Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic and in-plane ferromagnetic (FM) wave vectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using ${}^{75}\mathrm{As}$ NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of $T_{\mathrm{c}}$ and the shape of the superconducting dome in these and other iron-pnictide families.

Figure 1

(a) $T$ dependence of the NMR shift $K$ for a variety of indicated samples. (b) $T$ dependence of NMR spin-lattice relaxation rate $1/T_{1}T$ for the same samples. Here, and throughout, filled (open) symbols are used for $H\parallel ab$ ($H\parallel c$). (c) $K_i(T)$ vs ${\chi }_i(T)$ ($i=ab,c$) for a variety of samples. Data are from Refs. [14, 19, 23, 34, 35, 36, 37, 38]. For ${\mathrm{KFe}}_2{\mathrm{As}}_2$, $K_c$ is nearly $T$ independent making a $K$ vs $\chi$ analysis impossible. From the relative values of $K_{ab}$ and $K_c$, we estimate $K_{0,c}\sim 0.21\%$. For ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, $K_{0,ab}=0.14\%$ and $K_{0,c}=0.21\%$ are from Ref. [14]. For the 8% Co doped sample, $K_{0,c}=0.22\%$ from Ref. [19].

Figure 2

(a) Korringa ratios ${\alpha }_{\perp }$ (filled symbols) and ${\alpha }_{\parallel }$ (open symbols) as a function of $T$ in a variety of $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ and ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ samples. (b) Same for intraband Korringa ratios ${\alpha }_{\parallel }^{\text{intra}}$ and ${\alpha }_{\perp }^{\text{intra}}$, obtained by subtracting the interband (Curie-Weiss) contributions.

Figure 3

(a),(b): Competing sources of hyperfine field fluctuations along the $c$ axis. (c),(d): Competing sources of hyperfine field fluctuations in the $ab$ plane. Competition between (a) and (b) [(c) and (d)] determines the value of ${\alpha }_{\parallel }$ (${\alpha }_{\perp }$).

Figure 4

Potential relevance of FM spin fluctuations to iron pnictide phase diagram: (a) Doping dependence of the nearly $T$-independent values of ${\alpha }_{\parallel }^{\text{intra}}$ and ${\alpha }_{\perp }^{\text{intra}}$, which parametrize the strength of FM fluctuations [39]. We also show the doping dependence of the Curie-Weiss parameters $C_{\perp }$ and $C_{\parallel }$, which parametrize the strength of 2D AFM fluctuations. (b) The doping dependence of the Curie-Weiss temperature ${\mathrm{\Theta }}_{\mathrm{CW}}$. Solid lines are guides to the eye.