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ESR studies on high-Tc superconductor MgB2
Physica C 390 (2003) 197–203 www.elsevier.com/locate/physc
ESR studies on high-Tc superconductor MgB2 €seogßlu a,*, B. Aktasß b, F. Yildiz b, D.K. Kim c, Y. Ko M. Toprak c, M. Muhammed c a
Department of Physics, Fatih University, Hadimkoy Kampusu Buyukcekmece, 34900 Istanbul, Turkey b Gebze Institute of Technology, PK. 141, 41400 Kocaeli, Turkey c Materials Chemistry Division, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received 30 October 2002; received in revised form 18 February 2003; accepted 13 March 2003
Abstract MgB2 , a high-Tc superconductor, has been studied by electron spin resonance (ESR) techniques at the temperature range of 7–300 K. Polycrystalline powders consisting of MgB2 , MgO and MgB4 phases were diluted and oriented in paraffin by applying an external magnetic field of 15 kG. A very narrow (2.5 G), strong, and isotropic signal that corresponded to almost free electron g-values was observed at all temperatures. Both the signal intensity and line width were observed to exhibit strong temperature dependence below Tc . The intensity of the ESR spectra, which corresponds to dc susceptibility, generally obeys the Curie law in this temperature range. However, some critical temperatures (approximately 215, 190, 150, and 39 K) were evident from both intensity and line width curves. While the ESR line is strongly broadened, the signal intensity significantly start to increase just below T ¼ 39 K (corresponding to a transition temperature from normal to superconducting state), passes through a broad maximum around 30 K and then shows a sharp decrease as the temperature is decreased further. The origins of the minor changes both in the intensity and the line width curves at other (higher) critical temperatures are not clear yet. In fact, the change at 215 K was observed to be meta-stable. These minor changes might be taken as signs for changes of local crystalline field symmetry around weakly localized conduction electrons or holes, which are the sources of the ESR signal. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 74.72.)h; 74.60.)w; 76.30.)v Keywords: Electron spin resonance (ESR); Superconductivity
1. Introduction The recent announcement of a new binary intermetallic superconductor, magnesium diboride (MgB2 ), having a remarkably high transition
*
Corresponding author. Tel.: +90-212-8890810; fax: +90212-8890832. E-mail address: [email protected] (Y. K€ oseogßlu).
temperature [1,2] has attracted great scientific and technological interest. The observation of an boron isotope effect [3] shows that there is a shift in Tc from 39.2 K for Mg11 B2 to 40.2 K for Mg10 B2 . The result is consistent with electron–phonon mediated BCS superconductivity. Temperature dependence of magnetic parameters, as the Ginzburg–Landau parameter K 26 [4], the upper critical field Hc2 (T) and lower critical field Hc1 (T) [4–8] were determined from transport and magnetization
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(03)00896-7
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measurements. These parameters were consistent with the expectation for widely believed s-wave superconductivity in MgB2 [6,7]. A model is proposed for s-wave superconductivity, including the thermodynamic and optical response in sintered MgB2 wires [9]. Strong evidence for high intergranular critical current densities and large bulk magnetic flux pinning has also been observed [10]. However, thermal and electrical conductivity in the basal plane of single crystalline MgB2 give supporting evidence for two different gaps on different sheets of the Fermi surface in the superconducting state [11]. However, NMR measurements indicate a negligible s-character of the wave function of the conduction electrons at the Fermi level [12]. Results of penetration depth measurements are evidence for unconventional superconductivity [13]. Susceptibility measurements of MgB2 powder in tube wires [14] give systematic differences in the flux pinning in the wires, which is in very good agreement with direct high transport current measurements. On the other hand, degradation behavior of polycrystalline samples exposed to water [15] show that the material becomes amorphous-like and it exhibits strongly hydroscopic behavior. Also, water and moist air react with MgB2 and change into Mg(OH)2 , MgCO3 , and B2 O3 , even at room temperature. Therefore, in order to maintain the longterm stability of this material in applications, coating with a suitable protective layer would be necessary. Furthermore, high frequency conduction electron spin resonance (HC-ESR) studies [16] of fine powders suggest that MgB2 is a strongly anisotropic superconductor with the upper critical field, Hc2 , ranging between 2 and 16 T. In contrast to this, magnetic and electronic anisotropy measurements for polycrystalline and thin films [17,18] indicate that MgB2 exhibits nearly isotropic (or 3D) behavior. In addition, the temperature dependence of the ESR and surface resistance of MgB2 and Li-doped MgB2 give a single, narrow, nearly symmetric line having an isotropic g-value of 2.0036. This dependence is attributed to mobile spin 1=2 holes in the hexagonal boron planes and shows some deviation of the surface resistance from the linear temperature dependence
starting at 150 K. This suggests the possibility for the existence of a spin gap in the less thanoptimally-doped material. Large changes in surface resistance are observed as a function of dc magnetic field in the superconducting state, resulting from dissipation due to vortex movement [19,20]. Magnetic field dependent microwave absorption studies [20] show that the absorbed p miffiffiffiffi crowave power has been found to obey a H dependence with a change of slope indicating a transition from a strongly pinned flux lattice to the flux flow regime. However, there are still some contradictions between Refs. [16] and [17–19]. From this motivation, we have also studied polycrystalline MgBx powders by ESR technique in a temperature range of 6–300 K. We have tried a new method, namely a microwave heating technique, for sample preparation to determine if there would be any influence on the physical parameters of the MgBx crystals.
2. Experimental 2.1. Sample preparation (microwave processing of MgB2 ) Samples were prepared by microwave heating of the different constituents. Heat is generated from inside the material, in contrast with conventional heating methods where heat is transferred from inwards. This internal heat allows a reduction in processing time and energy cost, while allowing for new material synthesis to be possible. In the microwave synthesis technique, not only is heat transferred from the basic solution to the reaction mixture but, MgBx itself produced heat due to its dielectric properties. In conventional synthesis, heat can only be transferred to the reaction mixture from external heat sources. This can make microwave heat treatment much more effective than conventional thermal treatment. A commercially available domestic microwave oven model operating at a maximum power level of 800 W was used in this study. The oven was equipped with a thermocouple for temperature control. The domestic microwave oven could be operated over the entire power range by alternat-
Y. K€oseogßlu et al. / Physica C 390 (2003) 197–203
ing from maximum to zero power in timed cycles (duty cycles). A small tube furnace was specially designed from a high temperature insulating material (Kaowool, 1700 °C). A silicon carbide susceptor was placed inside the furnace. This material has a very high dielectric constant and microwave absorptivity. Therefore, it is employed for hybrid heating. This also creates a homogeneous field effect for heating of the sample. Metallic powders of Mg and B were mixed homogeneously with a stoichiometric ratio of 1:2. The mixture was placed in a quartz tube of diameter 8 mm, and the tube was then evacuated and sealed. The sealed-tube was placed in an alumina tube with a diameter of 2 cm. Then, this tube was placed in the designed Kaowool (alumina board) and the reaction was initiated by microwave heating. A K-type thermocouple was directly inserted into the vicinity of the reaction zone. The temperature was monitored and maintained at 850 °C for 1 h. 2.2. Sample characterization The polycrystalline structure was evidenced by X-ray diffraction (XRD) pattern taken by using a fully automated Regaku 2000 DMAX diffractometer. Actually sample seems to contain MgB2 , MgB4 and MgO phases as seen in Fig. 1. The main phase was determined as MgB2 , and secondary phases as MgO and MgB4 from the XRD analysis compared to conventionally synthesized MgB2 . This method reduced the reaction time to one third required for conventional synthesis. Normally, due
199
to the slow diffusion of volatile B into Mg phase, this reaction takes 3 h under the same conditions by the conventional method. If the reaction time had been kept longer in the microwave synthesis, all the impurity phases would have gone to MgB2 phase and oxide phase could be removed subsequently by treating the sample with washing process. 2.3. ESR measurements The polycrystalline powders placed in paraffin just above its melting temperature have been oriented in the presence of a strong magnetic field (15 kG). The samples were subsequently cooled down below the melting temperature of paraffin in this field to have magnetic orientation. A sample with dimensions 1.5 2 2.5 mm was cut from this ingot for ESR measurements. A conventional X-band (f ffi 9:7 GHz) Bruker EMX model spectrometer employing an ac magnetic field modulation technique was used to record the first derivative absorption signal. An Oxford continuous helium gas flow cryostat has been used, allowing the X-band microwave cavity to remain at ambient temperatures during ESR measurements at low temperatures. The temperature was controlled by a conventional Lakeshore 340 temperature controller within an accuracy of 1° between 4 and 300 K. A goniometer was used to rotate the sample with respect to the external magnetic field in order to observe angular variations of the ESR spectra.
3. Results and discussion
Fig. 1. XRD pattern of microwave synthesized MgB2 .
ESR measurements have been carried out by varying external magnetic field orientation and sample temperature. A strong and narrow microwave resonance absorption (with approximately g ¼ 2.009, which is close to g-value of the free electron 2.0023, and peak-to-peak line width ¼ 2.5 G) is observed at room temperature. The negative part of the first derivative resonance curve is slightly smaller than the positive part. The noticeable asymmetry implies conducting properties of the sample at ambient temperatures. These
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Fig. 2. ESR spectrum and Dysonian fit for MgBx at 263 K.
Fig. 3. ESR spectra at some selected temperatures.
asymmetric ESR spectra were observed to fit quite well to a Dysonian line proposed for conductors, as seen in Fig. 2. This suggests that the sample has conducting properties. We have not observed any meaningful angular dependence of the ESR spectra. That is, both the oriented and as prepared powder samples give almost the same spectra, indicating an isotropic nature of individual crystallites in terms of the paramagnetic centers. However, the temperature strongly influences the ESR signals. Fig. 3 shows some examples of temperature variation in ESR spectra taken 7–263 K. As can be seen from Fig. 3, the intensity of the signal increases steadily with decreasing temperature until 40 K, starts to increase at Tc ¼ 39 K, passes through a broad maximum at around 30 and then decrease quite sharply again until the lowest temperature (7 K in our case). The signal also becomes significantly noisy below 39 K, as the noise level increases as the temperature is decreased. It should be noted that both the strong decrease in signal and increase in amplitude of the noise start at the same temperature, Tc , corresponding to a transition to the superconducting state of MgB2 . Therefore, these effects are most likely related to the superconductivity. The very broad absorption at the zero-field region is not due to paramagnetic resonance; rather it should come from microwave absorption by supercon-
ducting electrons (from dissipative motions of the vortex due to alternating Lorentz forces in microwave region), usually observed in any superconducting sample [20]. This portion of the absorption shows hysteretic behavior with respect to sweep direction of the external dc field, as can be seen in Fig. 4. On the other hand, the position of the signal originating from paramagnetic electrons seems to remain almost unchanged in the field with respect
Fig. 4. Microwave absorption against external field.
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Fig. 5. ESR line width and intensity T 2 versus temperature. Fig. 6. Intensity and intensity T versus temperature.
to the temperature; i.e. g-values are not remarkably affected from the temperature. The values for the intensity obtained from second integration of the ESR signal, which corresponds to dc susceptibility, are plotted as a function of temperature in Fig. 5. Being well known [21], the field dependence of microwave power absorption (first integral of ESR spectra) by the sample is proportional to the imaginary component of transverse (perpendicular to the strong dc field) ac susceptibility of the sample. Some minor changes and deviation from the Curie law are noticeable at temperatures above Tc , as marked on the Fig. 5. In order to see the temperature behavior in more detail, the intensity has been multiplied by T and plotted the resultant intensity values together in the same figure (Fig. 6). There is a sharp change in intensity at Tc , seen in Fig. 6. The sudden bound in intensity around Tc is reminiscent to the similar bound that is usually observed in heat capacity curves due to an excess entropy gain [22] at the critical temperature. As can be seen from this figure, the temperature behavior of the intensity (susceptibility) essentially obeys the Curie law at the temperature above Tc . As mentioned above the signal starts to increase at Tc ¼ 39 K, passes through a broad maximum at around 30 and then decrease quite sharply again until lowest temperature (7 K in our case). It
should be recalled that the absorption intensity is proportional both to the microwave power and number of paramagnetic centers. One could say that the observed increase in the intensity just below Tc might be attributed to impurity phases. Signal from impurity grain phases tend to increase at Tc because of the increased microwave intensity over the impurity grains when the microwaves are excluded from the superconducting grains. But the decrement in absorption intensity below 30 K after this initial increment just below Tc could be expected, partly due to screening of microwave penetration into paramagnetic centers at the inner regions and partly due to a decrease in the number of paramagnetic conduction electrons as a result of continuous evolution of pairing with decreasing temperature below Tc of MgB2 . Also, included in Fig. 5 is the temperature behavior of the line width. The most striking change in the line width curve occurred in the temperature range below Tc . The line width drastically increased with decreasing temperature in this regime. This curve is very smooth and is almost constant minimum in the temperature range 40–125 K, consistently with the Li-doped sample [19]. It tends to slightly increase at higher temperatures. However, there are some small step-like changes in this curve at certain temperature values where changes are observed in intensity value, as mentioned above. The increments at about 150, 180 and 215 K
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are quite remarkable. It should be remembered that Li-doped MgB2 also shows some deviation of the surface resistance from the linear temperature dependence starting at 150 K. This was attributed to the possibility of the existence of a spin gap in the less-than-optimally-doped material [19]. In fact, we have observed some unstable changes both in line width and intensity curves at 215 K, but they were washed out after a few field sweeps. We could not yet clarify the origin of this effect. This effect will be studied in more detail.
4. Conclusions The sample seems to be a polycrystalline powder, as seen in Fig. 1. However, it contains two phases, approximately MgB2 ––75%, MgO––20% and MgB4 ––5%. In addition to usual microwave absorption from a superconducting sample, a paramagnetic signal was observed for almost all the temperature range. The broad absorptions starting from zero-field show very hysteretic behavior with respect to the sweep direction of a large external field. This is common behavior for high-Tc superconductors. A very sharp transition was observed from normal to superconducting states at Tc ¼ 39 K. The ESR line is very strong, narrow [18,19], isotropic, and asymmetric (Dysonian). However, the ESR line is much more symmetrical in our case when compared to the study at the Q-band [18], perhaps due to dilution of the powder. The source of the signal is nearly free conduction electrons or holes. One would expect significantly broad lines from a polycrystalline (powder) sample. However, the ESR signals from the present sample are very narrow (2.5 G) and isotropic compared to the other studies in the literature [17,18]. The line width ranges 20–25 G from [19] and 10 G at Tc and 110 G at 300 K [16]. From these results, one can say that the microcrystallites are very isotropic with respect to paramagnetic centers in terms of the spectroscopic splitting factor, g. The narrow line can also be caused by the exchange narrowing effect in our case. That is hopping of paramagnetic centers (conduction electrons or holes) should be much faster in our case.
Temperature behavior of the signal intensity below Tc is attributed partly to the decrease in the number of paramagnetic conducting carriers, due to pairing and screening of the microwave below Tc . In order to suppress the screening effect, the MgB2 crystalline powder was diluted in melted paraffin. Thus, we were able to observe an ESR signal from paramagnetic centers much below Tc , even at 6 K. The noise, inhomogeneous vortex field, and screening effects would have prevented the observation of ESR signal just below Tc . The secondary phases (MgO and MgB4 ) serve as a boundary for super currents, not allowing a very large vortex to distort resonance line, i.e. resonance signal, too much. Thus, the line becomes relatively narrower to be measurable below Tc . The significantly sharp changes in ESR line width and intensity curves at various temperatures above Tc point to some changes in electronic structure. Especially, the step-like change in line width curve at about 215 K is remarkably pronounced. Both intensity and line width curves are correlated and varied simultaneously at certain temperatures. These changes might be speculated to unknown modifications in either crystallographic or electronic structures. We are planning to investigate these effects in more details by preparing MgB2 in single crystal form with different dopants. The g-value, from our measurements, was observed to remain almost constant while Owens [19] has observed a very small shift in Lidoped samples. Since the line width is much smaller than those in literature, the uncertainty in spin Hamiltonian parameters should obviously be smaller in our case. The position of the paramagnetic resonance peak is not practically affected by the superconducting state, indicating a very small diamagnetic field. This might be caused by both dilution of the powder in paraffin and a relatively higher ratio of secondary phases, MgO and MgB4 , compared to the other studies in the literature.
References [1] J. Akimitsu, Symposium on Transition Metal Oxides, Sendai, Japan, 2001.
Y. K€oseogßlu et al. / Physica C 390 (2003) 197–203 [2] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [3] S.L. BudÕko, G. Lapertot, C. Petrovic, C.E. Cunningham, N. Anderson, P.C. Canfield, cond-mat/0101463v2, 3 February 2001. [4] A.G. Joshi, C.G.S. Pillai, P. Raj, S.K. Malik, Solid State Commun. 118 (2001) 445. [5] K.H. M€ uller, G. Fuchs, A. Handstein, K. Nenkov, V.N. Narozhnyi, D. Eckert, J Alloys Comp. 322 (2001) L10. [6] J.P. Joshi, S. Sarangi, A.K. Sood, S.V. Bhat, D. Pal, condmat/0103369 v2, 20 March 2001. [7] S.L. Li, H.H. Wen, Z.W. Zhao, Y.M. Ni, Z.A. Ren, G.C. Che, H.P. Yang, Z.Y. Liu, Z.X. Zhao, cond-mat/0103032 v3, 4 March 2001. [8] S.L. BudÕko, C. Petrovic, G. Lapertot, C.E. Cunningham, P.C. Canfield, cond-mat/0102413 v2, 23 February 2001. [9] S. Haas, K. Mahi, cond-mat/0104207 v1, 11 April 2001. [10] M. Kambara, N.H. Babu, E.S. Sadki, J.R. Cooper, H. Minami, D.A. Cardwell, A.M. Campbell, I.H. Inoue, Supercond. Sci. Technol. 14 (2001) L5. [11] A.V. Sologubenko, J. Jun, S.M. Kazakov, J. Karpinski, H.R. Ott, cond-mat/0111273, 15 November 2001. [12] J.K. Jung, S.H. Baek, F. Borsa, S.L. BudÕko, G. Lapertot, P.C. Canfield, cond-mat/0103040, 1 March 2001.
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[13] C. Panagopoulos, B.D. Rainford, T. Xiang, C.A. Scott, M. Kambara, I.H. Inoue, cond-mat/0103060, 2 March 2001. [14] B.A. Glowacki, M. Majoros, M. Bickers, J.E. Evetts, Y. Shi, I.M. Dougal, Supercond. Sci. Technol. 14 (2001) 193. [15] D.K. Aswal, K.P. Muthe, A. Singh, S. Sen, K. Shah, L.C. Gupta, S.K. Gupta, V.C. Sahni, Physica C 363 (2001) 208. [16] F. Simon, A. Janossy, T. Feher, F. Muranyi, S. Garaj, L. Forro, C. Petrovic, S.L. BudÕko, G. Lapertot, V.G. Kogan, P.C. Canfield, cond-mat/0104557, 28 April 2001. [17] O.F. de Lima, R.A. Riberio, M.A. Avila, C.A. Cardoso, A.A. Coelho, cond-mat/0103287, 13 March 2001. [18] S. Patnaik, L.D. Cooley, A. Gurevich, A.A. Polyanskii, J. Jiang, X.Y. Cai, A.A. Squitieri, M.T. Naus, M.K. Lee, J.H. Choi, L. Belenky, S.D. Bu, J. Letteri, X. Song, D.G. Schlom, S.E. Babcook, C.B. Eom, E.E. Hellstrom, D.C. Larbalester, Supercond. Sci. Technol. 14 (2001) 315. [19] F.J. Owens, Physica C 363 (2001) 202. [20] M.K. Bhide, R.M. Kadom, M.D. Sastry, A. Singh, S. Sen, D.K. Aswal, S.K. Gupta, V.C. Sahni, Supercond. Sci. Technol. 14 (2001) 572. [21] C.P. Schlichter, Principles of Magnetic Resonance, Harper & Row, New York, 1963. [22] S.B. Oseroff, S.W. Cheong, B. Aktasß, M.F. Hundley, Z. Fisk, L.W. Rupp Jr., Phys. Rev. Lett. 74-8 (1995) 1450.
ESR studies on high-Tc superconductor MgB2 €seogßlu a,*, B. Aktasß b, F. Yildiz b, D.K. Kim c, Y. Ko M. Toprak c, M. Muhammed c a
Department of Physics, Fatih University, Hadimkoy Kampusu Buyukcekmece, 34900 Istanbul, Turkey b Gebze Institute of Technology, PK. 141, 41400 Kocaeli, Turkey c Materials Chemistry Division, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received 30 October 2002; received in revised form 18 February 2003; accepted 13 March 2003
Abstract MgB2 , a high-Tc superconductor, has been studied by electron spin resonance (ESR) techniques at the temperature range of 7–300 K. Polycrystalline powders consisting of MgB2 , MgO and MgB4 phases were diluted and oriented in paraffin by applying an external magnetic field of 15 kG. A very narrow (2.5 G), strong, and isotropic signal that corresponded to almost free electron g-values was observed at all temperatures. Both the signal intensity and line width were observed to exhibit strong temperature dependence below Tc . The intensity of the ESR spectra, which corresponds to dc susceptibility, generally obeys the Curie law in this temperature range. However, some critical temperatures (approximately 215, 190, 150, and 39 K) were evident from both intensity and line width curves. While the ESR line is strongly broadened, the signal intensity significantly start to increase just below T ¼ 39 K (corresponding to a transition temperature from normal to superconducting state), passes through a broad maximum around 30 K and then shows a sharp decrease as the temperature is decreased further. The origins of the minor changes both in the intensity and the line width curves at other (higher) critical temperatures are not clear yet. In fact, the change at 215 K was observed to be meta-stable. These minor changes might be taken as signs for changes of local crystalline field symmetry around weakly localized conduction electrons or holes, which are the sources of the ESR signal. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 74.72.)h; 74.60.)w; 76.30.)v Keywords: Electron spin resonance (ESR); Superconductivity
1. Introduction The recent announcement of a new binary intermetallic superconductor, magnesium diboride (MgB2 ), having a remarkably high transition
*
Corresponding author. Tel.: +90-212-8890810; fax: +90212-8890832. E-mail address: [email protected] (Y. K€ oseogßlu).
temperature [1,2] has attracted great scientific and technological interest. The observation of an boron isotope effect [3] shows that there is a shift in Tc from 39.2 K for Mg11 B2 to 40.2 K for Mg10 B2 . The result is consistent with electron–phonon mediated BCS superconductivity. Temperature dependence of magnetic parameters, as the Ginzburg–Landau parameter K 26 [4], the upper critical field Hc2 (T) and lower critical field Hc1 (T) [4–8] were determined from transport and magnetization
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(03)00896-7
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measurements. These parameters were consistent with the expectation for widely believed s-wave superconductivity in MgB2 [6,7]. A model is proposed for s-wave superconductivity, including the thermodynamic and optical response in sintered MgB2 wires [9]. Strong evidence for high intergranular critical current densities and large bulk magnetic flux pinning has also been observed [10]. However, thermal and electrical conductivity in the basal plane of single crystalline MgB2 give supporting evidence for two different gaps on different sheets of the Fermi surface in the superconducting state [11]. However, NMR measurements indicate a negligible s-character of the wave function of the conduction electrons at the Fermi level [12]. Results of penetration depth measurements are evidence for unconventional superconductivity [13]. Susceptibility measurements of MgB2 powder in tube wires [14] give systematic differences in the flux pinning in the wires, which is in very good agreement with direct high transport current measurements. On the other hand, degradation behavior of polycrystalline samples exposed to water [15] show that the material becomes amorphous-like and it exhibits strongly hydroscopic behavior. Also, water and moist air react with MgB2 and change into Mg(OH)2 , MgCO3 , and B2 O3 , even at room temperature. Therefore, in order to maintain the longterm stability of this material in applications, coating with a suitable protective layer would be necessary. Furthermore, high frequency conduction electron spin resonance (HC-ESR) studies [16] of fine powders suggest that MgB2 is a strongly anisotropic superconductor with the upper critical field, Hc2 , ranging between 2 and 16 T. In contrast to this, magnetic and electronic anisotropy measurements for polycrystalline and thin films [17,18] indicate that MgB2 exhibits nearly isotropic (or 3D) behavior. In addition, the temperature dependence of the ESR and surface resistance of MgB2 and Li-doped MgB2 give a single, narrow, nearly symmetric line having an isotropic g-value of 2.0036. This dependence is attributed to mobile spin 1=2 holes in the hexagonal boron planes and shows some deviation of the surface resistance from the linear temperature dependence
starting at 150 K. This suggests the possibility for the existence of a spin gap in the less thanoptimally-doped material. Large changes in surface resistance are observed as a function of dc magnetic field in the superconducting state, resulting from dissipation due to vortex movement [19,20]. Magnetic field dependent microwave absorption studies [20] show that the absorbed p miffiffiffiffi crowave power has been found to obey a H dependence with a change of slope indicating a transition from a strongly pinned flux lattice to the flux flow regime. However, there are still some contradictions between Refs. [16] and [17–19]. From this motivation, we have also studied polycrystalline MgBx powders by ESR technique in a temperature range of 6–300 K. We have tried a new method, namely a microwave heating technique, for sample preparation to determine if there would be any influence on the physical parameters of the MgBx crystals.
2. Experimental 2.1. Sample preparation (microwave processing of MgB2 ) Samples were prepared by microwave heating of the different constituents. Heat is generated from inside the material, in contrast with conventional heating methods where heat is transferred from inwards. This internal heat allows a reduction in processing time and energy cost, while allowing for new material synthesis to be possible. In the microwave synthesis technique, not only is heat transferred from the basic solution to the reaction mixture but, MgBx itself produced heat due to its dielectric properties. In conventional synthesis, heat can only be transferred to the reaction mixture from external heat sources. This can make microwave heat treatment much more effective than conventional thermal treatment. A commercially available domestic microwave oven model operating at a maximum power level of 800 W was used in this study. The oven was equipped with a thermocouple for temperature control. The domestic microwave oven could be operated over the entire power range by alternat-
Y. K€oseogßlu et al. / Physica C 390 (2003) 197–203
ing from maximum to zero power in timed cycles (duty cycles). A small tube furnace was specially designed from a high temperature insulating material (Kaowool, 1700 °C). A silicon carbide susceptor was placed inside the furnace. This material has a very high dielectric constant and microwave absorptivity. Therefore, it is employed for hybrid heating. This also creates a homogeneous field effect for heating of the sample. Metallic powders of Mg and B were mixed homogeneously with a stoichiometric ratio of 1:2. The mixture was placed in a quartz tube of diameter 8 mm, and the tube was then evacuated and sealed. The sealed-tube was placed in an alumina tube with a diameter of 2 cm. Then, this tube was placed in the designed Kaowool (alumina board) and the reaction was initiated by microwave heating. A K-type thermocouple was directly inserted into the vicinity of the reaction zone. The temperature was monitored and maintained at 850 °C for 1 h. 2.2. Sample characterization The polycrystalline structure was evidenced by X-ray diffraction (XRD) pattern taken by using a fully automated Regaku 2000 DMAX diffractometer. Actually sample seems to contain MgB2 , MgB4 and MgO phases as seen in Fig. 1. The main phase was determined as MgB2 , and secondary phases as MgO and MgB4 from the XRD analysis compared to conventionally synthesized MgB2 . This method reduced the reaction time to one third required for conventional synthesis. Normally, due
199
to the slow diffusion of volatile B into Mg phase, this reaction takes 3 h under the same conditions by the conventional method. If the reaction time had been kept longer in the microwave synthesis, all the impurity phases would have gone to MgB2 phase and oxide phase could be removed subsequently by treating the sample with washing process. 2.3. ESR measurements The polycrystalline powders placed in paraffin just above its melting temperature have been oriented in the presence of a strong magnetic field (15 kG). The samples were subsequently cooled down below the melting temperature of paraffin in this field to have magnetic orientation. A sample with dimensions 1.5 2 2.5 mm was cut from this ingot for ESR measurements. A conventional X-band (f ffi 9:7 GHz) Bruker EMX model spectrometer employing an ac magnetic field modulation technique was used to record the first derivative absorption signal. An Oxford continuous helium gas flow cryostat has been used, allowing the X-band microwave cavity to remain at ambient temperatures during ESR measurements at low temperatures. The temperature was controlled by a conventional Lakeshore 340 temperature controller within an accuracy of 1° between 4 and 300 K. A goniometer was used to rotate the sample with respect to the external magnetic field in order to observe angular variations of the ESR spectra.
3. Results and discussion
Fig. 1. XRD pattern of microwave synthesized MgB2 .
ESR measurements have been carried out by varying external magnetic field orientation and sample temperature. A strong and narrow microwave resonance absorption (with approximately g ¼ 2.009, which is close to g-value of the free electron 2.0023, and peak-to-peak line width ¼ 2.5 G) is observed at room temperature. The negative part of the first derivative resonance curve is slightly smaller than the positive part. The noticeable asymmetry implies conducting properties of the sample at ambient temperatures. These
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Fig. 2. ESR spectrum and Dysonian fit for MgBx at 263 K.
Fig. 3. ESR spectra at some selected temperatures.
asymmetric ESR spectra were observed to fit quite well to a Dysonian line proposed for conductors, as seen in Fig. 2. This suggests that the sample has conducting properties. We have not observed any meaningful angular dependence of the ESR spectra. That is, both the oriented and as prepared powder samples give almost the same spectra, indicating an isotropic nature of individual crystallites in terms of the paramagnetic centers. However, the temperature strongly influences the ESR signals. Fig. 3 shows some examples of temperature variation in ESR spectra taken 7–263 K. As can be seen from Fig. 3, the intensity of the signal increases steadily with decreasing temperature until 40 K, starts to increase at Tc ¼ 39 K, passes through a broad maximum at around 30 and then decrease quite sharply again until the lowest temperature (7 K in our case). The signal also becomes significantly noisy below 39 K, as the noise level increases as the temperature is decreased. It should be noted that both the strong decrease in signal and increase in amplitude of the noise start at the same temperature, Tc , corresponding to a transition to the superconducting state of MgB2 . Therefore, these effects are most likely related to the superconductivity. The very broad absorption at the zero-field region is not due to paramagnetic resonance; rather it should come from microwave absorption by supercon-
ducting electrons (from dissipative motions of the vortex due to alternating Lorentz forces in microwave region), usually observed in any superconducting sample [20]. This portion of the absorption shows hysteretic behavior with respect to sweep direction of the external dc field, as can be seen in Fig. 4. On the other hand, the position of the signal originating from paramagnetic electrons seems to remain almost unchanged in the field with respect
Fig. 4. Microwave absorption against external field.
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Fig. 5. ESR line width and intensity T 2 versus temperature. Fig. 6. Intensity and intensity T versus temperature.
to the temperature; i.e. g-values are not remarkably affected from the temperature. The values for the intensity obtained from second integration of the ESR signal, which corresponds to dc susceptibility, are plotted as a function of temperature in Fig. 5. Being well known [21], the field dependence of microwave power absorption (first integral of ESR spectra) by the sample is proportional to the imaginary component of transverse (perpendicular to the strong dc field) ac susceptibility of the sample. Some minor changes and deviation from the Curie law are noticeable at temperatures above Tc , as marked on the Fig. 5. In order to see the temperature behavior in more detail, the intensity has been multiplied by T and plotted the resultant intensity values together in the same figure (Fig. 6). There is a sharp change in intensity at Tc , seen in Fig. 6. The sudden bound in intensity around Tc is reminiscent to the similar bound that is usually observed in heat capacity curves due to an excess entropy gain [22] at the critical temperature. As can be seen from this figure, the temperature behavior of the intensity (susceptibility) essentially obeys the Curie law at the temperature above Tc . As mentioned above the signal starts to increase at Tc ¼ 39 K, passes through a broad maximum at around 30 and then decrease quite sharply again until lowest temperature (7 K in our case). It
should be recalled that the absorption intensity is proportional both to the microwave power and number of paramagnetic centers. One could say that the observed increase in the intensity just below Tc might be attributed to impurity phases. Signal from impurity grain phases tend to increase at Tc because of the increased microwave intensity over the impurity grains when the microwaves are excluded from the superconducting grains. But the decrement in absorption intensity below 30 K after this initial increment just below Tc could be expected, partly due to screening of microwave penetration into paramagnetic centers at the inner regions and partly due to a decrease in the number of paramagnetic conduction electrons as a result of continuous evolution of pairing with decreasing temperature below Tc of MgB2 . Also, included in Fig. 5 is the temperature behavior of the line width. The most striking change in the line width curve occurred in the temperature range below Tc . The line width drastically increased with decreasing temperature in this regime. This curve is very smooth and is almost constant minimum in the temperature range 40–125 K, consistently with the Li-doped sample [19]. It tends to slightly increase at higher temperatures. However, there are some small step-like changes in this curve at certain temperature values where changes are observed in intensity value, as mentioned above. The increments at about 150, 180 and 215 K
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are quite remarkable. It should be remembered that Li-doped MgB2 also shows some deviation of the surface resistance from the linear temperature dependence starting at 150 K. This was attributed to the possibility of the existence of a spin gap in the less-than-optimally-doped material [19]. In fact, we have observed some unstable changes both in line width and intensity curves at 215 K, but they were washed out after a few field sweeps. We could not yet clarify the origin of this effect. This effect will be studied in more detail.
4. Conclusions The sample seems to be a polycrystalline powder, as seen in Fig. 1. However, it contains two phases, approximately MgB2 ––75%, MgO––20% and MgB4 ––5%. In addition to usual microwave absorption from a superconducting sample, a paramagnetic signal was observed for almost all the temperature range. The broad absorptions starting from zero-field show very hysteretic behavior with respect to the sweep direction of a large external field. This is common behavior for high-Tc superconductors. A very sharp transition was observed from normal to superconducting states at Tc ¼ 39 K. The ESR line is very strong, narrow [18,19], isotropic, and asymmetric (Dysonian). However, the ESR line is much more symmetrical in our case when compared to the study at the Q-band [18], perhaps due to dilution of the powder. The source of the signal is nearly free conduction electrons or holes. One would expect significantly broad lines from a polycrystalline (powder) sample. However, the ESR signals from the present sample are very narrow (2.5 G) and isotropic compared to the other studies in the literature [17,18]. The line width ranges 20–25 G from [19] and 10 G at Tc and 110 G at 300 K [16]. From these results, one can say that the microcrystallites are very isotropic with respect to paramagnetic centers in terms of the spectroscopic splitting factor, g. The narrow line can also be caused by the exchange narrowing effect in our case. That is hopping of paramagnetic centers (conduction electrons or holes) should be much faster in our case.
Temperature behavior of the signal intensity below Tc is attributed partly to the decrease in the number of paramagnetic conducting carriers, due to pairing and screening of the microwave below Tc . In order to suppress the screening effect, the MgB2 crystalline powder was diluted in melted paraffin. Thus, we were able to observe an ESR signal from paramagnetic centers much below Tc , even at 6 K. The noise, inhomogeneous vortex field, and screening effects would have prevented the observation of ESR signal just below Tc . The secondary phases (MgO and MgB4 ) serve as a boundary for super currents, not allowing a very large vortex to distort resonance line, i.e. resonance signal, too much. Thus, the line becomes relatively narrower to be measurable below Tc . The significantly sharp changes in ESR line width and intensity curves at various temperatures above Tc point to some changes in electronic structure. Especially, the step-like change in line width curve at about 215 K is remarkably pronounced. Both intensity and line width curves are correlated and varied simultaneously at certain temperatures. These changes might be speculated to unknown modifications in either crystallographic or electronic structures. We are planning to investigate these effects in more details by preparing MgB2 in single crystal form with different dopants. The g-value, from our measurements, was observed to remain almost constant while Owens [19] has observed a very small shift in Lidoped samples. Since the line width is much smaller than those in literature, the uncertainty in spin Hamiltonian parameters should obviously be smaller in our case. The position of the paramagnetic resonance peak is not practically affected by the superconducting state, indicating a very small diamagnetic field. This might be caused by both dilution of the powder in paraffin and a relatively higher ratio of secondary phases, MgO and MgB4 , compared to the other studies in the literature.
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