Slow magnetic fluctuations and superconductivity in fluorine-doped NdFeAsO

Among the widely studied superconducting iron-pnictide compounds belonging to the Ln1111 family (with Ln a lanthanide), a systematic investigation of the crossover region between the superconducting and the antiferromagnetic phase for the Ln = Nd case has been missing. We fill this gap by focusing on the intermediate doping regime of ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$ by means of dc-magnetometry and muon-spin spectroscopy ($\mu \mathrm{SR}$) measurements. The long-range order we detect at low fluorine doping is replaced by short-range magnetic interactions at $x=0.08$, where also superconductivity appears. In this case, longitudinal-field $\mu \mathrm{SR}$ experiments show clear evidence of slow magnetic fluctuations that disappear at low temperatures. This fluctuating component is ascribed to the glassy-like character of the magnetically ordered phase of NdFeAsO at intermediate fluorine doping.

Figure 1

Evolution of the $a$- and $c$-axis lengths of ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$ at 300 K as a function of the nominal fluorine content. Continuous lines are only guides to the eye.

Figure 2

Temperature dependence of the inverse dc susceptibility in ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$. Data were recorded in zero-field cooled (ZFC) mode in an applied field ${\mu }_{0}H=3$ T. Lines represent fits using the $1/[\chi \left(T\right)-{\chi }_0]$ model for $T>50$ K (for consistency among the normal and superconducting samples).

Figure 3

Temperature dependence of the volume magnetic susceptibility in ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$ for $x=0.08$ and 0.25 in an applied field of 1 mT, following zero-field cooling.

Figure 4

ZF-$\mu \mathrm{SR}$ asymmetry data for ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$ samples, collected at high and low temperatures. Solid lines represent numerical fits by means of Eqs. (1) and (2). See text for details.

Figure 5

Temperature dependence of the magnetic volume fraction for samples with $x\left(\mathrm{F}\right)=0.03$, 0.05, and 0.08. Solid lines represent numerical fits by means of Eq. (3).

Figure 6

Doping dependence of the internal field values as probed by implanted muons in sites A (filled symbols) and B (open symbols) at 5 K. For the low-doped samples, $x=0.03$ and 0.05, both the local-field value $B_{\mu }^A$ at site A and its width $\Delta B_{\mu }^A={\sigma }_A/{\gamma }_{\mu }$ (plotted as error bar) could be determined, whereas for the site B the field distribution width $\Delta B_{\mu }^B={\sigma }_B/{\gamma }_{\mu }$ is shown. For the $x=0.08$ sample, only $\Delta B_{\mu }^{A,B}={\lambda }_{A,B}/{\gamma }_{\mu }$ are displayed. The open square represents the internal field at site A for the undoped compound, as reported in Ref. [12]. The dotted and dashed curves are guides to the eye.

Figure 7

$x=0.08$ temperature dependence of the parameters extracted by fitting the time-domain $\mu \mathrm{SR}$ asymmetry data by means of Eq. (2). The dashed curves are guides to the eye.

Figure 8

Time-domain LF-asymmetry data taken at $T=30$ K in the $x=0.08$ sample for different values of the longitudinal field. Solid lines represent numerical fits by means of Eq. (2).

Figure 9

Longitudinal-field dependence of the parameters extracted by fitting the time-domain asymmetry data taken at $T=30$ K in the $x=0.08$ sample (see Fig. 8). The dashed curves are guides to the eye.

Figure 10

Magnetic ordering $T_{\mathrm{N}}$ and critical superconducting temperatures $T_c$ vs fluorine doping $x$ in the ${\mathrm{NdFeAsO}}_{1-x}{\mathrm{F}}_x$ family. The lines are guides to the eye. The $T_{\mathrm{N}}$ value for the $x=0$ case is taken from Ref. [12].

Figure 11

Muon asymmetry data taken at $T=30$ K in the $x=0.08$ sample in various applied longitudinal fields vs the scaled variable $t/B_{\mathrm{LF}}^{\gamma }$, with $\gamma =0.5$ for $x=0.08$.