Structural and magnetic phase transitions in Fe3Ge : A neutron diffraction study

A detailed neutron diffraction study of a stoichiometric ${\mathrm{Fe}}_{3}\mathrm{Ge}$ alloy with different initial states in a wide temperature range (up to 1000 K) made it possible to accurately reveal the features of magnetic and structural phase transformations in it. The use of complementary x-ray diffraction and analysis of neutron diffraction patterns by the Rietveld method allowed reliably separating the nuclear and magnetic contributions to the intensity of the diffraction peaks and performing refinement of the ${\mathrm{Fe}}_{3}\mathrm{Ge}$ magnetic state characteristics. Both main phases appearing in ${\mathrm{Fe}}_{3}\mathrm{Ge}$ are ferromagnetic with ${T}_{\mathrm{C}}=629$ K (hexagonal, $D{0}_{19}$) and ${T}_{\mathrm{C}}=714$ K (cubic, $L{1}_{2}$). In $D{0}_{19}$, the presence of a spin-flip transition (${T}_{\mathrm{sf}}=385$ K) was confirmed and the temperature dependence of the components of magnetic moment along the hexagonal axis and in the basal plane was obtained. It was shown that the transformation between ordered $L{1}_{2}$ and $D{0}_{19}$ structural states, predicted by the equilibrium-phase diagram, includes three steps: two diffusional stages ($L{1}_{2}\ensuremath{\rightarrow}A1, A3\ensuremath{\rightarrow}D{0}_{19}$) and one displacive stage ($A1\ensuremath{\rightarrow}A3$). The obtained structural data suggest that in Fe-Ge, as well as in Fe-Ga alloy, direct transitions between ordered phases are impossible. They should include a transition between disordered states.