- Email: [email protected]
Magnetic and transport properties of amorphous Ce-Al alloy
Physica B xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Magnetic and transport properties of amorphous Ce-Al alloy ⁎
Yusuke Amakaia,b, , Shigeyuki Murayamaa, Naoki Momonoa,b, Hideaki Takanoa, Tomohiko Kuwaic a b c
Graduate School of Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan Research Center for Environmentally Friendly Materials Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: Amorphous alloys Rare earth alloys Ce-Al Kondo effect Magnetic properties Transport properties
Amorphous (a-)Ce50Al50 has been prepared by DC high-rate sputter method. The structure of the obtained sample has been confirmed to have an amorphous structure because there are no Bragg peaks in the X-ray diffraction measurement and have a clear exothermic peak by the differential scanning calorimetry measurement. We have measured the resistivity ρ, magnetic susceptibility χ, specific heat Cp and thermoelectric power S for a-Ce50Al50. The temperature dependence of ρ exhibits a small temperature dependence less than 10% in the whole temperature region. χ follows a Curie-Weiss behavior in the high-temperature region of T > 90 K. The effective paramagnetic moment peff, estimated from C is 2.18 μB/Ce-atom. The low-temperature Cp/T increases rapidly with decreasing temperature and tends to a saturation. S(T) exhibits negative values in a wide temperature region. A minimum of S appear at around 60 K, and S decreases linearly with decreasing temperature down to 10 K. The low-temperature S is almost 0 μV/K down to 2 K. From these results, we have pointed out that present a-Ce50Al50 would be an incoherent Kondo material.
1. Introduction Ce-Al binary intermetallic compounds including CeAl3, CeAl2 and Ce3Al11 are well known as a heavy fermion compound [1–3]. The ground state of these compounds varies, and it is pointed out that competition between the Kondo effect and the Ruderman-KittelKasuya-Yoshida (RKKY) interaction is important for these phenomena. CeAl3 is a typical heavy-fermion compound without long range magnetic ordering. CeAl2 is a dense Kondo compound with antiferromagnetic order at low-temperature (TN = 3.8 K). Ce3Al11 is a Kondo compound with a ferromagnetic order in the temperature range 3.2 K < T < 6.2 K, and an incommensurate antiferromagnetic transition at T = 3.2 K. However, orthorhombic CeAl shows an antiferromagnetic order due to the RKKY interaction with no heavy-fermion behavior [4]. Recently, research on strongly correlated electron systems has evolved to not only high-quality single crystal samples but also quasicrystals, bulk-metallic-glasses and amorphous alloys with disordered structures. For example, in Ce-Al bulk-metallic-glasses which made by a single-roller melt-spinning, it is suggested the tunable competition between the Kondo effect and the RKKY interaction with the variation in Ce-concentration [5]. In addition, it has also been reported that two amorphous phases under high-pressure exist in Ce-
⁎
Al bulk-metallic-glasses [6–8], and the presence of a metastable phase of a novel cubic CeAl embedded in an amorphous matrix [9]. On the other hand, we have studied low-temperature properties of several binary amorphous Ce-alloys which made by a DC high-rate sputter method. Amorphous (a-)Ce-Mn and a-Ce-Ru alloys show a large electronic specific heat coefficient γ (> 200 mJ/molK2) and the T2 law with a large coefficient A (> 0.02 μΩcm/K2) in the low-temperature resistivity [10–14]. From these results, we have pointed out that an itinerant heavy-fermion state occurs at low-temperature as a Fermiliquid ground state in the structure-disordered system after formation of a dense Kondo state. In this study, in order to investigate the magnetic and electronic properties of a-Ce-Al alloys which made by sputter, we have prepared a-Ce50Al50 and measured the resistivity, magnetic susceptibility, specific heat and thermoelectric power. 2. Experimental Amorphous Ce50Al50 was prepared by a DC high-rate sputtering method from the arc-melted ingots onto the water-cooled Cu substrate (30 mmϕ). The purities of elements used were 3 N for Ce and 4 N for Al, respectively. The sample thickness was ~ 500 µm. The structure of the obtained samples was confirmed using X-ray diffraction measurements. Differential scanning calorimetry (DSC) was done between
Corresponding author at: Graduate School of Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan. E-mail address: [email protected] (Y. Amakai).
http://dx.doi.org/10.1016/j.physb.2017.09.111 Received 30 June 2017; Received in revised form 13 September 2017; Accepted 26 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Amakai, Y., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.111
Physica B xxx (xxxx) xxx–xxx
Y. Amakai et al.
Fig. 1. X-ray diffraction pattern for a-Ce50Al50. The inset shows DSC curve for aCe50Al50.
Fig. 3. Temperature dependence of the susceptibility χ and inverse susceptibility 1/χ for a-Ce50Al50.
300 K and 800 K with a heating rate of 30 K/min. The magnetic susceptibility was measured using a SQUID magnetometer (Quantum Design MPMS) from 2 to 300 K. The electrical resistivity was measured by using a standard four-probe method (Quantum Design PPMS) from 2 to 300 K. The specific heat was measured by using PPMS from 2 to 300 K. Thermoelectric power measurement was carried out in the differential method with a pair of Au-Fe/chromel fine thermocouples between 2 and 300 K.
times greater than that of the crystalline counterpart [4]. Furthermore, the ρ value for a-Ce50Al50 exhibits only a small temperature dependence, less than 10% in the whole temperature region, and it increases with decreasing temperature. Such a temperature dependence is one of the characteristics of disordered alloys. In the case of metal-metal based amorphous alloys exhibiting a relatively high electrical resistivity (> 200 μΩ cm), it is pointed out by Cote and Meisel that the temperature dependence of resistivity follows 1-αT2 in the low-temperature region, and proportional to -T in the high-temperature region [15]. ρ(T) of the present a-Ce50Al50 nicely follows -T at T > 100 K, and 1-αT2 at 20 K < T < 100 K. In the low-temperature region of T < 10 K, ρ(T) is characterized by a rapid enhancement which follows a –log T dependence as shown in the inset of Fig. 2. The behavior of the temperature dependence of ρ for the present a-Ce50Al50 is similar to that of Ce-Al bulk-metallic-glasses in Ref. [5]. Fig. 3 shows the temperature dependence of the magnetic susceptibility χ (left axis) and inverse susceptibility 1/χ (right axis) for aCe50Al50. χ(T) of a-Ce50Al50 increases monotonically with decreasing temperature, and a magnetic transition is not observed down to 2 K. 1/ χ exhibits a linear behavior at high-temperatures (T > 90 K), following the Curie-Weiss law, 1/χ = (T-θ)/C, where C is the Curie constant, and θ is the Weiss temperature. The value of θ is − 26 K. The effective paramagnetic moment peff, estimated from C is 2.18 μB/Ce-atom. This value is slightly smaller than 2.54 μB, which is the expected value for Ce3+ (J = 5/2). Fig. 4 shows the Cp/T versus T2 plot for a-Ce50Al50 alloy. The lowtemperature specific heat of a usual metal can be expressed by Cp = γT + βT3 where γT is the electronic contribution and βT3 the phonon contribution. This relation can also be used for amorphous alloys [16]. Cp/T for a-Ce50Al50 follows a linear relation as a function of T2 on T2 > 150 K2. The γ-value, which estimated by extrapolated to 0 K from linear T2 relation of T > 150 K2 is 90 mJ/molK2. In the low-temperature region of T2 < 100 K2, Cp/T increases rapidly with decreasing temperature and tends to a saturation at below 5 K2. The value of Cp/T at 4 K2 shows a very large value (557 mJ/molK2) compared with crystalline counterpart. Fig. 5 shows the temperature dependence of the thermoelectric power S for a-Ce50Al50. The S for a-Ce50Al50 exhibits negative in the wide temperature region. The absolute value of S is small due to having the high-resistivity for the amorphous structure [17]. The temperature dependence of S value for a-Ce50Al50 decreases monotonically with decreasing temperature in the high-temperature region of T > 150 K. However, in the temperature range of 70 K < T < 120 K, S is almost
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for the a-Ce50Al50. The inset shows the DSC trace of a-Ce50Al50. The diffraction exhibits two broad peaks at the center at about 32° and 49°; and definite Bragg peaks are not observed. The DSC trace of a-Ce50Al50 displayed clear exothermic peak around 615 K, being ascribed to the crystallization of the amorphous phase. We found that several peaks corresponding to the orthorhombic CeAl have been evolved from X-ray diffraction measurement on the sample after DSC measurement. Therefore, the present a-Ce50Al50 is expected as an amorphous version of orthorhombic CeAl. Fig. 2 shows the temperature dependence of the resistivity ρ for the a-Ce50Al50. The inset shows the resistivity on a logarithmic scale of temperature. The value of ρ at 300 K for the a-Ce50Al50 is about 10
Fig. 2. Temperature dependence of the resistivity ρ for a-Ce50Al50. The inset shows the resistivity for logarithmic scale of temperature.
2
Physica B xxx (xxxx) xxx–xxx
Y. Amakai et al.
electrons is also very large even in the amorphous structure, but the formation of a Fermi-liquid state and long-range magnetic order are suppressed. Orthorhombic CeAl, which is the crystalline counterpart of the present amorphous alloy, shows antiferromagnetic order arising from the RKKY interaction; the Kondo effect is not prominent [4]. Therefore, it is considered that the interaction changes by amorphization of Ce50Al50 from predominant RKKY to Kondo interaction. Longrange magnetic order disappears, and characteristics of incoherent Kondo effect are observed at low-temperature. 4. Conclusion We have succeed to prepare amorphous (a-)Ce50Al50 by a DC highrate sputter method. The structure of the obtained sample has been confirmed to have an amorphous structure because there are no Bragg peaks in the X-ray diffraction measurement; additionally, there is a clear exothermic peak at 615 K as obtained by DSC measurement. The temperature dependence of ρ is weak, less than 10% in the whole temperature region. The low-temperature ρ shows a rapid enhancement, followed by a –log T dependence. χ reevals a Curie-Weiss behavior in the high-temperature region (T > 90 K). The effective paramagnetic moment peff, estimated from C is 2.18 μB/Ce-atom. Cp/ T at low temperatures increases rapidly with decreasing temperature and tends to a saturation (Cp/T = 557 mJ/molK2, at 2 K). The Seebeck coefficient is exhibits negative in the wide temperature region. A minimum of S appear at around 60 K, and S decreases linearly with decreasing temperature down to 10 K. The low-temperature S is almost 0 μV/K down to 2 K. From these results, a-Ce50Al50 is a Kondo material that the hybridization of 4f-electrons and conduction electrons exist at low-temperature.
Fig. 4. Cp/T vs. T2 plot for a-Ce50Al50.
References [1] K. Andres, J.E. Graebner, H.R. Ott, Phys. Rev. Lett. 35 (1975) 1779. [2] Y. Onuki, Y. Furukawa, T. Komatsubara, J. Phys. Soc. Jpn. 53 (1984) 2734. [3] J.X. Boucherle, F. Givord, G. Lapertot, A. Mu ̃noz, J. Schweizer, J. Magn. Magn. Mater. 148 (1995) 397. [4] P.K. Das, A. Thamizhavel, J. Phys. Conf. Ser. 592 (2015) 012009. [5] Q.S. Zeng, C.R. Rotundu, W.L. Mao, J.H. Dai, Y.M. Xiao, P. Chow, X.J. Chen, C.L. Qin, H.-K. Mao, J.Z. Jiang, J. Appl. Phys. 109 (2011) 113716. [6] H.W. Sheng, H.Z. Liu, Y.Q. Cheng, J. Wen, P.L. Lee, W.K. Luo, S.D. Shastri, E. Ma, Nat. Mater. 6 (2007) 192. [7] Q. Zeng, Y. Ding, W.L. Mao, W. Yang, S.V. Sinogeikin, J. Shu, H.-k. Mao, J.Z. Jiang, Phys. Rev. Lett. 104 (2010) 105702. [8] L. Zhang, F. Sun, X. Hong, J. Wang, G. Liu, L. Kong, H. Yang, X. Liu, Y. Zhao, W. Yang, J. Alloy. Compd. 695 (2017) 1180. [9] B. Idzikowski, Z. Śniadecki, R. Puźniak, D. Kaczorowski, J. Magn. Magn. Mater. 421 (2017) 82. [10] Y. Amakai, S. Murayama, Y. Obi, H. Takano, K. Takanashi, Phys. Rev. B 78 (2009) 123705. [11] Y. Amakai, S. Murayama, Y. Obi, H. Takano, N. Momono, K. Takanashi, J. Phys. Soc. Jpn. 80 (2011) SA057. [12] Y. Obi, S. Murayama, Y. Amakai, Y. Okada, K. Asano, Physica B 378–380 (2006) 857. [13] Y. Amakai, S. Murayama, Y. Obi, H. Takano, N. Momono, K. Takanashi, J. Phys.: Conf. Ser. 150 (2009) 042004. [14] Y. Amakai, D. Yoshii, S. Murayama, H. Takano, N. Momono, Y. Obi, K. Takanashi, J. Phys.:Conf. Ser. 391 (2012) 012002. [15] P.J. Cote, L.V. Meisel, Phys. Rev. Lett. 159 (1967) 500. [16] W. Yang, H. Liu, X. Yang, L. Dou, J. Low. Temp. Phys. 160 (2010) 148. [17] J. Sakurai, F. Taniguchi, K. Nishimura, H. Amano, K. Suzuki, Physica B 186–188 (1993) 569. [18] G. Orveillon-Dubajic, B. Chevalier, S. Gorsse, J. Alloy. Compd. 463 (2008) 569. [19] K. Sumiyama, H. Amano, H. Yamauchi, T. Suzuki, J. Phys. Soc. Jpn. 61 (1992) 2359. [20] Y. Ōnuki, Y. Shimizu, M. Nishihara, Y. Machii, T. Komatsubara, J. Phys. Soc. Jpn. 54 (1985) 1964. [21] E. Gratz, E. Bauer, B. Barbara, S. Zemirli, F. Steglich, C.D. Bredl, W. Lieke, J. Phys. F: Met. Phys. 15 (1985) 1975.
Fig. 5. Temperature dependence of the thermoelectric power S for a-Ce50Al50.
independent of the temperature. With further decrease in temperature, a minimum of S appear at around 60 K, and S decreases linearly with decreasing temperature down to 10 K. In the below 10 K, S is almost 0 μV/K down to 2 K. In the case of the non-magnetic Ce-based amorphous alloy where Ce is tetravalent, S(T) is linear at low-temperature, and almost independent of temperature at high-temperature region [18]. The peak around 60 K of S for present a-Ce50Al50 is not attributable to a magnetic contribution as pointed out in Ref. [15], it may be due to the amorphous structure. However, since an increase in the lowtemperature of Cp/T and ρ is observed, the behavior of low temperature S may be related to the magnetic contribution of 4f-electrons of Ce. From the susceptibility measurement, a-Ce50Al50 has no magnetic transition and Ce is almost trivalent. Therefore, the increase in the Cp/ T and ρ at low-temperatures and the temperature dependence of S are attributed to the Kondo effect due to the hybridization of 4f-electrons and conduction electrons. Sumiyama et al. observed similar phenomena in amorphous Ce-Cu alloys [19]. Crystalline Ce-Cu intermetallic compounds are a typical heavy-fermion compounds such as CeCu6 (γ – 1500 mJ/molK2) and a Kondo lattice antiferromagnet CeCu2 (TN = 3.5 K) [20,21]. Sumiyama et al. pointed out that those amorphous CeCu alloys are incoherent Kondo systems, where the effective mass of
3
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Magnetic and transport properties of amorphous Ce-Al alloy ⁎
Yusuke Amakaia,b, , Shigeyuki Murayamaa, Naoki Momonoa,b, Hideaki Takanoa, Tomohiko Kuwaic a b c
Graduate School of Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan Research Center for Environmentally Friendly Materials Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: Amorphous alloys Rare earth alloys Ce-Al Kondo effect Magnetic properties Transport properties
Amorphous (a-)Ce50Al50 has been prepared by DC high-rate sputter method. The structure of the obtained sample has been confirmed to have an amorphous structure because there are no Bragg peaks in the X-ray diffraction measurement and have a clear exothermic peak by the differential scanning calorimetry measurement. We have measured the resistivity ρ, magnetic susceptibility χ, specific heat Cp and thermoelectric power S for a-Ce50Al50. The temperature dependence of ρ exhibits a small temperature dependence less than 10% in the whole temperature region. χ follows a Curie-Weiss behavior in the high-temperature region of T > 90 K. The effective paramagnetic moment peff, estimated from C is 2.18 μB/Ce-atom. The low-temperature Cp/T increases rapidly with decreasing temperature and tends to a saturation. S(T) exhibits negative values in a wide temperature region. A minimum of S appear at around 60 K, and S decreases linearly with decreasing temperature down to 10 K. The low-temperature S is almost 0 μV/K down to 2 K. From these results, we have pointed out that present a-Ce50Al50 would be an incoherent Kondo material.
1. Introduction Ce-Al binary intermetallic compounds including CeAl3, CeAl2 and Ce3Al11 are well known as a heavy fermion compound [1–3]. The ground state of these compounds varies, and it is pointed out that competition between the Kondo effect and the Ruderman-KittelKasuya-Yoshida (RKKY) interaction is important for these phenomena. CeAl3 is a typical heavy-fermion compound without long range magnetic ordering. CeAl2 is a dense Kondo compound with antiferromagnetic order at low-temperature (TN = 3.8 K). Ce3Al11 is a Kondo compound with a ferromagnetic order in the temperature range 3.2 K < T < 6.2 K, and an incommensurate antiferromagnetic transition at T = 3.2 K. However, orthorhombic CeAl shows an antiferromagnetic order due to the RKKY interaction with no heavy-fermion behavior [4]. Recently, research on strongly correlated electron systems has evolved to not only high-quality single crystal samples but also quasicrystals, bulk-metallic-glasses and amorphous alloys with disordered structures. For example, in Ce-Al bulk-metallic-glasses which made by a single-roller melt-spinning, it is suggested the tunable competition between the Kondo effect and the RKKY interaction with the variation in Ce-concentration [5]. In addition, it has also been reported that two amorphous phases under high-pressure exist in Ce-
⁎
Al bulk-metallic-glasses [6–8], and the presence of a metastable phase of a novel cubic CeAl embedded in an amorphous matrix [9]. On the other hand, we have studied low-temperature properties of several binary amorphous Ce-alloys which made by a DC high-rate sputter method. Amorphous (a-)Ce-Mn and a-Ce-Ru alloys show a large electronic specific heat coefficient γ (> 200 mJ/molK2) and the T2 law with a large coefficient A (> 0.02 μΩcm/K2) in the low-temperature resistivity [10–14]. From these results, we have pointed out that an itinerant heavy-fermion state occurs at low-temperature as a Fermiliquid ground state in the structure-disordered system after formation of a dense Kondo state. In this study, in order to investigate the magnetic and electronic properties of a-Ce-Al alloys which made by sputter, we have prepared a-Ce50Al50 and measured the resistivity, magnetic susceptibility, specific heat and thermoelectric power. 2. Experimental Amorphous Ce50Al50 was prepared by a DC high-rate sputtering method from the arc-melted ingots onto the water-cooled Cu substrate (30 mmϕ). The purities of elements used were 3 N for Ce and 4 N for Al, respectively. The sample thickness was ~ 500 µm. The structure of the obtained samples was confirmed using X-ray diffraction measurements. Differential scanning calorimetry (DSC) was done between
Corresponding author at: Graduate School of Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan. E-mail address: [email protected] (Y. Amakai).
http://dx.doi.org/10.1016/j.physb.2017.09.111 Received 30 June 2017; Received in revised form 13 September 2017; Accepted 26 September 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Amakai, Y., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.09.111
Physica B xxx (xxxx) xxx–xxx
Y. Amakai et al.
Fig. 1. X-ray diffraction pattern for a-Ce50Al50. The inset shows DSC curve for aCe50Al50.
Fig. 3. Temperature dependence of the susceptibility χ and inverse susceptibility 1/χ for a-Ce50Al50.
300 K and 800 K with a heating rate of 30 K/min. The magnetic susceptibility was measured using a SQUID magnetometer (Quantum Design MPMS) from 2 to 300 K. The electrical resistivity was measured by using a standard four-probe method (Quantum Design PPMS) from 2 to 300 K. The specific heat was measured by using PPMS from 2 to 300 K. Thermoelectric power measurement was carried out in the differential method with a pair of Au-Fe/chromel fine thermocouples between 2 and 300 K.
times greater than that of the crystalline counterpart [4]. Furthermore, the ρ value for a-Ce50Al50 exhibits only a small temperature dependence, less than 10% in the whole temperature region, and it increases with decreasing temperature. Such a temperature dependence is one of the characteristics of disordered alloys. In the case of metal-metal based amorphous alloys exhibiting a relatively high electrical resistivity (> 200 μΩ cm), it is pointed out by Cote and Meisel that the temperature dependence of resistivity follows 1-αT2 in the low-temperature region, and proportional to -T in the high-temperature region [15]. ρ(T) of the present a-Ce50Al50 nicely follows -T at T > 100 K, and 1-αT2 at 20 K < T < 100 K. In the low-temperature region of T < 10 K, ρ(T) is characterized by a rapid enhancement which follows a –log T dependence as shown in the inset of Fig. 2. The behavior of the temperature dependence of ρ for the present a-Ce50Al50 is similar to that of Ce-Al bulk-metallic-glasses in Ref. [5]. Fig. 3 shows the temperature dependence of the magnetic susceptibility χ (left axis) and inverse susceptibility 1/χ (right axis) for aCe50Al50. χ(T) of a-Ce50Al50 increases monotonically with decreasing temperature, and a magnetic transition is not observed down to 2 K. 1/ χ exhibits a linear behavior at high-temperatures (T > 90 K), following the Curie-Weiss law, 1/χ = (T-θ)/C, where C is the Curie constant, and θ is the Weiss temperature. The value of θ is − 26 K. The effective paramagnetic moment peff, estimated from C is 2.18 μB/Ce-atom. This value is slightly smaller than 2.54 μB, which is the expected value for Ce3+ (J = 5/2). Fig. 4 shows the Cp/T versus T2 plot for a-Ce50Al50 alloy. The lowtemperature specific heat of a usual metal can be expressed by Cp = γT + βT3 where γT is the electronic contribution and βT3 the phonon contribution. This relation can also be used for amorphous alloys [16]. Cp/T for a-Ce50Al50 follows a linear relation as a function of T2 on T2 > 150 K2. The γ-value, which estimated by extrapolated to 0 K from linear T2 relation of T > 150 K2 is 90 mJ/molK2. In the low-temperature region of T2 < 100 K2, Cp/T increases rapidly with decreasing temperature and tends to a saturation at below 5 K2. The value of Cp/T at 4 K2 shows a very large value (557 mJ/molK2) compared with crystalline counterpart. Fig. 5 shows the temperature dependence of the thermoelectric power S for a-Ce50Al50. The S for a-Ce50Al50 exhibits negative in the wide temperature region. The absolute value of S is small due to having the high-resistivity for the amorphous structure [17]. The temperature dependence of S value for a-Ce50Al50 decreases monotonically with decreasing temperature in the high-temperature region of T > 150 K. However, in the temperature range of 70 K < T < 120 K, S is almost
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for the a-Ce50Al50. The inset shows the DSC trace of a-Ce50Al50. The diffraction exhibits two broad peaks at the center at about 32° and 49°; and definite Bragg peaks are not observed. The DSC trace of a-Ce50Al50 displayed clear exothermic peak around 615 K, being ascribed to the crystallization of the amorphous phase. We found that several peaks corresponding to the orthorhombic CeAl have been evolved from X-ray diffraction measurement on the sample after DSC measurement. Therefore, the present a-Ce50Al50 is expected as an amorphous version of orthorhombic CeAl. Fig. 2 shows the temperature dependence of the resistivity ρ for the a-Ce50Al50. The inset shows the resistivity on a logarithmic scale of temperature. The value of ρ at 300 K for the a-Ce50Al50 is about 10
Fig. 2. Temperature dependence of the resistivity ρ for a-Ce50Al50. The inset shows the resistivity for logarithmic scale of temperature.
2
Physica B xxx (xxxx) xxx–xxx
Y. Amakai et al.
electrons is also very large even in the amorphous structure, but the formation of a Fermi-liquid state and long-range magnetic order are suppressed. Orthorhombic CeAl, which is the crystalline counterpart of the present amorphous alloy, shows antiferromagnetic order arising from the RKKY interaction; the Kondo effect is not prominent [4]. Therefore, it is considered that the interaction changes by amorphization of Ce50Al50 from predominant RKKY to Kondo interaction. Longrange magnetic order disappears, and characteristics of incoherent Kondo effect are observed at low-temperature. 4. Conclusion We have succeed to prepare amorphous (a-)Ce50Al50 by a DC highrate sputter method. The structure of the obtained sample has been confirmed to have an amorphous structure because there are no Bragg peaks in the X-ray diffraction measurement; additionally, there is a clear exothermic peak at 615 K as obtained by DSC measurement. The temperature dependence of ρ is weak, less than 10% in the whole temperature region. The low-temperature ρ shows a rapid enhancement, followed by a –log T dependence. χ reevals a Curie-Weiss behavior in the high-temperature region (T > 90 K). The effective paramagnetic moment peff, estimated from C is 2.18 μB/Ce-atom. Cp/ T at low temperatures increases rapidly with decreasing temperature and tends to a saturation (Cp/T = 557 mJ/molK2, at 2 K). The Seebeck coefficient is exhibits negative in the wide temperature region. A minimum of S appear at around 60 K, and S decreases linearly with decreasing temperature down to 10 K. The low-temperature S is almost 0 μV/K down to 2 K. From these results, a-Ce50Al50 is a Kondo material that the hybridization of 4f-electrons and conduction electrons exist at low-temperature.
Fig. 4. Cp/T vs. T2 plot for a-Ce50Al50.
References [1] K. Andres, J.E. Graebner, H.R. Ott, Phys. Rev. Lett. 35 (1975) 1779. [2] Y. Onuki, Y. Furukawa, T. Komatsubara, J. Phys. Soc. Jpn. 53 (1984) 2734. [3] J.X. Boucherle, F. Givord, G. Lapertot, A. Mu ̃noz, J. Schweizer, J. Magn. Magn. Mater. 148 (1995) 397. [4] P.K. Das, A. Thamizhavel, J. Phys. Conf. Ser. 592 (2015) 012009. [5] Q.S. Zeng, C.R. Rotundu, W.L. Mao, J.H. Dai, Y.M. Xiao, P. Chow, X.J. Chen, C.L. Qin, H.-K. Mao, J.Z. Jiang, J. Appl. Phys. 109 (2011) 113716. [6] H.W. Sheng, H.Z. Liu, Y.Q. Cheng, J. Wen, P.L. Lee, W.K. Luo, S.D. Shastri, E. Ma, Nat. Mater. 6 (2007) 192. [7] Q. Zeng, Y. Ding, W.L. Mao, W. Yang, S.V. Sinogeikin, J. Shu, H.-k. Mao, J.Z. Jiang, Phys. Rev. Lett. 104 (2010) 105702. [8] L. Zhang, F. Sun, X. Hong, J. Wang, G. Liu, L. Kong, H. Yang, X. Liu, Y. Zhao, W. Yang, J. Alloy. Compd. 695 (2017) 1180. [9] B. Idzikowski, Z. Śniadecki, R. Puźniak, D. Kaczorowski, J. Magn. Magn. Mater. 421 (2017) 82. [10] Y. Amakai, S. Murayama, Y. Obi, H. Takano, K. Takanashi, Phys. Rev. B 78 (2009) 123705. [11] Y. Amakai, S. Murayama, Y. Obi, H. Takano, N. Momono, K. Takanashi, J. Phys. Soc. Jpn. 80 (2011) SA057. [12] Y. Obi, S. Murayama, Y. Amakai, Y. Okada, K. Asano, Physica B 378–380 (2006) 857. [13] Y. Amakai, S. Murayama, Y. Obi, H. Takano, N. Momono, K. Takanashi, J. Phys.: Conf. Ser. 150 (2009) 042004. [14] Y. Amakai, D. Yoshii, S. Murayama, H. Takano, N. Momono, Y. Obi, K. Takanashi, J. Phys.:Conf. Ser. 391 (2012) 012002. [15] P.J. Cote, L.V. Meisel, Phys. Rev. Lett. 159 (1967) 500. [16] W. Yang, H. Liu, X. Yang, L. Dou, J. Low. Temp. Phys. 160 (2010) 148. [17] J. Sakurai, F. Taniguchi, K. Nishimura, H. Amano, K. Suzuki, Physica B 186–188 (1993) 569. [18] G. Orveillon-Dubajic, B. Chevalier, S. Gorsse, J. Alloy. Compd. 463 (2008) 569. [19] K. Sumiyama, H. Amano, H. Yamauchi, T. Suzuki, J. Phys. Soc. Jpn. 61 (1992) 2359. [20] Y. Ōnuki, Y. Shimizu, M. Nishihara, Y. Machii, T. Komatsubara, J. Phys. Soc. Jpn. 54 (1985) 1964. [21] E. Gratz, E. Bauer, B. Barbara, S. Zemirli, F. Steglich, C.D. Bredl, W. Lieke, J. Phys. F: Met. Phys. 15 (1985) 1975.
Fig. 5. Temperature dependence of the thermoelectric power S for a-Ce50Al50.
independent of the temperature. With further decrease in temperature, a minimum of S appear at around 60 K, and S decreases linearly with decreasing temperature down to 10 K. In the below 10 K, S is almost 0 μV/K down to 2 K. In the case of the non-magnetic Ce-based amorphous alloy where Ce is tetravalent, S(T) is linear at low-temperature, and almost independent of temperature at high-temperature region [18]. The peak around 60 K of S for present a-Ce50Al50 is not attributable to a magnetic contribution as pointed out in Ref. [15], it may be due to the amorphous structure. However, since an increase in the lowtemperature of Cp/T and ρ is observed, the behavior of low temperature S may be related to the magnetic contribution of 4f-electrons of Ce. From the susceptibility measurement, a-Ce50Al50 has no magnetic transition and Ce is almost trivalent. Therefore, the increase in the Cp/ T and ρ at low-temperatures and the temperature dependence of S are attributed to the Kondo effect due to the hybridization of 4f-electrons and conduction electrons. Sumiyama et al. observed similar phenomena in amorphous Ce-Cu alloys [19]. Crystalline Ce-Cu intermetallic compounds are a typical heavy-fermion compounds such as CeCu6 (γ – 1500 mJ/molK2) and a Kondo lattice antiferromagnet CeCu2 (TN = 3.5 K) [20,21]. Sumiyama et al. pointed out that those amorphous CeCu alloys are incoherent Kondo systems, where the effective mass of
3