Physica C 470 (2010) 1103–1105

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Physica C

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Critical current densities and flux creep rate in Co-doped BaFe2As2

with columnar defects introduced by heavy-Ion irradiation

Y. Nakajima a,b,*, Y. Tsuchiya a, T. Taen a, H. Yagyuda a, T. Tamegai a,b, S. Okayasu c, M. Sasase d, H. Kitamura e,T. Murakami e

a Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanb JST, Transformative Research-Project on Iron Pnictides (TRIP), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanc Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japand The Wakasa-wan Energy Research Center, Research and Development Group, 64-52-1 Nagatani, Tsuruga, Fukui 914-0192, Japane Radiation Measurement Research Section, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Available online 31 May 2010

Keywords:Iron-pnictide superconductorCritical current densityFlux creep

0921-4534/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.physc.2010.05.047

* Corresponding author at: Department of ApplieTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Jap

E-mail address: yasuyuki@ap.t.u-tokyo.ac.jp (Y. Na

We report the formation of columnar defects in Co-doped BaFe2As2 single crystals with different heavy-ion irradiations. The formation of columnar defects by 200 MeV Au ion irradiation is confirmed by trans-mission electron microscopy and their density is about 40% of the irradiation dose. Magneto-opticalimaging and bulk magnetization measurements reveal that the critical current density Jc is enhancedin the 200 MeV Au and 800 MeV Xe ion irradiated samples while Jc is unchanged in the 200 MeV Niion irradiated sample. We also find that vortex creep rates are strongly suppressed by the columnardefects. We compare the effect of heavy-ion irradiation into Co-doped BaFe2As2 and cupratesuperconductors.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Recently discovered iron-based superconductors with Tc � 26 K[1] have attracted great interest in the research for technologicalapplications. For the applications, it is necessary to improve theintrinsically low critical current density Jc. Although the transitiontemperature of the iron-based superconductors is still lower thancuprate superconductors, introduction of pinning centers can en-hance Jc. It is well known that a most efficient way to improvethe critical current density is to pin down vortices with columnardefects created by swift particle irradiation. In cuprate supercon-ductors, columnar defects enhance Jc dramatically [2,3].

Among the iron-based superconductors, intermetallic iron-arse-nide Ba(Fe1-xCox)2As2 with Tc � 24 K is one of the suitable systemsfor technological applications because the large single crystallinesamples can be readily obtained [4] and its Jc reaches 1 � 106 A/cm2 at T = 2 K [5–7]. Although the introduction of columnar defectscan increase Jc, it is an unsettled problem whether columnar de-fects can be introduced in iron-based superconductors because ofmany influencing factors to form the columnar defects, such asion species, ion energy, stopping power of incident ions, and ther-mal conductivity, etc.

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d Physics, The University ofan. Tel./fax: +81 3 5841 6848.kajima).

In this paper, we report the realization of columnar defects byseveral heavy-iron irradiations into Co-doped BaFe2As2 single crys-tals and the effect on Jc. The formation of columnar defects by200 MeV Au ion irradiation is confirmed by transmission electronmicroscopy (TEM). Magneto-optical imaging and bulk magnetiza-tion measurements reveal that Jc is enhanced in the 200 MeV Auand 800 MeV Xe ion irradiated samples while Jc is not increasedin the 200 MeV Ni ion irradiated sample. We also find that vortexcreep rates are strongly suppressed by the columnar defects.

2. Experimental

Single crystalline samples of Ba(Fe1-xCox)2As2 were grown byFeAs/CoAs self-flux method [7]. Co concentration was determinedby EDX measurements. 200 MeV Au ions and 200 MeV Ni ions wereirradiated into Ba(Fe0.93Co0.07)2As2 with Tc = 24 K along c-axis usingthe TANDEM accelerator in JAEA to create columnar defects.800 MeV Xe ions were irradiated into Ba(Fe0.925Co0.075)2As2 withTc = 24 K along c-axis at NIRS-HIMAC. Field-equivalent defect densi-ties, matching field, is BU = 20 kG in all ion irradiations. It should benoted that Tc is not affected by the heavy-ion irradiations. Plan viewand cross-sectional images of the irradiated Ba(Fe0.93Co0.07)2As2

were obtained by a high resolution and scanning TEM (JEOL,JEM-3000F). Magnetization was measured by a commercial SQUIDmagnetometer (MPMS-XL5, Quantum Design). Magneto-opticalimages were obtained by using the local-field-dependent Faraday

1104 Y. Nakajima et al. / Physica C 470 (2010) 1103–1105

effect in the in-plane magnetized garnet indicator film employing adifferential method [8,9].

3. Results and discussion

A plan view of the irradiated Ba(Fe0.93Co0.07)2As2 along c-axisreveals defects with the average density 4.2 ± 1.2 � 1010 cm�2,which is about 40% of the expected value. Inset of Fig. 1 showsthe high-resolution TEM image of the defect. The size of defectsis �2–5 nm with a large fluctuation, and the lattice image is sus-tained with some displacements of atoms. These results in iron-based superconductor make a good contrast to those in high tem-perature cuprate superconductors, where amorphous defects withdiameters �10 nm are created [10].

Enhancements of vortex pinning introduced by the irradiationare directly visualized by magneto-optical imaging. The inset ofFig. 2 shows an optical image of Ba(Fe0.93Co0.07)2As2 for the200 MeV Au ion irradiation. For comparison, we irradiated only ahalf the crystal by covering another half of the crystal by Au foilwith a thickness of 100 lm. The left area inside the rectangle isthe irradiated region. Main panel of Fig. 2 shows a magneto-opticalimage in the remanent state of Ba(Fe0.93Co0.07)2As2 at 5 K afterincreasing field up to 800 Oe for 5 s and reducing it to zero. Bright-er areas represent the regions with stronger field. Prominent differ-

Fig. 1. Cross-sectional view of the 200 MeV Au-ion irradiated Ba(Fe0.93Co0.07)2As2.The arrow indicates the direction of the irradiation, which corresponds to c-axis.Inset shows high-resolution TEM image of a defect in the plan view.

Fig. 2. Magneto-optical image at 5 K in the remanent state of Ba(Fe0.93Co0.07)2As2.Inset shows optical micrograph of Ba(Fe0.93Co0.07)2As2.

ence of vortex penetrations between the irradiated andunirradiated regions is observed. While in the unirradiated region,vortices penetrate deep into the sample in addition to the penetra-tion from a defect on the top edge, the penetration in the irradiatedregion is much reduced than those in the unirradiated region. Thisresult indicates that defects introduced by the irradiation enhancethe vortex pinning in the irradiated region.

We also confirm the enhancement of vortex pinning to the irra-diation by the estimation of Jc. Fig. 3a and 3b show the field depen-dence of critical current density obtained from bulk magnetizationusing the Bean model in the unirradiated and 200 MeV Au-ion irra-diated samples, respectively. In the unirradiated sample, Jc is about1 � 106 A/cm2 at T = 2 K under zero field. In the 200 MeV Au-ionirradiated sample, Jc is about 6 � 106 A/cm2 at T = 2 K under zerofield, which indicates that columnar defects enhance the vortexpinning. We note that the drastic enhancement of Jc at �BU/3 ob-served in cuprate superconductors [11,12] is absent or stronglysuppressed in irradiated Ba(Fe0.93Co0.07)2As2.

The effect of ion species on the formation of columnar defects isconfirmed by estimation of Jc. Fig. 4a and 4b show the field depen-dence of critical current density obtained from bulk magnetizationusing the Bean model in the 200 MeV Ni ion irradiated and800 MeV Xe ion irradiated Ba(Fe1-xCox)2As2, respectively. Interest-ingly, in the 200 MeV Ni ion irradiated sample, Jc is about1 � 106 A/cm2 at T = 2 K under zero field, which is almost the sameas that of the unirradiated sample. In the 800 MeV Xe ion irradi-ated sample, Jc is about 1.3 � 106 A/cm2 at T = 2 K under zero field,which is slightly enhanced by the irradiation. Jc increases to2.3 � 106 A/cm2 at T = 2 K under zero field in the 800 MeV Xe ionirradiated sample with the matching field of 40 kG. It should benoted that the values of electronic stopping power of the200 MeV Au, 200 MeV Ni, and 800 MeV Xe ion irradiation intoBa(Fe1-xCox)2As2 calculated by SRIM program are 2.96, 1.25, and2.87 keV/Å, respectively. These results suggest that the 200 MeVNi ion irradiation with smaller electronic stopping power doesnot create columnar defects and the stopping power of about2.9 keV/Å may be marginal for the formation of columnar defects

Fig. 3. The field dependence of critical current density obtained by magnetizationusing the Bean model at several temperatures in (a) the unirradiated and (b)200 MeV Au-ion irradiated Ba(Fe0.93Co0.07)2As2.

Fig. 4. The field dependence of critical current density obtained by magnetizationusing the Bean model at several temperatures in (a) the 200 MeV Ni ion irradiatedBa(Fe0.93Co0.07)2As2 and (b) 800 MeV Xe ion irradiated Ba(Fe0.925Co0.073)2As2.

Fig. 5. Temperature dependence of normalized relaxation rate S in the unirradiatedsample and the 200 MeV Au irradiated Ba(Fe0.93Co0.07)2As2 in the remanent state.

Y. Nakajima et al. / Physica C 470 (2010) 1103–1105 1105

in Ba(Fe1-xCox)2As2. Slight enhancement of Jc in 800 MeV Xe ionirradiated Ba(Fe1-xCox)2As2 may be due to the smaller size of de-fects than those in the 200 MeV Au-ion irradiated Ba(Fe1-xCox)2As2.

Fig. 5 shows the normalized relaxation rate S defined by S =|dlnM/dln t|, where t is time, in the remanent state of the unirradiatedand 200 MeV Au irradiated samples. In the unirradiated sample, Sshows a broad peak around T � 8 K, which is qualitatively similarto that reported in Ref. [5]. By contrast, S in the 200 MeV Au irradi-ated sample is strongly suppressed at all temperatures, which indi-cates that the enhancement of pinning due to columnar defectsaffects flux creep. With increasing temperature, S in the irradiated

sample increases monotonically and approaches to that in theunirradiated sample at temperatures close to Tc, which indicatethat additional flux pining by columnar defects is reduced by ther-mal fluctuations. We note that in YBa2Cu3O7-d, temperature inde-pendent plateau in the normalized relaxation rate with values inthe range S = 0.020–0.035, similar to the present value, is observedand interpreted in terms of collective creep theory [13]. We alsonote that a large peak of S at T � Tc/2 observed in cuprate super-conductors, possibly associated with Bose-glass phase, is absentin Ba(Fe0.93Co0.07)2As2 [14,15]. This could be due to discontinuouscolumnar defects, which reduces the localization of vortices ontocolumnar defects.

4. Summary

In summary, we have introduced columnar defects in Co-dopedBaFe2As2 single crystals by several heavy-ion irradiations. Forma-tion of columnar defects by the 200 MeV Au ion irradiation is con-firmed by scanning TEM observation and their density correspondsto about 40% of the expected value. Magneto-optical imaging andbulk magnetization measurements reveal drastic enhancement ofJc in the 200 MeV Au-ion irradiated sample and slight enhance-ment in the 800 MeV Xe ion irradiated samples while no increasein the 200 MeV Ni ion irradiated sample. We also find that vortexcreep rates are strongly suppressed by the columnar defects. Weconclude that the columnar defects introduced by heavy-iron irra-diation strongly enhance the flux pining in Co-doped BaFe2As2.

Acknowledgements

This work is partly supported by a Grant-in-Aid for ScientificResearch from MEXT, Japan and is one of the research projects withheavy ions at NIRS-HIMAC.

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