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Temperature dependent dielectric and phonon study of polycrystalline SmFeO3
Physica B: Condensed Matter 570 (2019) 187–190
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Temperature dependent dielectric and phonon study of polycrystalline SmFeO3
T
Anjali Panchwanee, Akash Surampalli, V. Raghavendra Reddy∗ UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore, 452001, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Orthoferrites SmFeO3 Spin-phonon coupling
The present work reports the frequency dependent dielectric, ac-conductivity study across spin reorientation transition (TSRT) & Raman spectroscopy study across TSRT, Néel temperature (TN,Fe) of polycrystalline SmFeO3 (SFO). Anomalous variation is observed in phonon mode parameters for SFO across TSRT and TN,Fe indicating the presence of spin-phonon coupling similar to that of other RFeO3 compounds. Anomalous behavior is also observed in the temperature variation of dielectric constant and ac-conductivity across TSRT and frequency dispersion is observed at lower temperature side of TSRT.
1. Introduction The urge of coupled magnetism and ferroelectricity for next generation storage devices led to the discovery of new magneto-electric (ME)/multiferroic materials. Recently RFeO3 (R = rare earth element) compounds have attracted great attention with the discovery of ME coupling [1]. Among RFeO3, SmFeO3 (SFO) shows spin reorientation transition TSRT at high temperatures (450–480 K) and hence is considered to have practical device applications at room temperature [2]. The weak ferromagnetic moment of Fe3+ spins rotates from c-axis to aaxis with lowering the temperature which is known as spin reorientation transition (TSRT). Very high dielectric constant (ε′) values are reported in SFO ceramics [3,4]. Recently Khan et al., have reported an anomalous behavior of ε′ and conductivity across TSRT and shown that microstructure plays a significant role in the giant ε′ values of SFO ceramics [5]. Frequency dispersion and anomalous variation of ε′ across TSRT and Néel temperature (TN,Fe) is reported in 50 nm sized SFO, however in bulk SFO (500 nm) very weak signatures are seen [6]. In 500 nm SFO sample no change in slope (dε′/dT) is observed across TSRT, whereas an anomaly is observed in the Jonscher frequency exponent across TSRT, which is explained as the signature of incipient ferroelectric ordering in polycrystalline SFO [6]. Recently spin-phonon coupling has been argued experimentally as well as theoretically to have crucial role in microscopic origin of the multiferroicity in RCrO3, RMnO3, RFeO3 compounds [7–9]. For example., crucial role of spin phonon coupling have been theoretically demonstrated by Masahito et al., for RMnO3 compounds and is
∗
expected to be relevant for all the multiferroic materials [7]. Bhadram et al., [8] have reported that the spin-phonon coupling play important role for the origin of ferroelectric polarization in rare-earth orthochromites where R atom is magnetic. Weber et al., have studied the structural evolution of RFeO3 family with Raman spectroscopy and it is shown that the Raman modes related to the FeO6 octahedra rotation are soft modes [9]. It is further shown that frequency of these modes depends only on the size of the rare earth for a given temperature, which is suggested to be helpful for the investigation of structural changes related to the ferroelectricity [9]. In our recent study we have shown the signatures of spin-phonon coupling and local structural re-arrangement even up to antiferromagnetic to paramagnetic transition of Fe sublattice (TN,Fe) in GdFeO3 [10]. However the study of phonons in bulk SmFeO3 is reported only till TSRT [11]. In view of this, in the present work, the phonon dynamics in polycrystalline SFO are investigated using detailed temperature dependent Raman measurements across the magnetic transitions viz., TSRT, TN,Fe. Also the dielectric and ac-conductivity measurements are carried out across TSRT at different frequencies. 2. Experimental details The single-phase bulk SFO was prepared using conventional solidstate reaction method. The XRD pattern is refined considering orthorhombic (pbnm space group) structure and the refined lattice parameters are reported in Ref. [12]. The presence of magnetic ordering in bulk SFO is shown from temperature dependent 57Fe Mössbauer, magnetization measurements. The transition temperatures (TSRT and TN,Fe) of
Corresponding author. E-mail addresses: [email protected], [email protected] (V.R. Reddy).
https://doi.org/10.1016/j.physb.2019.06.035 Received 14 January 2019; Received in revised form 14 April 2019; Accepted 16 June 2019 Available online 18 June 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 570 (2019) 187–190
A. Panchwanee, et al.
whose position is almost independent of the frequency. The second peak (P2) is broader and is at lower temperature (420 K) than P1. The position of P2 seems to be shifting towards high temperature with increasing frequency and merging with P1, similar to Sahoo et al., [3]. A similar frequency dispersion is observed below TSRT in SFO nanoparticles, which is interpreted as ferroelectric relaxor-like behavior by Chaturvedi et al., and the results are attributed to the polar displacement of Sm3+ ions within lattice [13]. Sahoo et al., observed two dielectric peaks at 453 and 493 K which is explained to be associated with Maxwell-Wagner type and spin reorientation respectively for SFO nanoparticles [3]. On the contrary, Smita et al., reported a change in slope (dε′/dT) across TSRT in SFO nano particles and mentioned that the peak across TSRT as observed by Sahoo et al., [3] could be due to enhancement of electrical signal from the extrinsic contributions. In order to see the contribution of ac-conductivity to dielectric peak (P1) across TSRT, we have calculated the ac-conductivity (σac) for the present sample using the relation σac = ωε′ε0tanδ, where ε0, tanδ are permittivity of free space, dielectric loss respectively [14]. The frequency dependence of ac-conductivity (σac) at representative temperatures is shown in Fig. 2(a). It is clearly seen that (σac) is almost constant with the frequency at lower frequency range and then abruptly increases at frequencies higher than 105 Hz. The similar behavior is observed for the temperatures lower than TSRT. For more understanding of the variation of ac-conductivity with the temperature, we have plotted the σac versus temperature as shown in Fig. 2(b). The σac shows multiple inflection points, but one can broadly mention that there is a significant change in slope across TSRT. This observation can be understood in terms of change in conduction behavior across TSRT as reported in bulk SFO [5,6]. Therefore, the observed peak in ε′ is expected to have the contributions of both the conductivity and the signature of incipient ferroelectric ordering. It may be noted that because of high conductivity, the ferroelectric measurements are carried out at low temperatures to show the true switched charge density in SFO ceramics [12,15]. However, the ferroelectric ordering is reported in epitaxial thin films of SFO even at room temperature [16,17]. In order to understand the role of phonons associated with different atomic vibrations in the anomalous behavior of dielectric constant (ε′) and ac-conductivity, Raman measurements are carried out in the temperature range (300–750 K) and the data is shown in Fig. 3. SmFeO3 has an orthorhombic crystal structure (Pbnm space group) have 24 Raman active modes given by the following irreducible representation, ΓRaman = 7Ag + 7B1g + 5B2g + 5B3g. All the observed modes in the present work match with the literature [9]. The assignment of associated symmetry and main atomic motions of the observed Raman modes at room temperature are carried out following the previous reports [10,18] and the recent report on Raman study of RFeO3 compounds [9]. The vibration modes associated with the displacement of Rare earth ion are mainly below 200 cm−1 whereas above 200 cm−1 vibrations associated with Fe & O play dominant role. The recorded
Fig. 1. Frequency variation of dielectric constant ε′ at representative temperatures. Inset shows temperature variation of ε′ at indicated frequencies.
about 450–480 K and 708 K, respectively are obtained from results of 57 Fe Mössbauer as reported in Ref. [12]. The presence of true switched ferroelectric polarization is shown from detailed ferroelectric PUND measurements at 200 K as reported in Ref. [12]. In the present work, Raman spectroscopy measurements are performed in back scattering configuration with Jobin Yvon Horibra LABRAM spectrometer using He-Ne laser (632.8 nm). A charge-coupled device (CCD) detector was used to detect the scattered signal. Temperature dependent Raman measurements were carried out in THMS600 sample stage of Linkam Scientific instruments. Ltd N4L 1735 LCR meter was used to study dielectric properties and ac conductivity.
3. Results and discussions The frequency and temperature dependent dielectric properties of the studied sample (SFO) is shown in Figs. 1 and 2. The dielectric constant (ε′) decreases with frequency at all temperatures. However at temperatures around TN,Fe dielectric constant shows sudden change in slope in the high frequency region (≥105Hz). Moreover ε′ increases with temperature till TSRT (∼ 480 K) and with further increase in temperature ε′ starts decreasing till 600 K and then again starts increasing with temperature till the highest temperature (∼ 680 K) as shown in Fig. 1. To clearly understand this variation ε′ versus temperature is plotted as an inset of Fig. 1. Two peaks are observed in the dielectric constant (ε′) versus temperature at different frequencies as shown in the inset of Fig. 1. The first peak (P1) is around 480 K (TSRT)
Fig. 2. (a) Frequency variation of ac-conductivity at representative temperatures (b) Temperature variation of ac-conductivity at different frequencies. 188
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Fig. 3. (a) Raman spectra at representative temperature from (300–690 K) for bulk SmFeO3 corrected by Bose-Einstein (BE) thermal factor. Spectra are shifted vertically for the sake of clarity in presentation. (b) The integrated intensity I(T) at each temperature normalized by I(300 K), depicting the anomaly at TSRT, TN,Fe.
Raman spectra at different temperatures are first corrected by BoseEinstein (BE) thermal factor given by [n (ω, T ) + 1] = [1 − exp (−ℏω/ kβ T )]−1, which are shown in Fig. 3(a). The temperature variation of ratio of BE corrected integrated intensities (I(T)/I(300 K)) is shown in Fig. 3(b). As one can see that the integrated intensity shows anomalous behavior across TSRT and TN,Fe suggesting the role of local structural rearrangement and lattice vibrations at these temperatures similar to other RFeO3 compounds [10]. We have then systematically investigated the temperature variation of Raman shift and full width at half maximum (FWHM) by de-convolution of different Raman modes, considering Lorentzian shape, in the interested region of wavenumber (not shown here). The temperature variation of peak position and FWHM of Raman modes associated with Sm3+ motion (Ag(2)), FeO6 octahedral rotation (B1g(2)) and FeO (2) stretching (B1g(4)) are shown in Fig. 4(a,b,c) respectively. The phonon modes are observed to shift towards lower frequencies with increasing temperature indicating the increasing of inter-atomic distances in accordance with the thermal expansion. Noticeable changes in slope of the temperature variation of the phonon mode position and FWHM related to Sm3+ motion, FeO6 octahedral rotation is observed across TSRT and TN,Fe. However subtle changes are observed in the peak position and FWHM of the Raman mode associated with FeO(2) stretching across TSRT and TN,Fe. All these observations indicate the phonon mediated magnetic interactions exists between Sm3+ and Fe3+ ions suggesting the existence of spin-phonon coupling in bulk polycrystalline SmFeO3. Therefore, as reported for other similar compounds [7–9], the present results show the signatures of spin-phonon coupling in bulk SmFeO3, which might be responsible in stabilizing the ferroelectric ordering resulting in the anomalous variation of dielectric data.
4. Conclusions The dielectric properties and ac-conductivity of the bulk SmFeO3 sample are studied in the frequency range, covering six orders of magnitude and in the temperature range of 300–670 K and an anomalous variation is observed across spin re-orientation transition (TSRT). The signatures of spin-phonon coupling and local structural rearrangement are observed across TSRT and Néel temperature (TN,Fe) in bulk SmFeO3 from Raman data. The Raman modes related to Sm3+ motion and FeO6 deformation are observed to show changes across the magnetic transitions viz., TSRT, TN,Fe, which suggest the phonon mediated magnetic interactions between Sm and Fe sub-lattice.
Fig. 4. Temperature dependence of Raman shift and full width at half maximum (FWHM) as obtained by Lorentz fitting of Ag(2), B1g(2), B1g(4) Raman modes. Two vertical lines indicate the TSRT and TN,Fe transition temperatures.
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Acknowledgments
[7] Masahito Mochizuki, Nobuo Furukawa, Naoto Nagaosa, Phys. Rev. B 84 (2011) 144409. [8] Venkata Srinu Bhadram, B. Rajeswaran, A. Sundaresan, Chandrabhas Narayana, Europhys. Lett. 101 (2013) 17008. [9] M.C. Weber, M. Guennou, H.J. Zhao, J. Íñiguez, R. Vilarinho, A. Almeida, J.A. Moreira, J. Kreisel, Phys. Rev. B 94 (2016) 214103. [10] A. Panchwanee, V.R. Reddy, A. Gupta, V.G. Sathe, Mater. Chem. Phys. 196 (2016) 205. [11] S. Tyagi, V.G. Sathe, G. Sharma, M.K. Gupta, R. Mittal, V. Srihari, H.K. Poswal, Detail investigations of SmFeO3 under extreme condition, Mater. Chem. Phys. (2018), https://doi.org/10.1016/j.matchemphys.2018.05.066. [12] A. Panchwanee, V.R. Reddy, A. Gupta, J. Magn. Magn. Mater. 448 (2018) 38. [13] Smita Chaturvedi, Priyank Shyam, Amey Apte, Jitender Kumar, Arpan Bhattacharyya, A.M. Awasthi, Sulabha Kulkarni, Phys. Rev. B 93 (2016) 174117. [14] A.K. Jonscher, Dielectric Relaxations in Solids, Chelsea Dielectrics, London, 1993. [15] Chenyang Zhang, Mingyu Shang, Milan Liu, Tingsong Zhang, Lei Ge, Hongming Yuan, Shouhua Feng, J. Alloy. Comp. 665 (2016) 152. [16] Zhenxiang Cheng, Fang Hong, Yuanxu Wang, Kiyoshi Ozawa, Hiroki Fujii, Hideo Kimura, Yi Du, Xiaolin Wang, Shixue Dou, ACS Appl. Mater. Interfaces 6 (2014) 7356. [17] WenZhe Si, KeKe Huang, XiaoFeng Wu, ShouHua Feng, Sci. China 57 (2014) 803. [18] S. Venugopalan, M. Dutta, A.K. Ramdas, J.P. Remeika, Phys. Rev. B 31 (1985) 1490.
Dr. Vasant Sathe is acknowledged for temperature dependent Raman data. Authors thank Prof. S. Sen and his group for high temperature dielectric measurements. Prof. Ajay Gupta is thanked for discussions. AP is thankful to CSIR for SRF fellowship. References [1] Y. Tokunaga, N. Furukawa, H. Sakai, Y. Taguchi, T. Arima, Y. Tokura, Nat. Mater. 8 (2009) 558. [2] Shixun Cao, Huazhi Zhao, Baojuan Kang, Jincang Zhang, Wei Ren, Sci. Rep. 4 (2014) 5960. [3] S. Sahoo, P.K. Mahapatra, R.N.P. Choudhary, J. Phys. D Appl. Phys. 49 (2016) 035302. [4] B.V. Prasad, G.N. Rao, J.W. Chen, D.S. Babu, Mater. Res. Bull. 46 (2011) 1670. [5] A.A. Khan, S. Satapathy, Anju Ahlawat, Pratik Deshmukh, A.K. Karnal, Ceram. Int. 44 (2018) 12401. [6] Smita Chaturvedi, Priyank Shyam, Rabindranath Bag, Mandar M. Shirolkar, Jitender Kumar, Harleen Kaur, Surjeet Singh, A.M. Awasthi, Sulabha Kulkarni, Phys. Rev. B 96 (2017) 024434.
190
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Temperature dependent dielectric and phonon study of polycrystalline SmFeO3
T
Anjali Panchwanee, Akash Surampalli, V. Raghavendra Reddy∗ UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore, 452001, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Orthoferrites SmFeO3 Spin-phonon coupling
The present work reports the frequency dependent dielectric, ac-conductivity study across spin reorientation transition (TSRT) & Raman spectroscopy study across TSRT, Néel temperature (TN,Fe) of polycrystalline SmFeO3 (SFO). Anomalous variation is observed in phonon mode parameters for SFO across TSRT and TN,Fe indicating the presence of spin-phonon coupling similar to that of other RFeO3 compounds. Anomalous behavior is also observed in the temperature variation of dielectric constant and ac-conductivity across TSRT and frequency dispersion is observed at lower temperature side of TSRT.
1. Introduction The urge of coupled magnetism and ferroelectricity for next generation storage devices led to the discovery of new magneto-electric (ME)/multiferroic materials. Recently RFeO3 (R = rare earth element) compounds have attracted great attention with the discovery of ME coupling [1]. Among RFeO3, SmFeO3 (SFO) shows spin reorientation transition TSRT at high temperatures (450–480 K) and hence is considered to have practical device applications at room temperature [2]. The weak ferromagnetic moment of Fe3+ spins rotates from c-axis to aaxis with lowering the temperature which is known as spin reorientation transition (TSRT). Very high dielectric constant (ε′) values are reported in SFO ceramics [3,4]. Recently Khan et al., have reported an anomalous behavior of ε′ and conductivity across TSRT and shown that microstructure plays a significant role in the giant ε′ values of SFO ceramics [5]. Frequency dispersion and anomalous variation of ε′ across TSRT and Néel temperature (TN,Fe) is reported in 50 nm sized SFO, however in bulk SFO (500 nm) very weak signatures are seen [6]. In 500 nm SFO sample no change in slope (dε′/dT) is observed across TSRT, whereas an anomaly is observed in the Jonscher frequency exponent across TSRT, which is explained as the signature of incipient ferroelectric ordering in polycrystalline SFO [6]. Recently spin-phonon coupling has been argued experimentally as well as theoretically to have crucial role in microscopic origin of the multiferroicity in RCrO3, RMnO3, RFeO3 compounds [7–9]. For example., crucial role of spin phonon coupling have been theoretically demonstrated by Masahito et al., for RMnO3 compounds and is
∗
expected to be relevant for all the multiferroic materials [7]. Bhadram et al., [8] have reported that the spin-phonon coupling play important role for the origin of ferroelectric polarization in rare-earth orthochromites where R atom is magnetic. Weber et al., have studied the structural evolution of RFeO3 family with Raman spectroscopy and it is shown that the Raman modes related to the FeO6 octahedra rotation are soft modes [9]. It is further shown that frequency of these modes depends only on the size of the rare earth for a given temperature, which is suggested to be helpful for the investigation of structural changes related to the ferroelectricity [9]. In our recent study we have shown the signatures of spin-phonon coupling and local structural re-arrangement even up to antiferromagnetic to paramagnetic transition of Fe sublattice (TN,Fe) in GdFeO3 [10]. However the study of phonons in bulk SmFeO3 is reported only till TSRT [11]. In view of this, in the present work, the phonon dynamics in polycrystalline SFO are investigated using detailed temperature dependent Raman measurements across the magnetic transitions viz., TSRT, TN,Fe. Also the dielectric and ac-conductivity measurements are carried out across TSRT at different frequencies. 2. Experimental details The single-phase bulk SFO was prepared using conventional solidstate reaction method. The XRD pattern is refined considering orthorhombic (pbnm space group) structure and the refined lattice parameters are reported in Ref. [12]. The presence of magnetic ordering in bulk SFO is shown from temperature dependent 57Fe Mössbauer, magnetization measurements. The transition temperatures (TSRT and TN,Fe) of
Corresponding author. E-mail addresses: [email protected], [email protected] (V.R. Reddy).
https://doi.org/10.1016/j.physb.2019.06.035 Received 14 January 2019; Received in revised form 14 April 2019; Accepted 16 June 2019 Available online 18 June 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 570 (2019) 187–190
A. Panchwanee, et al.
whose position is almost independent of the frequency. The second peak (P2) is broader and is at lower temperature (420 K) than P1. The position of P2 seems to be shifting towards high temperature with increasing frequency and merging with P1, similar to Sahoo et al., [3]. A similar frequency dispersion is observed below TSRT in SFO nanoparticles, which is interpreted as ferroelectric relaxor-like behavior by Chaturvedi et al., and the results are attributed to the polar displacement of Sm3+ ions within lattice [13]. Sahoo et al., observed two dielectric peaks at 453 and 493 K which is explained to be associated with Maxwell-Wagner type and spin reorientation respectively for SFO nanoparticles [3]. On the contrary, Smita et al., reported a change in slope (dε′/dT) across TSRT in SFO nano particles and mentioned that the peak across TSRT as observed by Sahoo et al., [3] could be due to enhancement of electrical signal from the extrinsic contributions. In order to see the contribution of ac-conductivity to dielectric peak (P1) across TSRT, we have calculated the ac-conductivity (σac) for the present sample using the relation σac = ωε′ε0tanδ, where ε0, tanδ are permittivity of free space, dielectric loss respectively [14]. The frequency dependence of ac-conductivity (σac) at representative temperatures is shown in Fig. 2(a). It is clearly seen that (σac) is almost constant with the frequency at lower frequency range and then abruptly increases at frequencies higher than 105 Hz. The similar behavior is observed for the temperatures lower than TSRT. For more understanding of the variation of ac-conductivity with the temperature, we have plotted the σac versus temperature as shown in Fig. 2(b). The σac shows multiple inflection points, but one can broadly mention that there is a significant change in slope across TSRT. This observation can be understood in terms of change in conduction behavior across TSRT as reported in bulk SFO [5,6]. Therefore, the observed peak in ε′ is expected to have the contributions of both the conductivity and the signature of incipient ferroelectric ordering. It may be noted that because of high conductivity, the ferroelectric measurements are carried out at low temperatures to show the true switched charge density in SFO ceramics [12,15]. However, the ferroelectric ordering is reported in epitaxial thin films of SFO even at room temperature [16,17]. In order to understand the role of phonons associated with different atomic vibrations in the anomalous behavior of dielectric constant (ε′) and ac-conductivity, Raman measurements are carried out in the temperature range (300–750 K) and the data is shown in Fig. 3. SmFeO3 has an orthorhombic crystal structure (Pbnm space group) have 24 Raman active modes given by the following irreducible representation, ΓRaman = 7Ag + 7B1g + 5B2g + 5B3g. All the observed modes in the present work match with the literature [9]. The assignment of associated symmetry and main atomic motions of the observed Raman modes at room temperature are carried out following the previous reports [10,18] and the recent report on Raman study of RFeO3 compounds [9]. The vibration modes associated with the displacement of Rare earth ion are mainly below 200 cm−1 whereas above 200 cm−1 vibrations associated with Fe & O play dominant role. The recorded
Fig. 1. Frequency variation of dielectric constant ε′ at representative temperatures. Inset shows temperature variation of ε′ at indicated frequencies.
about 450–480 K and 708 K, respectively are obtained from results of 57 Fe Mössbauer as reported in Ref. [12]. The presence of true switched ferroelectric polarization is shown from detailed ferroelectric PUND measurements at 200 K as reported in Ref. [12]. In the present work, Raman spectroscopy measurements are performed in back scattering configuration with Jobin Yvon Horibra LABRAM spectrometer using He-Ne laser (632.8 nm). A charge-coupled device (CCD) detector was used to detect the scattered signal. Temperature dependent Raman measurements were carried out in THMS600 sample stage of Linkam Scientific instruments. Ltd N4L 1735 LCR meter was used to study dielectric properties and ac conductivity.
3. Results and discussions The frequency and temperature dependent dielectric properties of the studied sample (SFO) is shown in Figs. 1 and 2. The dielectric constant (ε′) decreases with frequency at all temperatures. However at temperatures around TN,Fe dielectric constant shows sudden change in slope in the high frequency region (≥105Hz). Moreover ε′ increases with temperature till TSRT (∼ 480 K) and with further increase in temperature ε′ starts decreasing till 600 K and then again starts increasing with temperature till the highest temperature (∼ 680 K) as shown in Fig. 1. To clearly understand this variation ε′ versus temperature is plotted as an inset of Fig. 1. Two peaks are observed in the dielectric constant (ε′) versus temperature at different frequencies as shown in the inset of Fig. 1. The first peak (P1) is around 480 K (TSRT)
Fig. 2. (a) Frequency variation of ac-conductivity at representative temperatures (b) Temperature variation of ac-conductivity at different frequencies. 188
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Fig. 3. (a) Raman spectra at representative temperature from (300–690 K) for bulk SmFeO3 corrected by Bose-Einstein (BE) thermal factor. Spectra are shifted vertically for the sake of clarity in presentation. (b) The integrated intensity I(T) at each temperature normalized by I(300 K), depicting the anomaly at TSRT, TN,Fe.
Raman spectra at different temperatures are first corrected by BoseEinstein (BE) thermal factor given by [n (ω, T ) + 1] = [1 − exp (−ℏω/ kβ T )]−1, which are shown in Fig. 3(a). The temperature variation of ratio of BE corrected integrated intensities (I(T)/I(300 K)) is shown in Fig. 3(b). As one can see that the integrated intensity shows anomalous behavior across TSRT and TN,Fe suggesting the role of local structural rearrangement and lattice vibrations at these temperatures similar to other RFeO3 compounds [10]. We have then systematically investigated the temperature variation of Raman shift and full width at half maximum (FWHM) by de-convolution of different Raman modes, considering Lorentzian shape, in the interested region of wavenumber (not shown here). The temperature variation of peak position and FWHM of Raman modes associated with Sm3+ motion (Ag(2)), FeO6 octahedral rotation (B1g(2)) and FeO (2) stretching (B1g(4)) are shown in Fig. 4(a,b,c) respectively. The phonon modes are observed to shift towards lower frequencies with increasing temperature indicating the increasing of inter-atomic distances in accordance with the thermal expansion. Noticeable changes in slope of the temperature variation of the phonon mode position and FWHM related to Sm3+ motion, FeO6 octahedral rotation is observed across TSRT and TN,Fe. However subtle changes are observed in the peak position and FWHM of the Raman mode associated with FeO(2) stretching across TSRT and TN,Fe. All these observations indicate the phonon mediated magnetic interactions exists between Sm3+ and Fe3+ ions suggesting the existence of spin-phonon coupling in bulk polycrystalline SmFeO3. Therefore, as reported for other similar compounds [7–9], the present results show the signatures of spin-phonon coupling in bulk SmFeO3, which might be responsible in stabilizing the ferroelectric ordering resulting in the anomalous variation of dielectric data.
4. Conclusions The dielectric properties and ac-conductivity of the bulk SmFeO3 sample are studied in the frequency range, covering six orders of magnitude and in the temperature range of 300–670 K and an anomalous variation is observed across spin re-orientation transition (TSRT). The signatures of spin-phonon coupling and local structural rearrangement are observed across TSRT and Néel temperature (TN,Fe) in bulk SmFeO3 from Raman data. The Raman modes related to Sm3+ motion and FeO6 deformation are observed to show changes across the magnetic transitions viz., TSRT, TN,Fe, which suggest the phonon mediated magnetic interactions between Sm and Fe sub-lattice.
Fig. 4. Temperature dependence of Raman shift and full width at half maximum (FWHM) as obtained by Lorentz fitting of Ag(2), B1g(2), B1g(4) Raman modes. Two vertical lines indicate the TSRT and TN,Fe transition temperatures.
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Acknowledgments
[7] Masahito Mochizuki, Nobuo Furukawa, Naoto Nagaosa, Phys. Rev. B 84 (2011) 144409. [8] Venkata Srinu Bhadram, B. Rajeswaran, A. Sundaresan, Chandrabhas Narayana, Europhys. Lett. 101 (2013) 17008. [9] M.C. Weber, M. Guennou, H.J. Zhao, J. Íñiguez, R. Vilarinho, A. Almeida, J.A. Moreira, J. Kreisel, Phys. Rev. B 94 (2016) 214103. [10] A. Panchwanee, V.R. Reddy, A. Gupta, V.G. Sathe, Mater. Chem. Phys. 196 (2016) 205. [11] S. Tyagi, V.G. Sathe, G. Sharma, M.K. Gupta, R. Mittal, V. Srihari, H.K. Poswal, Detail investigations of SmFeO3 under extreme condition, Mater. Chem. Phys. (2018), https://doi.org/10.1016/j.matchemphys.2018.05.066. [12] A. Panchwanee, V.R. Reddy, A. Gupta, J. Magn. Magn. Mater. 448 (2018) 38. [13] Smita Chaturvedi, Priyank Shyam, Amey Apte, Jitender Kumar, Arpan Bhattacharyya, A.M. Awasthi, Sulabha Kulkarni, Phys. Rev. B 93 (2016) 174117. [14] A.K. Jonscher, Dielectric Relaxations in Solids, Chelsea Dielectrics, London, 1993. [15] Chenyang Zhang, Mingyu Shang, Milan Liu, Tingsong Zhang, Lei Ge, Hongming Yuan, Shouhua Feng, J. Alloy. Comp. 665 (2016) 152. [16] Zhenxiang Cheng, Fang Hong, Yuanxu Wang, Kiyoshi Ozawa, Hiroki Fujii, Hideo Kimura, Yi Du, Xiaolin Wang, Shixue Dou, ACS Appl. Mater. Interfaces 6 (2014) 7356. [17] WenZhe Si, KeKe Huang, XiaoFeng Wu, ShouHua Feng, Sci. China 57 (2014) 803. [18] S. Venugopalan, M. Dutta, A.K. Ramdas, J.P. Remeika, Phys. Rev. B 31 (1985) 1490.
Dr. Vasant Sathe is acknowledged for temperature dependent Raman data. Authors thank Prof. S. Sen and his group for high temperature dielectric measurements. Prof. Ajay Gupta is thanked for discussions. AP is thankful to CSIR for SRF fellowship. References [1] Y. Tokunaga, N. Furukawa, H. Sakai, Y. Taguchi, T. Arima, Y. Tokura, Nat. Mater. 8 (2009) 558. [2] Shixun Cao, Huazhi Zhao, Baojuan Kang, Jincang Zhang, Wei Ren, Sci. Rep. 4 (2014) 5960. [3] S. Sahoo, P.K. Mahapatra, R.N.P. Choudhary, J. Phys. D Appl. Phys. 49 (2016) 035302. [4] B.V. Prasad, G.N. Rao, J.W. Chen, D.S. Babu, Mater. Res. Bull. 46 (2011) 1670. [5] A.A. Khan, S. Satapathy, Anju Ahlawat, Pratik Deshmukh, A.K. Karnal, Ceram. Int. 44 (2018) 12401. [6] Smita Chaturvedi, Priyank Shyam, Rabindranath Bag, Mandar M. Shirolkar, Jitender Kumar, Harleen Kaur, Surjeet Singh, A.M. Awasthi, Sulabha Kulkarni, Phys. Rev. B 96 (2017) 024434.
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