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
, Toshiya Doi b, Yoshinori Hakuraku b, Hitoshi Kitaguchi
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
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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  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
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
Z. Mori et al. / Physica C 445–448 (2006) 880–883
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.
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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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