Magnetic properties of UAu: An exemplary study by muon spin-rotation (μSR) spectroscopy

The results of a muon spin-rotation (μSR) study of single-crystalline UAu in the paramagnetically and antiferromagnetically (AF) ordered phases are reported. Transverse-field measurements for the paramagnetic phase reveal four components in the μSR signal, distinguished by their different Knight shifts, which can be associated with two crystallographically different sites: a magnetically unique site in the z = 1/2 plane near the position (0 0 1/2), i.e. between two U3 ions, and a site with a threefold symmetry near the centre of a triangle formed by three U1 ions, located in the z = 1/2 plane. It appears that the U3 ions are nearly but not entirely, non-magnetic also in the paramagnetic phase. The temperature dependence of the different μ Knight shifts follows a Curie-Weiss law but it involves significantly different Curie-Weiss temperatures as compared with the bulk susceptibility, which even show an anisotropy in the hexagonal a-b plane. This does not seem to be a μ-induced feature but is rather a consequence of selectively monitoring just the magnetic response of the U1 ions. In the AF state below T = 22 K, measurements reveal two different spontaneous internal magnetic fields which can be traced back to the two different sites, thus confirming the non-collinear complex AF structure recently found by neutron scattering.