Coexistence of antiferromagnetic and ferromagnetic spin correlations in Ca(Fe1−xCox)2As2 revealed by As75 nuclear magnetic resonance

Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped ${\mathrm{BaFe}}₂{\mathrm{As}}₂$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron-pnictide superconducting family, $\mathrm{Ca}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_x)}₂{\mathrm{As}}₂$. We reanalyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin correlations in the electron-doped ${\mathrm{CaFe}}₂{\mathrm{As}}₂$ system. These NMR data indicate that FM fluctuations exist in general in iron-pnictide superconducting families and thus must be included to capture the phenomenology of the iron pnictides.