Physica C 370 (2002) 13–16 www.elsevier.com/locate/physc
Negligible eﬀect of grain boundaries on the supercurrent density in polycrystalline MgB2 Kijoon H.P. Kim, W.N. Kang, Mun-Seog Kim, C.U. Jung, Hyeong-Jin Kim, Eun-Mi Choi, Min-Seok Park, Sung-Ik Lee * Department of Physics, National Creative Research Initiative Center for Superconductivity, Pohang University of Science and Technology, Pohang, Kyungbuk 790784, Republic of Korea Received 2 August 2001; received in revised form 5 November 2001; accepted 16 November 2001
Abstract We used dc magnetization and transport measurement to estimate the superconducting critical current densities (Jc ) of polycrystalline MgB2 sintered under high temperature and high pressure. We measured the current–voltage (I–V) characteristics and found the existence of a vortex-glass phase in the ﬁeld–temperature (H–T) plane. This is notable in that the vortex-glass phase can be observed even in a polycrystalline specimen, which suggests that the supercurrent is not sensitive to the grain boundaries. Moreover, the transport (intergrain) Jc seems to be comparable to a magnetic (intragrain) Jc . Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.70.Ad; 74.60.Ge; 74.25.Fy; 74.25.Bt Keywords: MgB2 ; Critical current density; Vortex glass
1. Introduction For high-current applications with the sintered cuprates, one of the drawbacks is the existence of the grain boundaries which shows the weak-link behavior and which diminishes the bulk supercurrent by several order of magnitude . Thus, the critical current density (Jctr ) obtained from the transport measurement is usually much smaller than the value of Jcm observed from the magnetic
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hysteresis curve MðH Þ, where Jcm only reﬂects the intra-granular eﬀect. In this aspect, the newly discovered metallic MgB2 superconductor  is remarkable since the grain boundary eﬀect in this material has been suggested to be small [3–6]. In our previous works, we prepared a highly dense MgB2 superconductor by using a high pressure, high temperature synthesis method . The transmission electron microscope analysis of the sample revealed that the grains were compactly connected without discernable empty spaces at the grain boundaries . Also, we found that the irreversible magnetization of our MgB2 sample was dominated by bulk pinning (i.e., the surface barrier eﬀect was negligible) and that the pinning
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mechanism did not change even when the temperature was increased up to T =Tc 0:9 . In this work, we measured the resistance RðT Þ and the current–voltage (I–V) characteristics of MgB2 sintered at high pressure and high temperature to elucidate its current-carrying capability and the vortex pinning mechanism. The transport Jctr was found to be nearly the same as the magnetic Jcm , thus providing an evidence of a strong linkage between grains. We observed the existence of a vortex-glass phase [10,11], and a wide region below the Hc2 line was covered by a vortex-glass phase in the H–T plane. 2. Experimental Bulk polycrystalline MgB2 samples (4.5 mm in diameter and 3.3 mm in height) were sintered at 950 °C under a pressure of 3 GPa. The details of sample preparation and its characterization are reported elsewhere . For the transport study, we cut the sample into a bar shape, 4 mm in length, 460 lm in width, 70 lm in thickness. The standard photolithography technique was adopted for fabricating the electrical contact pads. To obtain good ohmic contacts (<1 X), we used a Au coating as contact pads after cleaning the sample surface with an Ar ion beam. The voltage noise, which is detrimental to precise measurements, was successfully reduced to a lower level . The RðT Þ was measured with a bias current density J ¼ 3:1 A/cm2 .
Fig. 1. Resistive superconducting transition for H ¼ 0:0, 0.1, 0.2, 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 T with a bias current density J ¼ 3:1 A/cm2 . The zero-ﬁeld onset Tc is 38.5 K with a narrow transition width of 0.5 K, as judged from the 90% to 10% drop of the resistive transition.
3. Results and discussion Fig. 1 shows the superconducting resistive transition for 0 6 H 6 5 T. The zero-ﬁeld Tc onset is 38.5 K and has a narrow transition width of 0.5 K, as judged from the 90% to 10% superconducting transition. As the magnetic ﬁeld increases, the Tc decreases and the transition width becomes broader. Fig. 2 shows the temperature dependence of the Jc for H ¼ 0:5–5 T. The values (Jcm ) with open symbols were estimated from the hysteresis in the magnetization measurements by using Bean’s critical model  while those (Jctr ) with solid symbols were estimated from transport measure-
Fig. 2. Temperature dependence of the critical current density of a polycrystalline MgB2 sample for H ¼ 0:5, 1.0, 2.0, 3.0, and 5.0 T. The values with open symbols were estimated from the hysteresis in the magnetization measurements by using Bean’s critical model while those with solid symbols were estimated from transport measurements using a 1 lV/mm criterion. The inset shows the resistive superconducting transition for H ¼ 0:0, 0.1, 0.2, 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 T with a bias current density J ¼ 3:1 A/cm2 . The zero-ﬁeld onset Tc is 38.5 K with a narrow transition width of 0.5 K, as judged from the 90% to 10% drop of the resistive transition.
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ments using a 1 lV/mm criterion. Especially, the Jc value at 21 K for H ¼ 5 T is about 2:3 102 A/ cm2 . While Jctr and Jcm are estimated in diﬀerent temperature ranges, Fig. 2 shows that those values fall on a smooth curve at each ﬁeld. This suggests that Jctr ðT Þ will be the same as Jcm ðT Þ at low temperatures. This behavior is unusual in polycrystalline cuprates [14,15]. To understand the vortex transformation in polycrystalline MgB2 , we studied the I–V characteristics for ﬁelds of 0 6 H 6 5 T. Fig. 3(a) shows
Fig. 3. (a) I–V characteristics at T ¼ 26:8, 27.2, 27.6, 28.0, 28.4, 28.8, 29.2, 29.6, and 33.0 K for H ¼ 3 T. The dotted line was artiﬁcially inserted to represent the I–V curve at Tg . (b) I–V curves collapse onto one curve above and one curve below Tg through a scaling function after transforming I and V into two variables Isc ¼ I=T jT Tg j2m and Vsc ¼ V =IjT Tg jmðz1Þ , respectively. The scaling exponents, m ¼ 1:5 and z ¼ 2:3, are in good agreement with the theoretical predictions for a three dimensional system.
representative I–V curves for temperatures from 26.8 to 33.0 K at H ¼ 3 T, which are similar to those of the single crystalline cuprates around the vortex-glass to vortex-liquid transition point, Tg . Thus, an analysis based on the vortex-glass theory [10,11] was performed. According to the theory, the I–V curves collapse onto one curve above and one curve below Tg through a scaling function after ransforming I and V into two variables Isc ¼ 2m mðz1Þ I=T jT Tg j and Vsc ¼ V =IjT Tg j , respectively. The scaling function was obtained as shown in Fig. 3(b) and had critical exponents m ¼ 1:5 and z ¼ 2:3, which were in good agreement with the theoretical predictions for a three dimensional system . The scaling exponents were found to be almost independent of the magnetic ﬁeld in the range of 1 6 H 6 5 T with values of m ¼ 1:50 0:01 and z ¼ 2:25 0:07. Below H ¼ 1 T, the I–V data showed a somewhat poor scaling behavior. The existence of this scaling behavior in a polycrystalline sample is notable because normally this behavior is screened by the grain boundary eﬀect, as in polycrystalline cuprates [14,15]. Fig. 4 shows the phase diagram in the H–T plane. The diagram is based on the vortex-glass
Fig. 4. Vortex phase diagram for polycrystalline MgB2 . The values of Hg ðT Þ were obtained using scaling analysis, and the upper critical ﬁelds, Hc2 ðT Þ, were estimated from the RðT Þ curves using three diﬀerent criterions of 90%, 10% values of the normal-state resistance, and maximum slope in dR=dT .
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line, Hg ðT Þ, obtained from an analysis using the vortex-glass theory. The upper critical ﬁelds, Hc2 ðT Þ, were estimated from the RðT Þ curves using three diﬀerent criterions of 90%, 10% values of the normal-state resistance, and maximum slope in dR=dT . The magnetic ﬁeld dependence of Tg is well described by Hg / ð1 Tg =Tc Þn with n ¼ 1:38 and Tc ¼ 38:5 K. This value of n is almost the same as those reported in single crystal high-Tc cuprate superconductors, such as YBa2 Cu3 Oy , and is consistent with the theoretical value, n ¼ 4=3 .
4. Conclusion We have studied magnetization and transport properties in MgB2 superconductor to measure the critical current densities in high magnetic ﬁeld. We found that the transport critical current density as a function of temperature was smoothly connected to the magnetically estimated one. We also found a existence of the vortex-glass phase in polycrystalline MgB2 . Our observations imply the highly possible high-current applications of MgB2 in sintered forms.
Acknowledgements This work is supported by the Ministry of Science and Technology of Korea through the Creative Research Initiative Program.
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