Nuclear resonance and relaxation in intermediate-valent YbCuAl

Abstract

²⁷Al NMR shifts and spin‐lattice relaxation times have been measured in the intermediate‐valent compound YbCuAl over the temperature range 4.2 K to 295 K. Above T_max?25 K, the temperature for which the bulk susceptibility χ exhibits a maximum, a linear relation was observed between K and χ: K=7.5(5) ×10⁻⁴−0.19(1) χ (emu/mole). For Tmax this relation was no longer obeyed; at 4.2 K the shift was about 20% more positive than predicted from the linear relation and the measured χ. Satellite NMR in the Kondo alloy CuFe exhibits no such nonlinearity. It is tentatively attributed to a temperature‐dependent effect of intermediate valence on the transferred hyperfine coupling between Yb moments and ²⁷Al nuclei. Relaxation data yield a 4f spin fluctuation rate which decreases by about a factor of 2 between T_max and 100 K. A simple ionic model for the intermediate‐valent state accounts qualitatively for this observation. Further corroborating experiments are suggested.