Magnetic cooling based on the magnetocaloric effect is topical for energy efficiency and environmental reasons. The MnCoGe family of compounds is promising for magnetic cooling which do not contain higher cost rare earths and can provide a working temperature range near room temperature. MnCoGe has two stable crystallographic structures, the low-temperature TiNiSi-type orthorhombic structure (Pnma, martensitic phase) and the high-temperature Ni₂In-type hexagonal structure (P6₃/mmc, austenitic phase). In this work, this martensitic transformation was adjusted by substitution of Fe or Ni for Mn or Co. Variable temperature x-ray diffraction, neutron diffraction and differential scanning calorimetry were used to determine the nature and position of the adjusted transitions. The magnetic properties in these compounds were studied by magnetisation and neutron diffraction experiments. Magnetocaloric effects were also explored.In as-prepared (Mn₁₋ₓFeₓ)CoGe (x = 0.01 to 0.04) and as-prepared Mn(Co₀․₉₆Fe₀․₀₄)Ge, the Fe substitution reduces the martensitic transformation temperature TM to near room temperature. Ferromagnetic structures with magnetic moments on the Mn sublattice along the cₒᵣth-axis are found in both (Mn₁₋ₓFeₓ)CoGe and Mn(Co₀․₉₆Fe₀․₀₄)Ge compounds below TM. The analysis of magnetisation measurements, plus variable temperature x-ray and neutron diffraction indicate that a magneto-structural transition exists in (Mn₁₋ₓFeₓ)CoGe (x = 0.01 to 0.03) and Mn(Co₀․₉₆Fe₀․₀₄)Ge. Large entropy changes via the magnetocaloric effect were observed near room temperature.⁵⁷Fe Mössbauer spectroscopy studies indicated that Fe is distributed on both the Mn and the Co sites with greater preference to occupy the Co site in both as-prepared (Mn₁₋ₓFeₓ)CoGe and Mn(Co₁₋ₓFeₓ)Ge, while some of the Fe moves from the Co site to the Fe site during annealing.In annealed (Mn₁₋ₓNiₓ)CoGe (x= 0.03 to 0.07) and annealed Mn(Co₀․₈₆Ni₀․₁₄)Ge, ferromagnetic structures with a magnetic moment on the Mn sublattice along the cₒᵣth-axis are obtained. However, analyses of neutron diffraction measurements point to the cₒᵣth-axis easy ferromagnetic and incommensurate magnetic structures with increasing Ni concentration in annealed Mn(Co₁₋ₓNiₓ)Ge (x = 0.4 to 1.0) compounds. Magneto-structural transitions form in annealed (Mn₁₋ₓNiₓ)CoGe (x= 0.03 and 0.04) and Mn(Co₁₋ₓNiₓ)Ge (x = 0.3 to 0.6). Conventional and/or inverse magnetocaloric effects are observed in these compounds.
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