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 T_c=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, ρ_c, and longitudinal magnetoresitivity, LMR, is typical for a HF system. Above T_ch~50 K the rho_c is weakly T-dependent while below T_ch the ρ_c 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: ρ_c (ρ_ab) below 8 K (12 K) is nearly linear in T. While ρ_ab extrapolates to a finite value at 0 K, ρ_c for the same crystal extrapolates nearly to zero. Applying field along c-axis restores the FL behavior: in mK range ρ_c=ρ₀+AT² for B>=6 T. The coefficient A diverges as (B-B_cr)^-1.4. Contrary to ab-plane measurements, however, our results suggest that the critical field B_cr lays below B_c2. 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.

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