arX

iv:1

010.

5070

v3 [

cond

-mat

.str

-el]

28

Mar

201

1

High-field electron spin resonance spectroscopy study of GdO1−xF

xFeAs

superconductors

A. Alfonsov1, F. Muranyi2, V. Kataev1, G. Lang1, N. Leps1, L. Wang1, R. Klingeler3,

A. Kondrat1, C. Hess1, S. Wurmehl1, A. Kohler1, G. Behr1, S. Hampel1, M. Deutschmann1,

S. Katrych4, N. D. Zhigadlo4, Z. Bukowski4, J. Karpinski4, and B. Buchner11 IFW Dresden, Institute for Solid State Research, 01069 Dresden, Germany

2 Universitat Zurich - Physik-Institut, CH-8057 Zurich, Switzerland3 Kirchhoff Institute for Physics, Heidelberg University, D-69120 Heidelberg, Germany

4 Laboratory for Solid State Physics, ETH Zurich, CH-8093 Zurich, Switzerland

We report a detailed investigation of GdO1−xFxFeAs (x = 0, 0.07 and 0.14) samples by means ofhigh-field/high-frequency electron spin resonance (HF-ESR) together with measurements of thermo-dynamic and transport properties. The parent GdOFeAs compound exhibits Fe long-range magneticorder below 128K, whereas both doped samples do not show such order and are superconductingwith Tc = 20K (x = 0.07) and Tc = 45K (x = 0.14). The Gd3+ HF-ESR reveals an appreciableexchange coupling between Gd and Fe moments, through which the static magnetic order is clearlyseen in the parent compound. Owing to this coupling, HF-ESR can probe sensitively the evolutionof the magnetism in the FeAs planes upon F doping. It is found that in both superconducting sam-ples, where the Fe long-range order is absent, there are short-range, static on the ESR time scalemagnetic correlations between Fe spins. Their occurrence on a large doping scale may be indicativeof the ground states’ coexistence.

I. INTRODUCTION

Iron-pnictide superconductors1 with superconductingcritical temperatures up to 55K2–4 have attracted ahuge interest due to striking similarities to supercon-ducting cuprates as well as due to their original prop-erties. Indeed, most families of these layered materialsfeature an antiferromagnetically (AFM) ordered parentcompound, and the evolution of superconductivity con-comitantly with suppression of AFM order upon doping.However there are important differences which render theFe-pnictides a separate new class of superconducting ma-terials. Most striking of them are semi-metallicity andthe spin density wave (SDW) character of the AFM or-der in the undoped pnictides contrasted with the Mott-insulating AFM state in the cuprates, as well as a multi-band versus single-band electronic structure in the Fe-pnictide and cuprate high-temperature superconductors,respectively.

Beyond study of the superconducting ground state,and of the magnetic and associated structural transitionsseen in the parent compound, much attention has beendevoted to the issue of the ground states’ coexistence.Discrepancies on this issue have been found between dif-ferent families3,5–11, with the variation of the boundaryof the two ground states and different length scales of co-existence, especially in the so-called 1111 family. In thisfamily, which has the composition ROFePn (R - rareearth, Pn - pnictide), the superconductivity evolves withthe substitution of fluorine for oxygen. Here, replace-ment of one rare earth element with another can cause asignificant variation of properties. Whereas in La-basedsuperconducting samples there is evidence against staticmagnetic order in the FeAs planes6, in the case of su-perconducting samples based on different magnetic rare

earths (R = Sm, Nd, Ce) evidence of remanent staticmagnetism is found3,11. The situation appears compli-cated due to the fact that the magnetism then tends tobe of a short-range order or disordered, possibly evendynamic12, which calls for the use of local probe tech-niques. These two different pictures complicate the es-tablishment of the unified phase diagram for 1111 pnic-tides, necessary for the full understanding of these mate-rials. In addition, as was shown by NMR13,14 and µSR15

studies, there is a magnetic coupling between 4f (Ce, Prand Sm) and 3d (Fe) moments. Such coupling of the rareearth to the FeAs plane might give an additional contri-bution to the difference in physical properties of different1111-type superconductors.

In the present work we investigate the evolution of themagnetism upon fluorine doping in Gd-based 1111 com-pound by means of high field/high frequency electronspin resonance (HF-ESR) complemented with measure-ments of thermodynamic and transport properties. TheESR data reveal a significant exchange coupling of Gd-and Fe-moments in the parent GdOFeAs sample whichenables the Gd3+ HF-ESR to probe sensitively the for-mation of the static SDW magnetic order in the FeAsplanes. Interestingly, it is found that the signatures ofsuch an order are still observed in the ESR spectra af-ter doping. In particular, though long-range SDW orderpresent at very low doping is suppressed at doping lev-els where superconductivity appears, our results implystatic on the ESR time scale, likely short-range, magneticcorrelations between Fe spins. This result suggests thatGdO1−xFxFeAs compounds may feature coexistence ofquasi-static magnetism and superconductivity on a largedoping range.

2

II. EXPERIMENTAL

A. Setups

The magnetization has been studied by means of acommercial SQUID magnetometer (MPMS-XL5, Quan-tum Design). For the thermal expansion measurementsa capacitance dilatometer was utilized, which allows avery accurate study of crystal length changes. We mea-sured the macroscopic length changes dL/L of polycrys-talline samples. The linear thermal expansion coeffi-cient α was calculated as the first temperature deriva-tive of dL/L, while the volume thermal expansion co-efficient is given by βvol = 3α for our polycrystallinesamples. The specific heat was studied in a QuantumDesign PPMS calorimeter by means of a relaxation tech-nique. In the electrical transport experiments the sam-ples were investigated by four-probe ρ measurements us-ing an alternating DC-current. The ESR measurementsat a frequency of ν = 9.6GHz were carried out in astandard Bruker EMX system. The HF-ESR experi-ments were performed with a home-made spectrometer16

at frequencies ν = 83 − 348GHz and magnetic fieldsB = 0 − 15T. All ESR measurements were made in atemperature range of 5− 300K.

B. Sample preparation

The polycrystalline samples GdO1−xFxFeAs (x =0, 0.15, 0.17, nominal content) were prepared by two dif-ferent routes. Route 1, which is similar to that describedin Ref. [17], starts with FeAs, Gd, Gd2O3 and GdF3 ina stoichiometric ratio. All materials were homogenizedby grinding in a mortar. Route 2 uses GdAs, Fe, Fe2O3

and FeF3 as starting materials in a stoichiometric ratio.Here, the starting materials were homogenized by grind-ing in a ball mill. In either case, the resulting powderswere pressed into pellets under Ar atmosphere, and sub-sequently annealed in an evacuated quartz tube either ina two step synthesis at 940◦C for 12 h and at 1150◦Cfor 48 h (60 h) or in a one step synthesis at 940◦C for168 h. In order to confirm the single phase character ofthe polycrystals, powder x-ray diffraction was performedon a Rigaku diffractometer (Cu Kα-radiation, graphitemonochromator). The samples were either phase pure orcontained insignificantly small amounts of GdAs, GdOF,and Fe3O4. The microstructure and the compositionwere examined by scanning electron microscopy (SEM,XL30 Philipps, IN400) equipped with an electron mi-croprobe analyzer for semi-quantitative elemental analy-sis using the wave length dispersive x-ray (WDX) mode.The analysis showed that the sample with x = 0.15 (nom-inal content) in fact contains ∼ 0.07 ± 0.02 of F andx = 0.17 contains ∼ 0.14± 0.02 of F. Further on, we willuse the doping levels obtained by WDX in order to labelthe samples.For the c-axis alignment of the parent GdOFeAs sam-

40 50 60 70 80 90

** ****

(004)

Inte

nsity

(arb

. u.)

2 (0)

GdOFeAs

(003)

(006)

(005)

FIG. 1: Powder x-ray diffraction data of the c-axis alignedGdOFeAs sample.

ple, which was synthesized under high pressure18, thepowder was mixed with epoxy resin and hardened whilerotating in a magnetic field of 1.5T. The x-ray diffrac-tion data of the aligned powder samples were collectedat room temperature using a PANalytical X’Pert PROsystem (Philips) with Co Kα-radiation (Fig. 1). Thepresence of highly intense [00l] reflections (Fig. 1, ar-rows) which dominate the pattern points to a sufficientlygood quality of the alignment. Reflections with Millerindices different from [00l] (Fig. 1, asterisks) are visiblein the background, too, but their intensity is stronglysuppressed compared to the powder pattern.

III. THERMODYNAMIC AND TRANSPORTMEASUREMENTS

In Fig. 2(a), the temperature dependence of the staticsusceptibility χ = M/B of GdOFeAs is presented. Asχ(T ) is dominated by the response of the Gd moments,the results are very similar for the F-doped samples,which are not shown. In general, the data obey theCurie-Weiss law which is expected due to the presenceof paramagnetic Gd3+ ions. Note that the response ofthe FeAs-layers which is e.g. visible in LaOFeAs is about3 orders of magnitude smaller and hence masked by themagnetism of the rare earth ions.19 The linear temper-ature dependence of the inverse susceptibility demon-strates the Curie-Weiss-like behavior. Analyzing the datain terms of the Curie-Weiss-law yields the antiferromag-netic Weiss temperature Θ = −16± 1K and the effectivemagnetic moments peff = 7.81±0.04 µB which is close tothe magnetic moment of a free Gd3+ ion (peff = 7.94 µB).At a low temperature of about ∼ 5K there is a kink ofthe magnetization due to the AFM ordering of the Gdmoments.While the structural and Fe magnetic phase transitions

are not visible in the magnetization data, there are pro-nounced anomalies in the specific heat cp and the thermal

3

0 50 100 150 200 2500

20

40

60

80

0

10

20

2 4 6 8

-1.0

-0.5

0.0

0.5

1.0

0.0

0.1

0.2

0.3

(b)

c p (J

/ mol

K)

Temperature (K)

0Tx = 0 0.07 0.14

T (K)

cp (

J/m

olK

)

9T0T

(10-5 / K

)

peff= 7.81 ± 0.04 µB

= -16 ± 1K

0 = (4.8 ± 0.1) 10-4 erg / G2 mol

= M

/B (e

rg /

G2 m

ol)

B = 1T

0

10

20

30(a)

-1 (erg / G2 m

ol) -1FIG. 2: (Color online) (a) Static susceptibility χ = M/B(left axis), χ−1 (right axis); (b) specific heat cp (left axis)and thermal expansion coefficient α (right axis) of GdOFeAsvs. temperature; specific heat data for x = 0.07 and x = 0.14samples is artificially shifted down for clarity; inset in (b)panel presents specific heat anomaly associated with long-range antiferromagnetic ordering of the Gd-moments at B = 0and B = 9T.

expansion coefficient α (Fig. 2(b)) in the case of the par-ent GdOFeAs sample. There is one broad feature visiblein the specific heat data. In contrast, the thermal expan-sion coefficient exhibits two huge anomalies with oppositesign which can be attributed to the structural and SDWtransitions of the compound at TST = 136 ± 5K andTSDW = 128 ± 2K. In addition, the specific heat datareveal a sharp anomaly at TNGd = 3.8K which is as-sociated with the onset of long range antiferromagneticorder of the Gd moments, in accord with the magnetiza-tion data. Note that the anomaly is not present in ourthermal expansion data due to the restricted tempera-ture range T ≥ 6K. Upon application of external mag-netic fields, Gd order is strongly suppressed as shownin Fig. 2 (inset in the lower panel). TNGd is shifted to2.5K in an external magnetic field of B = 9T. Whileanomalies associated with Gd-ordering are still observedin the specific heat data of the F-doped samples withx = 0.07 and x = 0.14, there are no visible anomaliesat higher temperatures (Fig. 2(b)). This evidences theabsence of long-range SDW order in the doped, super-conducting samples.

Fig. 3(a) shows the temperature dependence of theelectrical resistivity ρ of the GdO1−xFxFeAs samples forall three doping levels: x = 0, 0.07 and 0.14. To geta better insight into the data we present the tempera-

0 50 100 150 200 250

0.0

0.2

0.4

0.6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00

0.05

0.10

d/d

T (1

0-7

m /

K)

x=0.07

Temperature (K)

x=0

x=0.14

(b)

x=0.07

x=0.14

(mcm

)

x=0(a) TSDW

d/d

T (1

0-7

m /

K)

FIG. 3: (Color online) (a) Electrical resistivity ρ ofGdO1−xFxFeAs samples, x = 0, 0.07, 0.14; (b) electrical re-sistivity derivative dρ/dT for x = 0 (left axis), x = 0.07, 0.14(right axis).

ture derivatives dρ/dT in the bottom panel (Fig. 3(b)).The resistivity of the undoped material exhibits featuresclosely connected to the structural and magnetic phasetransitions: a maximum close to TST and an inflec-tion point at TSDW = 128 K, which are characteristicfor all the 1111 parent compounds4,17,20. With dop-ing, the electrical resistivity drastically changes its be-havior, superconductivity emerges at low temperatureand the intermediate temperature maximum disappears.The SC temperatures Tc for x = 0.07 and x = 0.14samples amount to 20K and 45K, respectively. No pro-nounced features of the SDW phase are present in thesecompounds in the whole investigated temperature range.However, the normal state behavior of ρ(T ) for x = 0.07is very unusual. At high temperatures the resistivity islinear down to approx 200 K, then it develops a cur-vature and drops below the linear approximation of thehigh temperature part. With decreasing temperature fur-ther, ρ(T ) becomes linear again and develops a slightopposite curvature at T <

∼ 50 K, prior to the onset ofsuperconductivity. Upon increasing the F doping levelin the samples, namely to x = 0.14, this anomaly be-comes weaker. A similar drop of ρ(T ) at T <

∼ 200 Kas found here has previously been observed for other1111-type pnictide superconductors4,17,21 as well as forBa1−xKxFe2As2

22,23. The qualitative resemblance to thesharp drop at TSDW which is observed in the respectiveparent compounds suggests that the resistivity drop inthe superconducting samples is indicative of remnants ofthe SDW phase. In fact, a recent study of the Nernst

4

effect on LaO1−xFxFeAs provides strong evidence thatprecursors of the SDW phase develop in the vicinityof the resistivity anomaly despite the absence of staticmagnetism24.

IV. ELECTRON SPIN RESONANCE, RESULTSAND DISCUSSION

A. GdOFeAs, 9.6 GHz X-band measurements

The ESR measurements performed at a frequency of9.6GHz on the c-axis aligned GdOFeAs sample in thewhole temperature range of study and for both sampleorientations (Hext || or ⊥ c) reveal one broad line withthe g-factor ∼ 2 (Fig. 4(a), inset). Such ESR responseis typical for the systems where Gd3+ ions occupy reg-ular positions in the crystal lattice with short distancesbetween neighboring ions25. The Gd3+ is an S-state ionwith a half-filled 4f shell which yields an isotropic g-factor equal to 2 and a spin value of 7/2. The ratherbig spin value leads to strong magnetic dipole-dipole in-teractions which together with the unresolved fine struc-ture broaden the ESR line. This broadening mechanismshould lead to a gaussian line shape which is howevernot observed in the spectra. Instead, the lorentzian38

function had to be used to fit the spectra (thin line inthe inset in Fig. 4(a)) in order to obtain accurate val-ues of the resonance field (Hres) and the linewidth. Thelorentzian shape suggests that homogeneous narrowing ofthe line does take place, which can be caused by isotropicexchange interaction between Gd spins25,26. The temper-ature dependencies of Hres and the linewidth are shownon Fig. 4(a),(b). With lowering the temperature no dras-tic changes are seen in Hres down to ∼ 10K where thereis a strong shift of the line due to the ordering of Gdmoments (Fig. 4(a)). The linewidth, in contrast to theresonance field, shows clear change in the behavior atTSDW = 128K for both sample orientations (Fig. 4(b)).At temperatures above TSDW there is a gradual decreaseof the width of the ESR line upon cooling. This can beattributed to a Korringa-like behavior, with the linewidthhaving a linear in T contribution due to the relaxationof the Gd spins through interaction with the conduc-tion electrons, similar to EuFe2As2

27. The slope valueamounts to ∼ 0.9 · 10−4T/K which is one order of mag-nitude smaller than that in EuFe2As2. A strong broad-ening of the line below TSDW can be attributed to theformation of the SDW state in the FeAs layers. Similareffects in the Gd3+ ESR linewidth were observed beforein the case of Gd2BaCuO5 samples where exchange cou-pling of Gd- and Cu-moments enabled to probe by meansof Gd3+ ESR the magnetic ordering of the Cu layers28

(see the discussion below).

-50

-40

-30

-20

-10

0

10

0 50 100 150 200 250 300

0.2

0.3

0.4

0.5

0.6

100 150 2000.15

0.16

0.17

0.18

0.19

0.0 0.2 0.4 0.6 0.8 1.0

*

H c H||c

T = 70 KH||c

TSDW= 128 K(a)

(Hre

s-H0)/H

0 (%)

TSDW= 128K

(b)

Line

wid

th (T

)Temperature (K)

H c H||c

0H (T)

abso

rptio

n de

rriv

ativ

e (a

rb.u

.)

FIG. 4: (Color online) Results of the X-Band ESR (ν =9.6GHz) measurements performed on c-axis aligned powderGdOFeAs sample for two sample orientations in the magneticfield, open circles - field in ab plane, open squares - field par-allel to the c-axis; (a) temperature dependence of the res-onance field on a reduced field scale (H − H0)/H0. Hereµ0H0 = 0.323/0.328 T is resonance field at the highest tem-perature for H ⊥ c/H ‖ c; the inset shows the spectrum atT = 70K in H ‖ c configuration, the arrow points at a smallFe ESR signal which presence indicates a small amount of im-purities; (b) temperature dependence of the linewidth; insetshows the change of the T-dependence at the SDW transition

B. GdOFeAs, high-frequency/field measurements

In the measurements performed at 9.6GHz, thelinewidth of the Gd3+ ESR signal is comparable toits resonance field. This leads to complications in thespectra analysis and to a lack of resolution. In or-der to improve the spectral resolution we performedhigh-frequency/field measurements on GdOFeAs sam-ples. The high-temperature ESR spectra of the non-oriented GdOFeAs powder sample measured at a fre-quency of 328GHz (Fig. 5(a)) consist of a single broadlorentzian-shaped line with a g-factor of ∼ 2 and alinewidth of ∼ 0.2T, similar to the low-frequency mea-surements. However, the very small Korringa con-tribution detected in the low frequency measurementsis not visible in the HF-ESR spectra (Fig. 9a). Thelow-temperature HF-ESR spectra exhibit an inhomoge-neously broadened shape which is in contrast to X-banddata. As a measure of this broadening the full width at

5

-15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15

0H0 = 11.7 T

280 K

240 K

180 K

130 K

4 K

40 K

Abs

orpt

ion

(arb

.u.)

(H-H0)/H0 (%)

85 K

(a)

powder

0H0 = 11.7 T(b)

Abs

orpt

ion

(arb

.u.)

alignedc-axis

280 K

240 K

180 K

130 K

4 K

40 K

(H-H0)/H0 (%)

85 K

FIG. 5: (Color online) Temperature evolution of the high-frequency/field ESR spectra of GdOFeAs powder and c-axisaligned powder samples at a frequency of ν = 328GHz, shownon a reduced field scale (H − H0)/H0. Here µ0H0 = 11.7Tis the resonance field of the signal at high temperature; (a)non-oriented powder; (b) c-axis oriented powder.

0 50 100 150 200 250 300-4

-3

-2

-1

0

1

2

3

4

50H0 = 11.7 T

Hres||

Hres

(Hre

s-H0)/H

0 (%)

Temperature (K)

TSDW= 128 K

Tif

FIG. 6: (Color online) The shift of the minimum of absorptionof the spectra with temperature, measured at a frequency ofν = 328GHz, on a reduced field scale (Hres −H0)/H0. Hereµ0H0 = 11.7T is the resonance field of the signal at hightemperature; open circles - powder sample, open squares - c-axis aligned powder sample, solid lines - fits of the resonancefield using Eq. A.4.

the half maximum (FWHM) of the signal ∆H has beentaken (see Fig. 9(a)). As can be seen, with decreasingthe temperature there is only a weak broadening of thesignal down to a characteristic temperature Tif ∼ 150K(if denotes an internal field at the Gd ion, see the discus-sion below) where substantial inhomogeneous broadeningbegins to develop continuously down to the lowest mea-sured temperature of 4K. Concomitantly with the inho-mogeneous broadening there is a noticeable shift of theminimum of the absorption to lower fields, as shown onFig. 6 (open circles) on a reduced field scale (H−H0)/H0.Here µ0H0 = 11.7T is the resonance field of the signalat high temperature. The spectral shape at low tem-

perature (Fig. 5(a)) appears to be very similar to theshape of the ESR signal from a powder sample with ananisotropic g-factor. However, since the Gd3+ ion is apure S-state ion, it should have an isotropic g-factor veryclose to 2. Hence one can conjecture that the shape of theESR signal from the GdOFeAs sample is caused by theanisotropy of the internal field at the Gd site arising fromthe AFM-ordered Fe moments. In such an anisotropicpowder situation most of the spectral weight is comingfrom the grains whose c-axes are oriented perpendicularto the direction of the external field Hext. One shouldnote here that in the case of in-plane anisotropy there willbe an additional averaging effect due to the distributionof resonance fields of grains whose (ab)-planes are paralleltoHext. Therefore we assume here that the low-field min-imum of the absorption corresponds to the mean valueof the resonance field of the Gd3+ ESR response (Hres⊥)in the case of the external field applied perpendicular tothe c-axis. Correspondingly, the high-field shoulder ofthe spectra arises from grains whose c-axes make smallangles with respect to Hext.

In order to probe the Gd3+ response for the geometryHext ‖ c we have performed ESR measurements on the c-axis oriented GdOFeAs powder sample. Though, accord-ing to the x-ray diffraction analysis, the alignment of thepowder particles was not perfect, a substantial c-axis tex-turing of the sample has been achieved (Fig. 1). Similarlyto the non-oriented powder sample, the c-axis orientedsample at temperatures above ∼ 150K exhibits only asmall broadening of the ESR spectrum with decreasingtemperature (Fig. 5(b)). Below this temperature the sig-nal experiences strong inhomogeneous broadening wheremost of the spectral weight is shifted to higher fields,which is opposite to the finding in the non-oriented pow-der sample. In the oriented sample most of the spectralweight and, consequently, the minimum of the absorp-tion should correspond to the resonance field of the Gd3+

ESR response (Hres‖) in the geometry Hext ‖ c whereas anon-ideal powder alignment yields the low-field shoulderof the ESR signal.

From our measurements on non-oriented and c-axisoriented powder samples one can, therefore, extract thetemperature dependencies of Hres in two configurations,i.e., for fields aligned along the c-axis (Hres‖) and inthe (ab)-plane (Hres⊥), as summarized in Fig. 6. As canbe seen, the changes of both resonance fields Hres⊥ andHres‖ start upon cooling at Tif

<∼ 150K and the shifts

have opposite directions.

The qualitative difference in the high-field/frequencyand low-field/frequency measurements leads to the con-clusion that the shift of the resonance field and the in-homogeneous broadening of the spectra measured at afrequency of 328GHz is a field-induced effect. To in-vestigate it, we have measured the frequency ν versusmagnetic field Hres dependence of the GdOFeAs pow-der and the c-axis aligned samples, respectively, both atT = 280K and T = 4K (Fig. 7). At T = 280K thespectrum at all studied frequencies and fields consists of

6

0 2 4 6 8 10 12 140

100

200

300

400

500

Hres /Hres||

Hres Hres||

Hres Hres||

Hres Hres||

Freq

uenc

y (G

Hz)

0H (T)

T = 4 K

83GHz

166GHz

249GHzg=2.005

328GHz

FIG. 7: (Color online) Frequency dependence of the ESR sig-nal from GdOFeAs non-oriented powder and c-axis orientedpowder samples measured at T = 4K; data points correspondto the position of the minimum of absorption of the spectra atdifferent frequencies, open circles - non-oriented powder sam-ple, open squares - c-axis oriented powder sample, dashedlines - guides for the eyes, solid line - the frequency depen-dence of the position of the high-T Gd ESR line measuredat T = 280K; the lineshape of the spectra is shown as well,thin solid line - non-oriented powder, thick solid line - c-axisoriented powder.

a single lorentzian line with the same linewidth value. Alinear ν(Hres) dependence has been revealed yielding ag-factor g = hν/(µBHres) equal to 2.005 (straight solidline on Fig. 7). The spectra at T = 4K for the non-oriented powder (thin line) and c-axis oriented powder(thick line) samples at different frequencies together withthe frequency dependence of the resonance fields Hres⊥

and Hres‖ are shown on Fig. 7 as well. This measurementreveals that the difference between Hres⊥ and Hres‖ in-creases linearly with increasing the frequency and thefield strength.

C. GdOFeAs, discussion

Since the inhomogeneous broadening and the shift ofthe Gd ESR signal set in close to the SDW ordering tem-perature, one can associate them with the interaction ofthe Fe ordered moments with the Gd spins. The shiftsof Hres⊥ and Hres‖ from a common high-temperatureparamagnetic value H0 can hence be related to the oc-currence of an internal magnetic field (Hint⊥ and Hint‖)at the Gd site due to the formation of the static SDW inthe FeAs layer. Based on the dependence shown on Fig. 7we can conclude that the strength of these internal fieldsdepends on the strength of the applied magnetic field.When the FeAs planes are in the paramagnetic

state, at temperatures above TSDW = 128K, the ex-change/dipolar field at the Gd position is negligible as theapplied magnetic field cannot effectively polarize small

paramagnetic moments at elevated temperatures. Attemperatures below TSDW = 128K the Fe moments or-der statically in the (ab)-plane. The AFM structure ofthe magnetic order, which is similar for all R-based 1111-type pnictides can be found in Ref. 15. Due to symmetryreasons, without an applied magnetic field the internalfield at the Gd site can be only of dipolar nature and inthis case according to an estimate it should not exceed∼ 0.03T. We suggest that application of an external mag-netic field induces tilting of the Fe spins and hence createsan uncompensated magnetic moment (mFe) in the direc-tion of the external field39. This moment can interactwith the Gd3+ spins which would effectively lead to theoccurrence of an additional internal field. If Hext ⊥ c,then the Fe spins tilt in the ab plane as it is schemat-ically shown on Fig. 8(a,b) for the configuration whereHext makes an angle of 90◦ with the AFM ordered Fespins. The shift of the ESR line Hres⊥, measured at328GHz, yields an estimate of the internal field (Hint⊥)of about ≈ 0.4T parallel both to Hext and to the un-compensated moment mFe in the FeAs plane. IfHext ‖ c,then the Fe moments tilt out of the plane (Fig. 8(c,d)). Inthis case the shift of the ESR line, measured at 328GHz,yields an estimate of the internal field (Hint‖) of about≈ 0.65T antiparallel both to the Hext and to the uncom-pensated moment mFe. One should note here that anestimate of the dipolar field produced by the FeAs lay-ers at the Gd site yields a value which does not exceed≈ 0.05T even for full out-of-plane canting. This field isone order of magnitude smaller than the experimentallyobserved value which clearly implies the presence of anappreciable exchange interaction between Gd and cantedFe moments. The dependence of the sign of the internalfield on the direction of the applied magnetic field sug-gests that the sign of the exchange interaction with Gdspins is different for different directions of the uncompen-sated Fe moments, i.e., ferromagnetic for the in-plane andantiferromagnetic for the out-of-plane directions. Thissurprisingly strong anisotropy of the exchange might berelated with the multiband electronic structure of ironpnictides which might give rise to different pathways forinteractions between the Gd 4f orbitals and the in-planexy and out-of-plane xz and yz Fe 3d bands. Note that,in zero magnetic field and hence without Fe spin canting(mFe = 0), the exchange interaction between Fe and Gdmoments of an arbitrary sign is geometrically frustrated(see Fig. 8). The application of a field which tilts the Femoments thus removes this frustration.

Considering the exchange interaction, one might sup-pose that the temperature dependence of the internalfield at the Gd site should follow the behavior of theSDW order parameter in 1111 compounds6,15,29, whichincreases fast within ∼ 30K starting at TSDW and thenstays almost constant with further decreasing the tem-perature. Here, the internal field acting on the Gd3+

moments arises at a temperature Tif ≈ 150K which is∼ 20K higher than TSDW = 128K and keeps increas-ing upon cooling till the lowest measured temperature

7

FIG. 8: (Color online) Canting of the Fe moments due to theapplied magnetic field Hext; (a),(b) Hext⊥ c (angle betweenHext and Fe AFM ordered moments is 90◦); (c),(d) Hext‖c; arrows on the Fe site depict magnetic moments, whereasarrows on the Gd sites represent the induced internal field.

(see Fig. 6). When approaching the temperature of theSDW transition from high temperatures, the appearanceof the internal field well above TSDW can be explainedby growing quasi-static correlations between the Fe mo-ments seen in the time window of the high-frequencyESR. The development of the internal field below TSDW

is found to be similar to that of some other systems whereparamagnetic ions are coupled to magnetically-orderedmoments of another type30,31. To explain this evolutionof the internal field we use a simple model based on amean-field approximation25,32 (for details see Appendix).According to this model the internal field depends on theapplied magnetic field and on temperature as following(see Eq. A.5):

Hint ∼ ±A(J, α, CGd)Hext

(T −Θ)

Here A(J, α, CGd) is a parameter determined by the Gd-Fe exchange coupling energy J , by the susceptibility ofthe ordered Fe moments to the applied field α and bythe Gd Curie constant CGd. As can be seen, this depen-dence qualitatively obeys a Curie-Weiss law at a givenapplied field which agrees well with the measured data.The model enables to fit the experimental data pointsreasonably well (see Appendix and Fig. 6). Firstly, thefit yields an estimate of the energy of the exchange cou-pling between Gd and Fe spins which is in the rangeof |J | ≈ 15 − 20K. Secondly, it shows that the magni-tude of the uncompensated Fe moment depends on theGd magnetization which suggests that the Gd subsystemadditionally tilts or polarizes the SDW.

0 50 100 150 200 2500.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

x = 0.14(c)

Tif(x=0.14)

0H

(T)

Temperature (K)

x = 0.07(b)

Tif(x=0.07)

0H

(T)

x = 0(a) TSDW=128 K

0H

(T)

Tif(x=0)

FIG. 9: (Color online) The full width at the half maximum(FWHM) ∆H of the ESR lines, measured at a frequency ofν = 348GHz (x = 0.07) and ν = 328GHz (x = 0, 0.14), as afunction of temperature for the non-oriented GdO1−xFxFeAssamples; (a) x = 0; (b) x = 0.07; (c) x = 0.14.

To summarize this part, the ESR results on GdOFeAssamples show that the Gd subsystem is exchange-coupledto the magnetic FeAs planes. On approaching the AFMSDW transition from above, the growing correlations be-tween the Fe moments yield a shift of the Gd ESR line.At lower temperature, depending on the angle betweenHext and the c-axis of the sample, the signal shifts tohigher or to lower fields due to the uncompensated ex-change field which is transferred to the Gd site from theFe moments canted in an external magnetic field. Sincethe full width at the half maximum ∆H is proportionalto the difference between resonance fields Hres⊥−Hres‖,then the width of the ESR signal ∆H of the non-orientedpowder sample can be taken as a measure of this ex-change field (Fig. 9a).

D. GdO1−xFxFeAs (x = 0.07, 0.14)

The influence of the fluorine doping on the Gd ESR hasbeen studied on two powder samples of GdO1−xFxFeAswith 7% and 14% of fluorine. On Fig. 10(a),(b) the evo-lution of the respective Gd3+ ESR spectra is shown ona reduced field scale. Similarly to the undoped sample,at high temperature the ESR spectrum for both doped

8

-15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15

100 K

0H0 = 12.4 T

280 KA

bsor

ptio

n (a

rb.u

.)

(H-H0)/H0 (%)

240 K

180 K

130 K

4 K

40 K

x = 0.07

(a)

Abs

orpt

ion

(arb

.u.)

0H0 = 11.7 T

(H-H0)/H0 (%)

x = 0.14

280 K

240 K

180 K150 K

4 K

40 K

100 K

(b)

FIG. 10: (Color online) Temperature evolution of the high-frequency/field ESR spectra of GdO1−xFxFeAs powder sam-ples measured at a frequency of ν = 348GHz (x = 0.07)and ν = 328GHz (x = 0.14), shown on a reduced field scale(H −H0)/H0. Here H0 is the resonance field of the signal athigh temperature; (a) x = 0.07, µ0H0 = 12.4T; (b) x = 0.14,µ0H0 = 11.7 T.

0 50 100 150 200 250 300

-2

-1

0

1

Hres , x=0.07 Hres , x=0.14 Hres , x = 0 (fit)

(Hre

s-H0)/H

0 (%)

Temperature (K)

Tif(x=0.07)

Tif(x=0.14)

FIG. 11: (Color online) The shift of the minimum of absorp-tion of the spectra with temperature, measured at a frequencyν = 348GHz(x = 0.07)/328 GHz(x = 0.14), on a reduced fieldscale (Hres −H0)/H0. Here µ0H0 = 12.4T/11.7 T is the res-onance field of the signal at high temperature, open hexagons- x = 0.07, open triangles - x = 0.14, solid line - fit of theresonance field in the parent GdOFeAs sample.

samples consists of a single lorentzian-shaped line withg = 2.005. While lowering the temperature, the line re-mains almost unchanged until a characteristic tempera-ture Tif is reached. This temperature corresponds to theonset of an additional inhomogeneous contribution to thewidth of the ESR signal ∆H . This inhomogeneous con-tribution is shown by the shaded area on Fig. 9(b,c). Thetemperature Tif clearly depends on the fluorine dopinglevel. In the case of the 7% F-doped sample a noticeablebroadening of the line starts at Tif (x = 0.07) ∼ 125K,whereas for the 14% doped sample it starts at a lowertemperature Tif (x = 0.14) ∼ 100K. For both doped

2 4 6 8 10 12 140

100

200

300

400

500 x=0, 0.07, 0.14 at T=280 K x=0 at T=4 K

Freq

uenc

y (G

Hz)

0H (T)

328 GHz

348 GHz

249 GHz

249 GHz

166 GHz

166 GHz

x=0.07

x=0.14

83 GHz

FIG. 12: (Color online) Frequency dependence of the ESRsignal from GdO1−xFxFeAs powder samples measured at atemperature of T = 4K, in SC state; data points correspondto the position of the minimum of absorption of the spectra atdifferent frequencies, open hexagons - x = 0.07, open triangles- x = 0.14, dashed line - guide for the eyes, solid line - thefrequency dependence of the position of the high-T Gd ESRline measured at T = 280K for both F-doped samples; thelineshape of the spectra is shown as well, thin solid line -x = 0.07, thick solid line - x = 0.14.

samples there is a shift of the minimum of the absorp-tion (Hres⊥) to lower magnetic fields below Tif (Fig. 11).Qualitatively this shift is similar to that of the undopedsample (solid line on Fig. 11), but it is less pronounced.In addition, the inhomogeneous broadening and the shiftof Hres⊥ to lower magnetic fields exhibit a magnetic fielddependence similar to that of the undoped GdOFeAssample (Fig. 12).

E. Superconducting GdO1−xFxFeAs, discussion

The remarkable similarities of the Gd ESR behaviorbetween the fluorine-doped samples and the undoped onestrongly suggest that, even in the superconducting sam-ples where the phase transition to the AFM SDW state isnot observed in the thermodynamics and transport prop-erties, quasi static (on the time scale of the ESR mea-surement) magnetic correlations in the FeAs planes arepresent below the characteristic temperature Tif . Suchcorrelations may explain the peculiar features in the re-sistivity data shown in Sec. III.

The unified phase diagram for the iron pnictides, es-pecially for the 1111 materials, is not fully establishedso far since the issue of coexistence of superconductivityand magnetism remains controversial. The nonmagneticrare earth based LaO1−xFxFeAs material exhibits no ev-idence for the presence of a static magnetic order for anysuperconducting composition, but rather reveals SDW-like spin fluctuations seen in the transport24 and inelastic

9

neutron scattering experiments33,34. On the other hand,the magnetic rare earth based systems (Sm, Nd, Ce)studied so far demonstrate coexistence of superconduc-tivity and static magnetism at least in the underdopedregion3,11. Our high-field ESR results show that thereis yet another 1111 system comprising a strongly mag-netic rare earth (Gd) subsystem where the coexistenceof quasi-static magnetism and superconductivity is stillvisible in large doping range. Here, the increase of thefluorine content and correspondingly the rise of Tc leadsto the suppression of magnetic correlations indicating apossible interplay between these two states. All this sug-gests that the coexistence and possible interplay of thestatic or quasi-static magnetism and superconductivitymay be a generic property of 1111-type compounds. Inthis regard, a remaining question yet to be answered isthe extent to which the R-Fe magnetic interaction in-fluences the magnetic correlations in the FeAs planes.Moreover, the issue of the coexistence of magnetism andsuperconductivity is frequently discussed in the literaturefor other pnictide families as well. Extending ESR exper-iments to compounds of these families would be of greatinterest.

V. CONCLUSION

Our HF-ESR study of polycrystalline samples of theGdO1−xFxFeAs superconductor reveals a magnetic cou-pling between the Gd subsystem and the FeAs layers.This coupling, most probably of the anisotropic exchangetype, is visible in the Gd ESR response in the undopedGdOFeAs in the SDW state, in form of a field-induced in-homogeneous broadening and shift of the ESR spectrum.This effect is caused by the interaction of the Gd spinswith the uncompensated Fe moments due to the cantingof the Fe moments in magnetic field. Furthermore, thedata suggest that the Gd moments additionally tilt theordered Fe moments. Surprisingly, the broadening andthe shift of the spectrum are present also in the dopedsuperconducting samples where there is no evidence oflong range magnetic order. This points to the presence ofshort range, static on the ESR time scale, magnetic cor-relations. This may be relevant to the interplay of mag-netism and superconductivity in these materials, whereon doping with fluorine there is a simultaneous increaseof the superconducting critical temperature and suppres-sion of the magnetic correlations. The possible relevanceof the exchange interaction between the magnetic rare-earth subsystem and the FeAs planes to the propertiesof this novel class of superconductors remains to be elu-cidated.

Acknowledgments

We thank S. Muller-Litvanyi, R. Muller, J. Werner,and S. Pichl for assistance in the sample preparation.

We thank U. Stockert and J. E. Hamann-Borrero forassistance in the sample characterization. The workat the IFW Dresden was supported by the DeutscheForschungsgemeinschaft through Grants No. BE1749/12and BE1749/13, the Research Unit FOR538 (Grant No.BU887/4) and the Priority Programme SPP1458 (GrantNo. GR3330/2). Work at the ETH was supported by theSwiss National Science Foundation through the NationalCenter of Competence in Research MaNEP (Materialswith Novel Electronic Properties).

Appendix

Here we provide a calculation of the internal field onthe Gd ion using a simple model based on the mean-fieldapproximation25,32. To simplify the calculations we as-sume that a magnetic field and magnetic moment vectorsare collinear. In this model, the internal field at the Gdsite Hint is proportional to the magnitude of the uncom-pensated Fe moment mFe with a coefficient λ:

Hint = ±λmFe (A.1)

Hereafter the sign depends on the type of interaction, be-ing ”+” for ferromagnetic and ”−” for antiferromagneticexchange. Neglecting the weak dipolar contribution, themagnetization normalized to the single ion mGd of theGd subsystem is proportional to the sum of the appliedmagnetic field Hext and internal field Hint:

mGd = χGd(T ) (Hext ±Hint), (A.2)

where χGd(T ) = CGd/(T − Θ) is the Gd magnetic sus-ceptibility, CGd is the Gd Curie constant, Θ is the GdCurie temperature. Due to the exchange interaction, theuncompensated magnetic moment mFe is proportionalnot only to the applied field but also to the internal fieldcreated by the Gd moments:

mFe = α(Hext + λmGd) (A.3)

Here α is the susceptibility of the ordered Fe moments tothe external magnetic field. Using Eq. A.1, Eq. A.2 andEq. A.3 one can obtain the equation for the internal fieldHint at the Gd site:

Hint = ±λ2αCGd + λα(T −Θ)

(T −Θ)− λ2αCGd

Hext (A.4)

Eq. A.4 enables to fit the measured temperature de-pendence of the internal field (see Fig. 6). The resonancefield of the Gd is determined by the applied field and bythe internal field H0 = Hext ± Hint. At high tempera-tures when there is no internal field at the Gd site at anystrength of the applied magnetic field (Hint = 0) one canmeasure the resonance field H0 = 11.7T (for measure-ment frequency ν = 328GHz). Assuming that the reso-nance field of the Gd ions H0 = 11.7T stays constant atall measured temperatures one obtains the expression for

10

the applied field Hext = H0∓Hint (H0 = 11.7T). The fitfor two measurement configurations (Hres⊥ and Hres‖)is shown on Fig. 6 by solid lines. The parameters CGd, Θand α can be taken from different experiments. The GdCurie constant CGd and Curie temperature Θ are knownfrom the susceptibility data of GdOFeAs samples (seeSec. III). As it is shown in Ref. 19 the bulk Fe suscep-tibility of LaOFeAs samples is determined by the spinsusceptibility. Therefore the parameter α can be esti-mated from this measurement yielding a value of ∼ 10−4

ergG2 mol

19,35–37. The λ value resulting from the fit is equal

to ∼ 19.7 G2 molerg

for Hres⊥ and ∼ 25.3 G2 molerg

for Hres‖.

According to the mean field theory32, these values yield

an estimate of the exchange interaction energy J for twoconfigurations amounting to |J | ∼ 15K for Hres⊥ and|J | ∼ 19K for Hres‖. In addition, Eq. A.1 enables to cal-culate the uncompensated moment mFe. Its value growswith decreasing the temperature and increasing the Gdsusceptibility until it reaches∼ 0.03µB at the lowest mea-sured temperature.Since α is very small compared to CGd and λ, Eq. A.4

can be simplified to the form:

Hint ∼ ±λ2αCGd

(T −Θ)Hext (A.5)

1 Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J.Am. Chem. Soc. 130, 32963297 (2008).

2 X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, and D. F.Fang, Nature 453, 761 (2008).

3 A. J. Drew, C. Niedermayer, P. J. Baker, F. L. Pratt, S. J.Blundell, T. Lancaster, R. H. Liu, G. Wu, X. H. Chen,I. Watanabe, et al., Nat. Mater. 8, 310 (2009).

4 C. Hess, A. Kondrat, A. Narduzzo, J. E. Hamann-Borrero,R. Klingeler, J. Werner, G. Behr, and B. Buchner, EPL(Europhysics Letters) 87, 17005 (2009).

5 J. Zhao, Q. Huang, C. de la Cruz, S. Li, J. W. Lynn,Y. Chen, M. A. Green, G. F. Chen, G. Li, Z. Li, et al.,Nat. Mater. 7, 953 (2008).

6 H. Luetkens, H.-H. Klauss, M. Kraken, F. J. Litterst,T. Dellmann, R. Klingeler, C. Hess, R. Khasanov, A. Am-ato, C. Baines, et al., Nat. Mater. 8, 305 (2009).

7 Y. Laplace, J. Bobroff, F. Rullier-Albenque, D. Colson,and A. Forget, Phys. Rev. B 80, 140501 (2009).

8 M.-H. Julien, H. Mayaffre, M. Horvatic, C. Berthier, X. D.Zhang, W. Wu, G. F. Chen, N. L. Wang, and J. L. Luo,EPL (Europhysics Letters) 87, 37001 (5pp) (2009).

9 S. Sanna, R. De Renzi, T. Shiroka, G. Lamura, G. Prando,P. Carretta, M. Putti, A. Martinelli, M. R. Cimberle,M. Tropeano, et al., Phys. Rev. B 82, 060508(R) (2010).

10 R. R. Urbano, E. L. Green, W. G. Moulton, A. P. Reyes,P. L. Kuhns, E. M. Bittar, C. Adriano, T. M. Garitezi,L. Bufaical, and P. G. Pagliuso, Phys. Rev. Lett. 105,107001 (2010).

11 J. P. Carlo, Y. J. Uemura, T. Goko, G. J. MacDougall,J. A. Rodriguez, W. Yu, G. M. Luke, P. Dai, N. Shannon,S. Miyasaka, et al., Phys. Rev. Lett. 102, 087001 (2009).

12 C. Bernhard, A. J. Drew, L. Schulz, V. K. Malik,M. Roessle, C. Niedermayer, T. Wolf, G. D. Varma, G. Mu,H. H. Wen, et al., New Journal of Physics 11, 055050(2009), 0902.0859.

13 Y. Nakai, K. Ishida, Y. Kamihara, M. Hirano, andH. Hosono, J. Phys. Soc. Jpn. 77, 073701 (2008).

14 G. Prando, P. Carretta, A. Rigamonti, S. Sanna, A. Palen-zona, M. Putti, and M. Tropeano, Phys. Rev. B 81, 100508(2010).

15 H. Maeter, H. Luetkens, Y. G. Pashkevich, A. Kwadrin,R. Khasanov, A. Amato, A. A. Gusev, K. V. Lamonova,D. A. Chervinskii, R. Klingeler, et al., Phys. Rev. B 80,094524 (2009).

16 C. Golze, A. Alfonsov, R. Klingeler, B. Buchner,

V. Kataev, C. Mennerich, H.-H. Klauss, M. Goiran, J.-M.Broto, H. Rakoto, et al., Phys. Rev. B 73, 224403 (2006).

17 A. Kondrat, J. E. Hamann-Borrero, N. Leps, M. Kosmala,O. Schumann, A. Kohler, J. Werner, G. Behr, M. Braden,R. Klingeler, et al., Eur. Phys. J. B 70 (4), 461-468 (2009).

18 N. D. Zhigadlo, S. Katrych, Z. Bukowski, S. Weyeneth,R. Puzniak, and J. Karpinski, Journal of Physics: Con-densed Matter 20, 342202 (2008).

19 R. Klingeler, N. Leps, I. Hellmann, A. Popa, U. Stockert,C. Hess, V. Kataev, H. Grafe, F. Hammerath, G. Lang,et al., Phys. Rev. B 81, 024506 (2010).

20 H.-H. Klauss, H. Luetkens, R. Klingeler, C. Hess, F. J.Litterst, M. Kraken, M. M. Korshunov, I. Eremin, S.-L. Drechsler, R. Khasanov, et al., Phys. Rev. Lett. 101,077005 (2008).

21 N. D. Zhigadlo, S. Katrych, S. Weyeneth, R. Puzniak,P. J. W. Moll, Z. Bukowski, J. Karpinski, H. Keller, andB. Batlogg, Phys. Rev. B 82, 064517 (2010).

22 M. Rotter, M. Pangerl, M. Tegel, and D. Johrendt, Angew.Chem. 47, 7949 (2008), 0807.4096.

23 A. Koitzsch, D. S. Inosov, D. V. Evtushinsky, V. B.Zabolotnyy, A. A. Kordyuk, A. Kondrat, C. Hess,M. Knupfer, B. Buchner, G. L. Sun, et al., Phys. Rev.Lett. 102, 167001 (2009).

24 A. Kondrat, G. Behr, B. Buchner, and C. Hess (2010),1006.0715.

25 S. E. Barnes, Adv. Phys. 30, 801 (1981).26 A. Abragam and B. Bleaney, Electron Paramagnetic Res-

onance of Transition Ions (1970).27 E. Dengler, J. Deisenhofer, H.-A. Krug von Nidda,

S. Khim, J. S. Kim, K. H. Kim, F. Casper, C. Felser, andA. Loidl, Phys. Rev. B 81, 024406 (2010).

28 G. F. Goya, R. C. Mercader, L. B. Stere, R. D. Sanchez,M. T. Causa, and M. Tovar, J. Phys.: Condens. Matter 8,4529 (1996).

29 C. de la Cruz, Q. Huang, J. W. Lynn, J. Li, W. Ratcliff II,J. L. Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L.Wang, et al., Nature 453, 899 (2008).

30 A. Fainstein, A. Butera, and M. Tovar, Phys. Rev. B 50,16708 (1994).

31 S. B. Oseroff, D. Rao, F. Wright, D. C. Vier, S. Schultz,J. D. Thompson, Z. Fisk, S.-W. Cheong, M. F. Hundley,and M. Tovar, Phys. Rev. B 41, 1934 (1990).

32 C. Kittel, Introduction to Solid State Physics (WILEY,2005).

11

33 S. Shamoto, M. Ishikado, S. Wakimoto, K. Kodama,R. Kajimoto, M. Arai, T. Fukuda, H. Nakamura,M. Machida, and H. Eisaki, Physica C: Superconductiv-ity 470, S284 (2009).

34 S. Wakimoto, K. Kodama, M. Ishikado, M. Matsuda,R. Kajimoto, M. Arai, K. Kakurai, F. Esaka, A. Iyo,H. Kito, et al., J. Phys. Soc. Jpn. 79, 074715 (2010).

35 M. A. McGuire, A. D. Christianson, A. S. Sefat, B. C.Sales, M. D. Lumsden, R. Jin, E. A. Payzant, D. Mandrus,Y. Luan, V. Keppens, et al., Phys. Rev. B 78, 094517(2008).

36 T. Nomura, S. W. Kim, Y. Kamihara, M. Hirano, P. V.Sushko, K. Kato, M. Takata, A. L. Shluger, and H. Hosono,

Supercond. Sci. Technol. 21, 125028 (2008), 0804.3569.37 Y. Kohama, Y. Kamihara, M. Hirano, H. Kawaji,

T. Atake, and H. Hosono, Phys. Rev. B 78, 020512 (2008).38 Note here that a dysonian ESR line shape, which is typical

for metallic samples in a cavity, is not observed here dueto the fine grinding of the sample and further mixing withepoxy. This procedure reduces the microwave dispersionwhich is the origin of the dysonian line shape.

39 For simplicity which does not affect the conclusions of thefollowing qualitative discussion, we assume hereafter thatthe uncompensated magnetic moments and thereby cre-ated internal fields are collinear with the applied magneticfield.