Metal-insulator transition in antiferromagnetic Ba_(1-x)K_xMn_2As_2 (0 ≤ x ≤ 0.4) single crystals studied by ~(55)Mn and ~(75)As NMR

摘要

The magnetic structure and metal-insulator transition in antiferromagnetic (AFM) BaMn_2As_2 and Ba_(1-x)K_xMn_2As_2 single crystals have been investigated by ~(55)Mn and ~(75)As nuclear magnetic resonance (NMR) measurements. In the parent AFM insulator BaMn_2As_2 with a N6el temperature T_N = 625 K, we observed a ~(55)Mn zero-field NMR (ZFNMR) spectrum and confirmed the G-type AFM structure from the field dependence of the ~(55)Mn spectra and ~(75)As NMR spectra below T_N. In hole-doped crystals with x ＞ 0.01, similar ~(55)Mn ZFNMR spectra were observed and the AFM state was revealed to be robust up to x = 0.4 with the ordered moment nearly independent of x. The nuclear spin-lattice relaxation rates (1/T_1) for both nuclei in the doped samples follow the Korringa relation T_1T = const, indicating a metallic state. This confirms the coexistence of AFM ordered localized Mn spins and conduction carriers from a microscopic point of view. From the x dependence of (T_1T)~(-1/2) for both nuclei, we conclude that this transition is caused by vanishing of the hole concentration as the transition is approached from the metallic side.