Anisotropic vortex pinning in the β-pyrochlore oxide superconductor KOs2O6

Physica C 471 (2011) 801–803

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Anisotropic vortex pinning in the b-pyrochlore oxide superconductor KOs2O6 Y. Ishii a,b,⇑, J. Yamaura b, Y. Okamoto b, A. Maeda a, Z. Hiroi b a b

Department of Basic Science, University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan

a r t i c l e

i n f o

Article history: Available online 13 May 2011 Keywords: Vortex pinning Peak effect Magnetic torque b-pyrochlore oxide KOs2O6

a b s t r a c t Vortex pinning in the b-pyrochlore oxide superconductor KOs2O6 with Tc = 9.6 K is investigated by measuring magnetic torque. A large anisotropy of magnetic torque is observed in the superconducting state below Tp = 7.6 K, where a first-order structural transition takes place, in spite of the inherent isotropic nature of the structural and electronic properties. Magnetic torque is enhanced at external magnetic fields parallel to the [1 1 1] and [0 0 1] directions. Moreover, a pronounced peak effect is also observed in the magnetic field dependence of the torque in these two directions. We consider that the observed anisotropy is related to a microstructure associated with the structural transition. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction b-pyrochlore oxide superconductor AOs2O6 (A = K, Rb, Cs) [1–3] possesses a highly symmetric atomic cage, that consists of Os and O atoms and includes a small A cation compared to the cage size. Similar with other cage compounds such as Si/Ge clathrates and filled skutterudites the b-pyrochlore oxides exhibit an anharmonic oscillation of the A cation called rattling. The rattling phenomena and superconductivity have been widely investigated thus far, which suggests that the superconductivity in the b-pyrochlore oxides is induced by the rattling vibration. Particularly, the strongcoupling superconductivity with the highest Tc = 9.6 K in KOs2O6 is due to strong electron–rattler interactions. As expected from the highly symmetric cubic structure with  space group Fd3m, the b-pyrochlore superconductors exhibit isotropic superconducting properties, which is in contrast to the case for compounds with anisotropic crystal structures such as cuprate superconductors. Previous studies on the penetration depth [4–6] and the superconducting gap [7,8] have revealed that they are completely three-dimensional superconductors. Moreover, there is little anisotropy in Hc2 [9]. On the other hand, anisotropic re-entrant behavior has been reported in the temperature dependence of resistivity in low magnetic fields of 1–2 T for a KOs2O6 single crystal, which strongly depends on the direction of applied magnetic field. This is probably caused by anisotropic vortex pinning, but the origin has not yet been clarified [10].

⇑ Corresponding author at: Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan. Tel.: +81 4 7136 3447; fax: +81 4 7136 3446. E-mail address: [email protected] (Y. Ishii). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.059

Only KOs2O6 exhibits a structural phase transition at Tp = 7.6 K [11]. This transition is a weak first-order phase transition [12] and an isomorphic transition without symmetry breaking [13]. Superconductivity changes its character dramatically at Tp from strong-coupling in the high temperature phase to weak-coupling in the low temperature phase. Interestingly, the second peak effect has been observed in the magnetic hysteresis at 7.1 K below Tp [9]. Since it is observed for H // [1 1 1] while not for H // [1 1 0], a certain anisotropic vortex pinning mechanism seems to be working below Tp. In this paper, vortex pinning in KOs2O6 has been investigated by means of magnetic torque in order to clarify the anisotropy below Tp. Based on the observed angle, temperature and magnetic field dependences of the magnetic torque, the mechanism of the vortex pinning is discussed. 2. Experiments A single crystal of KOs2O6 was prepared by the following procedures. KOsO4 and OsO2 precursors were mixed in a molar ratio of 1.3:1, pelletized by an uniaxial press, and placed in a fused silica tube under vacuum. After heating at 475 °C for 24 h, a plate-like single crystals of KOs2O6 with (1 1 1) facets were obtained. A crystal used for magnetic torque measurements was 0.33 mm in width, 0.14 mm thick and 74 lg in weight. The Tc of this crystal was 9.6 K from the temperature dependence of magnetic susceptibility measured by a SQUID magnetometer. The magnetic torque was measured using a commercial torque magnetometer set in a Quantum Design Physical Property Measurement System (PPMS), where two piezoresistive sensors on a sample holder detect a torque  surface of the crystal was attached on the samforce. The flat ð1 1 1Þ ple holder with a small amount of Apiezon N grease. After cooling in

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Y. Ishii et al. / Physica C 471 (2011) 801–803

 zero field, magnetic fields up to 10 T were applied along the ½1 1 1 direction. Then, as depicted in Fig. 1, the sample was rotated by angle  0 axis so that it could be exposed to magnetic fields h around the ½1 1 to major crystallographic directions. Background torque signals obtained without the crystal were subtracted from the raw data.

-8

(10 N m)

1

(a)

6.9 K 7T

0

3. Results

[111]

H

[111]

θ

[110] Fig. 1. Drawing of the sample setting. The directions of applied magnetic field, the crystal axes and the rotation axis are shown.

2.0 1.5

-8

(10 N m)

-1 2.5

(b)

[111]

6.9 K 7T

[112]

[110]

1.0

[001]

0.5 0

-60

0

60

120

(o)

180

240

300

Fig. 2. (a) Angular dependences of magnetic torque (s) and (b) the hysteresis width (Ds) measured at T = 6.9 K and l0H = 7 T.

9

6.9 K

49.2o

-5.5o

8

-8

(10 N m)

Hysteretic angular dependence has been observed in the magnetic torque of the KOs2O6 single crystal measured at 6.9 K and 7 T, as shown in Fig. 2a, reflecting certain anisotropy in vortex pinning below Tp. Signals after 180° rotation are slightly smaller than before owing to a small sample misalignment. The hysteresis width (Ds) is also plotted against h in Fig. 2b. Two peaks are observed in the Ds–h plot, that are located at h = 5.5° and 49.2° near the [1 1 1] and [0 0 1] directions, respectively. In general, the magnetic torque is given by s = M  H so that it vanishes when M // H. In the case of superconductors, it gives information on the anisotropy in vortex pinning: it becomes large, when the field direction is close to but not exactly along the direction of magnetization for weakly anisotropic materials, while is almost perpendicular to the direction of magnetization for extremely anisotropic superconductors like copper oxides [14]. Since the present compound should be nearly isotropic, the observed two peaks imply that the vortex pinning is enhanced at fields along the [1 1 1] and [0 0 1] directions. Fig. 3 shows the magnetic field dependence of torque measured at 6.9 K and fixed angles of 40.7°, 5.5°, 14°, and 49.2°, which may reflect pinning forces in magnetic fields parallel to [1 1 0], [1 1 1], [1 1 2] and [0 0 1], respectively. A clear hysteresis is observed in magnetic fields of 6–7.3 T for h = 5.5°, while no hysteresis is observed for h = 40.7°. This agrees with the previous results on M–H curves measured at H // [1 1 0] and [1 1 1] [9]. Furthermore, we found in this study another small peak effect at h = 49.2°, where vortex pinning is also enhanced at H // [0 0 1]. The magnetic field dependences of torque at h = 5.5° and at T = 5, 6, 6.3, 6.7, 6.9 and 7.5 K are shown in Fig. 4, which should correspond to magnetization curves in H // [1 1 1]. The peak effect is not observed at 7.5 K, appears at 6.9 K below Tp and grows with decreasing temperature. Note that a peak field (Hpk) shifts toward higher fields and also that the width of the peak effect (Hp1Hp2) is enlarged with decreasing temperature. Such a peak effect has been observed in many superconductors and often ascribed to a matching effect, where a certain periodic array of structural defects effectively pins vortices forming an Abrikosov lattice [15]. However, this may not be the present case, because the Hpk strongly depends on temperature, as shown in Fig. 4.

= 14o 7

6

5

-40.7o

5

6 0

H (T)

7

8

Fig. 3. Magnetic field dependence of torque at four angles of h = 40.7°, 5.5°, 14° and 49.2° at 6.9 K. The background signal has not been extracted. These torque curves correspond to magnetization curves in magnetic fields parallel to [1 1 0], [1 1 1], [1 1 2] and [0 0 1], respectively.

The H–T phase diagram shown in Fig. 5 summarizes the region where the peak effect has been observed, in addition to the Hc2 and Tp lines determined by previous specific heat and magnetization measurements on another crystal prepared by the same method [9]. A low temperature phase below Tp exhibits smaller Hc2 than a high temperature phase. It is known, moreover, that the superconducting condensation energy in the low temperature phase is smaller than that in the high temperature phase [9]. The fact that the peak effect is observed only below Tp indicates that the structural phase transition at Tp causes the peak effect. Since this transition is of the first-order, it is likely that the two superconducting phases coexist below Tp in the supercooling state. There are two reasons to believe this: the phase transition does not involve any

Y. Ishii et al. / Physica C 471 (2011) 801–803

because of the difference in the condensation energy between the two phases; it may be effective just below Hc2 and, thus, the Hpk depends on temperature, as shown in Fig. 5. If the phase boundaries are selectively generated on planes parallel to the [1 1 1] and [0 0 1] directions, one can explain the observed angle dependence of the magnetic torque and the appearance of the peak effect.

T = 7.5 K 5 x 10

-8

6.9 K

(N m)

6.7 K

Hpk Hp2

Hp1

4. Conclusion

6.3 K

In the magnetic torque measurements on a KOs2O6 single crystal, a strongly temperature-dependent peak effect has been observed below the structural transition temperature Tp under magnetic fields parallel to the [1 1 1] and [0 0 1] directions. It is suggested that the phase boundaries between the low- and hightemperature phases in the supercooling state work as pinning centers to result in the anisotropic vortex pinning in the isotropic superconductor.

6K

5K = -5.5o 0

2

4

6

8

10

H (T)

Acknowledgments

0

Fig. 4. Magnetic field dependence of torque measured at various temperatures at h = 5.5° (H // [1 1 1]). The background signal has not been extracted. Each curve is shifted upward by 3.5  108 N m for clarity. Hp1, Hpk and Hp2 represent characteristic fields where a hysteresis begins, exhibits a peak and ends, respectively.

Hc2

Tp

H // [111]

12

Hp2

μ0H (T)

10 8

(M)

6

(Cp) Hp1

2 0

This work was partly supported by the Japan Society for the Promotion of Science under the Grant-in-Aid for JSPS fellows and by Grant-in-Aids for Scientific Research B (22340092) provided by MEXT, Japan. References

14

4

803

5

6

7

T (K)

8

9

10

Fig. 5. H–T phase diagram of KOs2O6 for H // [1 1 1]. The shaded area represents a region where the second peak effect is observed. The Hc2 and Tp lines determined by previous study [9] are also plotted. Solid lines are guide for the eye. Cp and M represent Hc2 (T) determined by the specific heat and magnetization measurements, respectively.

symmetry change [13] and the volume change is just 0.03% [16]. The phase boundary between them may work as a pinning center,

[1] S. Yonezawa, Y. Muraoka, Y. Matsushita, Z. Hiroi, J. Phys.: Condens. Matter 16 (2004) L9. [2] S. Yonezawa, Y. Muraoka, Y. Matsushita, Z. Hiroi, J. Phys. Soc. Jpn. 73 (2004) 819. [3] S. Yonezawa, Y. Muraoka, Z. Hiroi, J. Phys. Soc. Jpn. 73 (2004) 1655. [4] I. Bonalde, R. Ribeiro, W. Brämer-Escamilla, J. Yamaura, Y. Nagao, Z. Hiroi, Phys. Rev. Lett. 98 (2007) 227003. [5] Y. Shimono, T. Shibauchi, Y. Kasahara, T. Kato, K. Hashimoto, Y. Matsuda, J. Yamaura, Y. Nagao, Z. Hiroi, Phys. Rev. Lett. 98 (2007) 257004. [6] R. Khasanov, D.G. Eshchenko, D. Di Castro, A. Shengelaya, F. La Mattina, A. Maisuradze, C. Baines, H. Luetkens, J. Karpinski, S.M. Kazakov, H. Keller, Phys. Rev. B 72 (2005) 104504. [7] Y. Kasahara, Y. Shimono, T. Shibauchi, Y. Matsuda, S. Yonezawa, Y. Muraoka, Z. Hiroi, Phys. Rev. Lett. 96 (2006) 247004. [8] T. Shimojima, Y. Shibata, K. Ishizaka, T. Kiss, A. Chainani, T. Yokoya, T. Togashi, X.-Y. Wang, C.T. Chen, S. Watanabe, J. Yamaura, S. Yonezawa, Y. Muraoka, Z. Hiroi, T. Saitoh, S. Shin, Phys. Rev. Lett. 99 (2007) 117003. [9] Z. Hiroi, S. Yonezawa, Y. Nagao, J. Yamaura, Phys. Rev. B 76 (2007) 014523. [10] Z. Hiroi, S. Yonezawa, J. Phys. Soc. Jpn. 75 (2006) 043701. [11] Z. Hiroi, S. Yonezawa, J. Yamaura, T. Muramatsu, Y. Muraoka, J. Phys. Soc. Jpn. 74 (2005) 1682. [12] Z. Hiroi, S. Yonezawa, J. Yamaura, J. Phys.: Condens. Matter 19 (2007) 145283. [13] J. Yamaura, M. Takigawa, O. Yamamuro, Z. Hiroi, J. Phys. Soc. Jpn. 79 (2010) 043601. [14] A. Wisniewski, R. Puzniak, J. Karpinski, J. Hofer, R. Szymczak, M. Baran, F.M. Sauerzopf, R. Molinski, E.M. Kopnin, J.R. Thompson, Phys. Rev. B. 61 (2000) 791. [15] H. Raffy, J.C. Renard, E. Guyon, Solid State Commun. 11 (1972) 1679. [16] K. Sasai, M. Kofu, R.M. Ibberson, K. Hirota, J. Yamaura, Z. Hiroi, O. Yamamuro, J. Phys.: Condens. Matter 22 (2010) 015403.