- Email: [email protected]
Coexistence of superconductivity and magnetism in the Tm-based superconductor probed by muon spin relaxation
ARTICLE IN PRESS Physica B 404 (2009) 740–742
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Coexistence of superconductivity and magnetism in the Tm-based superconductor probed by muon spin relaxation Naoki Kase a,, Jun Akimitsu a, Yasuyuki Ishii b, Takao Suzuki b, Isao Watanabe b, Masanori Miyazaki c, Masatoshi Hiraishi c, Soshi Takeshita d, Ryosuke Kadono c,d a
Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 229-8558, Japan Advanced Meson Science Laboratory, RIKEN Nishina Center, Wako, Saitama 351-0198, Japan c Department of Materials Structure Science, The Graduate University for Advanced Studies, Tsukuba, Ibaraki 305-0801, Japan d Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan b
a r t i c l e in f o
Keywords: ðSn1x Tmx ÞTm4 Rh6 Sn18 Superconductivity Magnetism mSR
a b s t r a c t The magnetic property of a thulium-based superconductor, Tm5Rh6Sn18 (superconducting transition temperature T c ¼ 2:2 K), is investigated by muon spin relaxation ðmSRÞ. Below around 6 K, the development of a quasi-static local magnetic field is clearly inferred from the observation of spontaneous oscillation signal in the mSR spectra under zero external field, where the magnetism persists even below T c. The internal magnetic field at the muon site, Hm , is estimated to be approximately 175 Oe. The fractional volume of the magnetic component is estimated to be 100%, strongly suggesting that the magnetism coexists with superconductivity. & 2008 Elsevier B.V. All rights reserved.
1. Introduction ðSn1x Rx ÞR4 T6 Sn18 T ¼ transition metal) compounds belong to a large stannide series [1]. These stannides crystalize in a tetragonal structure with a space group I41 =acd, and exhibit magnetic and/or superconducting transitions [2]. The reentrant superconductor ðSn1x Erx ÞEr4 Rh6 Sn18 has attracted much interest for the magnetic and superconducting state, because the physical properties dramatically change due to the difference of the composition of x; x0 is superconducting below T ’ 1:3 K down to 50 mK; x0:3 is a reentrant superconducting with T c ’ 1:05 K and T M ’ 0:5 K; x0:75 has a magnetic order at 0.65 K and is not superconductor [2,3]. On the other hand, ðSn1x Tmx ÞTm4 Rh6 Sn18 becomes superconductor at 2.2 K and exhibits a reentrant superconducting behavior demonstrated by the temperature dependence of the resistivity at fields higher than 1.4 kOe (above 0.4 K). In addition, because the composition of x is a constant value of about 0.8, ðSn1x Tmx ÞTm4 Rh6 Sn18 can be nearly described as a chemical formula of Tm5Rh6Sn18 ðx ’ 0:8Þ [4]. However, compared to the other reentrant superconductors, Tm5Rh6Sn18 still has far less information on the magnetic ordering or superconducting properties from the previous literature. This motivated us to investigate the superconducting state and the
Corresponding author.
E-mail address: [email protected] (N. Kase). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.11.185
relationship between magnetism and superconductivity more preciously from the view point of microscopic measurements. We investigated at the microscopic level the coexistence between magnetism and superconducting state by means of the muon spin relaxation (mSR) method. The mSR measurement shows the development of a quasi-static local magnetic field is clearly inferred from the observation of a spontaneous oscillation signal in the mSR spectra under zero external field, where the magnetism persists even below T c. Our result establishes the coexistence of superconductivity and magnetic ordering in the Tm-based superconductor.
2. Experimental detail Single crystals of Tm5Rh6Sn18 were grown by the Sn flux method. The starting materials were 99.9%-Tm powder, 99.9%-Rh powder and 99.999%-Sn shot. These materials were sealed in an evacuated quartz tube, with off-stoichiometric composition of Tm:Rh:Sn ¼ 1: 1:2: 10. The quartz tube was heated up to 1050, maintained at this temperature for about 3 h, and cooled down to 200 at the rate of 5/h, taking 7 days in total. The excess flux was removed from the crystals by spinning the ampoule in the centrifuge. The single-crystalline nature has been verified by using backscattering X-ray technique. The powder X-ray diffraction patterns could be indexed as the Tm5Rh6Sn18 with the space group I41 =acd. The electrical resistivity was measured by the conventional DC four-probe method in the temperature range
ARTICLE IN PRESS N. Kase et al. / Physica B 404 (2009) 740–742
741
from 0.4 to 300 K under various (including zero) applied magnetic fields with a PPMS system (Quantum Design Co., Ltd.). The mSR measurements were carried out at the RIKEN-RAL Muon Facility in the Rutherford Appleton Laboratory, which provided a pulsed beam of nearly 100% spin-polarized muons. The polycrystalline samples obtained from single crystals crushed into a fine powder were pressed into pellets, and sintered at 900 for 24 h under a high vacuum condition of 4:0 103 Pa. The sample was glued onto a high purity silver (99.998%) holder to avoid a depolarizing background mSR signal, and mounted into a 3 He cryostat. During the measurement under zero field (ZF), residual magnetic field at the sample position was reduced below 105 kOe.
3. Experimental results and discussion 3.1. Electrical resistivity Superconductivity can be inferred from electrical resistivity measurements, as shown in Fig. 1. The onset temperature of the superconducting transition and the temperature of zero resistivity are observed to be 2.30 and 2.20 K. The transition width DT is considered as the temperature interval between 10% and 90% of the transition and is observed to be approximately 0.07 K. The electrical resistivity in the normal state increases with a decrease in the temperature, indicating abnormal metallic behavior. This anomaly can be described as ln T behavior, which strongly indicates that Kondo effect is significant in this material. Fig. 2(a) shows the temperature dependence of the electrical resistivity under several magnetic fields. Tm5Rh6Sn18 become superconducting at 2.2 K and does not return to the normal state down to 0.4 K under zero magnetic field. At fields higher than 1.3 kOe, the superconducting state is broken due to the occurrence of the magnetic ordering. In fields exceeding 1.70 kOe the superconducting state is no longer fully established. Fig. 2(b) shows the superconducting phase diagram of Tm5Rh6Sn18. The T M and T c are determined by the mid-point temperature of T zero and T onset , and T is the moderate drop of the resistivity. What is the origin of T is not understood, but the fact that T depends on magnetic fields indicates low probability of the impurity effect of Sn (Sn; Hc (0) ¼ 305 Oe) or magnetic ordering.
Fig. 1. Temperature dependence of the electrical resistivity of Tm5Rh6Sn18. The inset shows the expansion at low temperature region.
Fig. 2. (a) Temperature dependence of the electrical resistivity of Tm5Rh6Sn18 under several magnetic fields. (b) Phase diagram of Tm5Rh6Sn18. The closed symbol shows superconducting transition, the square symbol shows breaking superconducting transition, and the open symbol shows a moderate drop of the resistivity.
3.2. mSR measurement ZF-mSR is the most sensitive technique to examine magnetism in any form, where the development of local magnetic moment leads to either spontaneous oscillation or exponential damping of AðtÞ. Fig. 3 shows the time-dependent muon–positron decay asymmetry under zero external field at several temperatures in Tm5Rh6Sn18. Below around 6 K, development of a quasi-static local magnetic field is clearly inferred from the observation of a spontaneous oscillation signal in the mSR spectra. Surprisingly, the spontaneous oscillation signal was observed even below the superconducting transition temperature T c ¼ 2:2 K. The ZF-mSR time spectra below T m ¼ 6 K are analyzed by using the following formula for powder specimen, PðtÞ ¼ A1 expðl1 tÞ cosð2pft þ fÞ þ A2 expðl2 tÞ þ AB , where Ai refers to the asymmetry of muons stopped in the sample, AB is a background, li is the relaxation rate, and f (¼ gm Hm =2p, where gm is the muon gyromagnetic ratio of 2p 13:553 MHz=kOe) is the precession frequency with Hm being the spontaneous local field. The solid curves are the best fits of the data in the time domain, as shown Fig. 3. The model yields good fits to data as indicated by reasonably small values of reduced chi square: w2 /N f is mostly less than 1.3, with N f being the number of degrees of freedom. As expected for a polycrystalline sample, the 13 term of each component represents the fraction of the muons possessing an initial polarization along the same direction of the internal field.
ARTICLE IN PRESS 742
N. Kase et al. / Physica B 404 (2009) 740–742
Fig. 3. ZF-mSR time spectra in Tm5Rh6Sn18 at 1.6 and 20 K. Solids curves represent those obtained by fitting with equation PðtÞ.
The fractional volume of the magnetic component ½¼ A1 =ðA1 þ A2 Þ is estimated to be 0.67, strongly suggesting that the magnetism occurs in the whole volume of the sample and therefore coexisting with superconductivity. The muon spin precession frequency f is displayed as a function of temperature in Fig. 4. The dashed curve for f in the down panel of Fig. 4 is the fitting result with a form f ðtÞ ¼ f ð0Þð1 T=T m Þb , which yields T m ¼ 7:1 K, f ð0Þ ¼ 2:38 MHz. The obtained T m is almost consistent with the temperature at which the oscillation signals appear. The internal magnetic field at the muon site, Hm , is estimated to be approximately 175 Oe from the relationship between f ð0Þ and Hm , f ð0Þ ¼ gm Hm =2p. It is noticeable that these spectra also exhibit an exponential damping without oscillation, which is described as a tail in equation PðtÞ (see Fig. 3). This longitudinal spin relaxation might be due to dynamical fluctuation of internal fields. This strong spin fluctuation indicates that the magnetic order is not represented by a simple static antiferromagnetism.
Fig. 4. The temperature dependence of muon spin precession frequency f , and corrected asymmetry. The dashed curve is the fitting result of f ðTÞ ¼ f ð0Þð1 T=T m Þb .
below 6 K reveals a magnetic order invisible in bulk measurements. From the temperature dependence of f and corrected asymmetry, we confirmed the growth of a magnetic ordered state below 6 K. In the ordered state, the magnetic volume fraction is calculated to be nearly 100%. We conclude that reentrant superconductor Tm5Rh6Sn18 exhibits the coexistence of superconductivity and magnetism at the microscopic level.
Acknowledgments This work was partially supported by ‘‘High-Tech Research Center Project’’ for Private Universities and Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. One of the authors (N. Kase) acknowledges the support of the Iwanami Fujukai Foundation. References
4. Conclusions In summary, we studied the magnetic properties of the Tmbased superconductor, Tm5Rh6Sn18, by means of the ZF-mSR technique. The observation of clear muon spin precession signals
[1] J.P. Remeika, et al., Solid State Comm. 34 (1980) 923. [2] S. Miraglia, et al., Acta Cryst. B 43 (1987) 76. [3] H.R. Ott, et al., in: Proceedings of the International Conference on Ternary Superconductors, Lake Geneva, WI, USA, 1980, p. 251. [4] A. Rojek, et al., Phys. C 223 (1994) 111.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Coexistence of superconductivity and magnetism in the Tm-based superconductor probed by muon spin relaxation Naoki Kase a,, Jun Akimitsu a, Yasuyuki Ishii b, Takao Suzuki b, Isao Watanabe b, Masanori Miyazaki c, Masatoshi Hiraishi c, Soshi Takeshita d, Ryosuke Kadono c,d a
Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 229-8558, Japan Advanced Meson Science Laboratory, RIKEN Nishina Center, Wako, Saitama 351-0198, Japan c Department of Materials Structure Science, The Graduate University for Advanced Studies, Tsukuba, Ibaraki 305-0801, Japan d Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan b
a r t i c l e in f o
Keywords: ðSn1x Tmx ÞTm4 Rh6 Sn18 Superconductivity Magnetism mSR
a b s t r a c t The magnetic property of a thulium-based superconductor, Tm5Rh6Sn18 (superconducting transition temperature T c ¼ 2:2 K), is investigated by muon spin relaxation ðmSRÞ. Below around 6 K, the development of a quasi-static local magnetic field is clearly inferred from the observation of spontaneous oscillation signal in the mSR spectra under zero external field, where the magnetism persists even below T c. The internal magnetic field at the muon site, Hm , is estimated to be approximately 175 Oe. The fractional volume of the magnetic component is estimated to be 100%, strongly suggesting that the magnetism coexists with superconductivity. & 2008 Elsevier B.V. All rights reserved.
1. Introduction ðSn1x Rx ÞR4 T6 Sn18 T ¼ transition metal) compounds belong to a large stannide series [1]. These stannides crystalize in a tetragonal structure with a space group I41 =acd, and exhibit magnetic and/or superconducting transitions [2]. The reentrant superconductor ðSn1x Erx ÞEr4 Rh6 Sn18 has attracted much interest for the magnetic and superconducting state, because the physical properties dramatically change due to the difference of the composition of x; x0 is superconducting below T ’ 1:3 K down to 50 mK; x0:3 is a reentrant superconducting with T c ’ 1:05 K and T M ’ 0:5 K; x0:75 has a magnetic order at 0.65 K and is not superconductor [2,3]. On the other hand, ðSn1x Tmx ÞTm4 Rh6 Sn18 becomes superconductor at 2.2 K and exhibits a reentrant superconducting behavior demonstrated by the temperature dependence of the resistivity at fields higher than 1.4 kOe (above 0.4 K). In addition, because the composition of x is a constant value of about 0.8, ðSn1x Tmx ÞTm4 Rh6 Sn18 can be nearly described as a chemical formula of Tm5Rh6Sn18 ðx ’ 0:8Þ [4]. However, compared to the other reentrant superconductors, Tm5Rh6Sn18 still has far less information on the magnetic ordering or superconducting properties from the previous literature. This motivated us to investigate the superconducting state and the
Corresponding author.
E-mail address: [email protected] (N. Kase). 0921-4526/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.11.185
relationship between magnetism and superconductivity more preciously from the view point of microscopic measurements. We investigated at the microscopic level the coexistence between magnetism and superconducting state by means of the muon spin relaxation (mSR) method. The mSR measurement shows the development of a quasi-static local magnetic field is clearly inferred from the observation of a spontaneous oscillation signal in the mSR spectra under zero external field, where the magnetism persists even below T c. Our result establishes the coexistence of superconductivity and magnetic ordering in the Tm-based superconductor.
2. Experimental detail Single crystals of Tm5Rh6Sn18 were grown by the Sn flux method. The starting materials were 99.9%-Tm powder, 99.9%-Rh powder and 99.999%-Sn shot. These materials were sealed in an evacuated quartz tube, with off-stoichiometric composition of Tm:Rh:Sn ¼ 1: 1:2: 10. The quartz tube was heated up to 1050, maintained at this temperature for about 3 h, and cooled down to 200 at the rate of 5/h, taking 7 days in total. The excess flux was removed from the crystals by spinning the ampoule in the centrifuge. The single-crystalline nature has been verified by using backscattering X-ray technique. The powder X-ray diffraction patterns could be indexed as the Tm5Rh6Sn18 with the space group I41 =acd. The electrical resistivity was measured by the conventional DC four-probe method in the temperature range
ARTICLE IN PRESS N. Kase et al. / Physica B 404 (2009) 740–742
741
from 0.4 to 300 K under various (including zero) applied magnetic fields with a PPMS system (Quantum Design Co., Ltd.). The mSR measurements were carried out at the RIKEN-RAL Muon Facility in the Rutherford Appleton Laboratory, which provided a pulsed beam of nearly 100% spin-polarized muons. The polycrystalline samples obtained from single crystals crushed into a fine powder were pressed into pellets, and sintered at 900 for 24 h under a high vacuum condition of 4:0 103 Pa. The sample was glued onto a high purity silver (99.998%) holder to avoid a depolarizing background mSR signal, and mounted into a 3 He cryostat. During the measurement under zero field (ZF), residual magnetic field at the sample position was reduced below 105 kOe.
3. Experimental results and discussion 3.1. Electrical resistivity Superconductivity can be inferred from electrical resistivity measurements, as shown in Fig. 1. The onset temperature of the superconducting transition and the temperature of zero resistivity are observed to be 2.30 and 2.20 K. The transition width DT is considered as the temperature interval between 10% and 90% of the transition and is observed to be approximately 0.07 K. The electrical resistivity in the normal state increases with a decrease in the temperature, indicating abnormal metallic behavior. This anomaly can be described as ln T behavior, which strongly indicates that Kondo effect is significant in this material. Fig. 2(a) shows the temperature dependence of the electrical resistivity under several magnetic fields. Tm5Rh6Sn18 become superconducting at 2.2 K and does not return to the normal state down to 0.4 K under zero magnetic field. At fields higher than 1.3 kOe, the superconducting state is broken due to the occurrence of the magnetic ordering. In fields exceeding 1.70 kOe the superconducting state is no longer fully established. Fig. 2(b) shows the superconducting phase diagram of Tm5Rh6Sn18. The T M and T c are determined by the mid-point temperature of T zero and T onset , and T is the moderate drop of the resistivity. What is the origin of T is not understood, but the fact that T depends on magnetic fields indicates low probability of the impurity effect of Sn (Sn; Hc (0) ¼ 305 Oe) or magnetic ordering.
Fig. 1. Temperature dependence of the electrical resistivity of Tm5Rh6Sn18. The inset shows the expansion at low temperature region.
Fig. 2. (a) Temperature dependence of the electrical resistivity of Tm5Rh6Sn18 under several magnetic fields. (b) Phase diagram of Tm5Rh6Sn18. The closed symbol shows superconducting transition, the square symbol shows breaking superconducting transition, and the open symbol shows a moderate drop of the resistivity.
3.2. mSR measurement ZF-mSR is the most sensitive technique to examine magnetism in any form, where the development of local magnetic moment leads to either spontaneous oscillation or exponential damping of AðtÞ. Fig. 3 shows the time-dependent muon–positron decay asymmetry under zero external field at several temperatures in Tm5Rh6Sn18. Below around 6 K, development of a quasi-static local magnetic field is clearly inferred from the observation of a spontaneous oscillation signal in the mSR spectra. Surprisingly, the spontaneous oscillation signal was observed even below the superconducting transition temperature T c ¼ 2:2 K. The ZF-mSR time spectra below T m ¼ 6 K are analyzed by using the following formula for powder specimen, PðtÞ ¼ A1 expðl1 tÞ cosð2pft þ fÞ þ A2 expðl2 tÞ þ AB , where Ai refers to the asymmetry of muons stopped in the sample, AB is a background, li is the relaxation rate, and f (¼ gm Hm =2p, where gm is the muon gyromagnetic ratio of 2p 13:553 MHz=kOe) is the precession frequency with Hm being the spontaneous local field. The solid curves are the best fits of the data in the time domain, as shown Fig. 3. The model yields good fits to data as indicated by reasonably small values of reduced chi square: w2 /N f is mostly less than 1.3, with N f being the number of degrees of freedom. As expected for a polycrystalline sample, the 13 term of each component represents the fraction of the muons possessing an initial polarization along the same direction of the internal field.
ARTICLE IN PRESS 742
N. Kase et al. / Physica B 404 (2009) 740–742
Fig. 3. ZF-mSR time spectra in Tm5Rh6Sn18 at 1.6 and 20 K. Solids curves represent those obtained by fitting with equation PðtÞ.
The fractional volume of the magnetic component ½¼ A1 =ðA1 þ A2 Þ is estimated to be 0.67, strongly suggesting that the magnetism occurs in the whole volume of the sample and therefore coexisting with superconductivity. The muon spin precession frequency f is displayed as a function of temperature in Fig. 4. The dashed curve for f in the down panel of Fig. 4 is the fitting result with a form f ðtÞ ¼ f ð0Þð1 T=T m Þb , which yields T m ¼ 7:1 K, f ð0Þ ¼ 2:38 MHz. The obtained T m is almost consistent with the temperature at which the oscillation signals appear. The internal magnetic field at the muon site, Hm , is estimated to be approximately 175 Oe from the relationship between f ð0Þ and Hm , f ð0Þ ¼ gm Hm =2p. It is noticeable that these spectra also exhibit an exponential damping without oscillation, which is described as a tail in equation PðtÞ (see Fig. 3). This longitudinal spin relaxation might be due to dynamical fluctuation of internal fields. This strong spin fluctuation indicates that the magnetic order is not represented by a simple static antiferromagnetism.
Fig. 4. The temperature dependence of muon spin precession frequency f , and corrected asymmetry. The dashed curve is the fitting result of f ðTÞ ¼ f ð0Þð1 T=T m Þb .
below 6 K reveals a magnetic order invisible in bulk measurements. From the temperature dependence of f and corrected asymmetry, we confirmed the growth of a magnetic ordered state below 6 K. In the ordered state, the magnetic volume fraction is calculated to be nearly 100%. We conclude that reentrant superconductor Tm5Rh6Sn18 exhibits the coexistence of superconductivity and magnetism at the microscopic level.
Acknowledgments This work was partially supported by ‘‘High-Tech Research Center Project’’ for Private Universities and Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. One of the authors (N. Kase) acknowledges the support of the Iwanami Fujukai Foundation. References
4. Conclusions In summary, we studied the magnetic properties of the Tmbased superconductor, Tm5Rh6Sn18, by means of the ZF-mSR technique. The observation of clear muon spin precession signals
[1] J.P. Remeika, et al., Solid State Comm. 34 (1980) 923. [2] S. Miraglia, et al., Acta Cryst. B 43 (1987) 76. [3] H.R. Ott, et al., in: Proceedings of the International Conference on Ternary Superconductors, Lake Geneva, WI, USA, 1980, p. 251. [4] A. Rojek, et al., Phys. C 223 (1994) 111.