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Competition of local-moment ferromagnetism andsuperconductivity in Co-substituted EuFe2As2

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Competition of local-moment ferromagnetism and

superconductivity in Co-substituted EuFe2As2

M Nicklas M Kumar1 E Lengyel W Schnelle and A Leithe-JasperMaxPlanck Institute forChemical Physics of Solids Nothnitzer Str 40 01187Dresden Germany

E-mail nicklascpfsmpgde (Michael Nicklas)

Abstract In contrast to SrFe2As2 where only the iron possesses a magnetic moment inEuFe2As2 an additional large local magnetic moment is carried by Eu2+ Like SrFe2As2EuFe2As2 exhibits a spin-density wave transition at high temperatures but in addition themagnetic moments of the Eu2+ order at around 20 K The interplay of pressure-inducedsuperconductivity and the Eu2+ order leads to a behavior which is reminiscent of re-entrantsuperconductivity as it was observed for example in the ternary Chevrel phases or in the rare-earth nickel borocarbides Here we study the delicate interplay of the ordering of the Eu2+

moments and superconductivity in EuFe19Co01As2 where application of external pressuremakes it possible to sensitively tune the ratio of the magnetic (TC) and the superconducting(Tconset) critical temperatures We find that superconductivity disappears once TC gt Tconset

1 IntroductionThe discovery of high-temperature superconductivity in the iron-based superconductors hasstimulated an enormous interest in the study of this new class of materials One peculiar findingis the interplay of the local 4f moments of the Eu2+ ions and superconductivity in EuFe2As2under pressure which is reminiscent of re-entrant superconductivity [1 2] Like the (A=Ca SrBa) members of the AFe2As2 family EuFe2As2 exhibits a spin-density wave (SDW) transitionaround T0 = 190 K related to the Fe2As2 layers but in addition the magnetic moments ofthe localized Eu2+ order at TN = 19 K [3 4] EuFe2As2 has a similar crystallographic andelectronic structure compared to that of SrFe2As2 [3] Therefore SrFe2As2 can be considered anon-f homolog of EuFe2As2 In SrFe2minusxCoxAs2 Co-substitution in the Fe2As2 layer stabilizes asuperconducting (SC) phase (02 x 04) leading to the expectation that Co-substitution inEuFe2As2 does the same which in the meantime has been confirmed experimentally [6 7]

In this paper we present a pressure study on EuFe19Co01As2 by means of electrical-resistivity(ρ) and ac-susceptibility (χac) measurements on polycrystalline samples In EuFe19Co01As2 inour accessible pressure range we can sensitively tune the magnetic ordering temperature TC fromTC lt Tconset to TC gt Tconset allowing us to study the peculiar interplay of 4f magnetism andsuperconductivity

2 Experimental DetailsThe polycrystalline samples of EuFe2minusxCoxAs2 were synthesized by sintering stoichiometricamounts of the precursors EuAs Fe2As and Co2As The use of precursors minimizes the

1 Present address Institute for Solid State Research IFW Dresden D-01171 Dresden Germany

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

Published under licence by IOP Publishing Ltd 1

elemental impurities of As and Fe X-ray diffraction measurements confirmed the BaAl4 typestructure (space group I4mmm) for all the samples Electrical resistance and ac susceptibilitywere measured using an LR700 resistancemutual inductance bridge (Linear Research) Aminiature compensated coil system placed inside the pressure cell was utilized for the χac

experiments Temperatures down to 18 K were reached using a physical property measurementsystem (PPMS Quantum Design) Pressures up to 3 GPa have been achieved in a double-layerpiston-cylinder type pressure cell with silicone fluid as pressure transmitting medium The SCtransition of Pb which served as a pressure gauge remained sharp at all pressures indicatinggood hydrostatic conditions

3 Experimental ResultsFigure 1 shows the resistance normalized to the value at room temperature RR300K(T ) forEuFe2minusxCoxAs2 (x = 0 01 02 and 03) In EuFe2As2 RR300K(T ) exhibits two distinct fea-tures at T0 = 195 K and TN = 125 K corresponding to the SDW instability and the orderingof the localized Eu2+ moments respectively in good agreement with literature [3 4] Upon Cosubstitution T0 decreases rapidly to 103 K for x = 01 At x = 02 no signature of the SDWtransition is visible in the ρ(T ) data anymore TNC exhibits a weak concentration dependenceIn the concentration range between x asymp 01 divide 02 the type of magnetic order possibly changesfrom antiferromagnetic (AFM) to ferromagnetic (FM) In EuFe18Co02As2 a sharp drop of theresistance below asymp 10 K indicates the onset of superconductivity This observation is consistentwith studies on single crystals [6] However there a higher Tc and reentrant-like SC behaviorwas reported For x le 01 and x ge 03 no signature of superconductivity is visible in our dataevidencing a more narrow SC region than in the non-f homolog system SrFe2minusxCoxAs2 [5]

In the following we will focus on the results of our pressure study on EuFe19Co01As2 ρ(T )for different pressures is shown in Fig 2 Upon applying pressure the anomaly indicating theSDW transition at T0 broadens and shifts rapidly to lower temperatures At 161 GPa nofeature related to T0 can be resolved in ρ(T ) anymore suggesting that the SDW order is alreadysuppressed at this pressure At low temperatures displayed in Fig 3a a kink in ρ(T ) at 148 Kand 169 K indicates the ordering of the Eu2+ moments at 0 and 043 GPa respectively [9]At 101 GPa a first signature of a SC transition becomes evident ρ(T ) drops sharply belowTconset = 242 K followed by a small maximum indicating the magnetic ordering of the Eu2+

and a further decrease in ρ(T ) on lowering the temperature This behavior is similar to that

0 100 200 3000

02

04

06

08

10

TNT0

x = 0 01 02 03

T(K)

EuFe2-xCoxAs2

RR

300K

T0

TC

TN

Figure 1 Electrical resistivity normalized tothe value at room temperature (RR300K) forEuFe2minusxCoxAs2

0 100 200 3000

100

200

300

400

500

T0

p (GPa) 043 101 161

EuFe19Co01As2

(cm

)

T (K)

p

p (GPa) 2 241 283

Figure 2 ρ(T ) of EuFe19Co01As2 atdifferent external pressures

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

2

0

100

200

300

0 10 20 300

2

4

6

TC

Tconset

161

283

241

2

101

p (GPa)

043

EuFe19Co01As2

(cm

)

TNa)

TN

b)

Tm

p (GPa) 001 041 117 162 224

TC

ac (a

u)

T (K)

Figure 3 (a) Low-temperature resistivityand (b) ac susceptibility of EuFe19Co01As2at different external pressures For clarity theχac(T ) data at different pressures have beenshifted with respect to each other

0 05 10 15 20 25 300

20

40

60

80

100

120

EuFe19Co01As2

TNC

Tconset

T (K

)

p (GPa)

T0

Tm

p

Figure 4 Temperature-pressure phasediagram of EuFe19Co01As2

observed previously but at higher pressures in EuFe2As2 [1] The drop in ρ(T ) as well as thefollowing maximum are much more pronounced at p = 161 GPa but are hardly visible anymoreat p = 2 GPa At even higher pressures no indication of superconductivity is present in theresistivity data anymore

The results of the χac(T ) experiments are presented in Fig 3b The χac(T ) curve atthe lowest pressure (p = 001 GPa) and the curves at higher pressures (p ge 101 GPa) arequalitatively different While the shape of χac(T ) at p = 001 GPa is reminiscent of an AFMphase transition the shape at p ge 101 GPa is typical for a FM transition A change of themagnetic groundstate from AFM to FM has been found in EuFe2As2 on P-doping on the As-site too [10] P doping corresponds to the application of positive chemical pressure thereforeour finding is not surprising and in agreement with the doping studies On increasing pressurethe anomaly in χac(T ) at TNC shifts to higher temperatures [9] At pressures p ge 101 GPaa second feature appears in χac(T ) below TC at Tm [9] We attribute this kink to a change inthe magnetic structure from FM to AFM No feature in ρ(T ) is visible at Tm A detailed studyof the magnetic properties will be presented elsewhere It is important to note that we do notobserve a clear signature of superconductivity in our χac results

In our study we observe a less robust superconductivity than reported in literature [6 7]This might be related to different sample preparation procedures or to internal strain in thesamples An important influence of internalexternal residual strain on the observed physicalproperties appears also in our experiments After releasing the pressure we still find reentrantSC behavior in ρ(T ) similar to the data at 161 GPa (not shown)

The Tminusp phase diagram in Fig 4 summarizes our results While Tc(p) decreases only slightlywith increasing pressure TNC(p) increases strongly by a factor of nearly 2 from 148 K at ambientpressure to 276 K at 283 GPa the highest pressure of our experiment We find a good agreement

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

3

between TNC(p) obtained from the ρ(T ) and χac(T ) experiments The strong increase of TNC(p)with pressure has not been reported in EuFe2As2 [1] There the magnetic ordering temperatureis nearly independent of pressure The different pressure dependencies of Tconset(p) and TNC(p)in EuFe19Co01As2 lead to a crossing of both temperature lines [Tconset(p) = TC(p)] in the T minuspphase diagram at a pressure plowast slightly higher than 2 GPa Above plowast once TC gt Tc no indicationof any SC transition is visible in the resistivity data Already at 2 GPa where Tc = 219 K andTC = 203 K get rather close The feature associated to the SC transition is considerably reducedcompared to the previous pressure

4 Discussion and SummaryIn summary our results show that in the substitution series EuFe2minusxCoxAs2 the SDW transitiontemperature T0 is suppressed rapidly upon increasing Co concentration while the orderingtemperature of the Eu2+ moments is almost constant For x = 02 we observe the onset ofsuperconductivity in ρ(T ) at asymp 10 K For x lt 02 and x gt 02 no indication of superconductivityis present The strong suppression of T0 as function of the Co concentration and the appearanceof superconductivity in the T minus x phase diagram is consistent with the expectation from thecomparison with the homolog non-f Co-substitution series SrFe2minusxCoxAs2 [5] However theresuperconductivity is stable in a broader concentration range The presence of only a narrow SCregime in the Eu system might be related to the presence of the local moment Eu2+ magnetism

Pressure rapidly suppresses the SDW instability at T0 in EuFe19Co01As2 In the samepressure region where the signature of T0 is lost in ρ(T ) the onset of superconductivity isobserved At low temperatures our results suggest a strong detrimental effect of the Eu2+

magnetism on the superconductivity In an intermediate pressure range (101 GPa le T le 2 GPa)the system is on the verge to a SC state indicated by the drop in the resistivity belowTconset However the formation of the SC state is inhibited by the formation of long rangemagnetic order of the Eu2+ moments A similar result has been reported previously in undopedEuFe2As2 [1] The important difference in the present study on EuFe19Co01As2 is that uponincreasing pressure TNC(p) increases while Tconset(p) decreases and therefore TC(p) becomesgreater than Tconset(p) above a certain pressure plowast Once TC(p) gt Tconset(p) no signature ofsuperconductivity is found anymore We take this as an evidence for the strong detrimentaleffect of the Eu2+ magnetism on the the formation of the SC state As our ac-susceptibilityresults indicate Eu2+ orders ferromagnetically under pressure Thus we speculate that theinternal magnetic fields due to the ferromagnetically ordered Eu2+ moments suppress thesuperconductivity in the iron-arsenide layers

AcknowledgementsWe gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG) for financial supportthrough SPP 1458

References[1] Miclea C F et al 2009 Phys Rev B 79 212509[2] Terashima T et al 2009 J Phys Soc Jpn 78 083701[3] Jeevan H S Hossain Z Kasinathan D Rosner H Geibel C and Gegenwart P 2008 Phys Rev B 78 052502[4] Ren Z Zhu Z Jiang S Xu X Tao Q Wang C Feng C Cao G and Xu Z 2008 Phys Rev B 78 052501[5] Leithe-Jasper A Schnelle W Geibel C and Rosner H 2008 Phys Rev Lett 101 207004[6] Jiang S Xing H Xuan G Ren Z Wang C Xu Z and Cao G 2009 Phys Rev B 80 184514[7] Zheng Q J et al 2009 arXiv09075547 (to be published)[8] Kumar M et al 2008 Phys Rev B 78 184516[9] TNC and Tconset were defined from ρ(T ) by taking the position of the corresponding minimum in d2ρ(T )dT 2

TNC and Tm from χac(T ) were determined accordingly[10] Ren Z Tao Q Jiang S Feng C Wang C Dai J Cao G and Xu Z 2009 Phys Rev Lett 102 137002

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

4

elemental impurities of As and Fe X-ray diffraction measurements confirmed the BaAl4 typestructure (space group I4mmm) for all the samples Electrical resistance and ac susceptibilitywere measured using an LR700 resistancemutual inductance bridge (Linear Research) Aminiature compensated coil system placed inside the pressure cell was utilized for the χac

experiments Temperatures down to 18 K were reached using a physical property measurementsystem (PPMS Quantum Design) Pressures up to 3 GPa have been achieved in a double-layerpiston-cylinder type pressure cell with silicone fluid as pressure transmitting medium The SCtransition of Pb which served as a pressure gauge remained sharp at all pressures indicatinggood hydrostatic conditions

3 Experimental ResultsFigure 1 shows the resistance normalized to the value at room temperature RR300K(T ) forEuFe2minusxCoxAs2 (x = 0 01 02 and 03) In EuFe2As2 RR300K(T ) exhibits two distinct fea-tures at T0 = 195 K and TN = 125 K corresponding to the SDW instability and the orderingof the localized Eu2+ moments respectively in good agreement with literature [3 4] Upon Cosubstitution T0 decreases rapidly to 103 K for x = 01 At x = 02 no signature of the SDWtransition is visible in the ρ(T ) data anymore TNC exhibits a weak concentration dependenceIn the concentration range between x asymp 01 divide 02 the type of magnetic order possibly changesfrom antiferromagnetic (AFM) to ferromagnetic (FM) In EuFe18Co02As2 a sharp drop of theresistance below asymp 10 K indicates the onset of superconductivity This observation is consistentwith studies on single crystals [6] However there a higher Tc and reentrant-like SC behaviorwas reported For x le 01 and x ge 03 no signature of superconductivity is visible in our dataevidencing a more narrow SC region than in the non-f homolog system SrFe2minusxCoxAs2 [5]

In the following we will focus on the results of our pressure study on EuFe19Co01As2 ρ(T )for different pressures is shown in Fig 2 Upon applying pressure the anomaly indicating theSDW transition at T0 broadens and shifts rapidly to lower temperatures At 161 GPa nofeature related to T0 can be resolved in ρ(T ) anymore suggesting that the SDW order is alreadysuppressed at this pressure At low temperatures displayed in Fig 3a a kink in ρ(T ) at 148 Kand 169 K indicates the ordering of the Eu2+ moments at 0 and 043 GPa respectively [9]At 101 GPa a first signature of a SC transition becomes evident ρ(T ) drops sharply belowTconset = 242 K followed by a small maximum indicating the magnetic ordering of the Eu2+

and a further decrease in ρ(T ) on lowering the temperature This behavior is similar to that

0 100 200 3000

02

04

06

08

10

TNT0

x = 0 01 02 03

T(K)

EuFe2-xCoxAs2

RR

300K

T0

TC

TN

Figure 1 Electrical resistivity normalized tothe value at room temperature (RR300K) forEuFe2minusxCoxAs2

0 100 200 3000

100

200

300

400

500

T0

p (GPa) 043 101 161

EuFe19Co01As2

(cm

)

T (K)

p

p (GPa) 2 241 283

Figure 2 ρ(T ) of EuFe19Co01As2 atdifferent external pressures

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

2

0

100

200

300

0 10 20 300

2

4

6

TC

Tconset

161

283

241

2

101

p (GPa)

043

EuFe19Co01As2

(cm

)

TNa)

TN

b)

Tm

p (GPa) 001 041 117 162 224

TC

ac (a

u)

T (K)

Figure 3 (a) Low-temperature resistivityand (b) ac susceptibility of EuFe19Co01As2at different external pressures For clarity theχac(T ) data at different pressures have beenshifted with respect to each other

0 05 10 15 20 25 300

20

40

60

80

100

120

EuFe19Co01As2

TNC

Tconset

T (K

)

p (GPa)

T0

Tm

p

Figure 4 Temperature-pressure phasediagram of EuFe19Co01As2

observed previously but at higher pressures in EuFe2As2 [1] The drop in ρ(T ) as well as thefollowing maximum are much more pronounced at p = 161 GPa but are hardly visible anymoreat p = 2 GPa At even higher pressures no indication of superconductivity is present in theresistivity data anymore

The results of the χac(T ) experiments are presented in Fig 3b The χac(T ) curve atthe lowest pressure (p = 001 GPa) and the curves at higher pressures (p ge 101 GPa) arequalitatively different While the shape of χac(T ) at p = 001 GPa is reminiscent of an AFMphase transition the shape at p ge 101 GPa is typical for a FM transition A change of themagnetic groundstate from AFM to FM has been found in EuFe2As2 on P-doping on the As-site too [10] P doping corresponds to the application of positive chemical pressure thereforeour finding is not surprising and in agreement with the doping studies On increasing pressurethe anomaly in χac(T ) at TNC shifts to higher temperatures [9] At pressures p ge 101 GPaa second feature appears in χac(T ) below TC at Tm [9] We attribute this kink to a change inthe magnetic structure from FM to AFM No feature in ρ(T ) is visible at Tm A detailed studyof the magnetic properties will be presented elsewhere It is important to note that we do notobserve a clear signature of superconductivity in our χac results

In our study we observe a less robust superconductivity than reported in literature [6 7]This might be related to different sample preparation procedures or to internal strain in thesamples An important influence of internalexternal residual strain on the observed physicalproperties appears also in our experiments After releasing the pressure we still find reentrantSC behavior in ρ(T ) similar to the data at 161 GPa (not shown)

The Tminusp phase diagram in Fig 4 summarizes our results While Tc(p) decreases only slightlywith increasing pressure TNC(p) increases strongly by a factor of nearly 2 from 148 K at ambientpressure to 276 K at 283 GPa the highest pressure of our experiment We find a good agreement

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

3

between TNC(p) obtained from the ρ(T ) and χac(T ) experiments The strong increase of TNC(p)with pressure has not been reported in EuFe2As2 [1] There the magnetic ordering temperatureis nearly independent of pressure The different pressure dependencies of Tconset(p) and TNC(p)in EuFe19Co01As2 lead to a crossing of both temperature lines [Tconset(p) = TC(p)] in the T minuspphase diagram at a pressure plowast slightly higher than 2 GPa Above plowast once TC gt Tc no indicationof any SC transition is visible in the resistivity data Already at 2 GPa where Tc = 219 K andTC = 203 K get rather close The feature associated to the SC transition is considerably reducedcompared to the previous pressure

4 Discussion and SummaryIn summary our results show that in the substitution series EuFe2minusxCoxAs2 the SDW transitiontemperature T0 is suppressed rapidly upon increasing Co concentration while the orderingtemperature of the Eu2+ moments is almost constant For x = 02 we observe the onset ofsuperconductivity in ρ(T ) at asymp 10 K For x lt 02 and x gt 02 no indication of superconductivityis present The strong suppression of T0 as function of the Co concentration and the appearanceof superconductivity in the T minus x phase diagram is consistent with the expectation from thecomparison with the homolog non-f Co-substitution series SrFe2minusxCoxAs2 [5] However theresuperconductivity is stable in a broader concentration range The presence of only a narrow SCregime in the Eu system might be related to the presence of the local moment Eu2+ magnetism

Pressure rapidly suppresses the SDW instability at T0 in EuFe19Co01As2 In the samepressure region where the signature of T0 is lost in ρ(T ) the onset of superconductivity isobserved At low temperatures our results suggest a strong detrimental effect of the Eu2+

magnetism on the superconductivity In an intermediate pressure range (101 GPa le T le 2 GPa)the system is on the verge to a SC state indicated by the drop in the resistivity belowTconset However the formation of the SC state is inhibited by the formation of long rangemagnetic order of the Eu2+ moments A similar result has been reported previously in undopedEuFe2As2 [1] The important difference in the present study on EuFe19Co01As2 is that uponincreasing pressure TNC(p) increases while Tconset(p) decreases and therefore TC(p) becomesgreater than Tconset(p) above a certain pressure plowast Once TC(p) gt Tconset(p) no signature ofsuperconductivity is found anymore We take this as an evidence for the strong detrimentaleffect of the Eu2+ magnetism on the the formation of the SC state As our ac-susceptibilityresults indicate Eu2+ orders ferromagnetically under pressure Thus we speculate that theinternal magnetic fields due to the ferromagnetically ordered Eu2+ moments suppress thesuperconductivity in the iron-arsenide layers

AcknowledgementsWe gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG) for financial supportthrough SPP 1458

References[1] Miclea C F et al 2009 Phys Rev B 79 212509[2] Terashima T et al 2009 J Phys Soc Jpn 78 083701[3] Jeevan H S Hossain Z Kasinathan D Rosner H Geibel C and Gegenwart P 2008 Phys Rev B 78 052502[4] Ren Z Zhu Z Jiang S Xu X Tao Q Wang C Feng C Cao G and Xu Z 2008 Phys Rev B 78 052501[5] Leithe-Jasper A Schnelle W Geibel C and Rosner H 2008 Phys Rev Lett 101 207004[6] Jiang S Xing H Xuan G Ren Z Wang C Xu Z and Cao G 2009 Phys Rev B 80 184514[7] Zheng Q J et al 2009 arXiv09075547 (to be published)[8] Kumar M et al 2008 Phys Rev B 78 184516[9] TNC and Tconset were defined from ρ(T ) by taking the position of the corresponding minimum in d2ρ(T )dT 2

TNC and Tm from χac(T ) were determined accordingly[10] Ren Z Tao Q Jiang S Feng C Wang C Dai J Cao G and Xu Z 2009 Phys Rev Lett 102 137002

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

4

0

100

200

300

0 10 20 300

2

4

6

TC

Tconset

161

283

241

2

101

p (GPa)

043

EuFe19Co01As2

(cm

)

TNa)

TN

b)

Tm

p (GPa) 001 041 117 162 224

TC

ac (a

u)

T (K)

Figure 3 (a) Low-temperature resistivityand (b) ac susceptibility of EuFe19Co01As2at different external pressures For clarity theχac(T ) data at different pressures have beenshifted with respect to each other

0 05 10 15 20 25 300

20

40

60

80

100

120

EuFe19Co01As2

TNC

Tconset

T (K

)

p (GPa)

T0

Tm

p

Figure 4 Temperature-pressure phasediagram of EuFe19Co01As2

observed previously but at higher pressures in EuFe2As2 [1] The drop in ρ(T ) as well as thefollowing maximum are much more pronounced at p = 161 GPa but are hardly visible anymoreat p = 2 GPa At even higher pressures no indication of superconductivity is present in theresistivity data anymore

The results of the χac(T ) experiments are presented in Fig 3b The χac(T ) curve atthe lowest pressure (p = 001 GPa) and the curves at higher pressures (p ge 101 GPa) arequalitatively different While the shape of χac(T ) at p = 001 GPa is reminiscent of an AFMphase transition the shape at p ge 101 GPa is typical for a FM transition A change of themagnetic groundstate from AFM to FM has been found in EuFe2As2 on P-doping on the As-site too [10] P doping corresponds to the application of positive chemical pressure thereforeour finding is not surprising and in agreement with the doping studies On increasing pressurethe anomaly in χac(T ) at TNC shifts to higher temperatures [9] At pressures p ge 101 GPaa second feature appears in χac(T ) below TC at Tm [9] We attribute this kink to a change inthe magnetic structure from FM to AFM No feature in ρ(T ) is visible at Tm A detailed studyof the magnetic properties will be presented elsewhere It is important to note that we do notobserve a clear signature of superconductivity in our χac results

In our study we observe a less robust superconductivity than reported in literature [6 7]This might be related to different sample preparation procedures or to internal strain in thesamples An important influence of internalexternal residual strain on the observed physicalproperties appears also in our experiments After releasing the pressure we still find reentrantSC behavior in ρ(T ) similar to the data at 161 GPa (not shown)

The Tminusp phase diagram in Fig 4 summarizes our results While Tc(p) decreases only slightlywith increasing pressure TNC(p) increases strongly by a factor of nearly 2 from 148 K at ambientpressure to 276 K at 283 GPa the highest pressure of our experiment We find a good agreement

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

3

between TNC(p) obtained from the ρ(T ) and χac(T ) experiments The strong increase of TNC(p)with pressure has not been reported in EuFe2As2 [1] There the magnetic ordering temperatureis nearly independent of pressure The different pressure dependencies of Tconset(p) and TNC(p)in EuFe19Co01As2 lead to a crossing of both temperature lines [Tconset(p) = TC(p)] in the T minuspphase diagram at a pressure plowast slightly higher than 2 GPa Above plowast once TC gt Tc no indicationof any SC transition is visible in the resistivity data Already at 2 GPa where Tc = 219 K andTC = 203 K get rather close The feature associated to the SC transition is considerably reducedcompared to the previous pressure

4 Discussion and SummaryIn summary our results show that in the substitution series EuFe2minusxCoxAs2 the SDW transitiontemperature T0 is suppressed rapidly upon increasing Co concentration while the orderingtemperature of the Eu2+ moments is almost constant For x = 02 we observe the onset ofsuperconductivity in ρ(T ) at asymp 10 K For x lt 02 and x gt 02 no indication of superconductivityis present The strong suppression of T0 as function of the Co concentration and the appearanceof superconductivity in the T minus x phase diagram is consistent with the expectation from thecomparison with the homolog non-f Co-substitution series SrFe2minusxCoxAs2 [5] However theresuperconductivity is stable in a broader concentration range The presence of only a narrow SCregime in the Eu system might be related to the presence of the local moment Eu2+ magnetism

Pressure rapidly suppresses the SDW instability at T0 in EuFe19Co01As2 In the samepressure region where the signature of T0 is lost in ρ(T ) the onset of superconductivity isobserved At low temperatures our results suggest a strong detrimental effect of the Eu2+

magnetism on the superconductivity In an intermediate pressure range (101 GPa le T le 2 GPa)the system is on the verge to a SC state indicated by the drop in the resistivity belowTconset However the formation of the SC state is inhibited by the formation of long rangemagnetic order of the Eu2+ moments A similar result has been reported previously in undopedEuFe2As2 [1] The important difference in the present study on EuFe19Co01As2 is that uponincreasing pressure TNC(p) increases while Tconset(p) decreases and therefore TC(p) becomesgreater than Tconset(p) above a certain pressure plowast Once TC(p) gt Tconset(p) no signature ofsuperconductivity is found anymore We take this as an evidence for the strong detrimentaleffect of the Eu2+ magnetism on the the formation of the SC state As our ac-susceptibilityresults indicate Eu2+ orders ferromagnetically under pressure Thus we speculate that theinternal magnetic fields due to the ferromagnetically ordered Eu2+ moments suppress thesuperconductivity in the iron-arsenide layers

AcknowledgementsWe gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG) for financial supportthrough SPP 1458

References[1] Miclea C F et al 2009 Phys Rev B 79 212509[2] Terashima T et al 2009 J Phys Soc Jpn 78 083701[3] Jeevan H S Hossain Z Kasinathan D Rosner H Geibel C and Gegenwart P 2008 Phys Rev B 78 052502[4] Ren Z Zhu Z Jiang S Xu X Tao Q Wang C Feng C Cao G and Xu Z 2008 Phys Rev B 78 052501[5] Leithe-Jasper A Schnelle W Geibel C and Rosner H 2008 Phys Rev Lett 101 207004[6] Jiang S Xing H Xuan G Ren Z Wang C Xu Z and Cao G 2009 Phys Rev B 80 184514[7] Zheng Q J et al 2009 arXiv09075547 (to be published)[8] Kumar M et al 2008 Phys Rev B 78 184516[9] TNC and Tconset were defined from ρ(T ) by taking the position of the corresponding minimum in d2ρ(T )dT 2

TNC and Tm from χac(T ) were determined accordingly[10] Ren Z Tao Q Jiang S Feng C Wang C Dai J Cao G and Xu Z 2009 Phys Rev Lett 102 137002

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

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between TNC(p) obtained from the ρ(T ) and χac(T ) experiments The strong increase of TNC(p)with pressure has not been reported in EuFe2As2 [1] There the magnetic ordering temperatureis nearly independent of pressure The different pressure dependencies of Tconset(p) and TNC(p)in EuFe19Co01As2 lead to a crossing of both temperature lines [Tconset(p) = TC(p)] in the T minuspphase diagram at a pressure plowast slightly higher than 2 GPa Above plowast once TC gt Tc no indicationof any SC transition is visible in the resistivity data Already at 2 GPa where Tc = 219 K andTC = 203 K get rather close The feature associated to the SC transition is considerably reducedcompared to the previous pressure

4 Discussion and SummaryIn summary our results show that in the substitution series EuFe2minusxCoxAs2 the SDW transitiontemperature T0 is suppressed rapidly upon increasing Co concentration while the orderingtemperature of the Eu2+ moments is almost constant For x = 02 we observe the onset ofsuperconductivity in ρ(T ) at asymp 10 K For x lt 02 and x gt 02 no indication of superconductivityis present The strong suppression of T0 as function of the Co concentration and the appearanceof superconductivity in the T minus x phase diagram is consistent with the expectation from thecomparison with the homolog non-f Co-substitution series SrFe2minusxCoxAs2 [5] However theresuperconductivity is stable in a broader concentration range The presence of only a narrow SCregime in the Eu system might be related to the presence of the local moment Eu2+ magnetism

Pressure rapidly suppresses the SDW instability at T0 in EuFe19Co01As2 In the samepressure region where the signature of T0 is lost in ρ(T ) the onset of superconductivity isobserved At low temperatures our results suggest a strong detrimental effect of the Eu2+

magnetism on the superconductivity In an intermediate pressure range (101 GPa le T le 2 GPa)the system is on the verge to a SC state indicated by the drop in the resistivity belowTconset However the formation of the SC state is inhibited by the formation of long rangemagnetic order of the Eu2+ moments A similar result has been reported previously in undopedEuFe2As2 [1] The important difference in the present study on EuFe19Co01As2 is that uponincreasing pressure TNC(p) increases while Tconset(p) decreases and therefore TC(p) becomesgreater than Tconset(p) above a certain pressure plowast Once TC(p) gt Tconset(p) no signature ofsuperconductivity is found anymore We take this as an evidence for the strong detrimentaleffect of the Eu2+ magnetism on the the formation of the SC state As our ac-susceptibilityresults indicate Eu2+ orders ferromagnetically under pressure Thus we speculate that theinternal magnetic fields due to the ferromagnetically ordered Eu2+ moments suppress thesuperconductivity in the iron-arsenide layers

AcknowledgementsWe gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG) for financial supportthrough SPP 1458

References[1] Miclea C F et al 2009 Phys Rev B 79 212509[2] Terashima T et al 2009 J Phys Soc Jpn 78 083701[3] Jeevan H S Hossain Z Kasinathan D Rosner H Geibel C and Gegenwart P 2008 Phys Rev B 78 052502[4] Ren Z Zhu Z Jiang S Xu X Tao Q Wang C Feng C Cao G and Xu Z 2008 Phys Rev B 78 052501[5] Leithe-Jasper A Schnelle W Geibel C and Rosner H 2008 Phys Rev Lett 101 207004[6] Jiang S Xing H Xuan G Ren Z Wang C Xu Z and Cao G 2009 Phys Rev B 80 184514[7] Zheng Q J et al 2009 arXiv09075547 (to be published)[8] Kumar M et al 2008 Phys Rev B 78 184516[9] TNC and Tconset were defined from ρ(T ) by taking the position of the corresponding minimum in d2ρ(T )dT 2

TNC and Tm from χac(T ) were determined accordingly[10] Ren Z Tao Q Jiang S Feng C Wang C Dai J Cao G and Xu Z 2009 Phys Rev Lett 102 137002

International Conference on Strongly Correlated Electron Systems (SCES 2010) IOP PublishingJournal of Physics Conference Series 273 (2011) 012101 doi1010881742-65962731012101

4