Tuneable structure and magnetic properties in Fe₃₋ₓVₓGe alloys

Abstract

We report a detailed experimental and theoretical study of the effects of V substitution for Fe atom on the structural, magnetic, transport, electronic and mechanical properties of an off-stoichiometric Fe₃₋ₓVₓGe intermetallic alloy series (0<= x <= 1). Single phase microstructures are observed for x < 0.75 , whereas higher V content alloys x >= 0.75 are multi-phased. Vanadium substitution is observed to induce a diffusionless martensitic phase transformation from a Heusler-like L2₁ structure to hexagonal DO₁₉ structure, as corroborated by Differential Scanning Calorimetry results. The vanadium substitution is also found to decreases the grain size, inhibiting the grain growth by pinning the grain boundary migration. All the alloys in the series are found to be soft ferromagnets at 5 K with saturation magnetic moment and Curie temperature decreasing as V concentration increases. The low temperature saturation magnetic moment is in close agreement with the expected Slater-Pauling values for the L2₁ phases, while the hexagonal samples have markedly higher values of saturation moments. First-principle calculations agree with the experimental findings and reveal that V substitution energetically favours one of the Fe sites in Fe₃Ge. The electrical resistivity measured over the temperature range from 5 K to 400 K shows negative temperature coefficient of resistivity at high temperatures with increasing the V concentration. Relatively high mechanical hardness values are also observed, with the values increasing with increasing V content. Vanadium substitution is found to play a central role in tuning the mechanical properties, stabilising the L2₁ structure, and shifting the martensitic transformation temperature to higher values from that of parent Fe₃Ge.