Neutron diffraction studies on heavy fermion superconducting and antiferromagnetic compounds CeRh 1– x Co x In 5

CeCoIn 5 is a heavy fermion superconductor with the transition temperature T c = 2.3 K, while in a related material CeRhIn 5 the superconductivity is highly suppressed and an incommensurate antiferromagnetic (AFM) order characterized by the propagation vector q h = (1/2, 1/2, 0.297) appears below T N = 3.8 K. From the elastic neutron diffraction measurements on mixed compounds CeRh 1– x Co x In 5 , it was found that the superconductivity is strongly suppressed by the incommensurate AFM order while it coexists with the commensurate AFM order with q c = (1/2, 1/2, 1/2) in the intermediate x region. This is in sharp contrast to the CeRh 1– x Ir x In 5 system, where both the commensurate and incommensurate AFM orders coexist with the superconductivity. These results reveal that particular areas on the Fermi surface nested by q h play an active role in forming the superconducting state in CeCoIn 5 . (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)