We have measured the Knight shifts $\left(K\right)$ and nuclear relaxation times $\left(T_1\right)$ for ${}^{59}\text{C}\text{o}$ in ${\text{CeCoIn}}_5$ under external fields along $a$ and $c$ axes with the goal of establishing the anisotropy of antiferromagnetic (AFM) spin fluctuations (SF). In our approach, we revisit the problem of interpreting anomalies in the relationship between Knight shift $K_{a,c}$ and static susceptibility ${\chi }_{a,c}$: assuming a single component susceptibility implies a temperature-dependent hyperfine coupling $A_{a,c}\left(T\right)$. Once adopted, a known discrepancy between the behaviors of $T_1^{-1}$ for ${}^{59}\text{C}\text{o}$ and ${}^{115}\text{I}\text{n}\left(1\right)$ sites is eliminated to within experimental uncertainties and the variation with temperature is analyzed within the framework of two-dimensional AFM SF in proximity to a quantum-critical point. Moreover, the ratio $\left[T_{1a}/T_{1c}\right]$ indicates easy-plane anisotropy for $T<T^{\ast }\sim 40\text{K}$, for the heavy-fermion state.

• Received 5 March 2010

DOI:https://doi.org/10.1103/PhysRevB.82.020501