Electronic and magnetic properties of Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ studied by $^{75}$As NMR*

Recently much attention has been paid to CaFe$_{2}$As$_{2}$ because the magnetic and electronic properties of the system can be controlled by changing the heat treatment conditions. CaFe$_{2}$As$_{2}$ annealed at 400 C for 24 hours undergoes a phase transition from a high-temperature tetragonal paramagnetic state to a low temperature orthorhombic antiferromagnetic state at T$_{\mathrm{N}}$ $\sim$ 160K. On the other hand, CaFe$_{2}$As$_{2}$ quenched from 960 C to room temperature shows a transition to a collapsed tetragonal non-magnetic phase below T$_{\mathrm{s}}$ $\sim$ 90 K. In order to investigate the difference in electronic and magnetic properties of the two different CaFe$_{2}$As$_{2}$ samples from a microscopic point of view, we have carried $^{75}$As-NMR spectra and spin-lattice relaxation measurements. We also performed $^{75}$As-NMR measurements on Co-doped CaFe$_{2}$As$_{2}$ superconductor. Based on our NMR data, we will discuss similarities and difference in magnetic fluctuations in the systems, and compare the NMR data with inelastic neutron scattering data.