Enhanced Susceptibilities and NMR Relaxation Rates in Co-Rh Alloys

Abstract

Dilute alloys of Co in Rh show a weakly temperature-dependent susceptibility increase Δχ (Co) ∼20×10−4 emu/mole of Co solute as compared with χ∼1×10−4 emu/mole for pure Rh. This effect is thought to be associated with Coulomb enhancement of the d-electron spin susceptibility in the neighborhood of the Co impurity. By plotting the 9Co NMR shift vs Δχ (Co) one finds an effective core-polarization hyperfine field considerably smaller than that of Co metal. Spin-echo double-resonance studies of these alloys show that the nearest-neighbor 103Rh nuclei to the Co impurity undergo a negative NMR shift of ∼0.56% relative to the bulk 103Rh nuclei. The origin of this shift is not clearly understood. In particular, attributing such a shift to d-spin polarization is not consistent with the short-range polarization effects expected. Measurements of T1 and T2 show strongly enhanced hyperfine field fluctuations at the 59Co nucleus, presumably due to local Coulomb interactions. We find T1T = 39.5 msec°K and T2T = 3.0 msec°K for temperatures ranging from 1.2° to 20.4°K. The discrepancy between T1 and T2 (spin lattice) is apparently a consequence of strong first-order quadrupolar broadening of the 59Co Zeeman levels for | m | >12.