Nuclear resonance and susceptibility study of Pt-Rh alloys

Abstract

Low-temperature [$T\ensuremath{\approx}(1\ensuremath{-}4)$\ifmmode^\circ\else\textdegree\fi{}K] measurements of the Knight shift and spin-lattice relaxation time for $^{103}\mathrm{Rh}$ and $^{195}\mathrm{Pt}$ in ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}$ for $0\ensuremath{\le}x\ensuremath{\le}1.0$ have been carried out using transient NMR methods. Bulk magnetic susceptibilities of these alloys as a function of temperature were also measured. The magnitude of the $^{103}\mathrm{Rh}$ Knight shift and relaxation rate increased monotonically with decreasing $x$ to the point that exchange enhancement effects dominate the NMR at $x=0.03$. In this low Rh content or impurity region the local susceptibility at the Rh site was approximately five times the Pt susceptibility. The localization is gradually increased with concentration as evidenced by the smooth monotonic increase in the ratio of $^{103}\mathrm{Rh}$ to $^{195}\mathrm{Pt}$ Knight shift as $x$ decreased. The magnitude of the $^{195}\mathrm{Pt}$ resonance shift was found to track the bulk susceptibility with both quantities exhibiting maxima near $x\ensuremath{\approx}0.2$. Here, in contrast to the Rh NMR, rigid-band considerations are qualitatively applicable in that the maxima reflect the peak in density of states of Pt known to occur just below the Fermi energy. Extended polarization of the Pt sublattice around Rh impurities probably accounts for the rigid-band nature of the Pt NMR. The experimental nuclear-nuclear exchange constants for $^{195}\mathrm{Pt}$ were approximately 4.5 kHz and very weakly concentration dependent.