Chapter 1 Magnetism of ternary intermetallic compounds of uranium

The magnetization $M(T,H)$, specific heat ${C}_{p}(T,H)$, ${}^{195}$Pt and ${}^{119}$Sn NMR Knight shifts $K(T)$, electrical resistivity $\ensuremath{\rho}(T)$, and magnetoresistance $\text{MR}(T,H)$ measurements were performed on a polycrystalline sample of antiferromagnetic compound U${}_{2}$Pt${}_{2}$Sn with the N\'eel temperature ${T}_{N}$ $=$ 15.7 K. It is found that the ${C}_{p}(T)$ and $\ensuremath{\rho}(T)$ data in the antiferromagnetic state may be interpreted with help of the spin-wave theory with a magnon gap $\ensuremath{\Delta}/{k}_{B}\ensuremath{\sim}6$\char21{}10 K. Furthermore, analysis of the experimental data allows us to conclude that U${}_{2}$Pt${}_{2}$Sn is a dense Kondo system, in which the exchange interaction, Kondo effect, and crystalline electric field compete with each other on the energy scale ${T}_{N}l{T}_{K}l{\ensuremath{\Delta}}_{\text{CEF}}$. The Kondo-lattice component of the ${}^{195}$Pt and ${}^{119}$Sn NMR Knight shift has been fitted to the two-fluid model for heavy fermions ${K}_{\text{KL}}\ensuremath{\propto}(1\ensuremath{-}T/{T}^{*})\mathrm{ln}({T}^{*}/T)$ with a characteristic temperature ${T}^{*}\ensuremath{\sim}$ 80 K. Electronic properties of heavy-fermion state in U${}_{2}$Pt${}_{2}$Sn are discussed in the context of the Wilson ratio (${R}_{W}$ $=$ 2.8), Kadowaki-Woods-Tsujii ratio [$A/{\ensuremath{\gamma}}^{2}\ensuremath{\sim}$ 2.15 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{\ensuremath{-}5}\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\ensuremath{\Omega}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}/(\mathrm{mJ}/{\mathrm{mol}}_{\mathrm{U}}\phantom{\rule{0.16em}{0ex}}\mathrm{K})}^{2}$ with $N$ $=$ 2], and an empirical ratio $R={T}^{*}/{T}_{K}\ensuremath{\sim}2$.

Electronic properties of the Kondo lattice U 2 Pt 2 Sn

We have performed the 119 Sn NMR, magnetic susceptibility, electrical resistivity and Hall effect measurements on a recently discovered heavy-electron compound U 3 Au 3 Sn 4 . The system is found to be in the incoherent Kondo state above about 40 K. Even under the presence of a large residual resistivity, the onset of coherence in the electronic system becomes evident below about 20 K in both ρ( T ) and R H ( T ), but it does not manifest itself in T 1 -1 ( T ). Observed almost T-independent T 1 -1 ( T ) below about 20 K, however, is not considered as due to development of strongly q -dependent spin fluctuation (SF), since an analysis in terms of the Wilson ratio rather suggests a localized character of the SF.

119Sn NMR Studies of the Heavy-Electron Compound U3Au3Sn4

The 121 Sb Knight shift and the 121,123 Sb nuclear spin-lattice relaxation time T 1 in U 3 Ni 3 Sb 4 have been measured. In the temperature range between 25 and 250 K, 1/ T 1 , which has been measured by nuclear quadrupole resonance (NQR), follows an activation law with an activation energy of 21.1 meV. The relaxation process is dominated by the magnetic contribution. The Knight shift is represented as the sum of a strongly temperature-dependent anisotropic term and a temperature-independent term. The transferred hyperfine field from the U 5 f electrons through the U 5 f -Sb 5 p hybridization makes an important contribution to the Knight shift.

NMR and NQR studies of a semiconducting compound U3Ni3Sb4

The 119Sn and 195Pt Knight shifts are anisotropic and exhibit Curie-Weiss-like temperature behavior above TN = 60 K, indicating the localized nature of 5f electrons. The 119Sn NMR is observable down to 4.2 K in spite of disappearance of the 195Pt NMR below TN. This suggests a spin structure which results in a cancellation of the transferred hyperfine fields at a Sn site.

NMR study of UPt2Sn

The 119 Sn Knight shift K and nuclear spin-lattice relaxation time T 1 in a heavy fermion compound U 3 Ni 3 Sn 4 have been measured between 1.5 and 270 K. The K vs the susceptibility plot is linear and yields a hyperfine field of 53.2 kOe/µ B . The nuclear relaxation rate due to U 5f moments (1/ T 1 ) f decreases markedly below about 20 K and is proportional to temperature below about 4 K, where the system is in the Fermi liquid state. The relaxation rate of 5f moments obtained from (1/ T 1 ) f increases linearly with temperature above about 120 K, indicating that the Korringa mechanism is dominant in the fluctuation of localized 5f moments.

Yutaka Hukuda

Papers

119Sn NMR Study of Heavy Fermion Compound U3Ni3Sn4