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Field-induced valence transition of EuNi2(Si1−xGex)2 in ultra-high magnetic fields
Physica B 294}295 (2001) 262}266
Field-induced valence transition of EuNi (Si Ge ) \V V in ultra-high magnetic "elds M. Shiga *, A. Mitsuda , H. Wada , V.V. Platonov, O.M. Tatsenko, V.D. Selemir, R.Z. Levitin Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan RFNC } VNIEF, Sarov, Nizhni Novorod Region, 607189, Russia Moscow State University, Vorobyvy Gory, Moscow, Russia
Abstract The EuNi (Si Ge ) system exhibits a temperature- and "eld-induced valence transition from Eu> to Eu> in the \V V concentration range 0.75)x)0.85. In this study, we performed magnetization measurements on compounds with 0.5)x)0.75 in ultra-high "elds up to 300 T generated by explosive compression of magnetic #ux. We observed a jump of the magnetization, indicating a "eld-induced valence change from Eu> to Eu> for each compound. The critical "eld increases linearly with decreasing x and reaches a value of 225 T for x"0.5. 2001 Elsevier Science B.V. All rights reserved. Keywords: EuNi Si ; EuNi Ge ; Valence transition; Metamagnetism
1. Introduction Some Ce-, Sm-, Eu-, Tm- and Yb-based compounds exhibit intermediate valence or non-integral valence behavior. Such behavior arises from the presence of the 4f level near the Fermi level. Among these systems, some Eu-based compounds are known to exhibit a valence transition, which may be introduced by "eld, pressure or temperature. EuPd Si [1] and EuNi (Si Ge ) [2], \V V
* Corresponding author. Tel.: #81-75-753-5465; fax: #8175-753-4861. E-mail address: [email protected] (M. Shiga). Present address: Institute for Solid State Physics, the University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
which crystallize in the ThCr Si type structure, are typical examples. Especially, the latter system is suitable for investigating the valence transition since it is possible to shift the valence state toward both divalent and trivalent states by increasing and decreasing the Ge concentration x. For x'0.82, Eu is in the divalent state (4f) with a stable magnetic moment at all temperatures studied and below 35}40 K antiferromagnetic ordering is observed. On the other hand, for x*0.82, at low temperatures Eu is almost in the non-magnetic trivalent (4f) state. With increasing temperature, a valence transition occurs toward the divalent state. The transition temperature ¹ , increases with decreasing Ge concen tration x. Recently, we have found that a magnetic "eld also induces the valence transition in
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 6 5 5 - 4
M. Shiga et al. / Physica B 294}295 (2001) 262}266
EuNi (Si Ge ) for 0.75)x)0.82, by using a \V V pulsed magnet ((43 T) and a destructive singleturn coil ((90 T) [2]. The transition "eld, H , increases with decreasing x. H versus ¹ plots, revealing that these values have a linear relation. However, the sample with x"0.70 shows no evidence of a "eld-induced valence transition up to 90 T. Since ¹ for x"0.70 is higher than that for x"0.75, it is expected that higher magnetic "elds would be necessary for inducing the valence transition. In this study, we report magnetization measurements in ultra-high "elds on EuNi (Si Ge ) . \V V with 0.50)x)0.75. We investigated whether the "eld-induced valence transition occurs also in this composition range.
2. Experiments Polycrystalline samples of EuNi (Si Ge ) \V V (x"0.5, 0.6, 0.65, 0.7 and 0.75) were prepared by melting the constituent elements in an argon arc furnace as explained in Ref. [2]. The purities of the starting elements are 99.9% for Eu metal, 99.95% for Ni metal, '5N for Si and Ge. Using X-ray di!raction, it was con"rmed that all samples are single phase with the ThCr Si type of structure. In the present study, as-cast samples were used, because in an earlier study [2] it was found that these samples show a sharper valence transition. The magnetic "elds up to 500 T were generated in a magnetic cumulative generator (MC-1) by compression of magnetic #ux. The layout [3] is shown in Fig. 1. Fig. 2 shows the magnetic "eld as a function of time. The magnetic susceptibility at 4.2 K was measured by an inductive method using a couple of compensation coils [4]. The sample was placed inside one of the coils. The signal from a pickup coil is proportional to the derivative of the magnetic moment, dM/dt. Since the coils are not absolutely identical, there exists a background signal that is proportional to the derivative dB/dt. In some cases, the sample with x"0.75, whose critical "eld has been established earlier [2], was inserted in one coil and a sample to be investigated in the other coil in order to improve the accuracy and reliability of the critical "eld determination.
263
Fig. 1. Layout of the magneto-cumulative generator.
3. Results Fig. 3 shows an example of a signal induced in the detection coils. In this case, EuNi (Si Ge ) was inserted in one of the detection coils as the standard sample and EuNi (Si Ge ) was in serted in another coil as the sample to be measured. In principle, it could be possible to obtain a M}H curve from these data by integrating the dM/dH signal. However, the sample coil picks up a non-negligible amount of noise signal in such an explosive-type of generator and furthermore the compensation of the two detection coils is not perfect. Thus, we only pay attention to the detection of anomalous changes of the magnetization, which is expected in the present system. We observe three anomalous points in dM/dt curve as indicated by the arrows in Fig. 3. By comparison with the signal from a "eld detection coil, the "eld strengths at the anomalies were determined to be 80, 150 and 220 T at the positions a, b and c, respectively. It is clear that the anomaly at a is attributable to the "eldinduced valence change of the EuNi (Si Ge ) sample because the critical "eld coincides with a previous measurement [2]. The anomaly at b is characteristic for the present measuring system and not a signal from the sample. By examining the reproducibility and comparison with the results on other samples, we have determined that the dip at the point c is attributable to the "eld-induced valence change of the EuNi (Si Ge ) sample. It is reasonable that the rapid increase of magnetization in the sample is detected as a dip in this measurement because the relevant sample is located in the opposite coil.
264
M. Shiga et al. / Physica B 294}295 (2001) 262}266
Fig. 2. Trace of signal of "eld detection coil.
Fig. 3. Trace of the signals induced in detection coils. A EuNi (Si Ge ) sample is inserted in one coil and a EuNi (Si Ge ) sample in the other.
Fig. 4 shows the valence-transition "elds of EuNi (Si Ge ) thus determined, together with \V V previous results for x*0.75 [2]. It is clearly seen in
the "gure that the critical "eld H increases linearly with decreasing x in the wide concentration range of 0.5)x)0.8.
M. Shiga et al. / Physica B 294}295 (2001) 262}266
265
4. Discussion A signal indicating the valence-transition "eld H was detected for all the samples with x)0.5. H increases almost linearly with decreasing x and the highest H in the present measurements is 220 T for x"0.5. The origin of this "eld-induced valence transition can naively be explained as follows: The Eu> ground state with J"0 has no magnetic moment and the Eu> excited state with J"S" 7/2 has a large moment. Therefore, an applied "eld stabilizes the latter state. The Eu> state has higher entropy and so a thermally induced valence change is also expected. A quantitative analysis was given on the basis of the inter-con"gurational #uctuation (ICF) model by Wada et al. [2] whose description is summarized as follows. They propose an energy level scheme as shown in Fig. 5. The ground state of Eu is Eu> with J"0 and excited multiplets with J"1 and 2 are taken into account. The energy level of Eu> with J"S"7/2 is located E above the Fermi level E . It is assumed that E depends $ on the population of the exited state as E "E (1!p ) [5], where p is the occupation probability of the Eu> state (p is that of Eu> state). The populations of p and p are given by Boltzmann statistics as
Fig. 4. Concentration dependence of the critical "eld of the "eld-induced valence transition in EuNi (Si Ge ) com \V V pounds at 4.2 K for increasing "eld. Circles: present results; Squares: after Wada et al. [2].
p " exp[!E (1!p ) p (X \
#g J H/k¹H] X
; 1# exp[ (!480k (X \ \ #g J H)/k¹*] , X
Fig. 5. The energy-level scheme for Eu in magnetic "elds.
(1)
where the e!ective temperature de"ned as ¹H"(¹#¹ (2) is introduced in order to take account of broadening of the 4f level by the c}f hybridization. By solving Eq. (1) using appropriate parameters E , and ¹ , Wada et al. explained semi-quantitatively the temperature and "eld dependence of EuNi (Si Ge ) for x*0.75. They have shown \V V
that (1) a "eld-induced transition has a trend to exhibit a "rst-order phase transition, and on the other hand, that (2) a thermally induced transition is rather gradual except for a limited range of parameters, and (3) the transition "eld H is linearly proportional to the transition temperature ¹ . We extended the calculation for a wide range of the parameter E (the bare excitation energy of the Eu> state), "xing other parameters (¹ is not an independent parameter but is determined as a
266
M. Shiga et al. / Physica B 294}295 (2001) 262}266
function of E , see Ref. [2]). The results (Fig. 6) indicate that a large E gives rise to a high H and, at the same time, the transition becomes continuous although it is still fairly sharp in the case of H &200 T. From the experiments with the pres ent apparatus, it is not possible to establish whether the valence change induced by a "eld is a "rst-order transition and exhibits hysteresis for increasing and decreasing "eld. References
Fig. 6. Calculated results for p , the fraction of ions in the Eu> state, which is proportional to the magnetization. The same parameters have been used as in Ref. [2].
[1] [2] [3] [4] [5]
E.V. Sampathkumaran et al., J. Phys. C 14 (1981) L237. H. Wada et al., J Phys.: Condens. Matter 9 (1997) 7913. A.I. Bykov et al., Physica B 216 (1996) 215. I.S. Dubenko et al., JETP Lett. 64 (1996) 2025. M. Croft et al., Phys. Rev. Lett. 48 (1982) 826.
Field-induced valence transition of EuNi (Si Ge ) \V V in ultra-high magnetic "elds M. Shiga *, A. Mitsuda , H. Wada , V.V. Platonov, O.M. Tatsenko, V.D. Selemir, R.Z. Levitin Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan RFNC } VNIEF, Sarov, Nizhni Novorod Region, 607189, Russia Moscow State University, Vorobyvy Gory, Moscow, Russia
Abstract The EuNi (Si Ge ) system exhibits a temperature- and "eld-induced valence transition from Eu> to Eu> in the \V V concentration range 0.75)x)0.85. In this study, we performed magnetization measurements on compounds with 0.5)x)0.75 in ultra-high "elds up to 300 T generated by explosive compression of magnetic #ux. We observed a jump of the magnetization, indicating a "eld-induced valence change from Eu> to Eu> for each compound. The critical "eld increases linearly with decreasing x and reaches a value of 225 T for x"0.5. 2001 Elsevier Science B.V. All rights reserved. Keywords: EuNi Si ; EuNi Ge ; Valence transition; Metamagnetism
1. Introduction Some Ce-, Sm-, Eu-, Tm- and Yb-based compounds exhibit intermediate valence or non-integral valence behavior. Such behavior arises from the presence of the 4f level near the Fermi level. Among these systems, some Eu-based compounds are known to exhibit a valence transition, which may be introduced by "eld, pressure or temperature. EuPd Si [1] and EuNi (Si Ge ) [2], \V V
* Corresponding author. Tel.: #81-75-753-5465; fax: #8175-753-4861. E-mail address: [email protected] (M. Shiga). Present address: Institute for Solid State Physics, the University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
which crystallize in the ThCr Si type structure, are typical examples. Especially, the latter system is suitable for investigating the valence transition since it is possible to shift the valence state toward both divalent and trivalent states by increasing and decreasing the Ge concentration x. For x'0.82, Eu is in the divalent state (4f) with a stable magnetic moment at all temperatures studied and below 35}40 K antiferromagnetic ordering is observed. On the other hand, for x*0.82, at low temperatures Eu is almost in the non-magnetic trivalent (4f) state. With increasing temperature, a valence transition occurs toward the divalent state. The transition temperature ¹ , increases with decreasing Ge concen tration x. Recently, we have found that a magnetic "eld also induces the valence transition in
0921-4526/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 6 5 5 - 4
M. Shiga et al. / Physica B 294}295 (2001) 262}266
EuNi (Si Ge ) for 0.75)x)0.82, by using a \V V pulsed magnet ((43 T) and a destructive singleturn coil ((90 T) [2]. The transition "eld, H , increases with decreasing x. H versus ¹ plots, revealing that these values have a linear relation. However, the sample with x"0.70 shows no evidence of a "eld-induced valence transition up to 90 T. Since ¹ for x"0.70 is higher than that for x"0.75, it is expected that higher magnetic "elds would be necessary for inducing the valence transition. In this study, we report magnetization measurements in ultra-high "elds on EuNi (Si Ge ) . \V V with 0.50)x)0.75. We investigated whether the "eld-induced valence transition occurs also in this composition range.
2. Experiments Polycrystalline samples of EuNi (Si Ge ) \V V (x"0.5, 0.6, 0.65, 0.7 and 0.75) were prepared by melting the constituent elements in an argon arc furnace as explained in Ref. [2]. The purities of the starting elements are 99.9% for Eu metal, 99.95% for Ni metal, '5N for Si and Ge. Using X-ray di!raction, it was con"rmed that all samples are single phase with the ThCr Si type of structure. In the present study, as-cast samples were used, because in an earlier study [2] it was found that these samples show a sharper valence transition. The magnetic "elds up to 500 T were generated in a magnetic cumulative generator (MC-1) by compression of magnetic #ux. The layout [3] is shown in Fig. 1. Fig. 2 shows the magnetic "eld as a function of time. The magnetic susceptibility at 4.2 K was measured by an inductive method using a couple of compensation coils [4]. The sample was placed inside one of the coils. The signal from a pickup coil is proportional to the derivative of the magnetic moment, dM/dt. Since the coils are not absolutely identical, there exists a background signal that is proportional to the derivative dB/dt. In some cases, the sample with x"0.75, whose critical "eld has been established earlier [2], was inserted in one coil and a sample to be investigated in the other coil in order to improve the accuracy and reliability of the critical "eld determination.
263
Fig. 1. Layout of the magneto-cumulative generator.
3. Results Fig. 3 shows an example of a signal induced in the detection coils. In this case, EuNi (Si Ge ) was inserted in one of the detection coils as the standard sample and EuNi (Si Ge ) was in serted in another coil as the sample to be measured. In principle, it could be possible to obtain a M}H curve from these data by integrating the dM/dH signal. However, the sample coil picks up a non-negligible amount of noise signal in such an explosive-type of generator and furthermore the compensation of the two detection coils is not perfect. Thus, we only pay attention to the detection of anomalous changes of the magnetization, which is expected in the present system. We observe three anomalous points in dM/dt curve as indicated by the arrows in Fig. 3. By comparison with the signal from a "eld detection coil, the "eld strengths at the anomalies were determined to be 80, 150 and 220 T at the positions a, b and c, respectively. It is clear that the anomaly at a is attributable to the "eldinduced valence change of the EuNi (Si Ge ) sample because the critical "eld coincides with a previous measurement [2]. The anomaly at b is characteristic for the present measuring system and not a signal from the sample. By examining the reproducibility and comparison with the results on other samples, we have determined that the dip at the point c is attributable to the "eld-induced valence change of the EuNi (Si Ge ) sample. It is reasonable that the rapid increase of magnetization in the sample is detected as a dip in this measurement because the relevant sample is located in the opposite coil.
264
M. Shiga et al. / Physica B 294}295 (2001) 262}266
Fig. 2. Trace of signal of "eld detection coil.
Fig. 3. Trace of the signals induced in detection coils. A EuNi (Si Ge ) sample is inserted in one coil and a EuNi (Si Ge ) sample in the other.
Fig. 4 shows the valence-transition "elds of EuNi (Si Ge ) thus determined, together with \V V previous results for x*0.75 [2]. It is clearly seen in
the "gure that the critical "eld H increases linearly with decreasing x in the wide concentration range of 0.5)x)0.8.
M. Shiga et al. / Physica B 294}295 (2001) 262}266
265
4. Discussion A signal indicating the valence-transition "eld H was detected for all the samples with x)0.5. H increases almost linearly with decreasing x and the highest H in the present measurements is 220 T for x"0.5. The origin of this "eld-induced valence transition can naively be explained as follows: The Eu> ground state with J"0 has no magnetic moment and the Eu> excited state with J"S" 7/2 has a large moment. Therefore, an applied "eld stabilizes the latter state. The Eu> state has higher entropy and so a thermally induced valence change is also expected. A quantitative analysis was given on the basis of the inter-con"gurational #uctuation (ICF) model by Wada et al. [2] whose description is summarized as follows. They propose an energy level scheme as shown in Fig. 5. The ground state of Eu is Eu> with J"0 and excited multiplets with J"1 and 2 are taken into account. The energy level of Eu> with J"S"7/2 is located E above the Fermi level E . It is assumed that E depends $ on the population of the exited state as E "E (1!p ) [5], where p is the occupation probability of the Eu> state (p is that of Eu> state). The populations of p and p are given by Boltzmann statistics as
Fig. 4. Concentration dependence of the critical "eld of the "eld-induced valence transition in EuNi (Si Ge ) com \V V pounds at 4.2 K for increasing "eld. Circles: present results; Squares: after Wada et al. [2].
p " exp[!E (1!p ) p (X \
#g J H/k¹H] X
; 1# exp[ (!480k (X \ \ #g J H)/k¹*] , X
Fig. 5. The energy-level scheme for Eu in magnetic "elds.
(1)
where the e!ective temperature de"ned as ¹H"(¹#¹ (2) is introduced in order to take account of broadening of the 4f level by the c}f hybridization. By solving Eq. (1) using appropriate parameters E , and ¹ , Wada et al. explained semi-quantitatively the temperature and "eld dependence of EuNi (Si Ge ) for x*0.75. They have shown \V V
that (1) a "eld-induced transition has a trend to exhibit a "rst-order phase transition, and on the other hand, that (2) a thermally induced transition is rather gradual except for a limited range of parameters, and (3) the transition "eld H is linearly proportional to the transition temperature ¹ . We extended the calculation for a wide range of the parameter E (the bare excitation energy of the Eu> state), "xing other parameters (¹ is not an independent parameter but is determined as a
266
M. Shiga et al. / Physica B 294}295 (2001) 262}266
function of E , see Ref. [2]). The results (Fig. 6) indicate that a large E gives rise to a high H and, at the same time, the transition becomes continuous although it is still fairly sharp in the case of H &200 T. From the experiments with the pres ent apparatus, it is not possible to establish whether the valence change induced by a "eld is a "rst-order transition and exhibits hysteresis for increasing and decreasing "eld. References
Fig. 6. Calculated results for p , the fraction of ions in the Eu> state, which is proportional to the magnetization. The same parameters have been used as in Ref. [2].
[1] [2] [3] [4] [5]
E.V. Sampathkumaran et al., J. Phys. C 14 (1981) L237. H. Wada et al., J Phys.: Condens. Matter 9 (1997) 7913. A.I. Bykov et al., Physica B 216 (1996) 215. I.S. Dubenko et al., JETP Lett. 64 (1996) 2025. M. Croft et al., Phys. Rev. Lett. 48 (1982) 826.