Antiferromagnetic Spin Fluctuations and Pseudogap Behavior in Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ Studied by$^{5}$As NMR*

$^{75}$As NMR measurements of single-crystalline Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ have been carried out for four different doping concentration crystals (x $=$ 0.023, 0.028, 0.033, 0.059) annealed at 350$^{\circ}$C [1]. Co-doped CaFe$_{2}$As$_{2}$ is a compound in 122 family of iron-pnictide superconductors with three principle phases exhibited: paramagnetic (PM), antiferromagnetic (AFM) and superconducting (SC) states. The magnetic phase transition to AFM state occurs at T$_{\mathrm{N}}=$180K at x$=$0 and is suppressed to T$_{\mathrm{N}}=$53K for x$=$0.028, which is accompanied by a structural phase transition from tetragonal to orthorhombic phases. $^{75}$As NMR was used to study the low energy spin dynamics via Knight shift (K) and spin-lattice relaxation rate (1/T$_{1})$ measurements. From our analysis of the temperature dependence of both K and (T$_{1}$T)$^{-1}$ in x$=$0.028 (T$_{\mathrm{N}}=$53K), 0.033 (T$_{\mathrm{c}}=$9K) and 0.059 (T$_{\mathrm{c}}=$10K), we found a gradual decrease of AFM spin fluctuations below T$^{\mathrm{\ast }}=$88K for x$=$0.028, 72K for x$=$0.033 and 41K for x$=$0.059, respectively, indicating the possible pseudogap behavior in spin excitation spectrum in the system. \\[4pt] [1] S. Ran, et al., Phys. Rev. B 85, 224528 (2012)