Superconductivity induced by phosphorus doping and its coexistence with ferromagnetism in EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$

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by  Zhi Ren

Superconductivity induced by phosphorus doping and its coexistence with
ferromagnetism in EuFe2(As0.7P0.3)2
Zhi Ren,1 Qian Tao,1 Shuai Jiang,1 Chunmu Feng,2 Cao Wang,1 Jianhui Dai,1 Guanghan Cao1∗ and Zhu’an Xu†1
1Department of Physics, Zhejiang University, Hangzhou 310027, China
2Test and Analysis Center, Zhejiang University, Hangzhou 310027, China
(Dated: January 20, 2009)
We have studied EuFe2(As0.7P0.3)2 by the measurements of x-ray diffraction, electrical resistiv-
9
ity,thermopower,magneticsusceptibility,magnetoresistance andspecificheat. Partial substitution
0
of As with P results in the shrinkage of lattice, which generates chemical pressure to the system.
0
It is found that EuFe2(As0.7P0.3)2 undergoes a superconducting transition at 26 K, followed by
2 ferromagnetic ordering of Eu2+ momentsat 20 K. This findingis thefirst observation of supercon-

n ductivitystabilized byinternal chemical pressure, and suppliesa rare exampleshowing coexistence
a of superconductivity and ferromagnetism in the ferro-arsenide family.
J

9 PACSnumbers: 74.70.Dd;74.25.Ha;74.62.Dh;74.10.+v
1

] Recently, high-temperature superconductivity has resistivity could be achieved, which was ascribed to the
n been discovered in a family of materials containing AFM ordering of the Eu2+ moments[17].
o
c FeAs layers.[1, 2, 3, 4] The superconductivity is in- While hetero-valent substitution generally induces
- duced by doping charge carriers into a parent com- chargecarriers,iso-valentsubstitutionmaysupplychem-
r
p pound, characterized by an antiferromagnetic (AFM) ical pressure. The latter substitution is of particular in-
u spin-density-wave (SDW) transition associated with the terest in EuFe2As2 when As is partially replaced by P,
s FeAs layers.[5, 6] Electron doping has been realized by assuggestedtheoreticallyinordertosearchforthemag-
.
t the partial substitutions of F-for-O,[1] vacancy-for-O,[3] neticquantumcriticalitywithoutchangingthenumberof
a
m Th-for-Ln,[4]Co/Ni-for-Fe,[7,8,9,10]. Examplesofhole Fe 3d-electrons[23]. In this Letter, we demonstrate bulk
- doping include the partial substitutions of Sr-for-Ln[11] superconductivity at26K inEuFe2(As0.7P0.3)2. For the
d and K-for-Ba/Sr/Eu[12, 13, 14]. All the chemical dop- first time, superconductivity has been realized through
n ing suppresses the long-range SDW order, eventually re- the doping at the As site in the iron arsenide system.
o
sulting in the emergence of superconductivity. Up to Strikingly, we observe coexistence of ferromagnetic or-
c
[ now, no superconductivity has been reported through dering of Eu2+ moments with superconductivity below
dopingattheAssite. Apartfromcarrierdoping,applica- 20 K.
3
v tion of hydrostatic pressure is also capable of stabilizing PolycrystallinesamplesofEuFe2(As0.7P0.3)2weresyn-
0 superconductivity.[15, 16, 17] thesized by solid state reaction with EuAs, Fe2As and
9 Fe2P.EuAswaspresynthesizedbyreactingEugrainsand
Among the parent compounds in ferro-arsenide fam-
3 As powders at 873 K for 10 h, then 1073 K for 10 h and
.2 milye,nEtsuFoef2EAus22+exihoinbsitosrpdeecrualinatrifbererhoamviaogrnbeetcicaaulslyetahtermeloa-- finally 1223 K for another 10 h. Fe2As was prepared by
1 reactingFepowersandAspowdersat873Kfor10hthen
1 tively high temperature of 20 K.[18, 19, 20] Magnetore-
1173Kfor 0.5h. Fe2P waspresynthesizedbyheating Fe
8 sistance measurements on EuFe2As2 crystals[21] suggest powdersandPpowdersveryslowlyto873Kandholding
0 a strong coupling between the magnetism of Eu2+ ions
: for10h. PowdersofEuAs,Fe2AsandFe2Pwereweighed
v and conduction electrons, which may affect or even de-
accordingtothestoichiometricratio,groundandpressed
i stroy superconductivity. For example, though Ni dop-
X intopelletsinanargon-filledglove-box. Thepelletswere
r ing in BaFe2As2 leads to superconductivity up to 21 sealed in evacuated quartz tubes and annealed at 1273
a K,[10]ferromagnetismratherthansuperconductivitywas K for 20h then cooledslowly to roomtemperature. The
found in EuFe2As2 by a systematic Ni doping[22]. An- phase purity of the samples was investigated by powder
otherrelevantexampleisthatthe superconductingtran-
X-raydiffraction,using a D/Max-rAdiffractometerwith
sitiontemperatureof(Eu,K)Fe2As2[14]is32K,substan- Cu-K radiation and a graphite monochromator. The
tially lower than those of (Ba,K)Fe2As2 (Tc=38 K)[12] XRDαdata were collected with a step-scan mode in the
and (Sr,K)Fe2As2 (Tc=37 K)[13]. Resistivity measure- 2θ range from 10◦ to 120◦. The structural refinement
ment under hydrostatic pressures[17] on EuFe2As2 crys- was performed using the programme RIETAN 2000.[24]
talsshowedaresistivitydropat29.5K.However,nozero
The electrical resistivity was measured on bar-shaped
samples using a standard four-probe method. The ap-
plied current density was 0.5 A/cm2. The measure-
∼
∗Electronicaddress: [email protected] ments of magnetoresistance, specific heat, ac magnetic
†Electronicaddress: [email protected] susceptibility and thermopower were performed on a
2

1.2 m)3
c
1.0 Ω
ounts) 01..80 OCDaibfflsceeurrelvanetceded cm)0.8 ρ-4 (10m12
c Ω 0.6 0
4 0 0.6 Rwp=10.5%, Rp=7.7% m 14 16T 1 (8K)20 22
y (1 0.4 a=0.3889(1) nm, c=1.1831(3) nm ρ (0.4
sit 0.2
en 0.2 (a)
nt 0.0
I 0
0.0
) -5
-0.2 K
20 40 60 80 100 120 V/
2 (degree) µ -10
(

FIG. 1: (color online). X-ray powder diffraction pattern S-15
at room temperature and the Rietveld refinement profile for
-20
EuFe2(As0.7P0.3)2. (b)
Quantum Design Physical Property Measurement Sys- -1205 b. unit)23 0 .00’ (arb∆χ 4500 1/(χ
toSenymsate(QmPuP(aMMntSPu-M9m)S.D-D5e)sC.igmnaMgnaegtniectpicroPpreorpteierstywMereeamsueraesmureendt u/mol) 68 χ’ (ar0120 25 30 35--00..5205. unit) 300-) (eχ
Figure 1 show the XRD pattern of EuFe2(As0.7P0.3)2 em 4 T ( K) 20 mu
at room temperature, together with the profile of the ( /m
χ
Rietveld refinement using the space group of I4/mmm. 2 10 o
(c) l)
No additional diffraction peak is observed. The refined
0
lattice parameters are a=3.889(1)˚A and c=11.831(3)˚A. 0
0 50 100 15 0 200 250 300
Cthoemap-aarxeids iwsitdhecrtehaosseedobfyt0h.e35u%n,dotpheedc-EaxuiFse2isAss2h[o1r9t]-, T (K)
ened by 1.8% and the cell volume shrinks by 3.2% for FIG. 2: (color online). Temperature dependence of resistiv-
ity (a), thermopower (b), and magnetic susceptibility (c) for
EuFe2(As0.7P0.3)2. These results suggest that the iso-
valent substitution of As with P indeed generates chem- the EuFe2(As0.7P0.3)2 sample. The inset of (a) shows an ex-
panded plot. The inset of (c) shows the real part of ac sus-
ical pressure to the system. In addition, the As(P) posi- ceptibility χ′ (left axis) for the same sample. A diamagnetic
tion z decreases, indicating that the As(P) atoms move signal (right axis) is obtained after subtraction of paramag-
towardtheFeplanes. Asaconsequence,theFe-As(P)-Fe netic contribution of Eu2+ moments.
angle increases from 110.15◦ to 111.48◦.
Figure 2(a) shows the temperature dependence of re-
mopower (S). The thermopower in the whole temper-
sistivity (ρ) for EuFe2(As0.7P0.3)2 under zero field. The ature range is negative, indicating that the dominant
anomalyassociatedwiththeSDWtransitioninundoped
charge carriers are electron-like. S decreases sharply
EuFe2As2[19] is completely suppressed. The resistivity below 29 K, corresponding to the s|up|erconducting tran-
is linear with temperature down to 90 K and shows
sition. However,the S valuedoesnotdroptozerountil
∼
upward deviation from the linearity at lower tempera- | |
13 K, in relation with the ferromagnetic ordering of
tures. The resistivity ratio of R(300K)/R(30K) is 5.2, ∼the Eu2+ moments.
indicating high quality of the present sample. Below 29
In figure 2(c), we show the temperature depen-
K, the resistivity drops steeply, suggesting a supercon-
dence of field-cooling dc magnetic susceptibility for
ducting transition. The midpoint of the transition is 26
EuFe2(As0.7P0.3)2 under µ0Hdc=0.1 T. The obvious de-
K.OncloserexaminationshownintheinsetofFig. 1(a), viation of linearity in χ−1 above 230 K is probably due
however, a small resistivity peak is observed around 16
to the presence of trace amount of ferromagnetic Fe2P
K, which coincides with the ferromagnetic ordering of
impurity[26]. The χ data between 50 K and 200 K can
Eu2+ moments (to be shown below). This observation
be well described by the modified Curie-Weiss law,
is reminiscent of the reentrant superconducting behav-
ior as observed, for example, in RNi2B2C2 (R=Tm, Er, χ=χ0+ C , (1)
Ho)[25]. T θ
−
Figure2(b)showsthetemperaturedependenceofther- where χ0 denotes the temperature-independent term, C
3

C
0.25 1K)9 14.0C-
m)0.20 (T)H4680 1 K)9200 111 (J/mol C678 Tc 111233...826ph (J/mol K)
(m c00..1105 0216 18 2T0 (2 K2)24 26 001 . T5T T C (J/mol60 1522 24T 2(K6) 28 J/mol K)1250
8 T 23 TT TCurie (C1ph0 01 TT 02. 5T T
0.05 4 T 30 -C 6 T
5
6 T 10 20 30 40
0 T 8 T T ( K)
0.00 0
0 10 20 30 40 0 50 100 150 200
T (K)
T (K)
FIG. 4: (color online). Temperature dependence of specific
FIG.3: (coloronline). Fielddependenceofresistivetransition
heatbefore(solidsymbol)andafter(opensymbol)deduction
for EuFe2(As0.7P0.3)2 sample. The inset shows the upper
critical fields as a function of temperature. oflatticecontributionfortheEuFe2(As0.7P0.3)2 sample. The
solidcurverepresentsthelatticecontributionfittedbytheDe-
bye model. See text for details. The upper left panel shows
the specific heat anomaly around 26 K, which is ascribed to
the Curie-Weiss constant and θ the Weiss temperature.
the superconducting transition. The lower right panel shows
Thefitting yieldsC= 8.1(1)emuK/molandθ=22(1)K. themagnetic specific heat anomaly around 20 K undermag-
·
ThecorrespondingeffectivemomentP =8.0(1)µ per netic fields.
eff B
formula unit, close to the theoretical value of 7.94 µ
B
for a free Eu2+ ion. χ increases steeply with decreasing
temperature below 20 K and becomes gradually satu- EuFe2(As0.7P0.3)2 sample. Two anomalies below 30 K
rated. Field dependence of magnetization gives a sat- are identified. One is a λ-shape peak with the onset at
urated magnetic moment of 6.9(1)µ /f.u., as expected 20K,indicatingasecond-ordertransition. Withincreas-
B
for fully paralleled Eu2+ moments with S=7/2. There- ingmagneticfields, the anomalyshifts tohighertemper-
fore, it is concluded that the moments of Eu2+ order atures and becomes broadened, in accordance with fer-
ferromagnetically in EuFe2(As0.7P0.3)2, in analogy with romagnetic nature of the transition. The other anomaly
EuFe2P2[27]. Due to the proximity of superconducting is much weaker but detectable at 26 K (shown in the
∼
transitionandferromagneticordering,thesuperconduct- upper-left panel of Fig. 4), which coincides with the su-
ing diamagnetic signal is hard to observe unless the ap- perconducting transition.
plied magnetic field is very low. As shown in the inset To analyze the C(T) data further, it is assumed that
of Fig. 2(c), ac magnetic susceptibility measured under the total specific heat consists of the electronic, phonon
µ0Hac=1Oeshowsakinkaround26K.Aftersubtraction and magnetic components. At high temperatures, the
ofthe paramagneticcontributionfromEu2+ moments,a dominant contribution comes from the phonon compo-
cleardiamagnetismisseen,confirmingsuperconductivity nent, which can be well described by the Debye model
in EuFe2(As0.7P0.3)2. with only one adjustable parameter (i. e., Debye tem-
Thetemperaturedependenceofresistivityundermag- peratureΘD). The data fitting above100K givesΘD of
netic fields is shown in figure 3. With increasing mag- 345 K. Since ΘD 1/√M (M the molecular weight), the
∼
netic fields, the resistive transition shifts towards lower ΘD for EuFe2(As0.7P0.3)2 is calculated to be 348 K, in
temperature and becomes broadened, further affirming goodagreementwith the fittedvalue,using the ΘD data
the superconducting transition. The Tc(H), defined as of380KforEuFe2P2[27]. Theupperboundofelectronic
a temperature where the resistivity falls to 50% of the specificheatcoefficientisestimatedtobe10mJ/molK2,
·
normal state value, is plotted as a function of magnetic whichiscomparablewiththoseofotheriron-arsenidesu-
field in the inset of Fig. 3. The µ0Hc2-T diagram shows perconductors. The electronic specific heat contribution
a slight upward curvature, which is probably due to the islessthan2%ofthetotalspecificheatevenatlowtem-
multi-band effect[28]. The initial slope µ0∂Hc2/∂T near peratures,andthusisnottakenintoconsiderationinthe
Tc is-1.18T/K,givinganuppercriticalfieldofµ0Hc2(0) following analysis.
30 T by linear extrapolation. It is also noted that the By subtracting the lattice contribution, the spe-
∼
reentrant superconducting behavior is not obvious un- cific anomaly due to the superconducting transition is
der magnetic field, in contrast with that in RNi2B2C2 more prominent. The jump in specific heat ∆C 300
≈
superconductors[25]. mJ/molK at T , which is of the same order as that in
c
·
Figure 4 shows the specific heat (C) for the Ba0.55K0.45Fe2As2crystals[30]. Meanwhile,themagnetic
4
entropy associated with the ferromagnetic transition is ena. The interplay of superconductivity and ferromag-
16.5J/molK,whichamountsto95%ofRln(2S+1)with netism may bring about exotic properties and provide
S=7/2 for·Eu2+ ions. The thermodynamic properties cluestothesuperconductivitymechanism,whichrenders
indicate thatthe superconductingtransitionandthe fer- EuFe2(As1−xPx)2 worthy of further exploration.
romagnetic ordering are both of bulk nature.
A broad specific-heat hump below 90 K is evident af-
ter deduction of the lattice contribution, indicating exis- Thisworkissupportedbythe NSFofChina,National
tenceofadditionalmagneticcontribution. This anomaly Basic Research Program of China (No. 2007CB925001)
is accompanied with the upward deviation from the lin- and the PCSIRT of the Ministry of Education of China
ear temperature dependence in resistivity as shown in (IRT0754).
Fig. 1(a). Meanwhile, negative magnetoresistance was
observed in the same temperature range, and reaches -
6% at 30 K under µ0H=8 T (data not shown here). All
these features are probably attributed to the interaction
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[10] J. L. Li et al.,New J. Phys. in press.
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hydrostaticpressure. As amatter offact, the onsettem-
[22] Z. Ren et al., arXiv:0810.2595 (2008).
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In summary, we have found superconductivity at 26
[29] Anupam et al.,arXiv:0812.1131 (2008).
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