Synthesis and Structural/Physical Properties of U₃Fe₂Ge₇: A Single-Crystal Study

A single crystal of U₃Fe₂Ge₇ was synthesized by the tin-flux method, and its structural and electronic properties were studied. The compound crystallizes in the orthorhombic crystal structure of La₃Co₂Sn₇ type with two Wyckoff sites for the U atoms. U₃Fe₂Ge₇ displays a ferromagnetic order below T_C = 62 K. Magnetization measurements in static (up to 14 T) and pulsed (up to 60 T) magnetic fields revealed a strong two-ion uniaxial magnetic anisotropy. The easy magnetization direction is along the c axis and the spontaneous magnetic moment is 3.3 μ_B per formula unit at 2 K. The moment per Fe atom is 0.2 μ_B, as follows from Mössbauer spectroscopy. The magnetic moments are oriented perpendicular to the shortest inter-uranium distances that occur within the zigzag chains in the ab plane, contrary to other U-based isostructural compounds. The magnetization along the a axis reveals a first-order magnetization process that allows for a quantitative description of the magnetic anisotropy in spite of its enormous energetic strength. The strong anisotropy is reflected in the specific heat and electrical resistivity that are affected by a gap in magnon spectrum.