Long- to short-range magnetic order in fluorine-doped CeFeAsO

The evolution of the antiferromagnetic order parameter in CeFeAsO${}_{1-x}$F${}_x$ as a function of the fluorine content $x$ was investigated primarily via zero-field muon-spin spectroscopy. The long-range magnetic order observed in the undoped compound gradually turns into a short-range order at $x=0.04$, seemingly accompanied or induced by a drastic reduction of the magnetic moment of the iron ions. Superconductivity appears upon a further increase in doping ($x>0.04$) when, unlike in the cuprates, the Fe magnetic moments become even weaker. The resulting phase diagram evidences the presence of a crossover region, where the superconducting and the magnetic order parameters coexist on a nanoscopic range.

Figure 1

X-ray diffraction pattern for the sample $x=0.04$ and the corresponding Rietveld refinement plot.

Figure 2

Evolution of the $a$- and $c$-axis lengths of CeFeAsO${}_{1-x}$F${}_x$ as a function of the real fluorine content, $x$(F), at 290 K. Lines are best fits to the experimental data, while the estimated uncertainty $\Delta x/x$ is ca. 25$\%$.

Figure 3

Normalized resistivity vs temperature for a selection of CeFeAsO${}_{1-x}$F${}_x$ samples. For better visibility the curves have been shifted against each other by 1.5 units. Arrows indicate the maxima of $d\rho /dT$.

Figure 4

Linear fit of $1/(\chi -{\chi }_0$) for the $x=0.04$ sample. Inset: Ce free-ion magnetic moment for all the samples under test, as derived from Curie-Weiss fits of the dc magnetization data. The dotted line shows the theoretically expected value for free Ce${}^{3+}$ ions (2.54 ${\mu }_{\mathrm{B}}$).

Figure 5

ZFC magnetization data for the superconducting samples at ${\mu }_{0}H=0.5$ mT. Arrows denote the FeAs SC transitions and the Ce magnetic ordering temperatures.

Figure 6

Zero-field, time-domain $\mu$SR data of CeFeAsO${}_{1-x}$F${}_x$ for selected fluorine doping values $x$, recorded at $T=5$ K. The superconductivity appears for $x\gtrsim 0.06$, whereas the antiferromagnetic order is always present. The range of the AF order, however, changes from long to short, as shown by the absence of oscillations in samples with a high F content.

Figure 7

Magnetic volume fraction vs temperature as extracted from the longitudinal component of $\mu$SR data and fitted with an $\mathrm{erf}\left(T\right)$ function [Eq. (2)] for a selection of samples ($x=0$, 0.03, 0.06). The relevant fit values, including those of other samples, are reported in Table .

Figure 8

Internal magnetic field $B_{\mu }$ as probed by ZF-$\mu$SR at the so-called A sites for $x=0$ and $x=0.03$.

Figure 9

Dependence on fluorine doping of the average magnetic field value $\langle B_{\mu }\rangle$ (for $x<0.4$) and field width ${\langle \Delta B_{\mu }^2\rangle }^{1/2}$ (for $x\ge 0.4$), as measured by muons stopping in sites A and B. The dashed line provides a guide to the eye, emphasizing the sharp field drop at $x\sim 0.4$, in concomitance with the onset of the short-range magnetic order.

Figure 10

Phase diagram of CeFeAsO${}_{1-x}$F${}_x$ as determined from the current $\mu$SR measurements. M and SC coexist in the region labeled M&SC, while the hatched area indicates the presence of a short-range (SR) magnetic order. The point at $x=0.16$ was taken from Ref.9. See text for details.

Figure 11

SEM image of $x=0.06$ sample showing the grain morphology.

Figure 12

Longitudinal field scan at $T=26.5$ K for the $x=0.06$ sample. The solid line represents a fit using the polarization recovery function.[39, 40]