NQR and T1 studies of the high pressure phase in YbInCu4

The pressure and temperature phase diagram of YbInCu$_{4}$ has been investigated by nuclear quadrupolar resonance (NQR) and spin- lattice relaxation rate (1/T$_{1})$ experiments. The pressure dependence of the $^{63}$Cu NQR frequency suggests that the 1st-order valence transition temperature, T$_{v}$, does not vanish at the critical pressure P$_{c}$ = 23.7 kbar and thus there is no quantum critical point (T$_{v}$ = 0) in YbInCu$_{4}$. This is consistent with the 1/T$_{1}$ data, which show no evidence for non-Fermi-liquid behavior near P$_{c}$. For pressures P $\ge $ P$_{c}$, 1/T$_{1}$ increases sharply near 2.4 K, which suggests the presence of ferromagnetic (FM) ordering associated with critical fluctuations, as suggested by the ac susceptibility. We analyzed the 1/T$_{1}$, resistivity, and the pressure-enhanced susceptibility data in the mixed- valent state of YbInCu$_{4}$ and found no evidence to indicate that the pressured-induced FM phase can be described by the Stoner theory for itinerant ferromagnetism. This may suggest that the pressure-induced FM order is due to pressure-stabilized Yb$^{3+}$ local moments rather than the itinerant electrons from the mixed-valent state. We also examined the possibility of the FM order induced by an external field near P$_{c}$, but found no evidence down to 1.5 K.