{mu}{sup +}SR studies of magnetic properties of boron carbide superconductors

Positive-muon spin relaxation ({mu}{sup +}SR) has been carried out in the recently-discovered rare-earth boron carbide superconductors RNi{sub 2}B{sub 2}C, R = Ho, Er and Tm. For R = Ho and Er zero-field {mu}{sup +}SR measurements showed a well-defined internal field below the Neel temperatures of 5.5 K coexisting with the superconducting state down to 0.1 K. The observed temperature dependence of the order parameter for Ho is consistent with a 2-dimensional Ising model. For R = Tm a spontaneous internal field appears above 30 K, whose magnitude saturates below about 3 K at a value corresponding to a rare earth moment much smaller than for Ho and Er. Transverse-field {micro}{sup {minus}}SR measurements in R = Tm showed a superconducting penetration depth {lambda}, = 1,200 {angstrom}. The temperature dependence of {lambda} is consistent with conventional s-wave pairing.