Evidence For Ordered Moments On The Rh Sites In Cerhin5

The magnetic structure of the tetragonal heavy fermion compound CeRhIn5 (T-N=3.8 K) has been studied by zero-field muon spin rotation (muSR) in the range 1.8-4.3 K in a single crystal. In addition, the anisotropic muon (mu(+)) Knight shift has been measured in the paramagnetic regime with the aim to determine the mu(+) site or sites. It is found that about 70% of the implanted mu(+) are located at the a site (1/2,1/2,1/2). The site of the other 30% is not unambigeously determined but may be the f sites (0,1/2,0) and (1/2,0,0). The data also imply that a small negative moment is induced on the Rh sites. Below T-N we find five components in the muSR signal reflecting spontaneous, relatively sharp internal fields of zero, 13, 37, 69, and 98 G at T=0 K, which are confined to the surface of cones oriented along the c axis with apertures ranging from 14degrees to 90degrees. These results are not reproducible by the known incommensurate helical magnetic structure of the Ce sublattice but can be explained if, in addition, a small moment order of unusual type on the Rh sublattice is assumed.