Magnetism and structural chemistry of U1−xYxCu2±ySi2∓y, U1−xYxCr2Si2, and U(Cu1−xCrx)2Si2 alloys

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Magnetic behavior for the three series of alloys U1−xYxCu2±ySi2∓y, U1−xYxCr2Si2, and U(Cu1−xCrx)2Si2 has been studied in the temperature range from 2 to 550 K. In all cases a complete solid solution with the ThCr2Si2 type of structure was observed from x-ray powder analysis with no ordering among the substituting atom species. Whereas YCu2Si2 did not reveal any homogeneous region on Si/Cu substitution, uranium-rich alloys show significant changes in the unit cell dimensions when comparing U1−xYxCu2Si2 and U1−xYxCu2+ySi2−y. In both latter alloy series a negative deviation of the c parameter from Vegard’s rule is found almost compensated for by a positive deviation of a when Y is substituted for U. Substitution of Cu by Cr resulted in a linear dependence of the lattice dimensions. YCu2Si2 and YCr2Si2 are temperature independent Pauli paramagnets above 2 K, whereas the UCu2±ySi2∓y and the U(Cu1−xCrx)2Si2 systems are characterized by the onset of ferromagnetic ordering at TM =105 K. Above TM all samples are in the paramagnetic state in which the uranium atoms carry an effective moment larger than 2 μB. Saturation moments reveal a maximum for a copper deficiency of y∼0.1, whereas excess copper correlates with a strong magnetocrystalline anisotropy indicating the presence of narrow domain walls. Substitution of U by Y in U1−xYxCu2±ySi2∓y leads to a linear decrease of TM and for concentrations of 0.4≤x≤0.8 metamagnetic behavior is observed. The antiferromagnetic transition is followed by a ferromagnetic alignment of the U atoms below the Néel temperature. The maximum of the anisotropy energy coincides with the minimum in the c parameter at x∼0.65. The Y-rich alloys U1−xYxCu2∓ySi2±y display antiferromagnetism with a rather complex structure as indicated by the change of sign of the paramagnetic Curie–Weiss temperature θp. In the U1−xYxCr2Si2 alloys the antiferromagnetic coupling is destroyed by rather small amounts of Y.