Magnetic field induced non-Fermi liquid to Fermi liquid crossover at the quantum critical point of YbCu_(5-x)Au_(x)

The temperature (T) dependence of the muon and $^{63}$Cu nuclear spin-lattice relaxation rates $1/T_1$ in YbCu4.4Au0.6 is reported over nearly four decades. It is shown that for $T\to 0$ $1/T_1$ diverges following the behaviour predicted by the self-consistent renormalization (SCR) theory developed by Moriya for a ferromagnetic quantum critical point. On the other hand, the static uniform susceptibility $\chi_s$ is observed to diverge as $T^{-2/3}$ and $1/T_1T\propto \chi_s^2$, a behaviour which is not accounted for by SCR theory. The application of a magnetic field $H$ is observed to induce a crossover to a Fermi liquid behaviour and for $T\to 0$ $1/T_1$ is found to obey the scaling law $1/T_1(H)= 1/T_1(0)[1+(\mu_BH/k_BT)^2]^{-1}$.