Pauli-limited superconductivity and antiferromagnetism in the heavy-fermion compound CeCo(In1-xZnx)5

We report on the anisotropic properties of Pauli-limited superconductivity (SC) and antiferromagnetism (AFM) in the solid solutions CeCo(In1-xZnx)5 (x≤0.07). In CeCo(In1-xZnx)5, the SC transition temperature Tc is continuously reduced from 2.3 K (x=0) to ∼1.4 K (x=0.07) by doping Zn, and then the AFM order with the transition temperature of TN∼2.2K develops for x larger than ∼0.05. The present thermal, transport, and magnetic measurements under magnetic field B reveal that the substitution of Zn for In yields little change of low-temperature upper critical field μ0Hc2 for both the tetragonal a and c axes, while it monotonically reduces the SC transition temperature Tc. In particular, the magnitudes of μ0Hc2 at the nominal Zn concentration of x=0.05 (measured Zn amount of ∼0.019) are 11.8 T for B||a and 4.8 T for B||c, which are as large as those of pure compound though Tc is reduced to 80% of that for x=0. We consider that this feature originates from a combination of both an enhanced AFM correlation and a reduced SC condensation energy in these alloys. It is also clarified that the AFM order differently responds to the magnetic field, depending on the field directions. For B||c, the clear anomaly due to the AFM transition is observed up to the AFM critical field of ∼5 T in the thermodynamic quantities, whereas it is rapidly damped with increasing B for B||a. We discuss this anisotropic response on the basis of a rich variety of AFM modulations involved in the Ce115 compounds.