NMR (Nuclear Magnetic Resonance) is a local, site-selective microscopic experimental probe. We use NMR to investigate the physical properties of Fe-based high temperature superconductor Ba(Fe1-xCox)2As2 for the entire doping regime. Our results indicate that superconducting critical temperature Tc is optimized when antiferromagnetic spin fluctuations (AFSF) is just right, i.e. , can not be more, can not be less [1,2,3,4,5]. We then extend our research into the fabrication and characterization of bulk form diluted ferromagnetic semiconductors (DFS), i.e., Li(1+y)Zn(1-x)MnxP, that are iso-structural to the Fe-based superconductors, where Mn substitution for Zn and off-stoichiometry Li introduce spins and carriers, respectively. We successful identified new 7Li NMR peak induced by Mn doping in Li(Zn1-xMnx)P, which enabled us to probe local static and dynamic properties of Mn spins with NMR for the first time[6,7]. We present unequivocal experimental evidence that the ferromagnetic ordering is indeed caused by randomly substituted Mn in Zn sites, instead of Mn cluster or other magnetic impurities. This result is also confirmed by muon spin relaxation (muSR) measurement[8], which shows a nearly full ferromagnetic ordered volume at base temperature. Moreover, we present the first experimental information on Mn spin dynamics in DMS, and establish that Mn-Mn ferromagnetic interactions are not limited to the near-neighbor sites, but extend over many unit cells, mostly likely due to the p-d Zener interactions.

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8, H.Y. Man and F.L. Ning* et al, PRB (2014, Submitted).