Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES3) Downloaded from journals.jps.jp by on /6/7 JPS

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1 Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES3) Downloaded from journals.jps.jp by on /6/7 Proc. Int. Conf. Strongly Correlated Electron Systems (SCES3) JPS Conf. Proc. 3, 6 (4) Field-induced order in heavy-fermion compound YbCo Zn Koji KANEKO, Stéphane RAYMOND, Eric RESSOUCHE, Gerard LAPERTOT, Tetsuya TAKEUCHI 3, Yusuke HIROSE 4, Fuminori HONDA 5, Yoshichika ŌNUKI 6 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Ibaraki 39-95, Japan SPSMS, UMR-E 9, CEA-INAC/UJF-Grenoble, 3854 Grenoble, France 3 Low Temperature Center, Osaka University, Toyonaka 56-43, Japan 4 Department of Physics, Niigata University, Niigata 95-8, Japan 5 Institute for Materials Research, Tohoku University, Oarai, Ibaraki 3-33, Japan 6 Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 93-3, Japan kaneko.koji@gmail.com (Received October, 3) The field-induced ordered phase of heavy-fermion compound YbCo Zn was investigated by means of single crystal neutron diffraction. At an applied field of.8 T along the [ ] direction, a superlattice peak was found at ( 3 ) at mk, whereas no peak was detected at q=( ). In addition, broad diffuse scattering was revealed around induced superlattice peak. The present result evidences that the field-induced ordered phase has different character to the pressure induced antiferromagnetic ordered phase. KEYWORDS: YbCo Zn, neutron scattering, field-induced order, heavy-fermion. Introduction New ternary compounds RT X (R: rare earth, T : transition metal, and X:Al, Zn) with the cubic CeCr Al -type structure (space group F d 3m) exhibit wide variety of attractive physical properties, including multipole order, heavy-fermion behavior and superconductivity. Among them, 6 members of the YbT Zn family show non-magnetic heavy-fermion ground state []. In particular, YbCo Zn has the extremely large electronic specific heat coefficient of γ=8 J/mol K, indicating that YbCo Zn is one of the heaviest heavy-fermion compounds so far, and implying its vicinity to the quantum critical point [, ]. In fact, YbCo Zn reacts sensitively against external fields. Relatively small pressure of GPa is sufficient to induce an ordered state with T M =.5 K, which increases with pressure [3]. Single crystal neutron scattering experiments under pressure confirmed that an antiferromagnetic peak is induced at ( ) in the pressure-induced ordered state [4]. As similar to pressure, a magnetic field also gives strong influence on physical properties of YbCo Zn. An application of small fields of about. induces metamagnetic transitions [, 5]. With further increasing fields above the metamagnetic transitions, strong reduction of the effective mass m and correspondingly the A coefficient in the electrical resistivity was observed [, 6, 7]. Furthermore, when the field is applied close to, novel field-induced ordered state is realized above 6 T [5, 8]. In contrast to the pressure-induced magnetic phase, bulk measurements and meanfield analyses suggest the primary order parameter for the field-induced phase as antiferroquadrupole. Namely, the field-induced phase is stabilised by different mechanism to the pressure-induced one. In order to reveal nature of the field-induced ordered phase in YbCo Zn, single crystal neutron diffraction experiments under magnetic fields were carried out. 6-4 The Physical Society of Japan

2 Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES3) Downloaded from journals.jps.jp by on /6/7 JPS Conf. Proc. 3, 6 (4) 6- Intensity ( 3 counts / 8 min. ) Intensity ( 3 counts / 6 min. ) (a) 6 mk.8 T (b) mk.8 T Intensity ( 3 counts / 5 min. ) 6 4 YbCo Zn -3/ -/ / mk (c).8 T Fig.. (Color online) Rocking curves at (a) ( ), (b) ( ), and (c) ( 3 below mk. ) recorded at and.8 T. Experiment A single crystal of YbCo Zn was grown by the Zn-flux method. Details of sample preparation was published in ref. [9]. Neutron scattering experiments were carried out on diffractometer D3 installed at the Institute Laue-Langevin (ILL), Grenoble, France. Incident neutrons with a wavelength of.37 Å were provided by a PG monochromator. A PG filter was placed before the sample in order to eliminate higher-order contamination. A vertical-field superconducting magnet having a large accessible window of + 3 in the vertical direction was used. A large single crystal with the mass of.3 g was mounted in a 3 He- 4 He dilution refrigerator with the [,, ] direction as a vertical, parallel to the field. 3. Results and Discussions Figure shows rocking curves at representative symmetry points measured under the highest field of.8 T below mk. At first, we have measured at positions with q=( ) where the pressure-induced superlattice reflection was discovered [4]. As shown in the panels (a) and (b), no trace of superlattice reflection was observed at ( ) and ( ). On the other hand, a resolutionlimited superlattice peak was found at ( 3 ) as shown in panel (c). The induced peak disappeared at in the paramagnetic phase, indicating that the observed peak originates from the field-induced ordered phase. One should note that the tail of the superlattice peak does not reach to the background level, which is described later. In order to confirm the validity of the observed superlattice peak, reciprocal lattice positions ) were investigated as shown in Fig.. In the present experimental setup, 9 out ), similar intensities were observed ). On the other hand, no peak was detected at equivalent to ( 3 of 4 equivalent reflections were accessible. In addition to ( 3 at the inversion position ( 3 ), and (± ± 3 ±

3 Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES3) Downloaded from journals.jps.jp by on /6/7 JPS Conf. Proc. 3, 6 (4) 6-3. Intensity ( counts / min. ) / / -/ mk.8 T 3 counts /.5 Mmon..8.4 / 3/ / mk.8 T counts /.5 Mmon / -3/ -/ mk.8 T Intensity ( counts / 5 sec. ) / 3/ / mk counts / 3.75 Mmon / -/ -3/ mk Fig.. (Color online) Rocking curves at { 3 } recorded at and.8 T below mk. ( ± 3 ± 3 ) and ( ). As a result, we found 4 superlattice peaks out of the 9 equivalent positions of { 3 }. It should be pointed out that there exists additional intensity at the each tail of the rocking curve of all the observed superlattice peaks. An origin of the extra scattering at the tail was subsequently investigated by expanding scan range as displayed in Fig. 3. The scan revealed that the superlattice peak was, in fact, accompanied by broad response which has maxima at slightly off-centered positions of δω ±.5. In order to get further insights into the observed broad diffuse scattering, the anisotropy was measured with fine collimation. Figure 4 summarizes the line scan profiles along the three principal directions, namely ( ), ( ) and ( ), through the superlattice peak at ( 3 ). The diffuse scattering was observed for all three directions with similar profiles. A simple fitting for the diffuse scattering using off-centered symmetric Gaussians with respect to the central position at ( 3 ) indicates that local maximum for the diffuse scattering is almost isotropic with δ.9 r.l.u. Namely, the diffuse scattering exists spherically around ( 3 ). In the following, we discuss the observed results in the field-induced ordered phase. As for the ordering vector, the absence of the peak at both ( ) and ( ) excludes the possibility of q=( ) which is the ordering vector of the pressure-induced magnetic phase [4]. In contrast, the present study reveals the presence of the superlattice peak at ( 3 ) in the field-induced ordered phase. This result evidences that the field-induced phase has different character from the pressure-induced antiferromagnetic one from a microscopic point of view. This is consistent with the behavior in the pressure-magnetic field phase diagram in which pressure-induced ordered phase does not connect to the field-induced one [5, ]. One characteristic feature in the observed superlattice peak is that the peak does not exist at all 3

4 Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES3) Downloaded from journals.jps.jp by on /6/7 JPS Conf. Proc. 3, 6 (4) Intensity ( 3 counts / min. )..5 YbCo Zn -3/ -/ / T,.6 K.8 T,. K Intensity ( counts / ~8 s ) 4 YbCo Zn -3/ -/ /.8 T 5 mk ( ) ( ) ( ) Fig. 3. (Color online) Rocking curves measured with wide rage around ( 3 ) recorded at the paramagnetic state and inside the field-induced ordered phase. distance in unit of a* ( r. l. u. ) Fig. 4. (Color online) Line scan profile along the three principal axes of ( ), ( ), and ( ) centered at ( 3 ) measured at.8 T, 5 mk. equivalent positions of { 3 }. In particular, the peak is completely absent at several positions as shown in Fig.. One possible approach to account for this absence could be the particular ordered moment direction, since neutron scattering is only sensitive to magnetic component perpendicular to the scattering vector Q. In other words, no magnetic scattering arises if an ordered moment aligns parallel to Q. However, any moment direction cannot satisfy the observed intensity variation among { 3 }. This fact suggests that the difference among { 3 } may stem from the structure factor. One should note that the application of the magnetic field breaks the three-fold symmetry in the current setup. Namely, the [ ] direction is not equivalent to the [ ], [ ] and [ ] directions under the magnetic field. More systematic data on the superlattice peak position at various Q will help to reveal a correct propagation vector of the field-induced phase, which then leads to determination of the primary order parameter through the direction of the ordered moment, as done in the field-induced antiferroquadrupolar order in PrOs 4 Sb [, ]. In addition to the superlattice peak, the present study reveals that superlattice peaks are surrounded by spherical diffuse scattering. The absence of diffuse scattering at ( ± 3 ± 3 ) and ( ) without superlattice peak indicates a coupling between the diffuse scattering and the superlattice peak. Within the present data, one cannot discriminate whether the diffuse scattering and the superlattice peak coexist in the microscopic scale, or spatially separated. In addition, the origin is an open question as well. Detailed field variation of both the superlattice peak and the diffuse scattering as well as additional inputs from other microscopic probes such as NMR is indispensable to reveal the nature of the complex response in YbCo Zn. 4. Summary We have observed in neutron diffraction under the magnetic field an appearance of superlattice peak at ( 3 ) inside the field-induced ordered phase, whereas no peak was detected at q=( ). In addition to the superlattice peak, broad diffuse scattering was revealed around the induced superlattice peak. The present result evidences that the field-induced ordered phase has different character to the 4

5 Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES3) Downloaded from journals.jps.jp by on /6/7 JPS Conf. Proc. 3, 6 (4) 6-5 pressure-induced antiferromagnetic ordered phase. Acknowledgement The authors would like to thank R. Shiina and K. Kubo for stimulating discussions. The support of the ILL technical staff is greatly acknowledged. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas Heavy Electrons (No. 54), and on Priority Areas of New Materials Science Using Regulated Nano Spaces (No. 3), of The Ministry of Education, Culture, Sports, Science, and Technology, Japan, and Grant-in-Aids for Young Scientist (B) (No ) and Scientific Research (C) (No ) from the Japan Society of Promotion of Science. References [] M. S. Torikachvili, S. Jia, E. D. Mun, S. T. Hannahs, R. C. Black, W. K. Neils, D. Martien, S. L. Bud ko, and P. C. Canfield: Proc. Natl. Acad. Sci. 4 (7) 996. [] M. Ohya, M. Matsushita, S. Yoshiuchi, T. Takeuchi, F. Honda, R. Settai, T. Tanaka, Y. Kubo, and Y. Ōnuki; J. Phys. Soc. Jpn. 79 () 836. [3] Y. Saiga, K. Matsubayashi, T. Fujiwara, M. Kosaka, S. Katano, M. Hedo, T. Matsumoto, and Y. Uwatoko: J. Phys. Soc. Jpn. 77 (8) 537. [4] K. Matsubayashi: unpublished. [5] T. Takeuchi, S. Yoshiuchi, M. Ohya, Y. Taga, Y. Hirose, K. Sugiyama, F. Honda, M. Hagiwara, K. Kindo, R. Settai, and Y. Ōnuki: J. Phys. Soc. Jpn. 8 () 473. [6] Y. Saiga, K. Matsubayashi, T. Fujiwara, T. Matsumoto, M. Kosaka, S. Katano, and Y. Uwatoko: J. Phys.: Conf. Ser. 5 (9) 468. [7] K. Kaneko, S. Yoshiuchi, T. Takeuchi, F. Honda, R. Settai, and Y. Ōnuki: J. Phys.: Conf. Ser. 39 () 6. [8] Y. Shimura, T. Sakakibara, S. Yoshiuchi, F. Honda, R. Settai, and Y. Ōnuki: J. Phys. Soc. Jpn. 8 () [9] S. Yoshiuchi, M. Toda, M. Matsushita, S. Yasui, Y. Hirose, M. Ohya, K. Katayama, F. Honda, K. Sugiyama, M. Hagiwara, K. Kindo, T. Takeuchi, E. Yamamoto, Y. Haga, R. Settai, T. Tanaka, Y. Kubo, and Y. Ōnuki: J. Phys. Soc. Jpn. 78 (9) 37. [] Y. Taga, S. Yoshiuchi, M. Ohya, J. Sakaguchi, Y. Hirose, F. Honda, T. Takeuchi, R. Settai, and Y. Ōnuki: J. Phys. Soc. Jpn. 8 () SB64. [] M. Kohgi, K. Iwasa, M. Nakajima, N. Metoki, S. Araki, N. Bernhoeft, J.-M. Mignot, A. Gukasov, H. Sato, Y. Aoki, and H. Sugawara: J. Phys. Soc. Jpn. 7 (3). [] K. Kaneko, N. Metoki, R. Shiina, T. D. Matsuda, M. Kohgi, K. Kuwahara, and N. Bernhoeft: Phys. Rev. B 75 (7)