < 0.55 and then increases when x ≥ 0.55. The adjustment of TM leads to the occurrences of first-order FM-Orth/PM-Hex magneto-structural transitions and large values of the direct MCE in the samples with ~0.20 < x < ~0.60. Moreover, the spiral-AFM/FM magnetic transitions in the orthorhombic phase for samples with ~0.55 < x < ~0.75 result in an inverse MCE.
Coherent scattering, Diffraction, Elements, Metals, Physical properties, Scattering, Thermodynamic properties, Transition elements, Magneto-thermal effects
Ren, Q. Y., Hutchison, W. D., Wang, J. L., Studer, A. J., Cadogan, J. M., & Campbell, S. J. (2017). Magnetic and structural transitions in magnetocaloric Mn(Co1-xNix)Ge alloys. Paper presented to the 41st Annual Condensed Matter and Materials Meeting, Charles Sturt University, Wagga Wagga, NSW, Australia,31st January - 3rd February 2017, (pp. 36). Retrieved from: