Magnetic Studies of Ternary Germanides U3Co4Ge7 and U3Co2Ge7 with Strong Uniaxial Anisotropy

U 3 Co 4 Ge 7 and U 3 Co 2 Ge 7 have been prepared as single crystals by the solution growth method from tin flux and studied by X-ray diffraction, magnetic, transport, and specific-heat measurements; ab initio electronic-structure calculations have also been performed to study magnetism theoretically. U 3 Co 4 Ge 7 has a tetragonal structure ($I4/mmm$), while U 3 Co 2 Ge 7 crystallizes in en orthorhombic structure ($Cmmm$), which appears to be a distortion of a tetragonal structure. All measurements document ferromagnetism in U 3 Co 4 Ge 7 below $T_{\text{C}} = 21$ K with a strong magnetocrystalline anisotropy. The $c$-axis has a strong ferromagnetic contribution to magnetization and was found to be the easy-magnetization direction. U 3 Co 2 Ge 7 orders magnetically below 40 K; the magnetic structure is sensitive to the ratio $a/c \approx 1$, e.g., the deviation from the tetragonal axis.