Correlation between Spin-Phonon Coupling and Ferroelectricity in RCrO 3. (R=Y, Gd, Sm): A Raman Study

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1 Correlation between Spin-Phonon Coupling and Ferroelectricity in RCrO 3 (R=Y, Gd, Sm): A Raman Study Venkata Srinu Bhadram, B. Rajeswaran, A. Sundaresan and Chandrabhas Narayana* Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P. O., Bangalore , India. *cbhas@jncasr.ac.in Abstract We have carried out a temperature-dependent Raman study of orthochromites, RCrO 3 (R = Gd, Sm and Y) to probe the role of magnetic R-ions in inducing ferroelectricity at the Néel temperature (T N ) of chromium. Our results show that the phonon behavior at T N in compounds with magnetic R-ion (Gd and Sm) is remarkably different from that of non-magnetic R-ion (Y). A strong spin-phonon coupling is evidenced from the anomalies in all the observed phonon frequencies and their line-widths at T N for the compounds with magnetic R-ion. In the case of nonmagnetic R-ion, the anomalous behavior observed in certain phonon frequencies is due to mangetostriction. In addition, the anomalies in the low frequency modes related to R-atom motion in SmCrO 3 and GdCrO 3 accounts for the displacement of the magnetic R 3+ ion resulting in ferroelectric polarization. PACS Number(s): j, t, Lx, q Magnetoelectric multiferroic materials with their coupled ferroelectric and ferromagnetic order parameters are promising for developing a new generation of both electrically and magnetically controlled multifunctional devices. 1-5 Multiferroic materials are broadly classified into two types; in type I multiferroics, the ferroelectricity and magnetism occur at high temperature but with

2 different temperature scale. However, the coupling between the two order parameters are rather weak. Type II multiferroics are generally centrosymmetric and magnetic where the ferroelectricity is caused by certain type of magnetic ordering. In the well-known example of TbMnO 3, the manganese moments order antiferromagnetically at T N = 41 K and at 25 K it undergoes another magnetic transition below which a cycloidal spin structure breaks the inversion symmetry and thus induces ferroelectricity. 6 In case of HoMnO 3, 7 a collinear magnetic ordering with E-type magnetic structure gives rise to ferroelectricity. It has been shown recently that canted antiferromagnetic ordering with two non equivalent spin pairs in the orthoferrite, SmFeO 3 induces ferroelectric polarization at the magnetic ordering temperature of iron. 8 Rajeswaran et al 9 have reported ferroelectricity at the magnetic ordering temperature of chromium in the isostructural orthochromites, RCrO 3 with magnetic R ion, where the interactions between R 3+ and Cr 3+ ions have been suggested to be responsible for the cause of ferroelectricity. In these systems, the interplay between magnetic and electric order could be mediated through spin-lattice coupling. Thus, probing the local structure would provide a good understanding of the multiferroic behavior in this class of materials. Raman spectroscopy is an ideal technique to study the local structural changes due to magnetic ordering effects in materials. Recent Raman reports exemplify the ability of Raman to elucidate the physics of multiferroics Raman spectroscopy of rare-earth orthorhombic manganites, RMnO 3 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) have been studied extensively in past In case of hexagonal YMnO 20 3 and LuMnO 21 3 ferroelectric to paraelectric phase transition is accompanied by tilting of MnO 5 polyhedra which is correlated with the changes in Raman modes related to rare-earth atom displacements. In the case of orthorhombic TbMnO 3, new Raman modes have been observed at the Néel temperature suggesting their magnetoelectric origin. 22

3 Though orthorhombic manganites have been studied extensively in the past, very few reports of Raman studies exist in the case of orthohochromites. In the present work we report the temperature-dependent Raman studies of rare-earth orthochromites, RCrO 3 with magnetic (Gd and Sm) and nonmagnetic (Y) R-ions to elucidate the origin of the ferroelctricity in these materials. We observed anomalies in all the observed phonon mode frequencies and their line-widths in GdCrO 3 and SmCrO 3 indicating a strong spin-phonon coupling in these systems. In contrast, we have not observed any anomalies in phonon linewidths in YCrO 3 which infers that Cr ordering alone is not responsible for spin-phonon coupling. Moreover, the low frequency lattice modes related to R atom vibrations undergo anomalous changes below T N in SmCrO 3 and GdCrO 3 indicating the displacement of R atom which results in ferroelectric polarization. Such changes are not observed in the case of YCrO 3 indicating the role of magnetic interactions between magnetic R 3+ and Cr 3+ in spin-phonon coupling and ferroelctricity in RCrO 3. The details of sample preparation and characterization are reported elsewhere. 9 Raman spectroscopic measurements have been carried out in back scattering geometry using 532 nm excitation wavelength and a custom built spectrometer equipped with a SPEX TRIAX 550 monochromator and a liquid nitrogen cooled charge-coupled device (CCD; Spectrum One with CCD 3000 controller, ISA Jobin Yovn). 23 Laser power at the sample was kept at ~6mW and typical spectral acquisition time is 1 min. Temperature dependent measurements were done using heating stage unit (Linkam THMS 600) equipped with a temperature-controller (Linkam TMS 94) with temperature accuracy of ±1K. Lorentzian functions were used to fit the spectral profile. Figure 1 shows the room temperature Raman spectra of RCrO 3 (R=Y, Sm, Gd) in the spectral range cm -1. RCrO 3 has an orthorhombic (Pnma ) structure with 24 Raman active

4 modes which are classified as Γ Raman = 7A g + 5B 1g +7B 2g +5B 3g. 15 These modes are assigned by following the earlier Raman reports on orthorhombic RMnO 3 and the recent room-temperature Raman study on RCrO 3 systems. 14, 15, 24 The phonon modes below 200 cm -1 in SmCrO 3, GdCrO 3 and below 250 cm -1 in YCrO 3 are related to lattice modes involving R atom vibrations. Region above 200 cm -1 (250 cm -1 in the case of YCrO 3 ) consists of various modes involving R atom and oxygen motion. A g (1) mode is related to the anti-stretching vibrations of CrO 6 octahedra. B 1g (3), A g (2) are octahedral rotations around crystallographic y-axis and B 1g (4), A g (4) are rotations around x-axis (Pnma setting). The doublet (A g (7) and B 2g (7)) at around 300 cm -1 in YCrO 3, GdCrO 3 and a singlet (A g (7)) in SmCrO 3 are related R-O vibrations. B 3g (3) is bending mode of CrO 6 octahedra. Most of these phonon modes shift to low frequency with increase in ionic radii of R-ion (r Y >r Gd >r Sm ) as seen in the case of orthorhombic manganites. 15 A detailed spectral analysis of these systems has been reported recently. 24 Now we discuss the temperature dependence of various Raman modes. The temperature effects on the anti-stretching mode A g (1) frequency and corresponding line widths are plotted in Fig. 2 in the temperature range K. Temperature dependent behavior of phonon mode of frequency ω is given as: 25 where ω(0) is the frequency at zero K. The term includes the changes in phonon frequency due to the quasi-harmonic effect which corresponds to the change in the lattice volume. Whereas gives the intrinsic anharmonic contribution to the frequency. represents the change in phonon frequency due to spin-lattice (spin-phonon) coupling which is caused by the phonon modulation of spin-exchange integral. Effects of

5 electron-phonon coupling on phonon frequency is given by which can be neglected when the carrier concentration is low. As seen in Fig. 2, phonon frequencies increase upon decreasing temperature up to T N in all three systems which is attributed to the anharmonic effect. The dotted line in the Fig. 2 is the fit to the experimental data above T N with the equation 26, where ω 0 and C are adjustable parameters. It is clear from Fig. 2 that A g (1) phonon frequency deviates from the above relation below T N in all the three systems, which could arise from the quasi-harmonic effects or spin-phonon coupling. However, it is interesting to note the different behavior of phonons below T N for different R-ions. An anomalous hardening of the A g (1) mode is observed in YCrO 3 and GdCrO 3 whereas a softening is observed for SmCrO 3. The observed hardening of A g (1) mode in YCrO 3 and GdCrO 3 is consistent with the earlier observation where a reduction in unit cell volume is reported for YCrO 3 and is attributed to magnetostriction. 27 Though we are not aware of such studies on SmCrO 3 and GdCrO 3 it is possible that a similar magnetostriction effect can occur in all these samples due to magnetic ordering of Cr 3+ ions. The observation that the phonon frequency below T N hardens in GdCrO 3 and softens in SmCrO 3 may be related to different magnetic interactions between magnetic R 3+ and Cr 3+ ions as reported earlier. 9 Thus, the origin of anomalous behavior of the A g (1) mode frequency in GdCrO 3 and SmCrO 3 may be due to a combined effect of magnetostriction and spin-lattice coupling. To give more insight into the origin of anomalous behavior of A g (1) mode frequency, we have plotted the temperature dependence of the corresponding line width in Fig. 2 (bottom panel). We observe that the line width increases below T N in the case of SmCrO 3 and GdCrO 3 whereas no significant changes are observed in the case of YCrO 3. Indeed, Raman line widths are expected

6 to decrease monotonously with temperature due to anharmonicity 26 as seen in the case of YCrO 3 (see Fig. 2). In contrast, an anomalous increase in the line width below T N is observed in SmCrO 3 and GdCrO 3 which signifies the contribution from effects that change the phonon lifetime. Generally, phonon lifetimes are effected by various processes such as spin-phonon coupling 28, 29 and electron-phonon coupling. 30 In present case, we can unequivocally attribute the change in line width to spin-phonon coupling. We now discuss the temperature dependence of the in-phase and out-of-phase octahedral rotations about y-axis in the temperature range K which is shown in Fig. 3. We observed anomalous softening of the phonon frequencies (A g (2), B 1g (3)) below T N in the case of SmCrO 3 and GdCrO 3. Remarkably, no significant changes are observed in YCrO 3. This behavior reflects that the tilt angle (180- Cr-O-Cr ) is affected in the former due to magnetic interactions between R and Cr-ions whereas it is unaffected in the nonmagnetic R-ion (Y). Temperature dependence of in-phase and out-of-phase octahedral rotations about x-axis (A g (4), B 1g (4)) is shown in left panel of Fig. 4. Unlike rotation about y-axis, all the samples show softening in rotation about x-axis below T N which may be predominantly due to the effect of magnetostriction. The behavior of out-of-phase bending mode, B 3g (3) as shown in right panel of Fig. 4 is similar to that observed for rotational modes about y-axis. It should be noticed that the A g (4), B 1g (4) and B 3g (3) modes in SmCrO3 exhibit an additional anomaly near 140 K (T*). We suggest that this anomaly is the prelude to spin-reorientation transition occurring at low temperature (around 40 K) 9. To verify whether the anomalous behavior of phonon modes, A g (4) and B 1g (4) is associated with spin-phonon coupling in addition to the contribution from magnetostriction as seen in YCrO 3, we analyzed the behavior of temperature dependence of line width of modes as shown in Fig. 5. It is

7 obvious that only in the cases of SmCrO 3 and GdCrO 3 we observe anomalies in line width around T N (namely, broadening of the line width) suggesting the presence of spin-phonon coupling. From these observations in our studies, we can conclude that spin-phonon coupling is present only in RCrO 3 with magnetic R-ions. Temperature dependence of the A g (7) and B 2g (7) mode frequencies and the corresponding line widths are shown in Fig. 6 (top panels). These are lattice modes involving R-O vibrations. We notice from the left panel that all the samples show phonon anomalies at T N but the anomaly is relatively weak in YCrO 3. Such a small change in phonon frequency along with no change in phonon line width suggests that these may be due to magnetostriction observed in YCrO 3. However, a strong softening of these modes along with the anomalies in their line width (top right panel) in GdCrO 3 and SmCrO 3 confirms the presence of spin-phonon coupling. As discussed earlier for SmCrO3 (Fig. 4), we observe anomaly at T* in these lattice modes as well. The relatively strong softening of these low frequency modes including the B 2g (5) mode (bottom panel of Fig. 6) further reveals the displacement of R-ions due to spin-phonon coupling caused by the interaction between the weak ferromagnetism of Cr-ions and magnetic R-ions which results in ferroelectric polarization in these systems. 9,31,32 In conclusion, our Raman results clearly demonstrate the interplay between spin-phonon coupling and ferroelectricty in RCrO 3 with magnetic R-ions. The presence of spin-phonon coupling in RCrO 3 with magnetic R-ion is established from anomalies observed in phonon modes. The spin-phonon coupling is also evidenced from softening of the low frequency phonon modes which further demonstrates the displacement of magnetic R-ion due to its interaction with the weak ferromagnetism associated with Cr-ordering giving rise to ferroelectric polarization in these systems.

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11 Figure captions: Fig. 1. (Color online) Unpolarized Raman spectra of RCrO 3 (R= Y, Gd, Sm) with mode assignments which have been done based on Ref [14, 15, 24]. Fig. 2. (Color online) (Top panel) Temperature dependence of the A g (1) mode frequencies of (a) YCrO 3 (b) GdCrO 3 (c) SmCrO 3 in the temperature range K. Red dotted lines represent the anharmonic contribution to the phonon frequency as mentioned in the text. (Bottom panel) corresponding line widths. Fig. 3. (Color online) Temeprature dependence of in- phase A g (2) and out-of- phase B 1g (3) octahedral rotations with respect to y-axis in (a)ycro 3 (b) GdCrO 3 (c) SmCrO 3. Fig. 4. (Color online) (Left panel) Temperature dependence of in-phase (A g (4)) and out-ofphase (B 1g (4)) CrO 6 rotations with respect to x-axis, (Right panel) Temperature effects on bending mode B 3g (3) in RCrO 3 (R=Y, Gd, Sm). Solid lines are guide to the eye. Fig. 5. (Color online) Effect of the temperature on B 1g (4) and A g (4) phonon line widths in RCrO 3 (R= Y, Gd, Sm). Solid lines are guide to the eye.

12 Fig. 6. (Top panel) Temperature dependence of (left) frequencies of A g (7), B 2g (7) modes involving R-O vibrations. (right) corresponding line widths. Solid lines are guide to the eye. (bottom panel)temperature dependence of B 2g (5) mode frequency involving pure R atom vibrations in RCrO 3. Dotted lines are guide to the eye.

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