Magnetic hyperfine interactions on Cd sites of the rare-earth cadmium compounds RCd (R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er)

Abstract

This paper reports the investigation of the magnetic hyperfine field Bhf in a series of rare-earth (R) cadmium intermetallic compounds RCd and GdCd2 measured by perturbed angular correlation (PAC) spectroscopy using 111In/111Cd as probe nuclei at Cd sites as well as first-principles calculations of Bhf at Cd sites in the studied compounds. Vapor–solid state reaction of R metals with Cd vapor and the 111In radioisotope was found to be an appropriate route of doping rare-earth cadmium compounds with the PAC probe 111In/111Cd. The observation that the hyperfine parameters depend on details of the sample preparation provides information on the phase preference of diffusing 111In in the rare-earth cadmium phase system. The 111Cd hyperfine field has been determined in the compounds RCd for the R constituents Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er, in several cases as a function of temperature. For most R constituents, the temperature dependence Bhf (T ) of 111Cd:RCd is consistent with ferromagnetic order of the compound. DyCd, however, presents a remarkable anomaly: a finite magnetic hyperfine field is observed only in the temperature interval 35 K T 80 K which indicates a transition from ferromagnetic order to a spin arrangement where all 4f -induced contributions to the magnetic hyperfine field at the Cd site cancel. First-principles calculation results for DyCd show that the (π, π, 0) antiferromagnetic configuration is energetically more favorable than the ferromagnetic. The approach used in the calculations to simulate the RCd system successfully reproduces the experimental values of Bhf at Cd sites and shows that the main contribution to Bhf comes from the valence electron polarization. The de Gennes plot of the hyperfine field Bhf of 111Cd:RCd vs the 4f -spin projection (g − 1)J reflects a decrease of the strength of indirect 4f -4f exchange across the R series. Possible mechanisms are discussed and the experimental results indicate that the indirect coupling is provided by the intra-atomic 4f -5d exchange and interatomic 5d-5d interaction between the spin-polarized 5d electrons of neighboring R atoms. The ratio of the hyperfine fields of GdCd and GdCd2 scales with the number of nearest Gd neighbors. In the paramagnetic phases of the RCd compounds, the PAC spectra indicate the presence of a broad distribution of weak quadrupole interactions suggesting a perturbation of the cubic CsCl symmetry of the Cd site, most probably due to chemical disorder of the R and Cd sublattices. A substantial interchange of R and Cd atoms is also reflected in the temperature dependence of the linewidth of the magnetic hyperfine interaction in the magnetically ordered phase of RCd and GdCd2. Its critical increase towards the order temperature is evidence for a distribution of the order temperature with a width of about 10 K.