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NMR study of the ordered state in UGa3
Physica B 312–313 (2002) 902–903
NMR study of the ordered state in UGa3 b % S. Kambea,*, H. Katoa, H. Sakaia, R.E. Walstedta, Y. Hagaa, D. Aokib, Y. Onuki a
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan b Graduate School of Science, Osaka University, Toyonaka-Osaka 560-0043, Japan
Abstract We report Ga-NMR spectra recorded with magnetic field H along the (1 0 0), (1 1 0) and (1 1 1) directions in both the paramagnetic and the antiferromagnetically (AF) ordered state (TN ¼ 67 K) of single crystal UGa3 : In the paramagnetic state, resonance peaks are quite sharp with line width of B20 G as expected for a single crystal. All the peaks either disappear or become quite small in the ordered state when H8ð1 1 0Þ and (111). In contrast, clear splitting of the peaks due to the ordered moments has been observed when H8ð1 0 0Þ: r 2002 Elsevier Science B.V. All rights reserved. PACS: 76.60.k; 71.27.+a Keywords: NMR; UGa3
The UX3 ðX ¼ Al; Ga; In and Tl) series exhibits a wide variety of magnetic ground states i.e., Pauli paramagnet, itinerant antiferromagnetically (AF) and localized AF. UGa3 is an AF with TN ¼ 67 K; having a small ordered moment 0:8mB [1] and a rather large specific heat g C=T ¼ 50 mJ=K2 mol [2], indicating that this compound is a typical itinerant magnet with an enhanced effective mass due to f-electron correlations. Neutron scattering measurements reveal that the AF structure is a simple type-II with a propagation vector q ¼ ð0:5; 0:5; 0:5Þ [1]; however, the direction of the ordered spin moment is still unknown. At 40 K; an unidentified second transition is observed in magnetic susceptibility [3]. In the present study, Ga-NMR has been measured in the paramagnetic and ordered states of a high quality UGa3 single crystal [4] in order to clarify the nature of the ordered state. Fig. 1 shows 69 GaðI ¼ 32Þ NMR field sweep spectra for a fixed frequency 96:4 MHz in the paramagnetic state at 90 K for H8ð1 0 0Þ: Similar 71 Ga spectra will be presented elsewhere. In this field direction, two different Ga sites appear, i.e., H8nzz and H>nzz (nzz 8ð1 0 0Þ is the *Corresponding author. E-mail address: [email protected] (S. Kambe).
principal axis of the electric field gradient). The six peaks observed correspond to one central resonance (m ¼ 122 12) and two satellite resonances (m ¼ 7122732) for the two different Ga sites. These six peaks give Knight shifts K8ð1 0 0Þ ¼ 1:48% and K>ð1 0 0Þ ¼ 1:66%; and the nuclear quadruple resonance frequency nQ ¼ 19:8 MHz: The width of the resonance peaks is quite small B20 G; indicating that the mosaic spread and distribution of nQ are small in the crystal. We estimate the applied field orientation is within 1:51 from (1 0 0). Using the usual K–w plot which relies on the temperature dependence of K and static susceptibility w; the hyperfine coupling constants have been determined: A8ð1 0 0Þ ¼ 140 kOe=mB and A>ð1 0 0Þ ¼ 120 kOe=mB : Spectra for H8ð1 1 0Þ and ð1 1 1Þ in the normal state are not presented, since resonance peaks have been found at the expected positions from the relation KðyÞ ¼ Kiso þ ð3 cos2 y 1ÞKani ; where y is the angle between field and (1 0 0), Kiso ¼ 13ðK8ð1 0 0Þ þ 2K>ð1 0 0Þ Þ; and Kani ¼ 13ðK8ð1 0 0Þ K>ð1 0 0Þ Þ: Fig. 2 shows 69 Ga-NMR spectra in the ordered state at 45 K for H8ð1 0 0Þ: Each resonance for H8nzz is split into 4 resonances i.e., 12 peaks are observed. These four resonances (sites) give different Knight shifts but the same nQ as in the normal state. This indicates that the
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 5 2 1 - 6
S. Kambe et al. / Physica B 312–313 (2002) 902–903
903
Fig. 1. 69 Ga-NMR field sweep spectra in the paramagnetic state. The open arrows indicate the three peaks for H8nzz ; and closed ones indicate those for H>nzz : The low, center and high field peaks correspond to m ¼ 32212; m ¼ 122 12 and m ¼ 122 32 resonance, respectively.
phase transition is magnetic and no charge redistribution happens at TN : The induced field at Ga sites are small B1:5 kG; which is of order of the dipolar field from the ordered U moment(B0:8mB ), indicating that the large isotropic transferred hyperfine field is canceled out as expected for the type-II AF structure. The peaks for H>nzz were also observed, but these were quite broad. The position of the latter peaks remains near that of the paramagnetic state, in agreement with the cancellation of the isotropic hyperfine field. At 40 K; the four peaks split into six peaks. This additional splitting suggests a rearrangement of spin structure at 40 K: No NMR peaks have been clearly observed below TN for H8ð1 1 1Þ and H8ð1 1 0Þ; indicating extensive splitting or broadening of the resonances, or the appearance of a large hyperfine field. The observed dependence of Ga-NMR spectra on field direction and temperature reflect the spin structure, although a detailed analysis is necessary to clarify it. Previous NMR spectra at zero field were unusual and could not be interpreted straightforwardly [5]. A detailed description of the observed spectra and discussions of the spin structure based on the NMR results will be reported elsewhere.
Fig. 2. 69 Ga-NMR field sweep spectra in the ordered state (T ¼ 45 K and f ¼ 96:4 MHzÞ for H8nzz : (a) m ¼ 32212; (b) m ¼ 122 12 and (c) m ¼ 122 32: All resonances are split into four.
References [1] P. Dervenagas, D. Kaczorowski, F. Bourdarot, P. Burlet, A. Czopnik, G.H. Lander, Phys. B 269 (1999) 368. . [2] D. Kaczorowski, R. Tro!c, D. Badurski, A. Bohm, L. Shiyk, F. Steglich, Phys. Rev. B 48 (1993) 16 425. [3] D. Kaczorowski, P.W. Klamut, A. Czopnik, A. Je’zowski, J. Magn. Magn. Mater. 171–181 (1998) 41. [4] D. Aoki, et al., J. Phys. Soc. Jpn. 70 (2001) 538. % [5] K. Ikushima, D. Aoki, Y. Haga, E. Yamamoto, Y. Onuki, J. Magn. Magn. Mater. 226–230 (2001) 89.
NMR study of the ordered state in UGa3 b % S. Kambea,*, H. Katoa, H. Sakaia, R.E. Walstedta, Y. Hagaa, D. Aokib, Y. Onuki a
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan b Graduate School of Science, Osaka University, Toyonaka-Osaka 560-0043, Japan
Abstract We report Ga-NMR spectra recorded with magnetic field H along the (1 0 0), (1 1 0) and (1 1 1) directions in both the paramagnetic and the antiferromagnetically (AF) ordered state (TN ¼ 67 K) of single crystal UGa3 : In the paramagnetic state, resonance peaks are quite sharp with line width of B20 G as expected for a single crystal. All the peaks either disappear or become quite small in the ordered state when H8ð1 1 0Þ and (111). In contrast, clear splitting of the peaks due to the ordered moments has been observed when H8ð1 0 0Þ: r 2002 Elsevier Science B.V. All rights reserved. PACS: 76.60.k; 71.27.+a Keywords: NMR; UGa3
The UX3 ðX ¼ Al; Ga; In and Tl) series exhibits a wide variety of magnetic ground states i.e., Pauli paramagnet, itinerant antiferromagnetically (AF) and localized AF. UGa3 is an AF with TN ¼ 67 K; having a small ordered moment 0:8mB [1] and a rather large specific heat g C=T ¼ 50 mJ=K2 mol [2], indicating that this compound is a typical itinerant magnet with an enhanced effective mass due to f-electron correlations. Neutron scattering measurements reveal that the AF structure is a simple type-II with a propagation vector q ¼ ð0:5; 0:5; 0:5Þ [1]; however, the direction of the ordered spin moment is still unknown. At 40 K; an unidentified second transition is observed in magnetic susceptibility [3]. In the present study, Ga-NMR has been measured in the paramagnetic and ordered states of a high quality UGa3 single crystal [4] in order to clarify the nature of the ordered state. Fig. 1 shows 69 GaðI ¼ 32Þ NMR field sweep spectra for a fixed frequency 96:4 MHz in the paramagnetic state at 90 K for H8ð1 0 0Þ: Similar 71 Ga spectra will be presented elsewhere. In this field direction, two different Ga sites appear, i.e., H8nzz and H>nzz (nzz 8ð1 0 0Þ is the *Corresponding author. E-mail address: [email protected] (S. Kambe).
principal axis of the electric field gradient). The six peaks observed correspond to one central resonance (m ¼ 122 12) and two satellite resonances (m ¼ 7122732) for the two different Ga sites. These six peaks give Knight shifts K8ð1 0 0Þ ¼ 1:48% and K>ð1 0 0Þ ¼ 1:66%; and the nuclear quadruple resonance frequency nQ ¼ 19:8 MHz: The width of the resonance peaks is quite small B20 G; indicating that the mosaic spread and distribution of nQ are small in the crystal. We estimate the applied field orientation is within 1:51 from (1 0 0). Using the usual K–w plot which relies on the temperature dependence of K and static susceptibility w; the hyperfine coupling constants have been determined: A8ð1 0 0Þ ¼ 140 kOe=mB and A>ð1 0 0Þ ¼ 120 kOe=mB : Spectra for H8ð1 1 0Þ and ð1 1 1Þ in the normal state are not presented, since resonance peaks have been found at the expected positions from the relation KðyÞ ¼ Kiso þ ð3 cos2 y 1ÞKani ; where y is the angle between field and (1 0 0), Kiso ¼ 13ðK8ð1 0 0Þ þ 2K>ð1 0 0Þ Þ; and Kani ¼ 13ðK8ð1 0 0Þ K>ð1 0 0Þ Þ: Fig. 2 shows 69 Ga-NMR spectra in the ordered state at 45 K for H8ð1 0 0Þ: Each resonance for H8nzz is split into 4 resonances i.e., 12 peaks are observed. These four resonances (sites) give different Knight shifts but the same nQ as in the normal state. This indicates that the
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 5 2 1 - 6
S. Kambe et al. / Physica B 312–313 (2002) 902–903
903
Fig. 1. 69 Ga-NMR field sweep spectra in the paramagnetic state. The open arrows indicate the three peaks for H8nzz ; and closed ones indicate those for H>nzz : The low, center and high field peaks correspond to m ¼ 32212; m ¼ 122 12 and m ¼ 122 32 resonance, respectively.
phase transition is magnetic and no charge redistribution happens at TN : The induced field at Ga sites are small B1:5 kG; which is of order of the dipolar field from the ordered U moment(B0:8mB ), indicating that the large isotropic transferred hyperfine field is canceled out as expected for the type-II AF structure. The peaks for H>nzz were also observed, but these were quite broad. The position of the latter peaks remains near that of the paramagnetic state, in agreement with the cancellation of the isotropic hyperfine field. At 40 K; the four peaks split into six peaks. This additional splitting suggests a rearrangement of spin structure at 40 K: No NMR peaks have been clearly observed below TN for H8ð1 1 1Þ and H8ð1 1 0Þ; indicating extensive splitting or broadening of the resonances, or the appearance of a large hyperfine field. The observed dependence of Ga-NMR spectra on field direction and temperature reflect the spin structure, although a detailed analysis is necessary to clarify it. Previous NMR spectra at zero field were unusual and could not be interpreted straightforwardly [5]. A detailed description of the observed spectra and discussions of the spin structure based on the NMR results will be reported elsewhere.
Fig. 2. 69 Ga-NMR field sweep spectra in the ordered state (T ¼ 45 K and f ¼ 96:4 MHzÞ for H8nzz : (a) m ¼ 32212; (b) m ¼ 122 12 and (c) m ¼ 122 32: All resonances are split into four.
References [1] P. Dervenagas, D. Kaczorowski, F. Bourdarot, P. Burlet, A. Czopnik, G.H. Lander, Phys. B 269 (1999) 368. . [2] D. Kaczorowski, R. Tro!c, D. Badurski, A. Bohm, L. Shiyk, F. Steglich, Phys. Rev. B 48 (1993) 16 425. [3] D. Kaczorowski, P.W. Klamut, A. Czopnik, A. Je’zowski, J. Magn. Magn. Mater. 171–181 (1998) 41. [4] D. Aoki, et al., J. Phys. Soc. Jpn. 70 (2001) 538. % [5] K. Ikushima, D. Aoki, Y. Haga, E. Yamamoto, Y. Onuki, J. Magn. Magn. Mater. 226–230 (2001) 89.