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Neutron scattering study of heavy fermion antiferromagnet Ce7Ni3 under pressure
Journal of Magnetism and Magnetic Materials 226}230 (2001) 85}86
Neutron scattering study of heavy fermion antiferromagnet Ce Ni under pressure K. Motoya *, T. Kawasaki , H. Kadowaki, T. Osakabe, H. Okumura, K. Kakurai, K. Umeo, T. Takabatake
Faculty of Science and Technology, Department of Physics, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan Neutron Scattering Laboratory, Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106, Japan Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Abstract Magnetic excitation of the heavy-fermion compound Ce Ni has been studied by inelastic neutron scattering technique in antiferromagnetic, non-Fermi-liquid (NFL) and Fermi liquid (FL) states under various pressures. The excitation spectra observed below and above the critical pressure for the magnetic ordering show a characteristic feature of spin #uctuations expected for the heavy-fermion material around magnetic instability. 2001 Elsevier Science B.V. All rights reserved. Keywords: Non-Fermi liquid; Heavy fermions; Neutron scattering; Inelastic; Instability; Pressure induced
Ce Ni crystallizes in the Th Fe -type hexagonal structure with three non-equivalent Ce sites. Speci"c heat and magnetic susceptibility measurements showed that an antiferromagnetic order (T "1.8 K) coexists with , a heavy-fermion state ("9 J/K mol f.u.) in this compound [1]. The antiferromagnetic order is suppressed by extremely low pressure (P "0.33 GPa) and non-Fermi liquid (NFL) behavior appears around P"0.4 GPa in speci"c heat and magnetic susceptibility. Above P" 0.62 GPa, the normal Fermi liquid (FL) state recovers [2]. Taking advantage of the low P and the absence of lattice disorder introduced by the alloying, we made neutron scattering study of magnetic excitations of Ce Ni under pressure around the magnetic instability and NFL/FL crossover. Magnetic structure of Ce Ni under the ambient pres sure was studied by neutron di!raction using a single crystal [3]. It was shown that the two successive phase transitions at ¹ "1.8 K and ¹ "0.7 K take place in , , * Corresponding author. Fax: #81-471-23-9361. E-mail address: [email protected] (K. Motoya).
zero "eld. At ¹ ordering of a single-k type magnetic , structure with an incommensurate modulation vector, k "0.22c* holds. This structure is approximately a sinusoidally modulated structure of c-axis moments, which belongs to the and irreducible representa tions of the space group. The root mean squares of the moments at ¹"1.4 K are 0.46$0.07, 0.70$0.03 and 0.10$0.05 for the Ce-1 (2(b)), the Ce-2 (6(c)) and the Ce-3 (6(c)) sites, respectively. Below ¹ the incommen, surate modulation vector k becomes temperature inde pendent, and a commensurate ordering with k "()cH appears in addition to the incommensurate structure. The metamagnetic transition under magnetic "eld of H "0.16 T applied parallel to the c-axis at ¹"1.5 K was shown to be the "rst-order transition to a ferromagnetic state. Inelastic neutron scattering measurements were made on a high-energy-resolution triple-axis spectrometer utilizing a horizontally focused analyzer. The energy resolution is 95 eV (FWHM) at the elastic position. A singlecrystal sample (7 mm diameter;20 mm length) was mounted in a clamp-type piston-cylinder pressure cell attached to a dilution refrigerator. Magnetic excitation
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 1 2 5 - 2
86
K. Motoya et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 85}86
Fig. 2. Pressure variations of static susceptibility (Q ) and energy width of spin #uctuations . Curves are drawn to guide the eye.
dynamical susceptibility [5], (Q ) (q) ; Im (Q #q, )" 1#(q/) #(q) Fig. 1. Inelastic neutron scattering spectra of Ce7Ni3 under various applied pressure measured at Q"(1 0 0). Background noise was subtracted. The curves show the "t to the data described in the text.
spectra in the antiferromagnetic and the pressure-induced paramagnetic phases up to P"0.79 GPa were taken with constant-q scans. Fig. 1 shows the excitation spectra under various pressure at ¹"0.22}0.35 K measured at Q"(1 0 0). Subtraction of the background noise mainly caused by the pressure cell, pressure media (Fluorinert) and large incoherent scattering from Ni atoms was made by measuring the scattering intensity by mounting a standard incoherent scatterer (vanadium metal) in the pressure cell instead of the sample. The qualitative aspect of the inelastic scattering spectra observed in the antiferromagnetic (P"0.19 GPa), the NFL (P"0.5 GPa) and the FL states (P"0.79 GPa) is quite similar in the present energy range. The scattering intensity decreases whereas the peak energy increases as the applied pressure is increased. The analysis of the excitation spectra was made based on the self-consistent renormalization (SCR) theory for spin #uctuations around the antiferromagnetic instability of heavy fermions proposed by Moriya and Takimoto [4]. Following their theory we used a simpli"ed form of the imaginary part of the
(1)
with (q)" (q#), (2) where (Q ), and Q are the static susceptibility, the speci"c energy scale of spin #uctuations and the antiferromagnetic wave vector, respectively. The curves in Fig. 1 are the results of the least-squares "t to the observed excitation spectra including the instrumental resolution function. Fig. 2 shows the variations of (Q ) and as a function of applied pressure. The pressure variation behavior of (Q ) and , namely the increase of and the decrease of (Q ) with pressure, is consistent with macroscopic measurements. Quantitative analysis of spin-#uctuation parameters, together with the comparison of the results of macroscopic measurements, will be presented in the forthcoming paper [6].
References [1] [2] [3] [4] [5] [6]
G. Sereni et al., Physica B 199&200 (1994) 567. K. Umeo et al., Phys. Rev. B 55 (1997) R692. H. Kadowaki et al., J. Phys. Soc. Jpn. 69 (2000) 2269. T. Moriya, Y. Takimoto, J. Phys. Soc. Jpn. 64 (1995) 960. W. Bao et al., Phys. Rev. B 58 (1998) 12727. K. Motoya et al., in preparation.
Neutron scattering study of heavy fermion antiferromagnet Ce Ni under pressure K. Motoya *, T. Kawasaki , H. Kadowaki, T. Osakabe, H. Okumura, K. Kakurai, K. Umeo, T. Takabatake
Faculty of Science and Technology, Department of Physics, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba, 278-8510, Japan Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan Neutron Scattering Laboratory, Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106, Japan Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Abstract Magnetic excitation of the heavy-fermion compound Ce Ni has been studied by inelastic neutron scattering technique in antiferromagnetic, non-Fermi-liquid (NFL) and Fermi liquid (FL) states under various pressures. The excitation spectra observed below and above the critical pressure for the magnetic ordering show a characteristic feature of spin #uctuations expected for the heavy-fermion material around magnetic instability. 2001 Elsevier Science B.V. All rights reserved. Keywords: Non-Fermi liquid; Heavy fermions; Neutron scattering; Inelastic; Instability; Pressure induced
Ce Ni crystallizes in the Th Fe -type hexagonal structure with three non-equivalent Ce sites. Speci"c heat and magnetic susceptibility measurements showed that an antiferromagnetic order (T "1.8 K) coexists with , a heavy-fermion state ("9 J/K mol f.u.) in this compound [1]. The antiferromagnetic order is suppressed by extremely low pressure (P "0.33 GPa) and non-Fermi liquid (NFL) behavior appears around P"0.4 GPa in speci"c heat and magnetic susceptibility. Above P" 0.62 GPa, the normal Fermi liquid (FL) state recovers [2]. Taking advantage of the low P and the absence of lattice disorder introduced by the alloying, we made neutron scattering study of magnetic excitations of Ce Ni under pressure around the magnetic instability and NFL/FL crossover. Magnetic structure of Ce Ni under the ambient pres sure was studied by neutron di!raction using a single crystal [3]. It was shown that the two successive phase transitions at ¹ "1.8 K and ¹ "0.7 K take place in , , * Corresponding author. Fax: #81-471-23-9361. E-mail address: [email protected] (K. Motoya).
zero "eld. At ¹ ordering of a single-k type magnetic , structure with an incommensurate modulation vector, k "0.22c* holds. This structure is approximately a sinusoidally modulated structure of c-axis moments, which belongs to the and irreducible representa tions of the space group. The root mean squares of the moments at ¹"1.4 K are 0.46$0.07, 0.70$0.03 and 0.10$0.05 for the Ce-1 (2(b)), the Ce-2 (6(c)) and the Ce-3 (6(c)) sites, respectively. Below ¹ the incommen, surate modulation vector k becomes temperature inde pendent, and a commensurate ordering with k "()cH appears in addition to the incommensurate structure. The metamagnetic transition under magnetic "eld of H "0.16 T applied parallel to the c-axis at ¹"1.5 K was shown to be the "rst-order transition to a ferromagnetic state. Inelastic neutron scattering measurements were made on a high-energy-resolution triple-axis spectrometer utilizing a horizontally focused analyzer. The energy resolution is 95 eV (FWHM) at the elastic position. A singlecrystal sample (7 mm diameter;20 mm length) was mounted in a clamp-type piston-cylinder pressure cell attached to a dilution refrigerator. Magnetic excitation
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 1 2 5 - 2
86
K. Motoya et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 85}86
Fig. 2. Pressure variations of static susceptibility (Q ) and energy width of spin #uctuations . Curves are drawn to guide the eye.
dynamical susceptibility [5], (Q ) (q) ; Im (Q #q, )" 1#(q/) #(q) Fig. 1. Inelastic neutron scattering spectra of Ce7Ni3 under various applied pressure measured at Q"(1 0 0). Background noise was subtracted. The curves show the "t to the data described in the text.
spectra in the antiferromagnetic and the pressure-induced paramagnetic phases up to P"0.79 GPa were taken with constant-q scans. Fig. 1 shows the excitation spectra under various pressure at ¹"0.22}0.35 K measured at Q"(1 0 0). Subtraction of the background noise mainly caused by the pressure cell, pressure media (Fluorinert) and large incoherent scattering from Ni atoms was made by measuring the scattering intensity by mounting a standard incoherent scatterer (vanadium metal) in the pressure cell instead of the sample. The qualitative aspect of the inelastic scattering spectra observed in the antiferromagnetic (P"0.19 GPa), the NFL (P"0.5 GPa) and the FL states (P"0.79 GPa) is quite similar in the present energy range. The scattering intensity decreases whereas the peak energy increases as the applied pressure is increased. The analysis of the excitation spectra was made based on the self-consistent renormalization (SCR) theory for spin #uctuations around the antiferromagnetic instability of heavy fermions proposed by Moriya and Takimoto [4]. Following their theory we used a simpli"ed form of the imaginary part of the
(1)
with (q)" (q#), (2) where (Q ), and Q are the static susceptibility, the speci"c energy scale of spin #uctuations and the antiferromagnetic wave vector, respectively. The curves in Fig. 1 are the results of the least-squares "t to the observed excitation spectra including the instrumental resolution function. Fig. 2 shows the variations of (Q ) and as a function of applied pressure. The pressure variation behavior of (Q ) and , namely the increase of and the decrease of (Q ) with pressure, is consistent with macroscopic measurements. Quantitative analysis of spin-#uctuation parameters, together with the comparison of the results of macroscopic measurements, will be presented in the forthcoming paper [6].
References [1] [2] [3] [4] [5] [6]
G. Sereni et al., Physica B 199&200 (1994) 567. K. Umeo et al., Phys. Rev. B 55 (1997) R692. H. Kadowaki et al., J. Phys. Soc. Jpn. 69 (2000) 2269. T. Moriya, Y. Takimoto, J. Phys. Soc. Jpn. 64 (1995) 960. W. Bao et al., Phys. Rev. B 58 (1998) 12727. K. Motoya et al., in preparation.