(_1-xPu_x)Sb solid solutions. 1. Magnetic configurations

Neutron and resonant x-ray magnetic scattering studies have been performed on single crystals of three compositions, x=0.25, 0.50, and 0.75 of the (U1-xPux)Sb solid solution. Neutron diffraction has established the ordering wave vector (k=1 for x=0.25 and 0.50, as in x=0, USb, and k=0.25 for x=0.75), the average magnetic moment, and the temperature dependence of the magnetic order. For the x=0.75 sample the presence of a third-order harmonic indicates a substantial squaring of the 4+, 4- magnetic configuration. Experiments with a field applied to the sample have shown that the x=0.25 and 0.50 samples are triple k (as in x=0, USb), but that the x=0.75 sample is initially (at T-N) triple k, but at a lower temperature of T' starts to transform to a single-k configuration (as found in x=1, PuSb). The (H, T) phase diagram of this sample is presented. At T=10 K some 30% of the x=0.75 sample remains in the (high-temperature) 3k state. The resonant x-ray magnetic scattering experiments have been performed at the U M-4 (3.728 keV) and Pu M-4 (3.963 keV) edges, exploiting the element-specific nature of this probe. For the x=0.50 sample, conventional long-range magnetic order is found. However, in the x=0.75 sample the better momentum resolution of the x-ray technique (compared to neutrons) shows that the ordering is not truly long range. Both the U and Pu moments have a correlation length of some 400 Angstrom only, even at the lowest temperature. For higher harmonics, especially the third order, the correlation length is even shorter. The observation of a second-order resonance harmonic is linked to the multi-k nature of the configurations, and its unusual temperature dependence is explained by using the phase diagram established for the x=0.75 sample by neutron diffraction. We conclude with a discussion of the frustration effects that have been observed in the x=0.75 sample and some suggestions for further work.