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Pressure effect on the magnetic properties in YbXCu4 (X=In and Cu)
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 405–407 www.elsevier.com/locate/jmmm
Pressure effect on the magnetic properties in YbXCu4 (X ¼ In and Cu) Takeshi Mitoa,, Masahiro Shimoidea, Takehide Koyamaa, Masanori Nakamuraa, S. Wadaa, Marian Reiffersb, Bogdan Idzikowskic, John L. Sarraod, Tatsuo C. Kobarashie a
Department of Physics, Faculty of Science, Kobe University, 1-1, Rokkodai-cho, Nadu-Ku, Nada, Kobe 657-8501, Japan b Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 043 53 Kosice, Slovakia c Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan, Poland d Los Alamos National Laboratory, Mail Stop K 764, Los Alamos, New Mexico 87545, USA e Department of Physics, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700-8530, Japan Available online 16 December 2004
Abstract The pressure-induced magnetic ordering recently found in YbInCu4 has been confirmed by the nuclear quadrupole resonance (NQR) measurement under pressure above PC2.45 GPa. The obtained results indicate that the transition between a well-localized phase at high temperature and a valence fluctuating Fermi liquid state at low temperature is of the first-order up to the critical pressure of PC. We also report the results of the nuclear magnetic resonance (NMR) measurements on YbCu5 with the cubic structure prepared by melt spinning technique. r 2004 Elsevier B.V. All rights reserved. PACS: 74.62.Fj; 75.20.Hr; 75.30.Kz Keywords: High pressure; NMR; Valence transitions; Heavy-fermion compounds
In strongly correlated electron systems, such as heavy fermion (HF) compounds, the study of physical properties in the vicinity of a magnetic instability has attracted much interest. Recently intensive studies have been carried out in Ce- and U-based HF compounds. This is mainly due to the fact that, in some of the Ce- and Ubased materials, it is possible to tune the f-electron state close to the localized–nonlocalized boundary by applying relatively low pressure. In Yb-based compounds, it is considered that, pressure works in the opposite direction Corresponding author. Tel.: +81 78 803 5664; fax: +81 78 803 5644. E-mail address: [email protected] (T. Mito).
to that in the Ce-based materials due to different electronic configurations between the 4f13–Yb3+ and the 4f–Ce3+. Pressure-induced magnetic ordering has been found only at quite rather high pressures such as 5 GPa or more. This leads to difficulty in precise investigation of the pressure-controlled physical properties near the magnetic instability in Yb-based systems. YbXCu4 series with the AuBe5-type cubic structure, especially the compounds with X ¼ Cu, Ag, and In are considered as good candidates for the above-mentioned issue. Actually it was recently found that stoichiometric YbInCu4 has a magnetically ordered ground state above critical pressure of PC2.45 GPa [1]. At ambient pressure, this compound exhibits a first-order valence
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.486
ARTICLE IN PRESS T. Mito et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 405–407
30 YbInCu4 Temperature [K]
transition at TV42 K from a well-localized Yb3+ state to a valence fluctuating one with decreasing temperature [2,3]. TV is strongly suppressed by applying hydrostatic pressure. The recent AC-susceptibility measurement under pressure suggested that after the complete suppression of the valence transition above PC, the ground state of the pressure-induced high-temperature (HT) phase is ferromagnetically ordered [1,4]. For YbAgCu4, it is a weakly mixed valent and has a relatively large coefficient of the electronic specific heat g250 mJ/mol K2 at ambient pressure [5]. High-pressure resistivity measurements, however, indicated that this compound does not order magnetically up to 19 GPa [6]. YbCu5 shows the largest g of 550 mJ/mol K2 of the three compounds, and does not show any evidence for the magnetic ordering down to 60 mK [7]. It is therefore expected that this compound is located close to the border magnetic–nonmagnetic phase boundary. In this report, we focus our interest on the nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) studies of YbInCu4 and YbCu5. The single crystals of YbInCu4 were grown using a flux technique. For the NQR measurements, many small pieces of single crystals were mounted inside a NMR coil. To create high pressure above 2 GPa, we used a piston-cylinder pressure-cell made of nonmagnetic NiCrAl/BeCu alloy. Daphne oil 7373 was used as a pressure-transmitting medium. As to YbCu5, the previous report suggested that high-pressure technique is needed to synthesize YbCu5 sample with the cubic structure, otherwise the hexagonal YbCu5 is stabilized [7]. In order to solve this problem, we employed meltspinning technique [8,9]. The obtained polycrystalline samples are in a form of ribbons, and the crystalline structure examined by X-ray diffraction measurements showed a single phase with the AuBe5-type cubic structure. Fig. 1 shows the pressure–temperature phase diagram for YbInCu4 determined by the resistivity, AC-susceptibility and NQR measurements [1,4]. In the present CuNQR measurement, the long-range magnetic orderings could be detected by disappearance of the signal intensity. This is caused by change in the resonance condition due to the appearance of local internal field at Cu-sites. The three phases, i.e. the well-localized phase at HT, the valence fluctuating Fermi liquid phase at low temperature (LT), and the pressure-induced magnetically ordered phase meet at PC and TM of 2.4 K. In order to discuss the transition between the HT and LT phases near PC, we plot DnQ ¼ 63nQ(HT)–63nQ (LT) against TV(P) in Fig. 2 with pressure implicit parameter. Here, 63nQ is the resonance frequency of 63Cu-NQR and is in proportion to the electrical field gradient at the positions of 63Cu nuclei originating from on-site and offsite (neighboring ions’) charges. nQ is therefore a quite sensitive parameter to the valence transition which
TV (resistivity)
TV
20
TV (NQR) TM (ac-χ) TM (NQR)
10 TM 0 0.5
1.0
1.5 2.0 Pressure [GPa]
2.5
Fig. 1. The pressure–temperature phase diagram for YbInCu4 determined by the resistivity, AC-susceptibility and NQR measurements. The solid lines are guides to the eye.
0.8
∆νQ [MHz]
406
0.6
0.4 ∆νQ= 63νQ(HT)-63νQ(LT)
0.2
= 0.376 + 0.012 TV 0.0 0
10
20
30
40
50
TV [K] Fig. 2. DnQ ¼ 63nQ(HT)–63nQ (LT) is plotted against the valence transition temperature TV(P) with pressure implicit parameter (see text for details).
immediately changes the charge distribution and the lattice parameters. As seen in Fig. 2, DnQ shows a linear relationship with TV(P). When extrapolating DnQ to TV ¼ 0, it clearly gives the nonzero value of DnQ. This indicates that, even at a pressure very close to PC, the electronic configuration in the HT phase is microscopically distinguishable from that in the LT phase, that is to say, indicates the existence of a discontinuous change in the valence of Yb ions and/or the lattice parameters. Therefore the localized–nonlocalized phase boundary in YbInCu4 is probably of the first order in the whole pressure range from 0 to PC. Above PC, the welllocalized f-electrons state is directly transformed into the magnetically ordered state as lowering temperature without showing any sign of intermediate heavy Fermi liquid behavior [4]. In order to identify the magnitude of
ARTICLE IN PRESS T. Mito et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 405–407
Knight shift [%]
-0.4
-0.6
-0.8 YbCu5
-1.0
4c-site 63Cu-NMR -1.2 0
20
40 60 Temperature [K]
80
100
Fig. 3. Temperature dependence of the Knight shift of 63Cu 4c site measured at the frequency of 11.2352 MHz and around the magnetic field of 1 T.
the ordered moments and magnetic structure, we intend to carry out the NMR measurements in the pressureinduced magnetically ordered phase. Next, we present results of the recent NMR measurements on YbCu5 which has two crystallographically inequivalent Cu sites, i.e. 4c and 16e sites. Fig. 3 shows the temperature dependence of the Knight shift of 63Cu 4c site. The minimum at around 20 K is considered to correspond with the maximum observed in the spin susceptibility, which may be associated with a formation of Kondo-lattice state [7]. Since the temperature dependence of the Knight shift is mainly ascribed to the spin part of the Knight shift KS originating from the spin susceptibility wS, the present result is interpreted by supposing a negative hyperfine coupling constant which connects KS and wS. We also measured the spin-lattice relaxation rate 1/T1. It should be noted (not shown here) that the absolute value and the temperature dependence of 1/T1 are in good agreement with the previous reports
407
which were measured on the sample prepared by high pressure technique [10]. Therefore, the present results for our sample prepared by the melt-spinning technique reflect intrinsic physical properties of the cubic YbCu5. We have been now investigating the pressure effect on this compound. In summary, we have carried out high pressure NQR measurement on YbInCu4 and NMR measurement on YbCu5. Our results indicate that (a) for YbInCu4 the localized–nonlocalized boundary on the pressure–temperature phase diagram is of the first order in the whole pressure range from 0 to PC2.45 GPa. (b) The YbCu5 sample prepared by the melt spinning method without using high-pressure technique exhibits the intrinsic physical properties of the cubic YbCu5, and this finding will open a new possibility for more intensive studies in the field of Yb-based HF systems. This work is supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Grant no. 14740211, 16740200, and 16340105); the Slovak Grant VEGA no. 3195/23; the contract no. I/2/2003 of the Slovak Academy of Sciences for the Centres of Excellence.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
T. Mito, et al., Phys. Rev. B 67 (2003) 224409. I. Felner, I. Nowik, Phys. Rev. B 33 (1986) 617. I. Felner, et al., Phys. Rev. B 35 (1987) 6956. T. Koyama, et al., J. Magn. Magn. Mater. 272–276 (Suppl.) (2004) E43. C. Rossel, et al., Phys. Rev. B 35 (1987) 1914. T. Graf, et al., Phys. Rev. B 52 (1995) 3099. N. Tsujii, et al., Phys. Rev. B 56 (1997) 8103. M. Reiffers, et al., Phys. Stat. Sol. (a) 196 (2003) 286. M. Reiffers, et al., J. Magn. Magn. Mater. 272–276 (2004) 209. Y. Ihara, et al., unpublished.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 405–407 www.elsevier.com/locate/jmmm
Pressure effect on the magnetic properties in YbXCu4 (X ¼ In and Cu) Takeshi Mitoa,, Masahiro Shimoidea, Takehide Koyamaa, Masanori Nakamuraa, S. Wadaa, Marian Reiffersb, Bogdan Idzikowskic, John L. Sarraod, Tatsuo C. Kobarashie a
Department of Physics, Faculty of Science, Kobe University, 1-1, Rokkodai-cho, Nadu-Ku, Nada, Kobe 657-8501, Japan b Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 043 53 Kosice, Slovakia c Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan, Poland d Los Alamos National Laboratory, Mail Stop K 764, Los Alamos, New Mexico 87545, USA e Department of Physics, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700-8530, Japan Available online 16 December 2004
Abstract The pressure-induced magnetic ordering recently found in YbInCu4 has been confirmed by the nuclear quadrupole resonance (NQR) measurement under pressure above PC2.45 GPa. The obtained results indicate that the transition between a well-localized phase at high temperature and a valence fluctuating Fermi liquid state at low temperature is of the first-order up to the critical pressure of PC. We also report the results of the nuclear magnetic resonance (NMR) measurements on YbCu5 with the cubic structure prepared by melt spinning technique. r 2004 Elsevier B.V. All rights reserved. PACS: 74.62.Fj; 75.20.Hr; 75.30.Kz Keywords: High pressure; NMR; Valence transitions; Heavy-fermion compounds
In strongly correlated electron systems, such as heavy fermion (HF) compounds, the study of physical properties in the vicinity of a magnetic instability has attracted much interest. Recently intensive studies have been carried out in Ce- and U-based HF compounds. This is mainly due to the fact that, in some of the Ce- and Ubased materials, it is possible to tune the f-electron state close to the localized–nonlocalized boundary by applying relatively low pressure. In Yb-based compounds, it is considered that, pressure works in the opposite direction Corresponding author. Tel.: +81 78 803 5664; fax: +81 78 803 5644. E-mail address: [email protected] (T. Mito).
to that in the Ce-based materials due to different electronic configurations between the 4f13–Yb3+ and the 4f–Ce3+. Pressure-induced magnetic ordering has been found only at quite rather high pressures such as 5 GPa or more. This leads to difficulty in precise investigation of the pressure-controlled physical properties near the magnetic instability in Yb-based systems. YbXCu4 series with the AuBe5-type cubic structure, especially the compounds with X ¼ Cu, Ag, and In are considered as good candidates for the above-mentioned issue. Actually it was recently found that stoichiometric YbInCu4 has a magnetically ordered ground state above critical pressure of PC2.45 GPa [1]. At ambient pressure, this compound exhibits a first-order valence
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.486
ARTICLE IN PRESS T. Mito et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 405–407
30 YbInCu4 Temperature [K]
transition at TV42 K from a well-localized Yb3+ state to a valence fluctuating one with decreasing temperature [2,3]. TV is strongly suppressed by applying hydrostatic pressure. The recent AC-susceptibility measurement under pressure suggested that after the complete suppression of the valence transition above PC, the ground state of the pressure-induced high-temperature (HT) phase is ferromagnetically ordered [1,4]. For YbAgCu4, it is a weakly mixed valent and has a relatively large coefficient of the electronic specific heat g250 mJ/mol K2 at ambient pressure [5]. High-pressure resistivity measurements, however, indicated that this compound does not order magnetically up to 19 GPa [6]. YbCu5 shows the largest g of 550 mJ/mol K2 of the three compounds, and does not show any evidence for the magnetic ordering down to 60 mK [7]. It is therefore expected that this compound is located close to the border magnetic–nonmagnetic phase boundary. In this report, we focus our interest on the nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) studies of YbInCu4 and YbCu5. The single crystals of YbInCu4 were grown using a flux technique. For the NQR measurements, many small pieces of single crystals were mounted inside a NMR coil. To create high pressure above 2 GPa, we used a piston-cylinder pressure-cell made of nonmagnetic NiCrAl/BeCu alloy. Daphne oil 7373 was used as a pressure-transmitting medium. As to YbCu5, the previous report suggested that high-pressure technique is needed to synthesize YbCu5 sample with the cubic structure, otherwise the hexagonal YbCu5 is stabilized [7]. In order to solve this problem, we employed meltspinning technique [8,9]. The obtained polycrystalline samples are in a form of ribbons, and the crystalline structure examined by X-ray diffraction measurements showed a single phase with the AuBe5-type cubic structure. Fig. 1 shows the pressure–temperature phase diagram for YbInCu4 determined by the resistivity, AC-susceptibility and NQR measurements [1,4]. In the present CuNQR measurement, the long-range magnetic orderings could be detected by disappearance of the signal intensity. This is caused by change in the resonance condition due to the appearance of local internal field at Cu-sites. The three phases, i.e. the well-localized phase at HT, the valence fluctuating Fermi liquid phase at low temperature (LT), and the pressure-induced magnetically ordered phase meet at PC and TM of 2.4 K. In order to discuss the transition between the HT and LT phases near PC, we plot DnQ ¼ 63nQ(HT)–63nQ (LT) against TV(P) in Fig. 2 with pressure implicit parameter. Here, 63nQ is the resonance frequency of 63Cu-NQR and is in proportion to the electrical field gradient at the positions of 63Cu nuclei originating from on-site and offsite (neighboring ions’) charges. nQ is therefore a quite sensitive parameter to the valence transition which
TV (resistivity)
TV
20
TV (NQR) TM (ac-χ) TM (NQR)
10 TM 0 0.5
1.0
1.5 2.0 Pressure [GPa]
2.5
Fig. 1. The pressure–temperature phase diagram for YbInCu4 determined by the resistivity, AC-susceptibility and NQR measurements. The solid lines are guides to the eye.
0.8
∆νQ [MHz]
406
0.6
0.4 ∆νQ= 63νQ(HT)-63νQ(LT)
0.2
= 0.376 + 0.012 TV 0.0 0
10
20
30
40
50
TV [K] Fig. 2. DnQ ¼ 63nQ(HT)–63nQ (LT) is plotted against the valence transition temperature TV(P) with pressure implicit parameter (see text for details).
immediately changes the charge distribution and the lattice parameters. As seen in Fig. 2, DnQ shows a linear relationship with TV(P). When extrapolating DnQ to TV ¼ 0, it clearly gives the nonzero value of DnQ. This indicates that, even at a pressure very close to PC, the electronic configuration in the HT phase is microscopically distinguishable from that in the LT phase, that is to say, indicates the existence of a discontinuous change in the valence of Yb ions and/or the lattice parameters. Therefore the localized–nonlocalized phase boundary in YbInCu4 is probably of the first order in the whole pressure range from 0 to PC. Above PC, the welllocalized f-electrons state is directly transformed into the magnetically ordered state as lowering temperature without showing any sign of intermediate heavy Fermi liquid behavior [4]. In order to identify the magnitude of
ARTICLE IN PRESS T. Mito et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 405–407
Knight shift [%]
-0.4
-0.6
-0.8 YbCu5
-1.0
4c-site 63Cu-NMR -1.2 0
20
40 60 Temperature [K]
80
100
Fig. 3. Temperature dependence of the Knight shift of 63Cu 4c site measured at the frequency of 11.2352 MHz and around the magnetic field of 1 T.
the ordered moments and magnetic structure, we intend to carry out the NMR measurements in the pressureinduced magnetically ordered phase. Next, we present results of the recent NMR measurements on YbCu5 which has two crystallographically inequivalent Cu sites, i.e. 4c and 16e sites. Fig. 3 shows the temperature dependence of the Knight shift of 63Cu 4c site. The minimum at around 20 K is considered to correspond with the maximum observed in the spin susceptibility, which may be associated with a formation of Kondo-lattice state [7]. Since the temperature dependence of the Knight shift is mainly ascribed to the spin part of the Knight shift KS originating from the spin susceptibility wS, the present result is interpreted by supposing a negative hyperfine coupling constant which connects KS and wS. We also measured the spin-lattice relaxation rate 1/T1. It should be noted (not shown here) that the absolute value and the temperature dependence of 1/T1 are in good agreement with the previous reports
407
which were measured on the sample prepared by high pressure technique [10]. Therefore, the present results for our sample prepared by the melt-spinning technique reflect intrinsic physical properties of the cubic YbCu5. We have been now investigating the pressure effect on this compound. In summary, we have carried out high pressure NQR measurement on YbInCu4 and NMR measurement on YbCu5. Our results indicate that (a) for YbInCu4 the localized–nonlocalized boundary on the pressure–temperature phase diagram is of the first order in the whole pressure range from 0 to PC2.45 GPa. (b) The YbCu5 sample prepared by the melt spinning method without using high-pressure technique exhibits the intrinsic physical properties of the cubic YbCu5, and this finding will open a new possibility for more intensive studies in the field of Yb-based HF systems. This work is supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Grant no. 14740211, 16740200, and 16340105); the Slovak Grant VEGA no. 3195/23; the contract no. I/2/2003 of the Slovak Academy of Sciences for the Centres of Excellence.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
T. Mito, et al., Phys. Rev. B 67 (2003) 224409. I. Felner, I. Nowik, Phys. Rev. B 33 (1986) 617. I. Felner, et al., Phys. Rev. B 35 (1987) 6956. T. Koyama, et al., J. Magn. Magn. Mater. 272–276 (Suppl.) (2004) E43. C. Rossel, et al., Phys. Rev. B 35 (1987) 1914. T. Graf, et al., Phys. Rev. B 52 (1995) 3099. N. Tsujii, et al., Phys. Rev. B 56 (1997) 8103. M. Reiffers, et al., Phys. Stat. Sol. (a) 196 (2003) 286. M. Reiffers, et al., J. Magn. Magn. Mater. 272–276 (2004) 209. Y. Ihara, et al., unpublished.