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Spin wave excitations in single crystalline U3Pd20Si6
Physica B 312–313 (2002) 897–898
Spin wave excitations in single crystalline U3 Pd20Si6 N. Asoa,*, N. Metokib, M. Kohgic, K.A. McEwend, Y. Koikeb,e, Y. Hagab, N. Tateiwaf, N. Kimurag, H. Aokig, T. Komatsubarag, Y. Moriib a
Neutron Scattering Laboratory, Institute for Solid State Physics, The University of Tokyo, Shirakata 106-1, Tokai-mura, Ibaraki 319-1106, Japan b Advanced Science Research Center, Japan Atomic Energy Research Inst., Tokai, Ibaraki 319-1195, Japan c Graduate School of Sci., Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan d University College London, London WC1E 6BT, UK e Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan f Research Center for Materials Science at Extreme Condition, Osaka University, Toyonaka, Osaka 560-8531, Japan g Center for Low Temp. Sci., Tohoku University, Sendai 980-8545, Japan
Abstract We have performed inelastic neutron scattering experiments to examine the magnetic ordering in U3 Pd20 Si6 ; which exhibits antiferromagnetic ordering at TN =19 K and ferromagnetic ordering at TC =2 K. We observed that both antiferromagnetic spin wave excitations due to the magnetic moments at the 8c site and ferromagnetic ones due to the magnetic moments at the 4a site coexisted below TC =2 K. Our results indicate direct experimental evidence that U3 Pd20 Si6 is a rare localized 5f electron system in uranium intermetalic compounds, with coexistence of both antiferromagnetic and ferromagnetic ordering. The anomolous line width around the zone center is also discussed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: U3 Pd20 Si6 ; Local moment magnetism; Spin wave; Neutron scattering
New ternary intermetalic compounds R3 Pd20 X6 (R=rare earth: X=Si, Ge) system with the cubic Cr23 C6 -type crystal structure (space group Fm3m) have attracted much attention. In these compounds where the rare earth occupy the two inequivalent crystallographic sites 4a and 8c, both with cubic symmetry, it is common that the successive phase transitions are found and each ordering is derived from each rare earth atom site. Recently, Tateiwa et al. suceeded in growing a new isostructural uranium compound U3 Pd20 Si6 [1, 2]. The temperature dependence of susceptibility obeys the Curie–Weiss law above 50 K with an effective moment 3:30mB =U: Two successive magnetic phase transitions are found in the electrical resistivity, the specific heat and the magnetic susceptibility at 19 and 2 K. Furthermore, time-of-flight inelastic neutron scattering study [4] *Corresponding author. Tel.: +81-29-287-8906; fax: +8129-283-3922. E-mail address: [email protected] (N. Aso).
shows crystalline electric field excitations originated from at both 4a and 8c uranium magnetic moments, indicating that U3 Pd20 Si6 is a rare localized 5f electron system in uranium intermetalic compounds. Here we report on inelastic neutron scattering studies on the spin wave excitations in single crystalline U3 Pd20 Si6 to investigate the nature of the 5f electrons. The single crystal of U3 Pd20 Si6 were grown by the Czochralski pulling method under an Ar gas atmosphere using a tri-arc furnace. Details of the preparation were previously reported [1]. Inelastic neutron scattering measurements were performed at the triple-axis spectrometers TAS-1 and LTAS installed at the research reactor JRR-3M at JAERI, Japan. The temperature was cooled down to 50 mK by using the dilution refrigerator. Previous elastic neutron experiments [3] shows that antiferromagnetic (AF) Bragg peaks with a wave vector k ¼ ½1; 1; 1 coming from the uranium magnetic moment at the 8c site appear below TN and ferromagnetic (F)
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 2 6 6 - 2
898
N. Aso et al. / Physica B 312–313 (2002) 897–898 300
U3Pd20Si6 Q=(1,1,1) LTAS, Ef = 3.5 meV, 80'-80'-80'
Intensity (counts / 10800000 Mon)
250
Fig. 1. Contour plot of the scattering intensities in the direction [1+q; 1+q; 1+q] at T ¼ 0:05 K.
peaks from the magnetic moment at the 4a sites do below TC and both coexist below TC : Fig. 1 illustrates the contour plot of the scattering intensities obtained with the fixed final energy of 3.5 meV at T=0.05 K below TC : As can be clearly seen, the AF spin wave excitations derived from the 8c site magnetic moment can be recognized in the 0:5ooo3:0 meV region. We made such contour plots at major symmetry axes and its dispersion relations are rather well understood by the spin wave of the G-type AF structure of the simple cubic, with the nearest neighbor exchange interaction between the 8c site magnetic moment (JB0:2 meV) and the uniaxial anisotropy giving the energy gap of 1.0 meV at q=0, indicated by the solid line in the plot. Its line width was found to show the energy resolution limited behavior except for the AF zone center. Superposed in the Q-independent incoherent scattering at o=0, additional weak scattering intensities are found in the 0ooo1 meV region. These intensities correspond to the F spin wave excitations due to the magnetic moments at the 4a site as shown by the broken line. These characteristic features are very rare in the uranium compounds, and indicate that the uranium magnetic moment at both 4a and 8c sites show the localized character. Fig. 2 show the temperature dependence of constant Q scans at (1+q; 1+q; 1+q) with q=0 and 0.4. In the spectrum at q=0 shown in the upper panel, some continuum like intensities at lower energies below the AF spin wave peak at about 1.0 meV can be recognized
T=50mK T=2K
200
150
100
50
0
Q=(1.4,1.4,1.4) 100
50
0
0
1
2
3
4
Energy (meV) Fig. 2. Temperature dependence of constant Q scans at q=0 (upper), 0.4 (lower).
and furthermore strikingly enhance at 2 K. These anomalous line width around the zone center may be due to the occurence of the F ordering below TC =2 K. As can be seen in the lower panel, the AF at 3 meV and the F spin waves at 0.6 meV can be clearly recognized in the spectrum of q=0.4. The most interesting experimental feature is no difference of the F spin wave spectrum between T=0.05 K (open) and TC =2 K (closed), indicating the F spin wave spectrum clearly survives even above TC : These experimental findings might correspond to the anomalous behavior in the specific heat measurement [1], in which the specific heat jump at TC =2 K is not sharp but spreaded up to about 5 K [1].
References [1] [2] [3] [4]
N. N. N. K.
Tateiwa, et Tateiwa, et Tateiwa, et Kuwahara,
al., J. Phys. Soc. Japan. 69 (2000) 1517. al., Physica B 281& 282 (2000) 254. al., J. Phys. Soc. Japan. 70 (2001) 2425. et al., physica B, these proceedings.
Spin wave excitations in single crystalline U3 Pd20Si6 N. Asoa,*, N. Metokib, M. Kohgic, K.A. McEwend, Y. Koikeb,e, Y. Hagab, N. Tateiwaf, N. Kimurag, H. Aokig, T. Komatsubarag, Y. Moriib a
Neutron Scattering Laboratory, Institute for Solid State Physics, The University of Tokyo, Shirakata 106-1, Tokai-mura, Ibaraki 319-1106, Japan b Advanced Science Research Center, Japan Atomic Energy Research Inst., Tokai, Ibaraki 319-1195, Japan c Graduate School of Sci., Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan d University College London, London WC1E 6BT, UK e Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan f Research Center for Materials Science at Extreme Condition, Osaka University, Toyonaka, Osaka 560-8531, Japan g Center for Low Temp. Sci., Tohoku University, Sendai 980-8545, Japan
Abstract We have performed inelastic neutron scattering experiments to examine the magnetic ordering in U3 Pd20 Si6 ; which exhibits antiferromagnetic ordering at TN =19 K and ferromagnetic ordering at TC =2 K. We observed that both antiferromagnetic spin wave excitations due to the magnetic moments at the 8c site and ferromagnetic ones due to the magnetic moments at the 4a site coexisted below TC =2 K. Our results indicate direct experimental evidence that U3 Pd20 Si6 is a rare localized 5f electron system in uranium intermetalic compounds, with coexistence of both antiferromagnetic and ferromagnetic ordering. The anomolous line width around the zone center is also discussed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: U3 Pd20 Si6 ; Local moment magnetism; Spin wave; Neutron scattering
New ternary intermetalic compounds R3 Pd20 X6 (R=rare earth: X=Si, Ge) system with the cubic Cr23 C6 -type crystal structure (space group Fm3m) have attracted much attention. In these compounds where the rare earth occupy the two inequivalent crystallographic sites 4a and 8c, both with cubic symmetry, it is common that the successive phase transitions are found and each ordering is derived from each rare earth atom site. Recently, Tateiwa et al. suceeded in growing a new isostructural uranium compound U3 Pd20 Si6 [1, 2]. The temperature dependence of susceptibility obeys the Curie–Weiss law above 50 K with an effective moment 3:30mB =U: Two successive magnetic phase transitions are found in the electrical resistivity, the specific heat and the magnetic susceptibility at 19 and 2 K. Furthermore, time-of-flight inelastic neutron scattering study [4] *Corresponding author. Tel.: +81-29-287-8906; fax: +8129-283-3922. E-mail address: [email protected] (N. Aso).
shows crystalline electric field excitations originated from at both 4a and 8c uranium magnetic moments, indicating that U3 Pd20 Si6 is a rare localized 5f electron system in uranium intermetalic compounds. Here we report on inelastic neutron scattering studies on the spin wave excitations in single crystalline U3 Pd20 Si6 to investigate the nature of the 5f electrons. The single crystal of U3 Pd20 Si6 were grown by the Czochralski pulling method under an Ar gas atmosphere using a tri-arc furnace. Details of the preparation were previously reported [1]. Inelastic neutron scattering measurements were performed at the triple-axis spectrometers TAS-1 and LTAS installed at the research reactor JRR-3M at JAERI, Japan. The temperature was cooled down to 50 mK by using the dilution refrigerator. Previous elastic neutron experiments [3] shows that antiferromagnetic (AF) Bragg peaks with a wave vector k ¼ ½1; 1; 1 coming from the uranium magnetic moment at the 8c site appear below TN and ferromagnetic (F)
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 2 6 6 - 2
898
N. Aso et al. / Physica B 312–313 (2002) 897–898 300
U3Pd20Si6 Q=(1,1,1) LTAS, Ef = 3.5 meV, 80'-80'-80'
Intensity (counts / 10800000 Mon)
250
Fig. 1. Contour plot of the scattering intensities in the direction [1+q; 1+q; 1+q] at T ¼ 0:05 K.
peaks from the magnetic moment at the 4a sites do below TC and both coexist below TC : Fig. 1 illustrates the contour plot of the scattering intensities obtained with the fixed final energy of 3.5 meV at T=0.05 K below TC : As can be clearly seen, the AF spin wave excitations derived from the 8c site magnetic moment can be recognized in the 0:5ooo3:0 meV region. We made such contour plots at major symmetry axes and its dispersion relations are rather well understood by the spin wave of the G-type AF structure of the simple cubic, with the nearest neighbor exchange interaction between the 8c site magnetic moment (JB0:2 meV) and the uniaxial anisotropy giving the energy gap of 1.0 meV at q=0, indicated by the solid line in the plot. Its line width was found to show the energy resolution limited behavior except for the AF zone center. Superposed in the Q-independent incoherent scattering at o=0, additional weak scattering intensities are found in the 0ooo1 meV region. These intensities correspond to the F spin wave excitations due to the magnetic moments at the 4a site as shown by the broken line. These characteristic features are very rare in the uranium compounds, and indicate that the uranium magnetic moment at both 4a and 8c sites show the localized character. Fig. 2 show the temperature dependence of constant Q scans at (1+q; 1+q; 1+q) with q=0 and 0.4. In the spectrum at q=0 shown in the upper panel, some continuum like intensities at lower energies below the AF spin wave peak at about 1.0 meV can be recognized
T=50mK T=2K
200
150
100
50
0
Q=(1.4,1.4,1.4) 100
50
0
0
1
2
3
4
Energy (meV) Fig. 2. Temperature dependence of constant Q scans at q=0 (upper), 0.4 (lower).
and furthermore strikingly enhance at 2 K. These anomalous line width around the zone center may be due to the occurence of the F ordering below TC =2 K. As can be seen in the lower panel, the AF at 3 meV and the F spin waves at 0.6 meV can be clearly recognized in the spectrum of q=0.4. The most interesting experimental feature is no difference of the F spin wave spectrum between T=0.05 K (open) and TC =2 K (closed), indicating the F spin wave spectrum clearly survives even above TC : These experimental findings might correspond to the anomalous behavior in the specific heat measurement [1], in which the specific heat jump at TC =2 K is not sharp but spreaded up to about 5 K [1].
References [1] [2] [3] [4]
N. N. N. K.
Tateiwa, et Tateiwa, et Tateiwa, et Kuwahara,
al., J. Phys. Soc. Japan. 69 (2000) 1517. al., Physica B 281& 282 (2000) 254. al., J. Phys. Soc. Japan. 70 (2001) 2425. et al., physica B, these proceedings.