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Superconductivity of MgB2 thin films prepared by ion implantation and pulsed plasma treatment
ARTICLE IN PRESS
Vacuum 78 (2005) 123–129 www.elsevier.com/locate/vacuum
Superconductivity of MgB2 thin films prepared by ion implantation and pulsed plasma treatment J. Piekoszewskia,b,, W. Kempinskic, B. Andrzejewskic, Z. Trybu"ac, L. Piekara-Sadyc, J. Kaszyn´skic, J. Stankowskic, Z. Wernerb,d, E. Richtere, F. Prokerte, J. Stanis"awskib, M. Barlakb a
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-145 Warszawa, Poland b The Andrzej So!tan Institute for Nuclear Studies, 05-400 Otwock/S´wierk, Poland c Institute of Molecular Physics Polish Academy of Sciences, Mariana Smoluchowskiego 17, PL-60179 Poznan´, Poland d Institute of Physical Chemistry PAS, 44/52 Kasprzaka Street, 01-224 Warsaw, Poland e Forschungszentrum Rossendorf e.V. Institut fu¨r Ionenstrahlphysik und Materialforschung, Postfach 510119, D-01314 Dresden, Germany
Abstract The experiments to synthesize thin MgB2 inter-metallic compound with the use of ion implantation and plasma pulse treatment are presented. Mg was implanted with 3 1018 cm2 of 80 keV and 5 1018 cm2 of 100 keV B+ ions and next treated with hydrogen and argon plasma pulses of duration of about 1 ms and fluence between 2 and 4 J/cm2. Superconducting properties were examined by magnetically modulated microwave absorption (MMMA), magnetic moment and electrical conductivity measurements. The structural properties of the implanted and pulse-treated samples were examined by the X-ray diffraction (XRD) and Rutherford backscattering (RBS) methods. The main result consists in observation of MMMA hysteresis loop demonstrating the existence of superconducting regions with Tc as high as 32 K. However, the zero-resistance effect has not been obtained due to incomplete global connectivity between the superconducting regions. r 2005 Elsevier Ltd. All rights reserved. Keywords: MgB2 thin films; Ion implantation; Pulsed plasma treatment
1. Introduction
Corresponding author. The Andrzej So"tan Institute for Nuclear Studies, 05-400 Otwock/S´wierk, Poland. Tel.: +48 22 718 0603; fax: +48 22 779 3481. E-mail address: [email protected] (J. Piekoszewski).
The discovery of superconductivity in MgB2 with Tc as high as 39 K [1] has created great excitement about the possibility of using this material in many practical applications. Main reasons for this interest are: low price of the
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.014
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compound elements, ease of synthesis, relatively high values of critical temperature Tc and critical current density Jc, progress perspectives in advancing superconducting properties, broad perspectives of applications in various industries, including the power generation industry and microelectronic applications. Some authors deem the discovery as a breakthrough in superconductivity technologies. Soon after the discovery, the first superconducting MgB2 thin films were successfully demonstrated e.g. [2–6]. The study on both bulk and film properties and technology are still in increasing progress. Common features in most approaches presented so far in thin film fabrication are as follows: (1) sample is annealed at 600–900 1C in Mg vapor, which gives rise to synthesis of a superconducting MgB2 layer, (2) the synthesis occurs in the solid phase in the Mg–B system. In the present work an attempt is undertaken to synthesize for the first time, superconducting MgB2 film from liquid phase without annealing the B–Mg system in Mg vapor, using ion implantation (of one component) into the substrate (second component) followed by transient melting processes (TMP). For TMP one can use high intensity, short duration: laser, electron ion or plasma pulses. High intensity is defined here as the intensity which causes transient melting of the near-surface region of the substrate. Boron is used as an implanted species and magnesium as a substrate material. For TMT, high-intensity pulsed plasma beams (HIPPB) are adopted. Below, we describe the technological procedure as well as the methods and results of superconducting properties tests of the obtained samples. The less commonly known technological and analytical methods applied in this work are described in more detail.
2. Experimental 2.1. Sample processing The substrates were cut out from commercial magnesium ingot of 99.8 wt% purity in the form of highly polished discs of 3.8 mm diameter and 2 mm
thickness. They were implanted with boron ions using Danfysik Model 1009-200 implanter. Two batches of 25 samples each were implanted. In the 1st batch the ion energy was 100 keV and the dose 5 1018 B/cm2, in the 2nd batch the ion energy and the dose were 80 keV and 3 1018 B/cm2, respectively. In both cases the temperature of the substrate was kept at about 200–250 1C. Subsequent step of the adopted procedure was the transient melting process using HIPPBs. The plasma pulses were generated in a rod plasma injector (IBIS) type of accelerator described elsewhere [7]. Briefly, the plasma pulses are generated as a result of a low-pressure, high-current discharge between two concentric sets of electrodes. A high-voltage pulse delayed some time td with respect to the moment of the injection of the working gas into the inter-electrode space, ignites the discharge. If td is set long enough to allow the injected gas to expand over the whole interelectrode space, the plasma contains almost exclusively the elements of the working gas. This mode of operation is referred to as pulse implantation doping (PID). Typically it occurs for td of about 150 ms and 200 ms for hydrogen and argon as the working gases, respectively. For significantly shorter td ; when there is a steep gradient of the gas concentration in the inter-electrode space, effective erosion of the metallic electrodes occurs. Atoms and ions of intentionally selected electrode material are deposited and alloyed into the substrate. This regime is referred to as deposition by the pulsed erosion (DPE) process. Both sets of inner and outer electrodes in the IBIS plasma generator consist of 32 metallic rods of 2 mm diameter and 250 mm length. The electrode–substrate distance is 300 mm. Fluences of plasma pulses are in the range of 1–7 J/cm2. Pulse duration in the ms range corresponds to irradiance in the MW/cm2 range. In the present experiments, the 1st batch of samples were pulse treated with magnesium rods as electrodes and hydrogen as the working gas. The choice of electrode material was dictated by a precaution not to introduce any foreign atoms into the Mg–B system, whereas hydrogen was used to reduce (at least partly) oxygen always present in magnesium. In the 2nd batch, magnesium
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electrodes were replaced by molybdenum ones and hydrogen was replaced by argon working gas. These changes were introduced for two reasons. First, magnesium electrodes easily deform under electrodynamic forces exerted on them during the discharge, thus deteriorating the uniformity of the plasma beam. Second, argon atoms are much less mobile than hydrogen ones, which facilitates a more precise setting of the discharge conditions, especially with regard to the delay time td : Spectral diagnostics of the electrode erosion and the substrate material evaporation were done in the ‘‘Mo electrodes, Ar gas’’ setup. Optical spectra were observed using a Mechelles spectrometer coupled to a standard PC. By monitoring intensities of the Mo I 550.65 nm, Ar II 434.8 nm, and Ar II 480.6 nm lines vs. td it was possible to minimize emission of electrode material and its deposition on the irradiated substrates. On the other hand, by monitoring intensities of the Mg I 516.73 nm, Mg I 517.27 nm, Mg I 518.36 nm, and Mg II 448.1 nm lines it was possible to show that there was a substantial amount of magnesium atoms and ions just above the melted substrate surface. Intensities of the Mg lines were proportional to the pulse fluence in the 1–3 J/cm2 range. The latter finding qualitatively supports our expectation that a plum of Mg vapor/ions exerts a pressure on the liquid surface sufficient to prevent any significant loss of magnesium from the irradiated Mg–B system. In the present HIPPB treatment each sample was irradiated separately. The beam fluences were chosen from 1 to 3 J/cm2. The number of pulses ranged from 1 to 5. 2.2. Characterization 2.2.1. X-ray diffraction X-ray diffraction patterns (XRD) were obtained in the grazing incidence technique using a standard instrument (D5000, Bruker AXS) equipped with a thin film setup composed of a collimator and a secondary monochromator placed in front of the scintillation detector. At the used Cu Ka radiation and the applied incidence angle of 11, the 1/e penetration depth of the X-rays amounts to approximately 300 nm. Thus the recorded signals
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originate mainly in the pulse-modified thin surface layer, whereas the input from the magnesium substrate is diminished. The phase analysis was carried out using the PDF database implemented in the Bruker AXS evaluation code. 2.2.2. Rutherford backscattering (RBS) measurements In order to get insight into the composition of the processed layer, selected samples were subjected to RBS analysis. The spectra were recorded using 1.8 MeV a particle beam at normal incident to the sample. The backscattered particles were detected at 1701 angle using Si(Li) detector with 15 keV energy resolution. 2.2.3. Superconducting state detection Three methods were used to detect superconducting state (SC) in the processed samples: magnetic measurements (Oxford Instruments MagLab 200 System DC magnetometer/AC spectrometer), magnetically modulated microwave absorption (MMMA), and four-probe electric conductivity measurements. Microwave non-resonant dissipation observed around zero field (‘‘EPR-zero field line’’) is considered as a convenient method in characterizing SC. Since the discovery of high-temperature superconductors (HTS), the MMMA technique has been successfully established [8]. The MMMA signal in granular superconducting samples as well as in point superconductor–insulator–superconductor (SC–I–SC), weak contacts is the result of Josephson dissipation related to the fluctuation of Cooper pairs to electrons in the conducting band. A Josephson junctions system (JJS) can be considered as a separate phase embedded in the host surrounding. MMMA is a unique and very sensitive contact-less method to determine Tc for small amounts of SC phases dispersed below the percolation threshold. MMMA has already been applied to study intercalated fullerene, perovskitecomposite (superconductor–insulator), as well as MgB2. Thin films of MgB2 superconductor sample were placed in a microwave resonator of the EPR spectrometer with modulation of the sweeping magnetic field. In this procedure, the output signal
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has a differential form of absorbed power dP/dB [9]. A weak Josephson contact SC–I–SC is a basic element that interacts magnetically with the microwave field. The Josephson dissipation related to Cooper pairs breaking leads to voltage fluctuations like in the EPR spectroscopy. The Josephson hysteresis loop (JHL) is the confirmation that the zero field MMMA line is related to the SC. The JHL has an opposite circulation vs. magnetic field B in comparison to a typical magnetic hysteresis loop.
3. Results and discussion Fig. 1 presents grazing angle (o ¼ 1 ) XRD patterns for an as-implanted sample and for the sample treated after implantation with 2 hydrogen pulses of the fluence of about 3 J/cm2. The strongest MgB2 (1 0 1) peak for the plasma-treated sample is shifted toward the greater values of 2y with respect to the peak for the as-implanted sample. Such shift means that pulse treatment causes the shrinkage of the structure. This can be due to either a compressive strain resulting from the rapid re-solidification of the near-surface layer or non-stoichiometry. Obviously both these reasons can contribute simultaneously. It seems interesting to note at this point that the (1 0 1)
Fig. 1. X-ray diffraction patterns of the strongest MgB2 (1 0 1) reflection for the as-implanted sample and for the sample treated with 2 hydrogen plasma pulses of 2 J/cm2.
peak shifted towards greater 2y as in our case also in MgB2 films in which superconductivity was achieved after furnace annealing of the boronimplanted magnesium [4]. Similarly, in Ref. [10], the highest value of transition temperature Tc was observed for the sample in which the lattice constant c calculated from MgB2(0 0 2) peak position had the smallest value with Dc=c ¼ 0:2%: In view of these facts we could have expected that the procedure described in Section 2 may lead to formation of superconducting MgB2 phases. Fig. 2 presents the results of RBS measurements. It may be seen that in comparison to the spectrum of unimplanted sample (Mg), a pronounced valley centered at around channel 400 has appeared in the spectrum of the as-implanted sample (a). This feature is associated with a dilution of Mg matrix with boron during implantation. The Mg distribution after implantation exhibits a strange bimodal character, reflecting apparently the boron distribution. Such bimodal distributions in the as-implanted samples have already been reported in other cases [11], although at the moment we see no explanation for this effect. Samples (b) and (c) were treated with 2 hydrogen plasma pulses with growing fluence (2 and 3 J/cm2, respectively). It may be seen that the width of the valley (related to the width of the boron profile) grows with the pulse fluence. For sample (c) the boron profile seems to be spread over the greatest depth. On the other hand, the Mg signal value at the minimum of the spectrum does not change significantly. Rough estimations based on SIMNRA simulations indicate that the Mg content at the peak of boron profile, amounts to no more than 15–25%. In the absence of other components of the surface layer, it means that the attained level of boron concentration is not less than 75–85%—far in excess of the stoichiometric composition of MgB2. However, this excess of boron does not preclude the existence of MgB2 phase, as evidenced by the XRD spectra. The width of boron profile in sample (c) may be estimated to about 1 mm if the density of the boron-containing layer is taken as a composition-weighted average of boron and magnesium densities.
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Fig. 2. RBS spectra of the un-implanted sample (Mg), the as-implanted sample (a), the sample (b) treated with 2 hydrogen plasma pulses of 2 J/cm2 fluence, and the sample (c) treated with 2 hydrogen plasma pulses of 3 J/cm2 fluence.
It is interesting to compare the data described so far with those obtained in the superconductivity measurements. Fig. 3 shows the MMMA signals observed for samples (b) and (c) at temperatures 10 and 20 K. It is seen that MMMA signal hysteresis is comparable in both samples at 10 K. However, in sample (b) it disappears already at 20 K, while in sample (c) it is still present at that temperature. The critical temperatures as deduced from MMMA signal vs. temperature dependency amount to 17 and 31 K for samples (b) and (c), respectively. This means that spread boron profiles obtained at higher pulse fluences correlate with the formation of a more stable superconducting phase. The strongest MMMA signal hysteresis (higher by an order of magnitude with respect to samples (b) and (c)) has been obtained for sample (d) treated with 4 pulses of Ar plasma at the fluence of about 2 J/cm2 (Fig. 4, upper panel). However, Tc for that sample has a rather low value of about 12 K, as seen in the bottom panel in Fig. 4 presenting the magnetic moment after zero-field cooling (ZFC) induced by 10 Oe magnetic field vs.
temperature. This result demonstrates a possibility of creating a substantial amount of superconducting phase, with reduced Tc due to a lack of stoichiometry [12]. We do not attribute special importance to the plasma-pulse composition (hydrogen or argon). We rather suggest that it is the pulse energy and number of pulses which determine the final result of the treatment. The highest Tc has been obtained for high-energy pulses, but the highest superconducting phase content has been obtained for the largest number of pulses, equivalent to a long diffusion time in the molten phase. An unexpected although interesting result has been obtained in electrical conductivity measurements for the sample treated with 2 hydrogenplasma pulses of about 2 J/cm2 fluence (Fig. 5). When a four-point-probe measurement is applied in the direction perpendicular to the sample surface, the observed conductivity vs. temperature dependence resembles a typical dependence for semiconductors. However, when this measurement is performed in parallel to the surface, the dependence is typical of a metal. The course of
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Fig. 3. MMMA signal vs. external magnetic field at 10 and 20 K for the samples treated with 2 hydrogen plasma pulses of 2 J/cm2 fluence (left panel) and of 3 J/cm2 fluence (right panel). In both cases the gain was 105.
of current. The drop of parallel resistance below 32 K is obviously associated with the presence of superconducting MgB2 phase. However, this phase does not form a continuous layer since the resistivity does not fall down to zero. Apparently, separate islands of superconducting phase are connected through metallic Mg paths. The electrical properties of the latter give rise to the observed course of resistance at higher temperatures.
4. Conclusions
Fig. 4. MMMA signal vs. external magnetic field at 10 K (gain 104) for the sample treated with 4 argon plasma pulses of fluence 2 J/cm2 (upper panel). DC magnetic moment vs. temperature for the same sample at H ¼ 10 Oe (lower panel).
transverse conductance can probably be explained in terms of semiconductor-like boron layer located below the surface and blocking the transverse flow
We have demonstrated that boron implantation into magnesium, combined with plasma-pulse treatment leads to formation in a surface layer of superconducting grains with Tc as high as about 32 K. However, even in the samples showing a relatively large content of superconducting phase, no zero-resistance effect has been obtained due to incomplete global connectivity. In further research, we plan to investigate effects of reduced ion-implanted dose, increased pulse fluences, and pulse multiplicity. We hope these treatment modifications may help to spread and homogenize the superconducting phase.
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surements and Dr. W. Szymczyk (IPJ) for his valuable assistance and comments.
References
Fig. 5. Resistance vs. temperature for the sample treated with 2 hydrogen plasma pulses measured in the direction perpendicular to the sample plane (upper panel) and in the plane of the sample (lower panel). K and K0 denote geometrical factors.
Acknowledgements The authors wish to thank Dr. R. Gro¨tzschel (Forschungszentrum Rossendorf) for RBS mea-
[1] Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y, Akimitsu J. Nature 2001;410:63–4. [2] Bumel G, Kang DJ, Lee HN, Moon SH, Oh B. Appl Phys Lett 2001;79:3464–6. [3] Parantham M, Cantoni C, Zhai HY, Christen HM, Aytyng T, Sathyamurthy S, Specht ED, Thompson JR, Lowndews DH, Kerchner HR, Christen DK. Appl Phys Lett 2001;78:3669–71. [4] Zhai HY, Christen HM, White CW, Budai JD, Lowndes DH. Appl Phys Lett 2002;80:4786–8. [5] Shinde SR, Ogale SB, Greene RL, Venkatesan T, Canfield PC, Bud’ko SL, Laperlot G, Petrovic C. Appl Phys Lett 2001;79:227–9. [6] Berenov A, Lockman Z, Qi X, MacManus-Driskol JL, Bugoslavsky Y, Cohen LF, Jo M-H, Stelmashenko NA, Tsaneva VN, Kambara M, Hari Babu N, Cardwell DA, Blamire MG. Appl Phys Lett 2001;79:4001–3. [7] Piekoszewski J, Langner J. Nucl Instr and Meth 1991;B53: 148–60. [8] Durny R, Hautala J, Ducharme S, Lee B, Symko OG, Taylor PC, Zheng DJ, Xu JA. Phys Rev 1987;B36: 2361; Stankowski J, Kahol PK, Dalal NS, Moodera JS. Phys Rev 1987;B36:7126; Blazey KW, Mueller KA, Bednorz JG, Berlinger W, Amoretti G, Buluggiu E, Vera A, Matacotta FC. Phys Rev 1987;B36:7241. [9] Stankowski J, et al. In: Klamut J, editor. Recent developments in HTSC. New York: Springer; 1996. p. 167. [10] Yamazaki H, Hikita Y, Hori H, Takagi H. Appl Phys Lett 2003;83:3740–2. [11] Haglund RF, Yang Li, Magruder RH, White CW, Zuhr RA, Yang Lina, Dorsinville R, Alfano RR. Nucl Instr and Meth 1994;B91:493–504. [12] Okuma S, et al. J Low Temp Phys 2003;131:1111.
Vacuum 78 (2005) 123–129 www.elsevier.com/locate/vacuum
Superconductivity of MgB2 thin films prepared by ion implantation and pulsed plasma treatment J. Piekoszewskia,b,, W. Kempinskic, B. Andrzejewskic, Z. Trybu"ac, L. Piekara-Sadyc, J. Kaszyn´skic, J. Stankowskic, Z. Wernerb,d, E. Richtere, F. Prokerte, J. Stanis"awskib, M. Barlakb a
Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-145 Warszawa, Poland b The Andrzej So!tan Institute for Nuclear Studies, 05-400 Otwock/S´wierk, Poland c Institute of Molecular Physics Polish Academy of Sciences, Mariana Smoluchowskiego 17, PL-60179 Poznan´, Poland d Institute of Physical Chemistry PAS, 44/52 Kasprzaka Street, 01-224 Warsaw, Poland e Forschungszentrum Rossendorf e.V. Institut fu¨r Ionenstrahlphysik und Materialforschung, Postfach 510119, D-01314 Dresden, Germany
Abstract The experiments to synthesize thin MgB2 inter-metallic compound with the use of ion implantation and plasma pulse treatment are presented. Mg was implanted with 3 1018 cm2 of 80 keV and 5 1018 cm2 of 100 keV B+ ions and next treated with hydrogen and argon plasma pulses of duration of about 1 ms and fluence between 2 and 4 J/cm2. Superconducting properties were examined by magnetically modulated microwave absorption (MMMA), magnetic moment and electrical conductivity measurements. The structural properties of the implanted and pulse-treated samples were examined by the X-ray diffraction (XRD) and Rutherford backscattering (RBS) methods. The main result consists in observation of MMMA hysteresis loop demonstrating the existence of superconducting regions with Tc as high as 32 K. However, the zero-resistance effect has not been obtained due to incomplete global connectivity between the superconducting regions. r 2005 Elsevier Ltd. All rights reserved. Keywords: MgB2 thin films; Ion implantation; Pulsed plasma treatment
1. Introduction
Corresponding author. The Andrzej So"tan Institute for Nuclear Studies, 05-400 Otwock/S´wierk, Poland. Tel.: +48 22 718 0603; fax: +48 22 779 3481. E-mail address: [email protected] (J. Piekoszewski).
The discovery of superconductivity in MgB2 with Tc as high as 39 K [1] has created great excitement about the possibility of using this material in many practical applications. Main reasons for this interest are: low price of the
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.01.014
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compound elements, ease of synthesis, relatively high values of critical temperature Tc and critical current density Jc, progress perspectives in advancing superconducting properties, broad perspectives of applications in various industries, including the power generation industry and microelectronic applications. Some authors deem the discovery as a breakthrough in superconductivity technologies. Soon after the discovery, the first superconducting MgB2 thin films were successfully demonstrated e.g. [2–6]. The study on both bulk and film properties and technology are still in increasing progress. Common features in most approaches presented so far in thin film fabrication are as follows: (1) sample is annealed at 600–900 1C in Mg vapor, which gives rise to synthesis of a superconducting MgB2 layer, (2) the synthesis occurs in the solid phase in the Mg–B system. In the present work an attempt is undertaken to synthesize for the first time, superconducting MgB2 film from liquid phase without annealing the B–Mg system in Mg vapor, using ion implantation (of one component) into the substrate (second component) followed by transient melting processes (TMP). For TMP one can use high intensity, short duration: laser, electron ion or plasma pulses. High intensity is defined here as the intensity which causes transient melting of the near-surface region of the substrate. Boron is used as an implanted species and magnesium as a substrate material. For TMT, high-intensity pulsed plasma beams (HIPPB) are adopted. Below, we describe the technological procedure as well as the methods and results of superconducting properties tests of the obtained samples. The less commonly known technological and analytical methods applied in this work are described in more detail.
2. Experimental 2.1. Sample processing The substrates were cut out from commercial magnesium ingot of 99.8 wt% purity in the form of highly polished discs of 3.8 mm diameter and 2 mm
thickness. They were implanted with boron ions using Danfysik Model 1009-200 implanter. Two batches of 25 samples each were implanted. In the 1st batch the ion energy was 100 keV and the dose 5 1018 B/cm2, in the 2nd batch the ion energy and the dose were 80 keV and 3 1018 B/cm2, respectively. In both cases the temperature of the substrate was kept at about 200–250 1C. Subsequent step of the adopted procedure was the transient melting process using HIPPBs. The plasma pulses were generated in a rod plasma injector (IBIS) type of accelerator described elsewhere [7]. Briefly, the plasma pulses are generated as a result of a low-pressure, high-current discharge between two concentric sets of electrodes. A high-voltage pulse delayed some time td with respect to the moment of the injection of the working gas into the inter-electrode space, ignites the discharge. If td is set long enough to allow the injected gas to expand over the whole interelectrode space, the plasma contains almost exclusively the elements of the working gas. This mode of operation is referred to as pulse implantation doping (PID). Typically it occurs for td of about 150 ms and 200 ms for hydrogen and argon as the working gases, respectively. For significantly shorter td ; when there is a steep gradient of the gas concentration in the inter-electrode space, effective erosion of the metallic electrodes occurs. Atoms and ions of intentionally selected electrode material are deposited and alloyed into the substrate. This regime is referred to as deposition by the pulsed erosion (DPE) process. Both sets of inner and outer electrodes in the IBIS plasma generator consist of 32 metallic rods of 2 mm diameter and 250 mm length. The electrode–substrate distance is 300 mm. Fluences of plasma pulses are in the range of 1–7 J/cm2. Pulse duration in the ms range corresponds to irradiance in the MW/cm2 range. In the present experiments, the 1st batch of samples were pulse treated with magnesium rods as electrodes and hydrogen as the working gas. The choice of electrode material was dictated by a precaution not to introduce any foreign atoms into the Mg–B system, whereas hydrogen was used to reduce (at least partly) oxygen always present in magnesium. In the 2nd batch, magnesium
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electrodes were replaced by molybdenum ones and hydrogen was replaced by argon working gas. These changes were introduced for two reasons. First, magnesium electrodes easily deform under electrodynamic forces exerted on them during the discharge, thus deteriorating the uniformity of the plasma beam. Second, argon atoms are much less mobile than hydrogen ones, which facilitates a more precise setting of the discharge conditions, especially with regard to the delay time td : Spectral diagnostics of the electrode erosion and the substrate material evaporation were done in the ‘‘Mo electrodes, Ar gas’’ setup. Optical spectra were observed using a Mechelles spectrometer coupled to a standard PC. By monitoring intensities of the Mo I 550.65 nm, Ar II 434.8 nm, and Ar II 480.6 nm lines vs. td it was possible to minimize emission of electrode material and its deposition on the irradiated substrates. On the other hand, by monitoring intensities of the Mg I 516.73 nm, Mg I 517.27 nm, Mg I 518.36 nm, and Mg II 448.1 nm lines it was possible to show that there was a substantial amount of magnesium atoms and ions just above the melted substrate surface. Intensities of the Mg lines were proportional to the pulse fluence in the 1–3 J/cm2 range. The latter finding qualitatively supports our expectation that a plum of Mg vapor/ions exerts a pressure on the liquid surface sufficient to prevent any significant loss of magnesium from the irradiated Mg–B system. In the present HIPPB treatment each sample was irradiated separately. The beam fluences were chosen from 1 to 3 J/cm2. The number of pulses ranged from 1 to 5. 2.2. Characterization 2.2.1. X-ray diffraction X-ray diffraction patterns (XRD) were obtained in the grazing incidence technique using a standard instrument (D5000, Bruker AXS) equipped with a thin film setup composed of a collimator and a secondary monochromator placed in front of the scintillation detector. At the used Cu Ka radiation and the applied incidence angle of 11, the 1/e penetration depth of the X-rays amounts to approximately 300 nm. Thus the recorded signals
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originate mainly in the pulse-modified thin surface layer, whereas the input from the magnesium substrate is diminished. The phase analysis was carried out using the PDF database implemented in the Bruker AXS evaluation code. 2.2.2. Rutherford backscattering (RBS) measurements In order to get insight into the composition of the processed layer, selected samples were subjected to RBS analysis. The spectra were recorded using 1.8 MeV a particle beam at normal incident to the sample. The backscattered particles were detected at 1701 angle using Si(Li) detector with 15 keV energy resolution. 2.2.3. Superconducting state detection Three methods were used to detect superconducting state (SC) in the processed samples: magnetic measurements (Oxford Instruments MagLab 200 System DC magnetometer/AC spectrometer), magnetically modulated microwave absorption (MMMA), and four-probe electric conductivity measurements. Microwave non-resonant dissipation observed around zero field (‘‘EPR-zero field line’’) is considered as a convenient method in characterizing SC. Since the discovery of high-temperature superconductors (HTS), the MMMA technique has been successfully established [8]. The MMMA signal in granular superconducting samples as well as in point superconductor–insulator–superconductor (SC–I–SC), weak contacts is the result of Josephson dissipation related to the fluctuation of Cooper pairs to electrons in the conducting band. A Josephson junctions system (JJS) can be considered as a separate phase embedded in the host surrounding. MMMA is a unique and very sensitive contact-less method to determine Tc for small amounts of SC phases dispersed below the percolation threshold. MMMA has already been applied to study intercalated fullerene, perovskitecomposite (superconductor–insulator), as well as MgB2. Thin films of MgB2 superconductor sample were placed in a microwave resonator of the EPR spectrometer with modulation of the sweeping magnetic field. In this procedure, the output signal
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has a differential form of absorbed power dP/dB [9]. A weak Josephson contact SC–I–SC is a basic element that interacts magnetically with the microwave field. The Josephson dissipation related to Cooper pairs breaking leads to voltage fluctuations like in the EPR spectroscopy. The Josephson hysteresis loop (JHL) is the confirmation that the zero field MMMA line is related to the SC. The JHL has an opposite circulation vs. magnetic field B in comparison to a typical magnetic hysteresis loop.
3. Results and discussion Fig. 1 presents grazing angle (o ¼ 1 ) XRD patterns for an as-implanted sample and for the sample treated after implantation with 2 hydrogen pulses of the fluence of about 3 J/cm2. The strongest MgB2 (1 0 1) peak for the plasma-treated sample is shifted toward the greater values of 2y with respect to the peak for the as-implanted sample. Such shift means that pulse treatment causes the shrinkage of the structure. This can be due to either a compressive strain resulting from the rapid re-solidification of the near-surface layer or non-stoichiometry. Obviously both these reasons can contribute simultaneously. It seems interesting to note at this point that the (1 0 1)
Fig. 1. X-ray diffraction patterns of the strongest MgB2 (1 0 1) reflection for the as-implanted sample and for the sample treated with 2 hydrogen plasma pulses of 2 J/cm2.
peak shifted towards greater 2y as in our case also in MgB2 films in which superconductivity was achieved after furnace annealing of the boronimplanted magnesium [4]. Similarly, in Ref. [10], the highest value of transition temperature Tc was observed for the sample in which the lattice constant c calculated from MgB2(0 0 2) peak position had the smallest value with Dc=c ¼ 0:2%: In view of these facts we could have expected that the procedure described in Section 2 may lead to formation of superconducting MgB2 phases. Fig. 2 presents the results of RBS measurements. It may be seen that in comparison to the spectrum of unimplanted sample (Mg), a pronounced valley centered at around channel 400 has appeared in the spectrum of the as-implanted sample (a). This feature is associated with a dilution of Mg matrix with boron during implantation. The Mg distribution after implantation exhibits a strange bimodal character, reflecting apparently the boron distribution. Such bimodal distributions in the as-implanted samples have already been reported in other cases [11], although at the moment we see no explanation for this effect. Samples (b) and (c) were treated with 2 hydrogen plasma pulses with growing fluence (2 and 3 J/cm2, respectively). It may be seen that the width of the valley (related to the width of the boron profile) grows with the pulse fluence. For sample (c) the boron profile seems to be spread over the greatest depth. On the other hand, the Mg signal value at the minimum of the spectrum does not change significantly. Rough estimations based on SIMNRA simulations indicate that the Mg content at the peak of boron profile, amounts to no more than 15–25%. In the absence of other components of the surface layer, it means that the attained level of boron concentration is not less than 75–85%—far in excess of the stoichiometric composition of MgB2. However, this excess of boron does not preclude the existence of MgB2 phase, as evidenced by the XRD spectra. The width of boron profile in sample (c) may be estimated to about 1 mm if the density of the boron-containing layer is taken as a composition-weighted average of boron and magnesium densities.
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Fig. 2. RBS spectra of the un-implanted sample (Mg), the as-implanted sample (a), the sample (b) treated with 2 hydrogen plasma pulses of 2 J/cm2 fluence, and the sample (c) treated with 2 hydrogen plasma pulses of 3 J/cm2 fluence.
It is interesting to compare the data described so far with those obtained in the superconductivity measurements. Fig. 3 shows the MMMA signals observed for samples (b) and (c) at temperatures 10 and 20 K. It is seen that MMMA signal hysteresis is comparable in both samples at 10 K. However, in sample (b) it disappears already at 20 K, while in sample (c) it is still present at that temperature. The critical temperatures as deduced from MMMA signal vs. temperature dependency amount to 17 and 31 K for samples (b) and (c), respectively. This means that spread boron profiles obtained at higher pulse fluences correlate with the formation of a more stable superconducting phase. The strongest MMMA signal hysteresis (higher by an order of magnitude with respect to samples (b) and (c)) has been obtained for sample (d) treated with 4 pulses of Ar plasma at the fluence of about 2 J/cm2 (Fig. 4, upper panel). However, Tc for that sample has a rather low value of about 12 K, as seen in the bottom panel in Fig. 4 presenting the magnetic moment after zero-field cooling (ZFC) induced by 10 Oe magnetic field vs.
temperature. This result demonstrates a possibility of creating a substantial amount of superconducting phase, with reduced Tc due to a lack of stoichiometry [12]. We do not attribute special importance to the plasma-pulse composition (hydrogen or argon). We rather suggest that it is the pulse energy and number of pulses which determine the final result of the treatment. The highest Tc has been obtained for high-energy pulses, but the highest superconducting phase content has been obtained for the largest number of pulses, equivalent to a long diffusion time in the molten phase. An unexpected although interesting result has been obtained in electrical conductivity measurements for the sample treated with 2 hydrogenplasma pulses of about 2 J/cm2 fluence (Fig. 5). When a four-point-probe measurement is applied in the direction perpendicular to the sample surface, the observed conductivity vs. temperature dependence resembles a typical dependence for semiconductors. However, when this measurement is performed in parallel to the surface, the dependence is typical of a metal. The course of
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Fig. 3. MMMA signal vs. external magnetic field at 10 and 20 K for the samples treated with 2 hydrogen plasma pulses of 2 J/cm2 fluence (left panel) and of 3 J/cm2 fluence (right panel). In both cases the gain was 105.
of current. The drop of parallel resistance below 32 K is obviously associated with the presence of superconducting MgB2 phase. However, this phase does not form a continuous layer since the resistivity does not fall down to zero. Apparently, separate islands of superconducting phase are connected through metallic Mg paths. The electrical properties of the latter give rise to the observed course of resistance at higher temperatures.
4. Conclusions
Fig. 4. MMMA signal vs. external magnetic field at 10 K (gain 104) for the sample treated with 4 argon plasma pulses of fluence 2 J/cm2 (upper panel). DC magnetic moment vs. temperature for the same sample at H ¼ 10 Oe (lower panel).
transverse conductance can probably be explained in terms of semiconductor-like boron layer located below the surface and blocking the transverse flow
We have demonstrated that boron implantation into magnesium, combined with plasma-pulse treatment leads to formation in a surface layer of superconducting grains with Tc as high as about 32 K. However, even in the samples showing a relatively large content of superconducting phase, no zero-resistance effect has been obtained due to incomplete global connectivity. In further research, we plan to investigate effects of reduced ion-implanted dose, increased pulse fluences, and pulse multiplicity. We hope these treatment modifications may help to spread and homogenize the superconducting phase.
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surements and Dr. W. Szymczyk (IPJ) for his valuable assistance and comments.
References
Fig. 5. Resistance vs. temperature for the sample treated with 2 hydrogen plasma pulses measured in the direction perpendicular to the sample plane (upper panel) and in the plane of the sample (lower panel). K and K0 denote geometrical factors.
Acknowledgements The authors wish to thank Dr. R. Gro¨tzschel (Forschungszentrum Rossendorf) for RBS mea-
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