Investigation of the magnetic hyperfine field of 111Cd in the rare-earth Laves phases RCo2 and RNi2

The magnetic hyperfine field Bhf of the closed-shell probe nucleus 111Cd on the cubic R site of the C15 Laves phase RCo2 (R = Gd, Tb, Dy, Ho, Er, Sm, Nd and Pr) and RNi2 (R = Gd, Sm) has been investigated at 9 K by perturbed angular correlation (PAC) spectroscopy. In RCo2 the hyperfine field increases with increasing R spin, both for the heavy and the light R. The experimental trend suggests that the total hyperfine field has two contributions of opposite sign: Bhf = B3d-B4f , where B3d is induced by the 3d Co moment and B4f is due to indirect 4f-4f exchange. The 4f contribution can be estimated from the measurement of Bhf in RNi2 where the 3d moment vanishes. The 111Cd hyperfine fields in GdNi2 and SmNi2 at 9 K are Bhf = 7.5(5) and 2.0(2) T, respectively. Assuming a linear dependence of B4f on the projection (g-1)J of the R spin on the total angular momentum J, as expected from the Ruderman-Kittel-Kasuya-Yosida (RKKY) theory of indirect 4f-4f exchange, the variation of B3d across the R series can be determined from the experimental values of Bhf. In heavy RCo2 (R Gd, ... , Er) B3d follows very closely the variation of the Co moment μCo across the R series with B3d /μCo = 28.7(2) T μ-1B. With the same ratio for the light RCo2 (R = Pr, Nd, Sm) the experimental Bhf values indicate an increase of the 3d moment from μCo = 0.45 to 0.7 μB between PrCo2 and SmCo2. (author)