Single crystals of the U2(Fe1-xCox)13.6Si3.4 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) solid solutions (hexagonal crystal structure of the Th2Ni17 type) were prepared for first time and their magnetization was measured at 4.2 K along the principal axes. The alloys exhibit a large basal-plane type magnetocrystalline anisotropy. Strong non-linearity of hard-direction magnetization curves and especially first-order magnetization process (FOMP) at low x values needs for description a large second anisotropy constant K2 which cannot be provided by the Fe sublattice and is, therefore, a strong evidence for magnetic ordering of U sublattice. K2 turns from negative to positive with increasing x at x = 0.2 which is reflected in disappearance of FOMP. The molecular spontaneous magnetic moments Ms decreases from 23 to 14 μB/f.u. whereas the Curie temperatures TC increases from 530 K to 595 K with increasing x from 0 to 0.5.
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