The recent discovery of a new heavy-fermions (HF) family, CeMIn5 (M=Ir, Rh, Co), raises once again the long-lasting question about the relation between magnetic and superconducting (SC) interactions. CeCoIn5 has Tc=2.3 K and is believed to reside close to a quantum critical point (QCP). Although no long-range magnetic order is present, short-range AF spin fluctuations are observed. In order to understand the interplay between superconductivity and spin fluctuations in CeCoIn5 we have performed c-axis magnetotransport measurements between 40 mK and 300 K and in the fields up to 9 T, applied along c-axis.
Above 20 K the behavior of resistivity along c-axis, ρc, and longitudinal magnetoresitivity, LMR, is typical for a HF system. Above Tch~50 K the rhoc is weakly T-dependent while below Tch the ρc starts to decrease rapidly, indicating the development of coherence effects. After subtracting the LMR of the non-magnetic analog, LaCoIn5, LMR caused by the presence of Ce ions scales in the manner characteristic for single-Kondo-impurity regime. All LMR(B) curves between 50 K and 100 K can be superimposed onto a single unique curve when scaling them according to equation LMR(B,T)=f[B/(T+T*)] with T*=2 K.
At lower temperatures resistivity exhibits non-Fermi-liquid (nFL) behavior: ρc (ρab) below 8 K (12 K) is nearly linear in T. While ρab extrapolates to a finite value at 0 K, ρc for the same crystal extrapolates nearly to zero. Applying field along c-axis restores the FL behavior: in mK range ρc=ρ0+AT2 for B>=6 T. The coefficient A diverges as (B-Bcr)-1.4. Contrary to ab-plane measurements, however, our results suggest that the critical field Bcr lays below Bc2. This indicates that the critical behavior observed here is connected with the vicinity to an ordered phase other than the SC state and confirms that CeCoIn5 is close to a magnetic QCP that is masked by the occurrence of superconductivity.