Enhancement of Jc of MgB2 thin films by introduction of oxygen during deposition

Enhancement of Jc of MgB2 thin films by introduction of oxygen during deposition

Physica C 445–448 (2006) 880–883 www.elsevier.com/locate/physc Enhancement of Jc of MgB2 thin films by introduction of oxygen during deposition Zon Mo...

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Physica C 445–448 (2006) 880–883 www.elsevier.com/locate/physc

Enhancement of Jc of MgB2 thin films by introduction of oxygen during deposition Zon Mori a

a,*

, Toshiya Doi b, Yoshinori Hakuraku b, Hitoshi Kitaguchi

c

Yatsushiro National College of Technology, 2627 Hirayama-Shinmachi, Yatsushiro, Kumamoto 866-8501, Japan b Faculty of Engineering, Kagoshima University, Kagoshima 890-0065, Japan c National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan Available online 15 June 2006

Abstract The introduction of various pinning center are examined as the effective means for improvement of Jc of MgB2 thin films. We have investigated the effects of introduction of oxygen during deposition on the superconducting properties of MgB2 thin films. MgB2 thin films were prepared on polished sapphire C(0 0 0 1) single crystal substrates by using electron beam evaporation technique (EB) without any post-annealing. The background pressure was less than 1.3 · 106 Pa. The evaporation flux ratio of Mg was set at 30 times as high as that of B, and the growth rate of MgB2 film was 1 nm/s. The film thickness was typically 300 nm at 5 min deposition. The substrate temperature was 245 C. Under these conditions, we controlled the oxygen partial pressure (P O2 ) within the range from 1.3 · 106 to 1.3 · 103 Pa by using a quadrapole mass spectrometer. Although Tc of deposited thin film decreased in order of P O2 , DM in the magnetization hysteresis loops measured from 0 to 6 T at 4.2 K increased up to 1.3 · 105. On the other hand, thin film prepared under P O2 of 1.3 · 103 Pa does not show superconducting transition. Between these films, there is no difference in the crystal structure from X-ray diffraction (XRD). These results suggest that the pinning center in the thin films increased by introduction of oxygen. Extremely small amount of oxygen introduction has enabled the control of growth of oxide.  2006 Elsevier B.V. All rights reserved. PACS: 74.76 Keywords: MgB2 thin film; Flux pinning; Oxide

1. Introduction MgB2 superconductors are expected to play an important role for high current and high field applications because of its simple crystal structure, long coherence length, and high critical temperature. Since these devices for large current carriers can operate at temperatures attainable with a simple refrigerator, research for various concrete applications has already been advanced. For these purposes, not only the use of conventional technology in

*

Corresponding author. Tel.: +81 965 53 1280; fax: +81 965 53 1289. E-mail address: [email protected] (Z. Mori).

0921-4534/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.051

the fabrication of high-quality thin films but also introduction of defects or grain boundaries to pin the vortices is important. Defects, grain boundaries and impurities are known to act as effective flux pinning centers in MgB2 [1– 4]. However, these defects also affect the current flow or pairing interaction and often decrease Tc. To prevent the Tc from decreasing by defects is one of a problem to be solved. In order to introduce the defects for enhance pinning force in MgB2 thin films, the following three processes have mainly been adopted: (1) doping of substitutional impurities such as C, Si, Al and so on [4,5], (2) shift of compositional ratio of Mg/B [6] and (3) introduction of oxide [7,8]. The way of (1) provides a means of varying

Z. Mori et al. / Physica C 445–448 (2006) 880–883

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the superconducting properties in a controllable manner whereas it needs an extra evaporating source and process become complex. In addition, ways of (1) and (2) also might affect the electronic structure and vary the intrinsic properties in MgB2 films. These problems are expected to be evaded by introducing the oxide. It is reported that although the inter-grain connectivity was worsened by increasing the amount of MgO, well distributed low size MgO particles improved the pinning ability on powder in tube samples. In this paper we report that the MgB2 thin film fabrication by electron beam deposition and the effects of introduction of oxygen during deposition. 2. Experimental MgB2 thin films were deposited on a polished C-plane sapphire (0 0 1) single crystal by electron beam deposition technique. The basal pressure of the deposition chamber was typically 1.0 · 107 Pa (basal pressure was different in some samples). We controlled the oxygen partial pressure (P O2 ) by using a quadrapole mass spectrometer. A pure magnesium block (99.99%) shaped into the form of a crucible was used as the evaporation source. However, since a large boron block was hard to obtain, we used granular boron (99.95%) stuffed into the crucible. The flux of each atom species that arrived onto thse substrate was monitored using a quartz crystal monitor (QCM). The output of the QCM was feed back to the automatically controlled electron beam regulator. In this way precise control of atom flux was administered. Flux rate of Mg was adjusted so that the composition of these films should be equal to Mg:B = 1:2. Film growth rate was nearly 1 nm/ s, and film thickness was 300 nm (for a 5 min deposition). Substrate was heated to 240 C using a halogen lamp heater that enabled a quick response to changes in the substrate temperature. These deposition conditions were varied experimentally to discern optimal conditions. The composition of the thin films was examined by inductively coupled plasma (ICP) photoemission spectroscopy. Electric transport was ascertained by four-point probe geometry. For the transport measurement, the thin films were patterned with 1 mm line. The crystal structure was checked using X-ray diffraction (XRD). Magnetic field characteristic was checked by physically property measurement system (PPMS). The thin films’ surfaces were observed by atomic force microscopy (AFM). 3. Results and discussion The XRD patterns for the thin films deposited under various basal pressures (high vacuum of 8 · 108 Pa and low vacuum of 1.5 · 105 Pa) are shown in Fig. 1. Peak position of (0 0 2) based on JCPDS is displayed by solid line in this figure. Fig. 1 clearly shows that these films are fully c-axis oriented and any peak shifts correspond to the shift of Mg/B ratio was not observed. Composition of these thin films was checked by ICP. Fig. 2 shows the

Fig. 1. XRD patterns for the thin films deposited under various basal pressures (high vacuum of 8 · 108 Pa and low vacuum of 1.5 · 105 Pa).

Fig. 2. The magnetization hysteresis curves measured at 4.2 K for the MgB2 thin films prepared in the various basal pressures. The magnetic fields were applied perpendicular to the thin film surface.

magnetization hysteresis loops measured from 0 to 6 T at 4.2 K of these films. DM in the magnetization hysteresis loop (magnetic field was applied perpendicular to the thin film surface) of the film prepared under low vacuum is large in 2 T or more. These results indicate that pinning centers in the MgB2 thin film were increased by deposition under low vacuum. Because the crystal strain due to the Mg deficiency is not seen from Fig. 1, minute MgO that formed by the residual oxygen acts as pinning center. Oxygen has not been introduced here but superconducting properties of the thin films were changed by oxygen that slightly remained in the chamber. To examine the effects of MgO in more detail, we introduce the oxygen during deposition. First of all, we prepared MgB2 thin films without introducing the oxygen

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for 5 min. At this stage, film thickness was 300 nm. Afterwards, the pure oxygen gas (4 N) was introduced to the vacuum chamber and then followed the deposition of MgB2 thin film for 5 min. Oxygen partial pressure (P O2 ) was controlled at 1.3 · 105, 3.5 · 105 and 1.3 · 103 Pa by using a quadrapole mass spectrometer. Composition of these thin films were Mg:B = 1:1.5, 1:1.3 and 1:1.0, respectively. It is because oxidization of Mg occurred by introducing oxygen and adhesion rate of the Mg to the substrate rose. Actually, thickness of thin film prepared without oxygen was 300 nm, whereas the thickness of thin film prepared in P O2 ¼ 1:3  103 Pa was 600 nm. Fig. 3 shows the XRD patterns for the thin films deposited under various partial pressures of O2 (P O2 ). Fig. 3 clearly shows that these films are fully c-axis oriented and any peak shifts correspond to the Mg deficiency was not observed. This suggests that excess Mg in the thin film exists in the form of MgO or Mg and compositional shift of the MgB2 matrix was negligible. In addition, the change in the size of the grain by the oxygen introduction was not seen as shown in the AFM images of Fig. 4. Therefore, the influence of the pinning by the grain boundary was considered same level in both samples. We can make comparative study of the effect of MgO from these results. However, only thin films prepared under P O2 ¼ 1:3  103 Pa were insulating, and the peak was not observed. Fig. 5 shows the resistivity vs. temperature curves for these thin films. Zero resistivity temperature (Tc) of thin film prepared under 1.0 · 107 Pa (without oxygen) was 33 K. Tc decrease in order of P O2 and thin films prepared under P O2 ¼ 1:3  103 Pa were insulating. Fig. 6 shows the magnetization hysteresis loops measured from 0 to 6 T at 4.2 K of these films. One can see the rise of DM in all the measured range in the film prepared under oxygen. Especially, DM in the thin film of P O2 ¼ 1:3  105 Pa increased in 5.8 times at 1 T and 7.3 times at 4 T compared with the thin film without oxygen. These result suggested

Fig. 4. Comparison of the AFM images of the MgB2 thin films prepared in the different oxygen partial pressure.

Fig. 5. Resistivity vs. temperature curves of the MgB2 thin films prepared in the different oxygen partial pressure.

Fig. 6. Magnetization hysteresis curves measured at 4.2 K for the MgB2 thin films prepared in the different oxygen partial pressure. The magnetic fields were applied perpendicular to the thin film surface. Fig. 3. XRD patterns of the MgB2 thin films prepared in the different oxygen partial pressure (P O2 ) of 1.3 · 105, 3.5 · 105 and 1.3 · 103 Pa. Basal pressure of deposition chamber was 1.0 · 107 Pa (P O2 ¼ 0).

that the oxide (provably MgO) formed in the thin film has acted as an effective pinning center.

Z. Mori et al. / Physica C 445–448 (2006) 880–883

In summary, superconducting MgB2 thin films were prepared on C-plane sapphire substrate by electron beam deposition and the effects of introduction of oxygen during deposition were examined. As a result, although Tc decreased, remarkable increase of flux pinning was seen in the MgB2 thin films by formation of oxide. These oxide seem to be working as flux pinning sites. Acknowledgement A part of this work is supported by Research Promotion Bureau, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan under the contract No. 16-554.

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