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A μSR study of the magnetic properties of Ce3Pd20Ge6
ARTICLE IN PRESS
Physica B 374–375 (2006) 192–194 www.elsevier.com/locate/physb
A mSR study of the magnetic properties of Ce3Pd20Ge6 V.N. Duginova,, K.I. Gritsaja, V.Yu. Pomjakushina, A.N. Ponomarevb, A.A. Nezhivoyb, A.V. Gribanovc, V.N. Nikiforovc, Yu.D. Seropeginc a
Joint Institute for Nuclear Research, 141980 Dubna, Moscow region,Russia b RSC ‘‘Kurchatov Institute’’, Kurchatov sq.1, 123182 Moscow, Russia c Moscow State University, Moscow, 119899, Russia
Abstract The compounds Ce3 Pd20 X6 ðX ¼ Ge; SiÞ manifest unusual physical properties which would catalogue them as magnetic Kondo systems. Our zero-field mSR measurements were undertaken to gain information about the magnetic behaviour at low temperatures. The muon spin relaxation rate was found to increase up to value of 4 ms1 at 50 mK. Below 0.3 K an increase in the depolarization rate is believed to represent the development of quasi-static ordering of magnetic moments of electronic origin. A follow-up series of transverse-field mSR measurements were performed. The external fields were varied up to 5 kOe and the temperature dependence of the internal magnetic field was found to be similar to that found in Ce3 Pd20 Si6 . The mSR experiments were carried out at the PSI, Villigen, Switzerland. r 2005 Elsevier B.V. All rights reserved. PACS: 76.75.+i; 75.20.Hr; 75.50.Lk Keywords: Magnetism; Ce3Pd20Ge6; Heavy fermions
Ternary rare-earth compounds R–T–X (R, rare earth metal; T, transition element; X, Si or Ge) have recently received considerable attention due to their interesting properties such as heavy fermion state, non-Fermi liquid behaviour, superconductivity, mixed valence, Kondo phenomena, and anomalous magnetic ordering. Cerium phases are known to exhibit these features particularly frequently. The systems Ce3 Pd20 X6 ðX ¼ Ge; SiÞ manifest unusual physical properties which categorize them as Kondo magnetic. The competition between magnetic and Kondo interactions was traditionally considered to result in either the magnetic ground state with significant full suppression of Kondo features, or in a non-magnetic Kondo ground state. However, in the last decade it has been found that there are many f-electron compounds in which magnetic ordering coexists with Kondo behaviour. The Ce3 Pd20 X6 ðX ¼ Ge; SiÞ systems
Corresponding author. Fax: +749621 66666.
E-mail address: [email protected] (V.N. Duginov). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.11.052
belong to them. The results of our previous experiments with the compound Ce3 Pd20 Si6 have been published [1]. It is known, there are two relatively separated cerium subsystems in these compounds [2]. The Ce3 Pd20 Ge6 structure is presented in Fig. 1. Only the cerium positions are shown because the total number of atoms per unit cell is quite large(1 1 6). The sites of Ce1 form a face-centred ‘large’ cube with other atoms in the cell inside. The atoms of Ce2 make up a ‘small’ cube with only Pd atoms inside. One of the subsystems involves Ce3þ ions in Ce2 [3] positions which form ‘small’ cubes inside the unit cell. Within the framework of the ‘molecular magnetism’ model [5], Ce2 atoms should interact primarily only with other Ce atoms of the ‘small’ cube. Ions of each cube make up magnetic ‘molecules’ with a magnetic moment that increases with decreasing temperature and undergo antiferromagnetic-like ordering at T magn . The second cerium subsystem consists of Ce1 ions. These ions are less magnetically active because they have Ge as nearest neighbors. In this model, the Ce1 atoms may mostly
ARTICLE IN PRESS V.N. Duginov et al. / Physica B 374–375 (2006) 192–194
193
Ce2
Ce1
Fig. 2. The temperature dependence of the depolarization rate of the muon spin at ZF conditions.
Fig. 1. The unit cell of Ce3Pd20Ge6. Only Ce positions are shown.
play the role of Kondo scattering centres for the conduction electrons. Ordering in the Ce2 sublattice near T magn removes the Ce1–Ce2 interactions which are competitive with the Kondo ones. This could explain the enhanced increase in electrical resistivity due to Kondo scattering of the conducting electrons in Ce3 Pd20 Si6 after the magnetic transition. Though the crystal structures of Ce3 Pd20 Ge6 and Ce3 Pd20 Si6 are almost identical, no magnetic transition has been detected in the former compound [4,5]. It was found [5] that at temperatures down to 4.2 K and magnetic fields up to 4000 Oe magnetic properties of Ce3 Pd20 Ge6 may be described within the frame of usual paramagnetism theory including crystal electric field (CEF) effects. The polycrystalline samples of Ce3 Pd20 Ge6 were prepared by using a melting technique in an arc furnace in argon atmosphere as described in Ref. [3]. The crystal structure of the samples, determined by X-ray analysis, is the same as reported in Ref. [3]. Zero-field mSR measurements were undertaken to gain information about the magnetic ordering at low temperatures. The experiments were performed on the surface-mþ beamline pM3 using the LTF-set-up at the Paul Scherrer Institute (Villigen, Switzerland) [6]. The depolarization function was represented by an exponential function. The temperature dependence of the depolarization rate of the muon spin is shown in Fig. 2. Below 0.4 K the increase of the depolarization rate represents the development of quasi-static ordering of magnetic moments of electronic origin most probably randomly oriented. We observe a recovery of the polarization in longitudinal-field measurements, with fields up to 1 T (Fig. 3). This proves the dynamic nature of part of the muon spin depolarization. We also performed transverse-field mSR measurements on the muon decay channel mE1 with the GPD-set-up at the Paul Scherrer Institute (Villigen, Switzerland). The precession time spectra AðtÞ are described by the sum of two signals: AðtÞ ¼ As expðls tÞ cosðos t þ fÞþ Ab expðlb tÞ cosðob t þ fÞ, where the background signal
Fig. 3. The recovery of the muon spin polarization in longitudinal field.
Ab ðtÞ corresponds to the muon stopping in the sample holder with a low depolarization rate. The value of the external magnetic field during these measurements was controlled by the muon spin precession frequency of the background signal. The measurements were performed in an external field 3 kOe. A clear frequency shift was seen at temperatures below 60 K. The temperature dependence of the magnetic field seen by the muons (at 3 kOe) is shown in Fig. 4. The experimental points on the dashed line in this figure correspond to the external magnetic field B ¼ ob =gm . For other field values, a similar behaviour was observed. This behaviour may be ascribed to an increase of the total moments of the superparamagnetic cube (SPC) containing eight Ce2 atoms with decreasing temperature. The m Knight-shift on Pd is too small to explain our results. Direct evidence of the SPC formation would be proved by measuring the deviation of the frequency shift from the linear scaling with the external field due to the increase of the total moment of the cube. The temperature dependence of the muon spin depolarization is shown in Fig. 5. The sharp increase in the depolarization rate below 10 K is observed. In our opinion, Ce3 Pd20 Ge6 is a ‘typical’ anomalous rare-earth magnetic system, which demonstrates the whole
ARTICLE IN PRESS 194
V.N. Duginov et al. / Physica B 374–375 (2006) 192–194
variety of peculiarities of the Kondo-lattice magnets. The CEF analysis indicates a significant reduction of the magnetic moment of cerium ions in Ce1 positions in comparison to those in Ce2 positions. This reduction significantly increases at To15 K. It seems to confirm the suggestion [7,8] that Ce1 ions may play a major role in Kondo-like behaviour of Ce3 Pd20 Ge6 . We have obtained the reproducible results on two samples of Ce3Pd20Ge6. It seems that all magnetic anomalies can be observed only at sufficiently low magnetic field.
Fig. 4. The temperature dependence of the internal magnetic field acting on muon. The experimental points on the dashed line correspond to the external magnetic field.
The authors are grateful to D. Herlach, Ch. Baines, U. Zimmermann and to the Directorate of the Paul Scherrer Institute for the possibility of carrying out the experiments on the SmS facilities. The investigation was supported by Russian Foundation for Basic Research, project 01-02-18019. References [1] [2] [3] [4] [5] [6] [7]
Fig. 5. The temperature dependence of the depolarization rate of muon spin at TF conditions.
[8]
V.N. Duginov, et al., Physica B 289 (2000) 43. V.N. Nikiforov, et al., Low Temp. Phys. 24 (1998) 565. A.V. Gribanov, et al., J. Alloys Compounds 204 (1994) L9. Yu.P. Gajdukov, Yu.A. Koksharov, Yu.V. Kochetkov, J. Mirkovic´, V.N. Nikiforov, Pis’ma v ZhETF 61 (1995) 385. V.N. Nikiforov, Yu.A. Koksharov, J. Mirkovic´, Yu.V. Kochetkov, J. Magn. Magn. Mater. 163 (1996) 184. R. Abela, et al., Hyp. Int. 87 (1994) 1105. V.Yu. Irkhin, M.I. Katsnelson, Phys. Rev. B 56 (1997) 8109; V.Yu. Irkhin, M.I. Katsnelson, Phys. Rev. B 59 (1999) 9348; V.Yu. Irkhin, Yu.P. Irkhin, Electronic structure, correlation effects and physical properties in d- and f-transition metals and their compounds, cond-mat/9812072. Yu. Izumov, M. Katsnelson, Yu. Skrabin, Itinerant Electron Magnetism, Moscow, Nauka, 1994, p. 280.
Physica B 374–375 (2006) 192–194 www.elsevier.com/locate/physb
A mSR study of the magnetic properties of Ce3Pd20Ge6 V.N. Duginova,, K.I. Gritsaja, V.Yu. Pomjakushina, A.N. Ponomarevb, A.A. Nezhivoyb, A.V. Gribanovc, V.N. Nikiforovc, Yu.D. Seropeginc a
Joint Institute for Nuclear Research, 141980 Dubna, Moscow region,Russia b RSC ‘‘Kurchatov Institute’’, Kurchatov sq.1, 123182 Moscow, Russia c Moscow State University, Moscow, 119899, Russia
Abstract The compounds Ce3 Pd20 X6 ðX ¼ Ge; SiÞ manifest unusual physical properties which would catalogue them as magnetic Kondo systems. Our zero-field mSR measurements were undertaken to gain information about the magnetic behaviour at low temperatures. The muon spin relaxation rate was found to increase up to value of 4 ms1 at 50 mK. Below 0.3 K an increase in the depolarization rate is believed to represent the development of quasi-static ordering of magnetic moments of electronic origin. A follow-up series of transverse-field mSR measurements were performed. The external fields were varied up to 5 kOe and the temperature dependence of the internal magnetic field was found to be similar to that found in Ce3 Pd20 Si6 . The mSR experiments were carried out at the PSI, Villigen, Switzerland. r 2005 Elsevier B.V. All rights reserved. PACS: 76.75.+i; 75.20.Hr; 75.50.Lk Keywords: Magnetism; Ce3Pd20Ge6; Heavy fermions
Ternary rare-earth compounds R–T–X (R, rare earth metal; T, transition element; X, Si or Ge) have recently received considerable attention due to their interesting properties such as heavy fermion state, non-Fermi liquid behaviour, superconductivity, mixed valence, Kondo phenomena, and anomalous magnetic ordering. Cerium phases are known to exhibit these features particularly frequently. The systems Ce3 Pd20 X6 ðX ¼ Ge; SiÞ manifest unusual physical properties which categorize them as Kondo magnetic. The competition between magnetic and Kondo interactions was traditionally considered to result in either the magnetic ground state with significant full suppression of Kondo features, or in a non-magnetic Kondo ground state. However, in the last decade it has been found that there are many f-electron compounds in which magnetic ordering coexists with Kondo behaviour. The Ce3 Pd20 X6 ðX ¼ Ge; SiÞ systems
Corresponding author. Fax: +749621 66666.
E-mail address: [email protected] (V.N. Duginov). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.11.052
belong to them. The results of our previous experiments with the compound Ce3 Pd20 Si6 have been published [1]. It is known, there are two relatively separated cerium subsystems in these compounds [2]. The Ce3 Pd20 Ge6 structure is presented in Fig. 1. Only the cerium positions are shown because the total number of atoms per unit cell is quite large(1 1 6). The sites of Ce1 form a face-centred ‘large’ cube with other atoms in the cell inside. The atoms of Ce2 make up a ‘small’ cube with only Pd atoms inside. One of the subsystems involves Ce3þ ions in Ce2 [3] positions which form ‘small’ cubes inside the unit cell. Within the framework of the ‘molecular magnetism’ model [5], Ce2 atoms should interact primarily only with other Ce atoms of the ‘small’ cube. Ions of each cube make up magnetic ‘molecules’ with a magnetic moment that increases with decreasing temperature and undergo antiferromagnetic-like ordering at T magn . The second cerium subsystem consists of Ce1 ions. These ions are less magnetically active because they have Ge as nearest neighbors. In this model, the Ce1 atoms may mostly
ARTICLE IN PRESS V.N. Duginov et al. / Physica B 374–375 (2006) 192–194
193
Ce2
Ce1
Fig. 2. The temperature dependence of the depolarization rate of the muon spin at ZF conditions.
Fig. 1. The unit cell of Ce3Pd20Ge6. Only Ce positions are shown.
play the role of Kondo scattering centres for the conduction electrons. Ordering in the Ce2 sublattice near T magn removes the Ce1–Ce2 interactions which are competitive with the Kondo ones. This could explain the enhanced increase in electrical resistivity due to Kondo scattering of the conducting electrons in Ce3 Pd20 Si6 after the magnetic transition. Though the crystal structures of Ce3 Pd20 Ge6 and Ce3 Pd20 Si6 are almost identical, no magnetic transition has been detected in the former compound [4,5]. It was found [5] that at temperatures down to 4.2 K and magnetic fields up to 4000 Oe magnetic properties of Ce3 Pd20 Ge6 may be described within the frame of usual paramagnetism theory including crystal electric field (CEF) effects. The polycrystalline samples of Ce3 Pd20 Ge6 were prepared by using a melting technique in an arc furnace in argon atmosphere as described in Ref. [3]. The crystal structure of the samples, determined by X-ray analysis, is the same as reported in Ref. [3]. Zero-field mSR measurements were undertaken to gain information about the magnetic ordering at low temperatures. The experiments were performed on the surface-mþ beamline pM3 using the LTF-set-up at the Paul Scherrer Institute (Villigen, Switzerland) [6]. The depolarization function was represented by an exponential function. The temperature dependence of the depolarization rate of the muon spin is shown in Fig. 2. Below 0.4 K the increase of the depolarization rate represents the development of quasi-static ordering of magnetic moments of electronic origin most probably randomly oriented. We observe a recovery of the polarization in longitudinal-field measurements, with fields up to 1 T (Fig. 3). This proves the dynamic nature of part of the muon spin depolarization. We also performed transverse-field mSR measurements on the muon decay channel mE1 with the GPD-set-up at the Paul Scherrer Institute (Villigen, Switzerland). The precession time spectra AðtÞ are described by the sum of two signals: AðtÞ ¼ As expðls tÞ cosðos t þ fÞþ Ab expðlb tÞ cosðob t þ fÞ, where the background signal
Fig. 3. The recovery of the muon spin polarization in longitudinal field.
Ab ðtÞ corresponds to the muon stopping in the sample holder with a low depolarization rate. The value of the external magnetic field during these measurements was controlled by the muon spin precession frequency of the background signal. The measurements were performed in an external field 3 kOe. A clear frequency shift was seen at temperatures below 60 K. The temperature dependence of the magnetic field seen by the muons (at 3 kOe) is shown in Fig. 4. The experimental points on the dashed line in this figure correspond to the external magnetic field B ¼ ob =gm . For other field values, a similar behaviour was observed. This behaviour may be ascribed to an increase of the total moments of the superparamagnetic cube (SPC) containing eight Ce2 atoms with decreasing temperature. The m Knight-shift on Pd is too small to explain our results. Direct evidence of the SPC formation would be proved by measuring the deviation of the frequency shift from the linear scaling with the external field due to the increase of the total moment of the cube. The temperature dependence of the muon spin depolarization is shown in Fig. 5. The sharp increase in the depolarization rate below 10 K is observed. In our opinion, Ce3 Pd20 Ge6 is a ‘typical’ anomalous rare-earth magnetic system, which demonstrates the whole
ARTICLE IN PRESS 194
V.N. Duginov et al. / Physica B 374–375 (2006) 192–194
variety of peculiarities of the Kondo-lattice magnets. The CEF analysis indicates a significant reduction of the magnetic moment of cerium ions in Ce1 positions in comparison to those in Ce2 positions. This reduction significantly increases at To15 K. It seems to confirm the suggestion [7,8] that Ce1 ions may play a major role in Kondo-like behaviour of Ce3 Pd20 Ge6 . We have obtained the reproducible results on two samples of Ce3Pd20Ge6. It seems that all magnetic anomalies can be observed only at sufficiently low magnetic field.
Fig. 4. The temperature dependence of the internal magnetic field acting on muon. The experimental points on the dashed line correspond to the external magnetic field.
The authors are grateful to D. Herlach, Ch. Baines, U. Zimmermann and to the Directorate of the Paul Scherrer Institute for the possibility of carrying out the experiments on the SmS facilities. The investigation was supported by Russian Foundation for Basic Research, project 01-02-18019. References [1] [2] [3] [4] [5] [6] [7]
Fig. 5. The temperature dependence of the depolarization rate of muon spin at TF conditions.
[8]
V.N. Duginov, et al., Physica B 289 (2000) 43. V.N. Nikiforov, et al., Low Temp. Phys. 24 (1998) 565. A.V. Gribanov, et al., J. Alloys Compounds 204 (1994) L9. Yu.P. Gajdukov, Yu.A. Koksharov, Yu.V. Kochetkov, J. Mirkovic´, V.N. Nikiforov, Pis’ma v ZhETF 61 (1995) 385. V.N. Nikiforov, Yu.A. Koksharov, J. Mirkovic´, Yu.V. Kochetkov, J. Magn. Magn. Mater. 163 (1996) 184. R. Abela, et al., Hyp. Int. 87 (1994) 1105. V.Yu. Irkhin, M.I. Katsnelson, Phys. Rev. B 56 (1997) 8109; V.Yu. Irkhin, M.I. Katsnelson, Phys. Rev. B 59 (1999) 9348; V.Yu. Irkhin, Yu.P. Irkhin, Electronic structure, correlation effects and physical properties in d- and f-transition metals and their compounds, cond-mat/9812072. Yu. Izumov, M. Katsnelson, Yu. Skrabin, Itinerant Electron Magnetism, Moscow, Nauka, 1994, p. 280.