High-field electron spin resonance spectroscopy study of GdFeAsO${}_{1-x}$F${}_x$ superconductors

We report a detailed investigation of GdO${}_{1-x}$F${}_x$FeAs ($x=0$, $0.07$, and $0.14$) samples by means of high-field and high-frequency electron spin resonance (HF-ESR) together with measurements of thermodynamic and transport properties. The parent GdOFeAs compound exhibits Fe long-range magnetic order below $128\hspace{0.5em}$K, whereas both doped samples do not show such order and are superconducting with $T_c=20$ K ($x=0.07$) and $T_c=45$ K ($x=0.14$). The Gd${}^{3+}$ HF-ESR reveals an appreciable exchange coupling between Gd and Fe moments, through which the static magnetic order is clearly seen in the parent compound. Owing to this coupling, HF-ESR can probe sensitively the evolution of the magnetism in the FeAs planes upon F doping. It is found that in both superconducting samples, where the Fe long-range order is absent, there are short-range, static on the ESR time scale magnetic correlations between Fe spins. Their occurrence on a large doping scale may be indicative of the ground states’ coexistence.

Figure 1

Powder x-ray diffraction data of the $c$-axis aligned GdOFeAs sample.

Figure 2

(a) Static susceptibility $\chi =M/B$ (left axis), ${\chi }^{-1}$ (right axis); (b) specific heat $c_{\mathrm{p}}$(left axis) and thermal-expansion coefficient $\alpha$ (right axis) of GdOFeAs vs temperature; specific-heat data for $x=0.07$ and $x=0.14$ samples is artificially shifted down for clarity; inset in (b) presents specific-heat anomaly associated with long-range antiferromagnetic ordering of the Gd moments at $B=0$ and $B=9$ T.

Figure 3

(a) Electrical resistivity $\rho$ of GdO${}_{1-x}$F${}_x$FeAs samples, $x$ $=$ 0, 0.07, 0.14; (b) electrical resistivity derivative $d\rho /\mathit{dT}$ for $x$ $=$ 0 (left axis), $x$ $=$ 0.07, 0.14 (right axis).

Figure 4

Results of the $X$-band ESR ($\nu =9.6$ GHz) measurements performed on $c$-axis aligned powder GdOFeAs sample for two sample orientations in the magnetic field: open circles, field in $\mathit{ab}$ plane; open squares, field parallel to the $c$ axis. (a) Temperature dependence of the resonance field on a reduced field scale $(H-H_0)/H_0$. Here ${\mu }_0{H}_0=0.323/0.328\hspace{0.5em}$ T is resonance field at the highest temperature for $H\perp c$/$H\parallel c$; the inset shows the spectrum at $T=70$ K in $H\parallel c$ configuration; the arrow points at a small Fe ESR signal, which indicates a small amount of impurities. (b) Temperature dependence of the linewidth; inset shows the change of the $T$ dependence at the SDW transition.

Figure 5

Temperature evolution of the high-frequency–high-field ESR spectra of GdOFeAs powder and $c$-axis aligned powder samples at a frequency of $\nu =328$ GHz, shown on a reduced field scale $(H-H_0)/H_0$. Here ${\mu }_0{H}_0=11.7$ T is the resonance field of the signal at high temperature: (a) nonoriented powder; (b) $c$-axis oriented powder.

Figure 6

The shift of the minimum of absorption of the spectra with temperature, measured at a frequency of $\nu =328$ GHz, on a reduced field scale $(H_{\mathrm{res}}-H_0)/H_0$. Here ${\mu }_0{H}_0=11.7$ T is the resonance field of the signal at high temperature: open circles, powder sample; open squares, $c$-axis aligned powder sample; solid lines, fits of the resonance field using Eq. (A4).

Figure 7

Frequency dependence of the ESR signal from GdOFeAs nonoriented powder and $c$-axis oriented powder samples measured at $T=4$ K; data points correspond to the position of the minimum of absorption of the spectra at different frequencies: open circles, nonoriented powder sample; open squares, $c$-axis oriented powder sample; dashed lines, guides for the eyes; solid line, the frequency dependence of the position of the high-$T$ Gd ESR line measured at $T=280$ K. The line shape of the spectra is shown as well: thin solid line, nonoriented powder; thick solid line, $c$-axis oriented powder.

Figure 8

Canting of the Fe moments due to the applied magnetic field $H_{\mathrm{ext}}$; (a),(b) $H_{\mathrm{ext}}\perp c$ (angle between $H_{\mathrm{ext}}$and Fe AFM ordered moments is ${90}^{°}$); (c),(d) $H_{\mathrm{ext}}\parallel c$. Arrows on the Fe site depict magnetic moments, whereas arrows on the Gd sites represent the induced internal field.

Figure 9

The full width at half maximum (FWHM) $\Delta H$ of the ESR lines, measured at a frequency of $\nu =348$ GHz ($x=0.07$) and $\nu =328$ GHz ($x=0,0.14$), as a function of temperature for the nonoriented GdO${}_{1-x}$F${}_x$FeAs samples: (a) $x=0$; (b) $x=0.07$; (c) $x=0.14$.

Figure 10

Temperature evolution of the high-frequency–high-field ESR spectra of GdO${}_{1-x}$F${}_x$FeAs powder samples measured at a frequency of $\nu =348$ GHz ($x=0.07$) and $\nu =328$ GHz ($x=0.14$), shown on a reduced field scale $(H-H_0)/H_0$. Here $H_0$ is the resonance field of the signal at high temperature: (a) $x=0.07$, ${\mu }_0{H}_0=12.4$ T; (b) $x=0.14$, ${\mu }_0{H}_0=11.7$ T.

Figure 11

The shift of the minimum of absorption of the spectra with temperature, measured at a frequency $\nu =348$ GHz ($x=0.07$)/328 GHz($x=0.14$), on a reduced field scale $(H_{\mathrm{res}}-H_0)/H_0$. Here ${\mu }_0{H}_0=12.4$ T/11.7 T is the resonance field of the signal at high temperature: open hexagons, $x=0.07$; open triangles, $x=0.14$; solid line, fit of the resonance field in the parent GdOFeAs sample.

Figure 12

Frequency dependence of the ESR signal from GdO${}_{1-x}$F${}_x$FeAs powder samples measured at a temperature of $T=4$ K, in SC state. Data points correspond to the position of the minimum of absorption of the spectra at different frequencies: open hexagons, $x=0.07$; open triangles, $x=0.14$; dashed line, guide for the eyes; solid line, frequency dependence of the position of the high-$T$ Gd ESR line measured at $T=280$ K for both F-doped samples. The line shape of the spectra is shown as well: thin solid line, $x=0.07$; thick solid line, $x=0.14$.