Effect of magnetic order on the superfluid response of single-crystal ErNi2B2C:: A penetration depth study -: art. no. 214505

We report measurements of the in-plane magnetic penetration depth Delta lambda(T) in single crystals of ErNi2B2C down to similar to 0.1 K using a tunnel-diode based, self-inductive technique at 21 MHz. We observe four features: (1) a slight dip in Delta lambda(T) at the Neel temperature T-N=6.0 K, (2) a peak at T-WFM=2.3 K, where a weak ferromagnetic component sets in, (3) another maximum at 0.45 K, and (4) a final broad drop down to 0.1 K. Converting to superfluid density rho(s), we see that the antiferromagnetic order at 6 K only slightly depresses superconductivity. We seek to explain some of the above features in the context of antiferromagnetic superconductors, where competition between the antiferromagnetic molecular field and spin fluctuation scattering determines increased or decreased pair breaking. Superfluid density data show only a slight decrease in pair density in the vicinity of the 2.3 K feature, thus supporting other evidence against a pure ferromagnetic state in this temperature range.