Antiferromagnetic spin correlations and pseudogap-like behavior in Ca(Fe1-xCox)2As2 studied by 75As nuclear magnetic resonance and anisotropic resistivity

Abstract

We report ${}^{75}$As nuclear magnetic resonance (NMR) measurements of single-crystalline Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x$ = 0.023, 0.028, 0.033, and 0.059) annealed at 350~${}^{\circ }$C for 7 days. From the observation of a characteristic shape of ${}^{75}$As NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of $x$ = 0 ($T_{\mathrm{N}}$ = 170 K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in $x$ = 0.023 ($T_{\mathrm{N}}$ = 106 K) and $x$ = 0.028 ($T_{\mathrm{N}}$ = 53 K). Through the temperature dependence of the Knight shifts and the nuclear spin lattice relaxation rates (1/$T_1$), although stripe-type AFM spin fluctuations are realized in the paramagnetic state as in the case of other iron pnictide superconductors, we found a gradual decrease of the AFM spin fluctuations below a crossover temperature $T^{\ast }$ which was nearly independent of Co-substitution concentration, and is attributed to a pseudogap-like behavior in the spin excitation spectra of these systems. The $T^{\ast }$ feature finds correlation with features in the temperature-dependent inter-plane resistivity, ${\rho }_c(T)$, but not with the in-plane resistivity ${\rho }_a(T)$. The temperature evolution of anisotropic stripe-type AFM spin fluctuations are tracked in the paramagnetic and pseudogap phases by the 1/$T_1$ data measured under magnetic fields parallel and perpendicular to the $c$ axis. Based on our NMR data, we have added a pseudogap-like phase to the magnetic and electronic phase diagram of Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$.

14 pages, 12 figures, accepted for publication in Phys. Rev. B

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