Large orbital magnetic moment and its quenching in the itinerant uranium intermetallic compoundsUTGa5(T=Ni,Pd,Pt)

The crystal structure, lattice strain due to the antiferromagnetic ordering, and magnetic form factor in the itinerant $5f$ compounds $\mathrm{U}T{\mathrm{Ga}}_{5}(T=\mathrm{N}\mathrm{i},\mathrm{}\mathrm{P}\mathrm{d},\mathrm{}\mathrm{Pt})$ have been studied by neutron scattering. High-resolution powder diffraction revealed that the tetragonality of the U-Ga layers increases down to the series of the transition metal element T. The integrated intensities of the antiferromagnetic reflections can be well explained with the N'eel-type structure for ${\mathrm{UNiGa}}_{5},$ whereas ${\mathrm{UPtGa}}_{5}$ has the antiferromagnetic stacking of the ferromagnetically ordered uranium moments in the c plane. In both compounds the uranium moments orient along the c axis with moments of 0.75(5) and 0.32(5) ${\ensuremath{\mu}}_{\mathrm{B}}$ for ${\mathrm{UNiGa}}_{5}$ and ${\mathrm{UPtGa}}_{5},$ respectively. No magnetic peak could be observed in the powder diffraction pattern of ${\mathrm{UPdGa}}_{5}$ due to the small magnetic moment less than the experimental sensitivity. The orbital contributions in the magnetic form factor are reduced from the free-ion value, especially for ${\mathrm{UNiGa}}_{5}.$ This suppression shows a strong correlation with the bulk susceptibility. We observed lattice anomalies associated with the antiferromagnetic ordering. The tetragonality of the U-Ga layers is a sensitive measure of the nearest-neighbor interaction, the lattice anomaly and the orbital contribution suggest that orbital degrees of freedom may play an important role for the magnetic properties in these itinerant $5f$ antiferromagnets.