Raman scattering study of the geometrically frustrated pyrochlore Y2Ru2O7

Vibrational Spectroscopy 42 (2006) 284–287 www.elsevier.com/locate/vibspec

Raman scattering study of the geometrically frustrated pyrochlore Y2Ru2O7 J.S. Bae a, In-Sang Yang a,*, J.S. Lee b, T.W. Noh b, T. Takeda c, R. Kanno d a Department of Physics and Division of Nanosciences, Ewha Womans University, Seoul 120-750, Republic of Korea School of Physics and Research Center for Oxide Electronics, Seoul National University, Seoul 151-747, Republic of Korea c Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Hokkaido 060-8628, Japan d Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan b

Available online 27 June 2006

Abstract Geometrically frustrated pyrochlore Y2Ru2O7, which shows a spin-glass-like transition at TG  80 K, were investigated by temperaturedependent Raman scattering. Three discernable phonons appear around 315, 410, and 510 cm1 without any abrupt change in the number of Raman active modes within the temperature range of 10–300 K. Fitting each phonon with Lorentz oscillators, we analyzed the effects of temperature on the phonon frequencies and the linewidths. The temperature-dependence of the mode near 510 cm1 shows abnormal behavior below TG, while the other two phonons follow the usual thermal effect of lattice vibration. This behavior can be understood in terms of spin–phonon coupling. Considering the atomic modulations of each phonon mode, it is conjectured that the 510 cm1 phonon mode is isotropically coupled to the spin degree of freedom, while the other modes are not. # 2006 Elsevier B.V. All rights reserved. Keywords: Raman scattering; Geometrically frustrated; Phrochlore Y2Ru2O7

1. Introduction Compounds with pyrochlore structure have attracted much attention with their intriguing properties related with their geometrically frustrated spin configurations [1]. It is difficult for magnetic moments to order antiferromagnetically in the pyrochlore compound, A2B2O7, since magnetic atoms at A or B sites compose a tetragonal unit. Nevertheless, pyrochlore Y2Ru2O7 has shown many signatures of an antiferromagnetic ordering or a strong antiferromagnetic correlation [2,3]. Many pieces of evidence on strong spin–phonon coupling in Y2Ru2O7 were presented in our previous research on the infrared-active phonon modes [2]. Among seven infrared-active phonon modes, while three phonon modes exhibit normal temperature (T)-dependences, the other four modes exhibit strong anomalies below the spin-glass-like transition temperature TG  80 K [4]. We suggested that the strong spin–phonon coupling effect should play a crucial role in the intriguing

* Corresponding author. Fax: +82 232772372. E-mail address: [email protected] (I.-S. Yang). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.05.004

magnetic state of this compound. However, it is necessary to give an appropriate answer to the question why only specific phonons are involved in the anomaly of phonon dynamics in the spin-glass-like ground state below TG in order to understand the spin–phonon coupling mechanism in pyrochlore Y2Ru2O7. In these respects, we investigated T-dependent Raman spectra of Y2Ru2O7. We observed three discernable Ramanactive phonon modes around 315, 410, 510 cm1. By fitting the observed phonon peaks with the Lorentz oscillator function, we obtained T-dependences of phonon frequency and linewidth for each phonon. The phonon around 510 cm1 shows interesting T-dependent behavior below TG, while other modes show typical thermal effects. Considering lattice vibrations for each phonon, we propose that isotropic lattice modulation should be closely related to a specific spin configuration in the magnetic ground state of the geometrically frustrated system, Y2Ru2O7. 2. Experiments High-density Y2Ru2O7 polycrystals were synthesized at 3 Gpa with a solid state reaction method [5]. Raman-scattering

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measurements at low temperatures were performed using a closed cycle He refrigerator and a Jobin Yvon T64000 spectrometer equipped with a nitrogen-cooled charge-coupled device detector. The 514.5 nm line of an Ar-ion laser was used as the excitation source in the back scattering geometry. The laser power of the excitation source was kept low to avoid heating the sample surface. The diameter and power of the beam spot on the sample surface were about 5 mm and less than 0.4 mW, respectively. When necessary, the surfaces of the samples were cleaned using diamond paper. 3. Results and discussion Fig. 1 shows the T-dependent Raman spectra of Y2Ru2O7 from 10 to 300 K. For clarity, we displayed the spectra with an upward baseline shift, with T decreasing from 300 to 10 K. Three discernable phonon modes are observed around 315, 410, and 510 cm1. Additionally, two weak phonons seem to appear around 350 and 560 cm1. It should be noted that no additional peak appears below TG, which indicates that there is no abrupt change in the structural symmetry at TG, in agreement with the neutron scattering and infrared phonon data [2,3]. Factor group analysis (FGA) predicts six Raman active modes for the cubic pyrochlore with a space group Fd3m [6], i.e. G ðRamanÞ ¼ A1g þ Eg þ 4F 2g Comparing the results for Y2Ru2O7 with previous Raman experimental results on Y2Ru2O7, three phonon modes around 315, 410, and 510 cm1 can be assigned as Eg, F 2g, and A1g mode, respectively [7,8]. And the other two weak phonons around 350 and 560 cm1 should be assigned as F 2g modes. While the FGA predicts six Raman modes, the Raman spectra for Y2Ru2O7 show only five Raman modes up to around 800 cm1. Actually, we could find an additional phonon mode around 700 cm1 for Bi2xYxRu2O7 [9], and other pyrochlores, such as A2Mn2O7 (A = Y, Tl, In) [7], which possesses the same cubic structure as Y2Ru2O7. Therefore, one near 700 cm1 of four F 2g modes seems to be missing for Y2Ru2O7.

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Fig. 2(a)–(c) shows the T-dependences of the phonon modes around 315, 410, and 510 cm1, respectively. The T-dependent Raman spectra are displaced with upward baseline shifts, as T decreases from 300 to 10 K. We could notice that the mode near 510 cm1 and the modes near 315 and 410 cm1 show different T-dependent features. As T decreases, the latter two phonons show gradual blue-shifts down to 10 K. On the other hand, while the 510 cm1 phonon mode exhibit a similar blue-shift down to 100 K, it shows a red-shift below around TG  80 K. In order to address the T-dependent phonon dynamics in more detail, we analyzed the Raman spectra by fitting with a series of Lorentz oscillators of the spectral response SðvÞ ¼ ½1 þ nðvÞ

X i

Si G i v ðv2

2

 v2i Þ þ G 2i v2

where vi, Gi and Si represent the frequency, the linewidth, and the amplitude of the ith phonon mode, respectively. The quantity [1 + n(v)] is the Bose–Einstein thermal factor. Weak phonon features around 350 and 560 cm1 are discarded for analysis. Fig. 3 shows the T-dependences of the fitted parameters of each phonon mode. We evaluated Dv = v(T)  v(300 K) and DG = G(T)  G(300 K). As T decreases down to around TG, i.e., 80 K, all three phonons shift to higher frequencies and their linewidths become narrower as usual. However, going down below TG, unusual phonon behavior appears for the mode near 510 cm1. Its peak frequency drops abruptly around TG, as shown in Fig. 3(a), while its peak width shows gradual decrease through TG, as shown in Fig. 3(b). For the phonon modes around 315 and 410 cm1, their Tdependences could be well explained in terms of the usual thermal effect [10]. As T decreases, the anharmonic thermal motion should decrease, so it leads to the decrease of the lattice constants. Then the lattice vibration costs more energy, resulting in shift to higher phonon frequencies. Decrease of the anharmonic thermal motion should also make the linewidths of phonons narrower [11]. On the other hand, the T-dependences of the 510 cm1 mode cannot be simply understood in terms of the thermal effect. Since the phonon anomalies occur near the magnetic transition T, i.e., TG, we suppose that they should be closely related to the coupling between the lattice and the spin degrees of freedom [2]. It should be noted that these phonon anomalies near TG are similar to those for some infrared-active phonon modes. In our previous infrared phonon study [2], we proposed that decrease of several phonon frequencies below TG would be a clear evidence of a strong spin–phonon coupling effect. If an ion is displaced by x from an equilibrium position, the crystal potential is given by X 1 Sj U ¼ kx2 þ J i j ðxÞ~ Si~ 2 i; j

Fig. 1. T-dependence of Raman spectra of Y2Ru2O7. Three discernable peaks (315, 410, and 510 cm1) and small peaks (350 and 560 cm1) appear.

Here, k is the force constant. The latter term is from the exchange interaction. The exchange energy constant Jij(x) depends on the structural parameters, such as the lengths

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Fig. 2. T-dependences of each phonon mode. (a), (b), and (c) show T-dependences for the 315 cm1 phonon mode, the 410 cm1 phonon mode, and the 510 cm1 phonon mode, respectively. In each figure, the spectra from top to bottom are shown for temperatures of 10, 80, 200, and 300 K.

between the Ru4+ magnetic ions and the Ru–O–Ru angles. Its second derivative X  @2 J i j  @2 U ¼kþ hSi S j i @x2 @x2 i; j gives a harmonic force constant, where the second term represents spin–phonon coupling. As a result of such perturbation in the force constant, the Raman frequency can be affected by an additional contribution Dv  lhSiSji [12]. The spin–phonon coupling coefficient l is different for different phonons, and can have either a positive or a negative sign [13,14]. In order to get a more thorough understanding of the unusual behaviors of phonon near 510 cm1, we compared the atomic displacements of each phonon mode. Based on the previous work [15], we sketched atomic displacements of phonon modes and showed them in Fig. 4(a) for the phonon near 510 cm1 and in Fig. 4(b) for the phonon around 410 cm1 phonon. The eigen vectors for the 315 cm1 phonon, whose behavior is similar to the 410 cm1 phonon, are not shown. Each octahedron is RuO6 on the h1 1 1i plane. Magnetic ions, Ru4+, are marked by a bold dot, and oxygen atoms are at vertices of the octahedra. Note that both modes are related to displacements of only O ions. The directions in which the O ions should move are indicated by arrows. For the exchange interaction of the Y2Ru2O7 compound, we consider a super-exchange interaction between magnetic Ru4+ ions mediated by O ions. Therefore, the magnetic interaction in Y2Ru2O7 should be very sensitive to the Ru–O–Ru angle. Since the Ru–O–Ru bond angle is important to understand the spin–

phonon coupling mechanism, we paid attention to the modulations of the bond-angle due to the O displacements. Let the Ru–O–Ru angle without any atomic displacement be u0 about 1298 [5]. Under the lattice modulations, some of the Ru– O–Ru angles become larger than u0, some become smaller than u0, and others do not change. Each angle type is indicated by bold solid, dotted, and thin solid lines, respectively, connecting nearest neighboring Ru4+ ions. The atomic displacements for the 510 cm1 mode, depicted in Fig. 4(a), show increase of all the Ru–O–Ru angles under the lattice modulation. On the other hand, those for the 410 cm1 mode, depicted in Fig. 4(b), show three kinds of bond angle changes, i.e., increased, decreased, and almost unchanged Ru–O–Ru angles. These are summarized in Fig. 4(c) and (d), showing the hexagon of the Ru4+ ions and part of the next hexagons. Phonons are dynamic lattice vibrations, involving various changes in the bond angles and bond lengths between atoms. Fig. 4(c) shows that the phonon around 510 cm1 involves in-phase (isotropic) change in all the Ru–O–Ru angles resulting in net change in the superexchange energy. On the other hand, the phonon around 410 cm1 has mixture of increase, decrease, and non-change of the Ru–O–Ru angles, resulting in small net change in the superexchange energy. Thus, the 510 cm1 phonon mode could be isotropically coupled to the spin degree of freedom, while the 410 cm1 mode could not. Our Raman results show that the antiferromagnetic ordering below TG results in decrease of the frequency of the 510 cm1 mode, meaning that the spin–phonon coupling interaction, i.e., lhSiSjiis negative for this mode. The Tdependent value of hSiSji can be evaluated from the integrated

Fig. 3. T-dependences of (a) frequencies and (b) linewidths of the three phonon modes. Empty circles, triangles, and solid squares indicate parameters for the 315, 410 and 510 cm1 phonon mode, respectively.

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Fig. 4. Atomic displacements for the phonons at (a) 510 cm1 and (b) 410 cm1. In (a) and (b), each octahedron is RuO6. Magnetic ions, Ru4+, are marked by a bold dot. The directions in which the O ions should move to are indicated by arrows. In the hexagon consisted of Ru4+, bold solid lines indicate larger Ru–O–Ru angles than u0, thin solid lines indicate smaller Ru–O–Ru angle than u0, and dotted lines indicate mixed behavior of the Ru–O–Ru angle. (c) and (d) summarize the figures of (a) and (b) in the view of Ru4+ and Ru–O–Ru angle modulations, showing the hexagon of the Ru4+–Ru4+ ring and part of the next hexagon. Phonon mode around 510 cm1 involves in-phase changes of all the Ru–O–Ru angles, while phonon mode around 410 cm1 has mixed Ru–O–Ru angles modulations.

intensities of the neutron scattering peaks of magnetic origin as hSiSji  0.25 [3]. As shown in Fig. 3(c), Dv for the 510 cm1 phonon mode is around 2 cm1. Since Dv = lhSiSji, we estimate the value of l for the 510 cm1 mode as about 8 cm1, which is comparable to those of the infrared-active phonon modes located around 490 and 420 cm1 [2]. In conclusion, we measured the Raman spectra of Y2Ru2O7 and observed intriguing temperature-dependences for the phonon mode near 510 cm1, which shows abrupt decrease of the peak frequency below the spin-glass-like transition temperature, TG  80 K. Considering that there appears no additional peak below TG, the anomaly does not seem to be caused by a structural distortion. Rather, we suggest that the phonon mode around 510 cm1 is coupled with the spin degree of freedom in the magnetic ground state below TG. By assigning an atomic displacement and the corresponding spin configuration to each phonon, we proposed that spin-glass-like state below TG of the geometrically frustrated Y2Ru2O7 system actually takes an antiferromagnetic ordering, which couples with an isotropic lattice modulation. Acknowledgements ISY acknowledges the financial support by the 2003 Advanced Basic Research Laboratory (ABRL) program R142003-027-01001-0. This work was supported partly by the Ministry of Science and Technology through the Creative Research Initiative program.

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