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Griffiths phase-like behavior and origin of spin-phonon interaction in Eu0.75Y0.25MnO3
Journal of Magnetism and Magnetic Materials 482 (2019) 38–43
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Griffiths phase-like behavior and origin of spin-phonon interaction in Eu0.75Y0.25MnO3 Surbhi Guptaa, Gaurav Sharmab, V.R. Reddyb, V.G. Satheb, V. Siruguria, a b
T
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UGC-DAE Consortium for Scientific Research Mumbai Centre, 246C, CFB, BARC Campus, Mumbai 400085, India UGC-DAE Consortium for Scientific Research Indore Centre, 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: Manganites Magnetization Raman spectroscopy Mossobauer spectroscopy
The magnetic state and the spin-phonon coupling in the Eu0.75Y0.25MnO3 are investigated using temperature dependent magnetization, Raman spectroscopy and Mossbauer spectroscopy measurements. The magnetization data showed bifurcation in zero-field cooled and field-cooled warming curves and indicate two antiferromagnetic transitions at 47 K and 22 K. Short range ferromagnetic correlations are clearly observed well above the antiferromagnetic transition temperatures, which are attributed to the presence of Griffiths-like phase. The temperature dependent Raman spectroscopy measurements showed anomalous softening of phonon modes well above the antiferromagnetic ordering temperature of ∼47 K and also showed signatures of both the ordering temperatures. The Mossbauer spectra showed anomalous changes in hyperfine field as a function of temperature which match with the temperature at which B2g phonons showed softening.
1. Introduction Recent extensive studies show that the noncollinear magnetic structures such as a spin helix are essential to host the ferroelectricity in multiferroics [1,2]. In this type of multiferroics, the space inversion symmetry is broken by the magnetic order. Based on this strategy, many multiferroic materials have been found so far [3–6]. Here, a spiral magnetic order or a charge order along a direction results in alternate ferromagnetic and anti-ferromagnetic ordering that generates polarization. Strong candidates in this class are manganites like TbMnO3, DyMnO3 that show ferroelectricity due to spiral magnetic ordering [2,7–9]. Because of the orbital ordering of Mn3+ ions in orthorhombic charge ordered RMnO3, the exchange interaction among the neighbouring spins is ferromagnetic (FM) in the a–b plane and antiferromagnetic (AFM) along the c-axis. Consistently, spins in each a–b plane of LaMnO3 order ferromagnetically and the magnetization direction alternates along the c-axis (TN = 150 K). The replacement of La by smaller ions such as Tb or Dy increases the structural distortion, inducing next-nearest-neighbor AFM exchange in the a–b plane comparable to the nearest-neighbor FM exchange. This frustrates the FM ordering of spins in the a–b plane, and below ∼42 K, Tb(Dy)MnO3 shows an incommensurate magnetic ordering with a collinear sinusoidal modulation along the b-axis, which is paraelectric. However, as the temperature is lowered, the magnetization grows in magnitude and
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a spiral state with rotating spins set in at ∼28 K which is energetically more favorable, and induces ferroelectricity [10]. Eu0.75Y0.25MnO3 presents a unique opportunity to study the role of distortion induced due to rare earth ions. The composite ionic radii in this compound are nearly equal to that in TbMnO3. This compound also shows two magnetic transitions, one at 47 K and the other one at 22 K. Thus, Eu1−xYxMnO3 allows tuning of the lattice and magnetic properties, forming a multiferroic manganite compound without the magnetic interference of the rare-earth ions, and it is shown that magnetoelectric contribution to the dielectric permittivity can be observed in all the compositions in crystallographic a-direction only [11]. Spin-lattice coupling is important to judge the interaction among various degrees of freedom particularly magnetic and electric order in this case. In this compound where the ferroelectricity is generated due to frustrated (spiral) magnetic order, it is expected to show strong spin lattice coupling. The spin phonon coupling is reported previously in Eu1−xYxMnO3 series of compounds [12] and it is shown that the coupling exists predominantly within the MnO2 plane, manifesting itself by softening of the phonon modes in Raman spectroscopy. It is shown that even for increasing Y concertation, spin-phonon coupling exists without presence of proper ferromagnetic order in any direction. The spinphonon coupling is supposed to be linked with the magnetic ordering transition. According to Granado et al. [13], the phonon frequency renormalization is proportional to the spin–spin correlation
Corresponding author. E-mail address: [email protected] (V. Siruguri).
https://doi.org/10.1016/j.jmmm.2019.03.050 Received 26 October 2018; Received in revised form 31 January 2019; Accepted 7 March 2019 Available online 08 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 482 (2019) 38–43
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function < Si Sj > for the near neighbour spins localized at the Mn3+ ions. On the contrary, in most of the studies on Eu1−xYxMnO3 compounds, the spin-phonon coupling, i.e. the phonon softening is observed below ∼100 K which is well above the antiferromagnetic transition temperatures reported for this series of compounds. Therefore, in this report, we studied the various properties of Eu0.75Y0.25MnO3 compound in an attempt to understand the cause of spin-phonon coupling or phonon softening well above the observed magnetic ordering temperature. 2. Experimental details The polycrystalline Eu0.75Y0.25MnO3 sample was prepared using conventional solid-state route at ambient pressure. Stoichiometric ratios of high purity (≥99.9%) Eu2O3, Y2O3 and MnCO3 were mixed well, ground and calcined at 1100 °C for 24 h. The reacted mixture was again ground and heated at 1300 °C for further 24 h. The resulting powders were then pressed into pellets and sintered at 1300 °C. The sample purity was characterized by X-ray diffraction (XRD) [14]. The temperature and field dependence DC magnetisation measurements were carried out on a commercial vibrating sample magnetometer (VSM) coupled to a 90 kOe physical property measurement system (PPMS) (Model: 6000, Quantum Design, USA) in the temperature range of 3 K–300 K. Raman spectra were taken from 5 K to 300 K using Horiba JY HR800 micro-Raman system in unpolarized configuration. The system is equipped with an excitation source Ar+ laser (488 nm), 1800 grooves/ mm grating, an edge filter for Rayleigh line rejection and a Peltier cooled CCD detector. The overall spectral resolution of the system is ∼1 cm−1 and laser power on the sample was kept below 2 mW. For low temperature measurements, a Janis-make flow type liquid He cryostat was used with temperature stability of ± 1 K and for thermal contacts, GE-varnish was used. Before taking the spectra, each temperature point was given a stabilization time of 2 min. A pure silicon single crystal was also mounted as a reference along with the sample. The temperature dependence of the Raman shift of the Si single crystal followed the expected anharmonic behaviour which ensured temperature stability of the sample. Transmission 151Eu Mössbauer measurements were carried out using a conventional constant-acceleration spectrometer equipped with a WissEl velocity drive. The velocity scale was calibrated with a 57Co (Rh) source and a metallic iron foil at room temperature. The 151Eu source was kept at room temperature while the temperature of the absorber (300 to 25 K) placed inside Janis CCR was varied. The Mössbauer spectra were recorded at different temperatures so as to cover the transition temperature.
Fig. 1. Temperature dependence of magnetic susceptibility of Eu0.75Y0.25MnO3 measured using ZFC-FCW protocols under an applied field of 100 Oe, 10 kOe and 60 kOe.
magnetic field, the ZFC and FCW curves show bifurcation below T = 200 K before the system enters a magnetically ordered state as seen in Fig. 1. As the measuring field increases to 10 kOe, the ZFC-FCW bifurcation shifts to the temperature where the long range magnetic order sets in (TN1 = 47 K) and in 60 kOe magnetic field, ZFC-FCW curves merge up to ∼13 K. The bifurcation of ZFC-FCW curves below 13 K can be understood by considering presence of short range weak ferromagnetic correlations (FMC) at low temperatures (< 13 K). In order to deduce the nature of magnetic ordering, the inverse of magnetic susceptibility χ as a function of temperature is calculated from the FCW curve under 100 Oe magnetic field and is plotted in Fig. 2(a). For the same the Curie-Weiss (C-W) law χ = C/(T − θw) is employed, where C is the Curie constant, and θw the Curie temperature. It is observed that χ−1 follows Curie-Weiss (CW) law above a certain temperature, say TG ∼ 188 K, and the linear extrapolation of the CW law cuts the x-axis at θw = −161.32 K. The large negative value of θw confirms the existence of AFM ordering. We have also calculated the frustration factor f = |θw|/TN1 = 3.43 > 1 which implies the existence of frustrated magnetic structure in E0.75Y0.25MnO3 [15,16]. The linear high-T region of the inverse dc susceptibility starts deviating from the CW behaviour at temperature TG ∼ 188 K (Fig. 2(a)). It is also observed that when applied magnetic field is increased, the deviation of χ−1 from CW law is suppressed (Fig. 2(b) and (c)) and TG shows a shift from 188 K to 148 K and 128 K in the magnetic fields of 10 kOe and 60 kOe, respectively. Also, the magnitude of CW temperature decreases from −161.32 K to −15.56 K as field increases from 100 Oe to 60 kOe while the Néel temperature remains unchanged on application of field. This indicates that the phase between TG and TN1 (long range order) is not a pure paramagnetic phase, and some sort of local spin order sets in well above TN1. This feature in the present investigation may be a signature of Griffiths phase (GP) which has also been observed in variety of other systems as reported in literature [17–22]. A GP-like anomaly, i.e., the existence of short range ferromagnetic correlations (FMC) without any spontaneous magnetization, develops at TG ∼ 188 K, a temperature much higher than the ordering temperature of TN1. TG is indicative of non-analytical behaviour of magnetization where low field magnetization is dominated by FMC. The Griffiths singularity is characterized by a power law relation for inverse susceptibility, as obtained from following relation [23], χ −1 = (T − TNR )1 − λ , where λ is positive magnetic susceptibility exponent and its value should be in the limit of 0 ≤ λ ≤ 1 and ∼0.8 as per theoretical prediction of Castro Neto et al. [24] for the system exhibiting the Griffiths phase and TNR is the critical temperature of random
3. Results and discussion 3.1. Magnetic studies The DC magnetic susceptibility χ(T) measured over a temperature range of 3–300 K, under zero field cooled (ZFC) and field cooled warming (FCW) conditions, under three different magnetic fields H = 100 Oe, 10 kOe, and 60 kOe, is shown in Fig. 1. To obtain ZFC curve, the sample is cooled under zero magnetic field from 300 K and magnetization was measured during warming cycle. The FCW measurements were also carried out during the warming cycle, however, in this case, the sample is cooled in the presence of applied field. Both ZFC and FCW magnetization exhibit a peak at temperature TN1 = 47 K in all the three fields which denotes antiferromagnetic (AFM) ordering, and on further lowering the temperature, another AFM order with spiral spin arrangement sets in at TN2 = 22 K, in both 100 Oe and 10 kOe fields. Under 60 kOe magnetic field, the ZFC-FCW curves merge and the lower transition at TN2 = 22 K is suppressed, indicating that this field is able to suppress the spiral spin order. It is observed that under 100 Oe 39
Journal of Magnetism and Magnetic Materials 482 (2019) 38–43
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Fig. 2. Inverse of magnetic susceptibility as a function of temperature calculated from FCW curves measured under different magnetic fields (a) 100 Oe, (b) 10 kOe and (c) 60 kOe. Inset of (a) shows the plot of χ−1 vs {T/TNR ) − 1}at 100 Oe magnetic field.
indicating presence of FMC below and above TN1. However, the nonlinear behavior at 3 K is larger at higher fields indicating that the FM component dominates at low temperatures as inferred from the χ-T curve measured at 60 kOe.
ferromagnetism where susceptibility has a tendency to diverge. To determine λ, χ−1 is plotted as a function of the ({T/TNR ) − 1} (inset of the Fig. 2(a)) in logarithmic scale at 100 Oe. We followed the method described by Pramanik et al. [22] to estimate the value of TNR , which turns out to be 58 K. Using this TNR value, λ = λ GP is calculated. From slope of fitted linear region, the value of λ turns out to be λGP ∼ 0.842 ± 0.001 at 100 Oe which is in the limit of 0 ≤ λ ≤ 1 and in agreement with the theoretical prediction [24]. This indicates the likely presence of Griffiths phase between the TN1 and TG in Eu0.75Y0.25MnO3, which may be confirmed by further experiments. To further probe the magnetic state, the magnetization as a function of field M(H) was measured at T = 3 K, 8 K, 22 K, 35 K, and 50 K up to 90 kOe magnetic field and is shown in Fig. 3(a). The measurements were done after cooling the sample in zero field. The M (H) continues to increase with H and does not show any saturation even for 90 kOe magnetic field at all measured temperatures which shows that the compound is in predominantly antiferromagnetic state. However, a small hysteresis in M (H) curves is observed for temperatures below 50 K, the enlarged view of it is shown in the inset of Fig. 3(a). At low temperatures and low fields, a small opening of hysteresis loop is clearly seen below 50 K with very little coercive field Hc ∼ 0.5 kOe and remnant magnetization ∼0.1359 emu/gm at 3 K. The presence of hysteresis loop at low temperatures at low field and no saturation even in 90 kOe field indicate the coexistence of FMC and AFM phases in Eu0.75Y0.25MnO3. Fig. 3(b) gives the virgin curve of the M(H) measured at 3 K. The virgin curve passes through origin and shows a linear behavior up to 25 kOe indicating AFM ground state. As the field increases above 25 kOe, curves show non linearity up to 90 kOe field, indicating presence of FMC. The virgin curves measured up to 50 K showed similar nature
3.2. Raman spectroscopy Raman spectroscopy is an ideal tool to investigate the local structural changes due to magnetic ordering, defects, phase transition etc. [25–27]. It is also an effective tool to elucidate the coupling between spin and lattice orders. In order to investigate the changes in lattice due to magnetic order in Eu0.75Y0.25MO3, temperature dependent Raman spectroscopy was carried out in the spectral range of 320–720 cm−1 at various temperatures from 5 K to 300 K as shown in Fig. 4(a). All the spectra are corrected with Bose Einstein thermal correction factor. The overall spectra and positions of the various peaks matched well with the Raman spectra reported previously. No dramatic changes in the form of appearance and/or disappearance of Raman modes are seen in the spectra as a function of temperature. Thus, the temperature dependent Raman spectra negated any change in crystal symmetry in the measured temperature range. However, significant and anomalous shift in the mode positions is observed as a function of temperature. The strongest Raman modes in manganites occur for rotation, tilting or stretching of the MnO6 octahedra. The Raman modes are characterized following the previous reports [12,28]. Accordingly, the B2g(1) mode is assigned to a symmetric stretching of the MnO6 octahedra, the Ag(1) and Ag(3) to a mixture of an antisymmetric stretching and a MnO6 bending, Ag(4) to a MnO6 rotation, B2g(2) to a scissors like stretching and B2g(3) to a MnO6 bending. In addition a broad mode is observed at around 650 cm−1. Such a feature is reported in most of the
Fig. 3. (a) Field dependent magnetization measured at different temperatures, (b) Virgin curve plot for Eu0.75Y0.25MnO3 at 3 K. 40
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frequency obtained using a Lorentzian function are plotted as a function of temperature as shown in Fig. 5. It is clear from Fig. 5, the modes showed normal hardening as the temperature is reduced from room temperature till ∼200 K, below which the peak position showed a saturation before showing a observable softening behavior below 150–90 K temperature interval and significant softening at low temperatures. Granado et al. [30] reported the softening of B2g mode across TN in LaMnO3 which was attributed to the variation of lattice parameters around TN. However, in an another report, Granado et al. [13] considered the effect of variation in lattice parameter due to temperature and magnetostriction, spin-phonon coupling, and anharmonic potential on the temperature dependent Raman spectra. It is argued that due to phonons, the exchange interaction modulates and as the long range magnetic order sets in in the sample around the magnetic transition temperature, the exchange interactions in turn modulate the phonon frequencies. This modulation in phonon frequency across the magnetic transition temperature is regarded as spin phonon-coupling. From a comparison with lattice parameters, it was argued that anharmonic and magnetostriction effects do not play any role in the softening of the phonon mode. From the theoretical expressions, it was shown that spin–phonon coupling is mainly responsible for the observed softening. The peaks were fitted by Lorentz function and the peak position and width as a function of temperature are deduced. The T dependence of the various phonon modes is given in Fig. 5. The solid red line corresponds to the anharmonic function [31]:
2 ⎞ ω (T ) = ω (0) − C ⎛1 + x e − 1⎠ ⎝
Fig. 4. (a) Temperature dependent Raman spectra of Eu0.75Y0.25MnO3 compound recorded in warming condition from 5 K to 300 K, Lorentzian fit of Raman spectrum of Eu0.75Y0.25MnO3 compound recorded at (b) 5 K and (c) 200 K.
where ω (T) and ω(0) are modes frequency at Temperature T and 0 K ħω respectively, C is a constant and x = 2k 0T , ω0 is the Raman frequency B and T is the temperature in Kelvin, which describes the expected temperature dependence of the phonon frequency solely based on phonon–phonon decay. It is clear from the Fig. 5 that the phonon mode position deviates significantly from the expected anharmonic behavior at low temperatures, confirming softening of the phonons. The deviation for most of the modes is below 90 K where the phonon frequencies start softening. However, for B2g (1) mode, the deviation starts below 130 K. The
manganites and is assigned as a zone boundary one-phonon mode which becomes Raman active through local disorder [29]. Fig. 4(b) and (c) show fitted the Raman spectra in the spectral range of 320–720 cm−1 at 5 K and 200 K, respectively. All the Raman modes are fitted using the Lorentzian line shape function as shown by red and green lines in Fig. 4(b) and (c). The various fitted phonon modes
Fig. 5. Temperature dependence of the modes Ag(4), Ag(3), Ag(1) and B2g(1). The solid lines are the fitted curves as described in the text. 41
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Fig. 6. Mössbauer spectra collected at different temperatures above and below TN1.
phonon frequency shows a significant softening of around 3–4 cm−1 for most of the modes, indicating strong spin-phonon coupling in this system. It is worth noting that in the regime of softening, the mode position as a function of temperature showed a distinct deviation at TN1 and TN2 for all the modes (highlighted by dotted lines). This showed that the Raman mode frequency is sensitive to the antiferromagnetic order. Thus, we can conclude that most of the phonons show softening well above the magnetic ordering temperature. The significant phonon softening seen in the present experiment thus represents strong spinphonon coupling which is related to the magnetic correlations present in the compound. 3.3. Mössbauer study The Mössbauer spectra were recorded at different temperatures in the range 25–300 K and are shown in Fig. 6. The observed Mössbauer spectrum, which is almost a symmetric line, is similar to that observed by M. Lantieri et al. [32], but with relatively broader lines (Fig. 6). One may note that with temperature, there is a considerable change in the width of the lines. However, in view of the relatively lower statistics in the present work, we have adopted a simple method for analyzing the data by fitting with a Lorentzian line and by keeping the width and centre as free parameters. The variation in width and centre can be envisaged as the hyperfine field and centre shift variation, respectively. It is to be noted that as Y and Eu are expected to be non-magnetic, the observed hyperfine field at Eu site is essentially the transferred field from the magnetic ordering of Mn ions in the studied compound. The obtained variation of centre shift (CS) and the width (Γ) are plotted in Fig. 7(b) and (c), respectively, and for better comparison, the temperature dependence of the DC magnetic susceptibility is also plotted in the same figure (Fig. 7(a)). The observed CS values confirm the presence of Eu3+ in the studied sample, as expected. Usually, the measured centre shift is a sum of isomer shift and second order Doppler shift (δ ≡ δIS + δSOD). The observed CS variation is fitted with a straight line from the high temperature data and one can clearly see the anomalous variation of CS between 30 and 100 K. The CS is found to increase with decrease of temperature below 100 K, reaching a maximum at about 70 K with further decrease till the lowest studied
Fig. 7. (a) DC-χ measured under 100 Oe magnetic field as a function of temperature, (b) Temperature dependence of Centre Shift deduced from Mössbauer spectra, (c) temperature dependence of the line width (hyperfine field).
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temperature of 25 K. This observation is inferred as change in the electronic configuration of Eu3+ as temperature approaches that of TN1. It is worth noting here that the temperature where the peak width showed a maximum value is concomitant with the phonon softening temperature shown by the B2g mode. The further change in the peak width or the upturn below 40 K again matches with the change in slope seen in the phonon frequency vs temperature behaviour. The increase of the hyperfine field (taken as width of the spectrum, Γ) below 47 K due to the onset of antiferromagnetic ordering below TN1 is consistent with the phase diagram of the studied compound. As the Mössbauer spectra were measured only till 25 K, we could not see the signatures of TN2 in the present work. However, the observed hyperfine field shows some remarkable changes between the temperatures of 50–200 K. Below 200 K, the hyperfine field increases sharply, reaching a maximum at about 130 K, and then decreasing at a slower rate till 47 K, and again increasing below TN1. The sharp increase of hyperfine field below 200 K matches with the bifurcation between ZFC and FC magnetization data (TG ∼ 188 K) which showed signatures of Griffiths phase between TG and TN1. Further, the Raman spectra showed softening of phonon modes below 100 K where the hyperfine field shows a decrease. Both these observations clearly show that the spin-phonon coupling observed in present case has origin in magnetic states of the compound.
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4. Conclusion Detail investigations were carried out using magnetization (χ vs T, χ−1 vs T and M(H)), Mossbauer spectroscopy and Raman spectroscopy to determine the possible origin of spin-phonon interaction in Eu0.75Y0.25MnO3. The magnetization and Mossbauer analysis showed that the long range magnetic order (antiferromagnetic) sets in below 47 K and 22 K. However, a short range Griffiths like ferromagnetic correlation concomitant with magnetic frustration sets in well above TN1. These short range correlations are responsible for the phonon frequency renormalization observed well above TN1. Acknowledgements One of the authors (SG) would like to acknowledge UGC-DAE CSR for financial assistance. References [1] I.A. Sergienko, E. Dagotto, Phys. Rev. B 73 (2006) 094434.
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Research articles
Griffiths phase-like behavior and origin of spin-phonon interaction in Eu0.75Y0.25MnO3 Surbhi Guptaa, Gaurav Sharmab, V.R. Reddyb, V.G. Satheb, V. Siruguria, a b
T
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UGC-DAE Consortium for Scientific Research Mumbai Centre, 246C, CFB, BARC Campus, Mumbai 400085, India UGC-DAE Consortium for Scientific Research Indore Centre, 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: Manganites Magnetization Raman spectroscopy Mossobauer spectroscopy
The magnetic state and the spin-phonon coupling in the Eu0.75Y0.25MnO3 are investigated using temperature dependent magnetization, Raman spectroscopy and Mossbauer spectroscopy measurements. The magnetization data showed bifurcation in zero-field cooled and field-cooled warming curves and indicate two antiferromagnetic transitions at 47 K and 22 K. Short range ferromagnetic correlations are clearly observed well above the antiferromagnetic transition temperatures, which are attributed to the presence of Griffiths-like phase. The temperature dependent Raman spectroscopy measurements showed anomalous softening of phonon modes well above the antiferromagnetic ordering temperature of ∼47 K and also showed signatures of both the ordering temperatures. The Mossbauer spectra showed anomalous changes in hyperfine field as a function of temperature which match with the temperature at which B2g phonons showed softening.
1. Introduction Recent extensive studies show that the noncollinear magnetic structures such as a spin helix are essential to host the ferroelectricity in multiferroics [1,2]. In this type of multiferroics, the space inversion symmetry is broken by the magnetic order. Based on this strategy, many multiferroic materials have been found so far [3–6]. Here, a spiral magnetic order or a charge order along a direction results in alternate ferromagnetic and anti-ferromagnetic ordering that generates polarization. Strong candidates in this class are manganites like TbMnO3, DyMnO3 that show ferroelectricity due to spiral magnetic ordering [2,7–9]. Because of the orbital ordering of Mn3+ ions in orthorhombic charge ordered RMnO3, the exchange interaction among the neighbouring spins is ferromagnetic (FM) in the a–b plane and antiferromagnetic (AFM) along the c-axis. Consistently, spins in each a–b plane of LaMnO3 order ferromagnetically and the magnetization direction alternates along the c-axis (TN = 150 K). The replacement of La by smaller ions such as Tb or Dy increases the structural distortion, inducing next-nearest-neighbor AFM exchange in the a–b plane comparable to the nearest-neighbor FM exchange. This frustrates the FM ordering of spins in the a–b plane, and below ∼42 K, Tb(Dy)MnO3 shows an incommensurate magnetic ordering with a collinear sinusoidal modulation along the b-axis, which is paraelectric. However, as the temperature is lowered, the magnetization grows in magnitude and
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a spiral state with rotating spins set in at ∼28 K which is energetically more favorable, and induces ferroelectricity [10]. Eu0.75Y0.25MnO3 presents a unique opportunity to study the role of distortion induced due to rare earth ions. The composite ionic radii in this compound are nearly equal to that in TbMnO3. This compound also shows two magnetic transitions, one at 47 K and the other one at 22 K. Thus, Eu1−xYxMnO3 allows tuning of the lattice and magnetic properties, forming a multiferroic manganite compound without the magnetic interference of the rare-earth ions, and it is shown that magnetoelectric contribution to the dielectric permittivity can be observed in all the compositions in crystallographic a-direction only [11]. Spin-lattice coupling is important to judge the interaction among various degrees of freedom particularly magnetic and electric order in this case. In this compound where the ferroelectricity is generated due to frustrated (spiral) magnetic order, it is expected to show strong spin lattice coupling. The spin phonon coupling is reported previously in Eu1−xYxMnO3 series of compounds [12] and it is shown that the coupling exists predominantly within the MnO2 plane, manifesting itself by softening of the phonon modes in Raman spectroscopy. It is shown that even for increasing Y concertation, spin-phonon coupling exists without presence of proper ferromagnetic order in any direction. The spinphonon coupling is supposed to be linked with the magnetic ordering transition. According to Granado et al. [13], the phonon frequency renormalization is proportional to the spin–spin correlation
Corresponding author. E-mail address: [email protected] (V. Siruguri).
https://doi.org/10.1016/j.jmmm.2019.03.050 Received 26 October 2018; Received in revised form 31 January 2019; Accepted 7 March 2019 Available online 08 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
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function < Si Sj > for the near neighbour spins localized at the Mn3+ ions. On the contrary, in most of the studies on Eu1−xYxMnO3 compounds, the spin-phonon coupling, i.e. the phonon softening is observed below ∼100 K which is well above the antiferromagnetic transition temperatures reported for this series of compounds. Therefore, in this report, we studied the various properties of Eu0.75Y0.25MnO3 compound in an attempt to understand the cause of spin-phonon coupling or phonon softening well above the observed magnetic ordering temperature. 2. Experimental details The polycrystalline Eu0.75Y0.25MnO3 sample was prepared using conventional solid-state route at ambient pressure. Stoichiometric ratios of high purity (≥99.9%) Eu2O3, Y2O3 and MnCO3 were mixed well, ground and calcined at 1100 °C for 24 h. The reacted mixture was again ground and heated at 1300 °C for further 24 h. The resulting powders were then pressed into pellets and sintered at 1300 °C. The sample purity was characterized by X-ray diffraction (XRD) [14]. The temperature and field dependence DC magnetisation measurements were carried out on a commercial vibrating sample magnetometer (VSM) coupled to a 90 kOe physical property measurement system (PPMS) (Model: 6000, Quantum Design, USA) in the temperature range of 3 K–300 K. Raman spectra were taken from 5 K to 300 K using Horiba JY HR800 micro-Raman system in unpolarized configuration. The system is equipped with an excitation source Ar+ laser (488 nm), 1800 grooves/ mm grating, an edge filter for Rayleigh line rejection and a Peltier cooled CCD detector. The overall spectral resolution of the system is ∼1 cm−1 and laser power on the sample was kept below 2 mW. For low temperature measurements, a Janis-make flow type liquid He cryostat was used with temperature stability of ± 1 K and for thermal contacts, GE-varnish was used. Before taking the spectra, each temperature point was given a stabilization time of 2 min. A pure silicon single crystal was also mounted as a reference along with the sample. The temperature dependence of the Raman shift of the Si single crystal followed the expected anharmonic behaviour which ensured temperature stability of the sample. Transmission 151Eu Mössbauer measurements were carried out using a conventional constant-acceleration spectrometer equipped with a WissEl velocity drive. The velocity scale was calibrated with a 57Co (Rh) source and a metallic iron foil at room temperature. The 151Eu source was kept at room temperature while the temperature of the absorber (300 to 25 K) placed inside Janis CCR was varied. The Mössbauer spectra were recorded at different temperatures so as to cover the transition temperature.
Fig. 1. Temperature dependence of magnetic susceptibility of Eu0.75Y0.25MnO3 measured using ZFC-FCW protocols under an applied field of 100 Oe, 10 kOe and 60 kOe.
magnetic field, the ZFC and FCW curves show bifurcation below T = 200 K before the system enters a magnetically ordered state as seen in Fig. 1. As the measuring field increases to 10 kOe, the ZFC-FCW bifurcation shifts to the temperature where the long range magnetic order sets in (TN1 = 47 K) and in 60 kOe magnetic field, ZFC-FCW curves merge up to ∼13 K. The bifurcation of ZFC-FCW curves below 13 K can be understood by considering presence of short range weak ferromagnetic correlations (FMC) at low temperatures (< 13 K). In order to deduce the nature of magnetic ordering, the inverse of magnetic susceptibility χ as a function of temperature is calculated from the FCW curve under 100 Oe magnetic field and is plotted in Fig. 2(a). For the same the Curie-Weiss (C-W) law χ = C/(T − θw) is employed, where C is the Curie constant, and θw the Curie temperature. It is observed that χ−1 follows Curie-Weiss (CW) law above a certain temperature, say TG ∼ 188 K, and the linear extrapolation of the CW law cuts the x-axis at θw = −161.32 K. The large negative value of θw confirms the existence of AFM ordering. We have also calculated the frustration factor f = |θw|/TN1 = 3.43 > 1 which implies the existence of frustrated magnetic structure in E0.75Y0.25MnO3 [15,16]. The linear high-T region of the inverse dc susceptibility starts deviating from the CW behaviour at temperature TG ∼ 188 K (Fig. 2(a)). It is also observed that when applied magnetic field is increased, the deviation of χ−1 from CW law is suppressed (Fig. 2(b) and (c)) and TG shows a shift from 188 K to 148 K and 128 K in the magnetic fields of 10 kOe and 60 kOe, respectively. Also, the magnitude of CW temperature decreases from −161.32 K to −15.56 K as field increases from 100 Oe to 60 kOe while the Néel temperature remains unchanged on application of field. This indicates that the phase between TG and TN1 (long range order) is not a pure paramagnetic phase, and some sort of local spin order sets in well above TN1. This feature in the present investigation may be a signature of Griffiths phase (GP) which has also been observed in variety of other systems as reported in literature [17–22]. A GP-like anomaly, i.e., the existence of short range ferromagnetic correlations (FMC) without any spontaneous magnetization, develops at TG ∼ 188 K, a temperature much higher than the ordering temperature of TN1. TG is indicative of non-analytical behaviour of magnetization where low field magnetization is dominated by FMC. The Griffiths singularity is characterized by a power law relation for inverse susceptibility, as obtained from following relation [23], χ −1 = (T − TNR )1 − λ , where λ is positive magnetic susceptibility exponent and its value should be in the limit of 0 ≤ λ ≤ 1 and ∼0.8 as per theoretical prediction of Castro Neto et al. [24] for the system exhibiting the Griffiths phase and TNR is the critical temperature of random
3. Results and discussion 3.1. Magnetic studies The DC magnetic susceptibility χ(T) measured over a temperature range of 3–300 K, under zero field cooled (ZFC) and field cooled warming (FCW) conditions, under three different magnetic fields H = 100 Oe, 10 kOe, and 60 kOe, is shown in Fig. 1. To obtain ZFC curve, the sample is cooled under zero magnetic field from 300 K and magnetization was measured during warming cycle. The FCW measurements were also carried out during the warming cycle, however, in this case, the sample is cooled in the presence of applied field. Both ZFC and FCW magnetization exhibit a peak at temperature TN1 = 47 K in all the three fields which denotes antiferromagnetic (AFM) ordering, and on further lowering the temperature, another AFM order with spiral spin arrangement sets in at TN2 = 22 K, in both 100 Oe and 10 kOe fields. Under 60 kOe magnetic field, the ZFC-FCW curves merge and the lower transition at TN2 = 22 K is suppressed, indicating that this field is able to suppress the spiral spin order. It is observed that under 100 Oe 39
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Fig. 2. Inverse of magnetic susceptibility as a function of temperature calculated from FCW curves measured under different magnetic fields (a) 100 Oe, (b) 10 kOe and (c) 60 kOe. Inset of (a) shows the plot of χ−1 vs {T/TNR ) − 1}at 100 Oe magnetic field.
indicating presence of FMC below and above TN1. However, the nonlinear behavior at 3 K is larger at higher fields indicating that the FM component dominates at low temperatures as inferred from the χ-T curve measured at 60 kOe.
ferromagnetism where susceptibility has a tendency to diverge. To determine λ, χ−1 is plotted as a function of the ({T/TNR ) − 1} (inset of the Fig. 2(a)) in logarithmic scale at 100 Oe. We followed the method described by Pramanik et al. [22] to estimate the value of TNR , which turns out to be 58 K. Using this TNR value, λ = λ GP is calculated. From slope of fitted linear region, the value of λ turns out to be λGP ∼ 0.842 ± 0.001 at 100 Oe which is in the limit of 0 ≤ λ ≤ 1 and in agreement with the theoretical prediction [24]. This indicates the likely presence of Griffiths phase between the TN1 and TG in Eu0.75Y0.25MnO3, which may be confirmed by further experiments. To further probe the magnetic state, the magnetization as a function of field M(H) was measured at T = 3 K, 8 K, 22 K, 35 K, and 50 K up to 90 kOe magnetic field and is shown in Fig. 3(a). The measurements were done after cooling the sample in zero field. The M (H) continues to increase with H and does not show any saturation even for 90 kOe magnetic field at all measured temperatures which shows that the compound is in predominantly antiferromagnetic state. However, a small hysteresis in M (H) curves is observed for temperatures below 50 K, the enlarged view of it is shown in the inset of Fig. 3(a). At low temperatures and low fields, a small opening of hysteresis loop is clearly seen below 50 K with very little coercive field Hc ∼ 0.5 kOe and remnant magnetization ∼0.1359 emu/gm at 3 K. The presence of hysteresis loop at low temperatures at low field and no saturation even in 90 kOe field indicate the coexistence of FMC and AFM phases in Eu0.75Y0.25MnO3. Fig. 3(b) gives the virgin curve of the M(H) measured at 3 K. The virgin curve passes through origin and shows a linear behavior up to 25 kOe indicating AFM ground state. As the field increases above 25 kOe, curves show non linearity up to 90 kOe field, indicating presence of FMC. The virgin curves measured up to 50 K showed similar nature
3.2. Raman spectroscopy Raman spectroscopy is an ideal tool to investigate the local structural changes due to magnetic ordering, defects, phase transition etc. [25–27]. It is also an effective tool to elucidate the coupling between spin and lattice orders. In order to investigate the changes in lattice due to magnetic order in Eu0.75Y0.25MO3, temperature dependent Raman spectroscopy was carried out in the spectral range of 320–720 cm−1 at various temperatures from 5 K to 300 K as shown in Fig. 4(a). All the spectra are corrected with Bose Einstein thermal correction factor. The overall spectra and positions of the various peaks matched well with the Raman spectra reported previously. No dramatic changes in the form of appearance and/or disappearance of Raman modes are seen in the spectra as a function of temperature. Thus, the temperature dependent Raman spectra negated any change in crystal symmetry in the measured temperature range. However, significant and anomalous shift in the mode positions is observed as a function of temperature. The strongest Raman modes in manganites occur for rotation, tilting or stretching of the MnO6 octahedra. The Raman modes are characterized following the previous reports [12,28]. Accordingly, the B2g(1) mode is assigned to a symmetric stretching of the MnO6 octahedra, the Ag(1) and Ag(3) to a mixture of an antisymmetric stretching and a MnO6 bending, Ag(4) to a MnO6 rotation, B2g(2) to a scissors like stretching and B2g(3) to a MnO6 bending. In addition a broad mode is observed at around 650 cm−1. Such a feature is reported in most of the
Fig. 3. (a) Field dependent magnetization measured at different temperatures, (b) Virgin curve plot for Eu0.75Y0.25MnO3 at 3 K. 40
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frequency obtained using a Lorentzian function are plotted as a function of temperature as shown in Fig. 5. It is clear from Fig. 5, the modes showed normal hardening as the temperature is reduced from room temperature till ∼200 K, below which the peak position showed a saturation before showing a observable softening behavior below 150–90 K temperature interval and significant softening at low temperatures. Granado et al. [30] reported the softening of B2g mode across TN in LaMnO3 which was attributed to the variation of lattice parameters around TN. However, in an another report, Granado et al. [13] considered the effect of variation in lattice parameter due to temperature and magnetostriction, spin-phonon coupling, and anharmonic potential on the temperature dependent Raman spectra. It is argued that due to phonons, the exchange interaction modulates and as the long range magnetic order sets in in the sample around the magnetic transition temperature, the exchange interactions in turn modulate the phonon frequencies. This modulation in phonon frequency across the magnetic transition temperature is regarded as spin phonon-coupling. From a comparison with lattice parameters, it was argued that anharmonic and magnetostriction effects do not play any role in the softening of the phonon mode. From the theoretical expressions, it was shown that spin–phonon coupling is mainly responsible for the observed softening. The peaks were fitted by Lorentz function and the peak position and width as a function of temperature are deduced. The T dependence of the various phonon modes is given in Fig. 5. The solid red line corresponds to the anharmonic function [31]:
2 ⎞ ω (T ) = ω (0) − C ⎛1 + x e − 1⎠ ⎝
Fig. 4. (a) Temperature dependent Raman spectra of Eu0.75Y0.25MnO3 compound recorded in warming condition from 5 K to 300 K, Lorentzian fit of Raman spectrum of Eu0.75Y0.25MnO3 compound recorded at (b) 5 K and (c) 200 K.
where ω (T) and ω(0) are modes frequency at Temperature T and 0 K ħω respectively, C is a constant and x = 2k 0T , ω0 is the Raman frequency B and T is the temperature in Kelvin, which describes the expected temperature dependence of the phonon frequency solely based on phonon–phonon decay. It is clear from the Fig. 5 that the phonon mode position deviates significantly from the expected anharmonic behavior at low temperatures, confirming softening of the phonons. The deviation for most of the modes is below 90 K where the phonon frequencies start softening. However, for B2g (1) mode, the deviation starts below 130 K. The
manganites and is assigned as a zone boundary one-phonon mode which becomes Raman active through local disorder [29]. Fig. 4(b) and (c) show fitted the Raman spectra in the spectral range of 320–720 cm−1 at 5 K and 200 K, respectively. All the Raman modes are fitted using the Lorentzian line shape function as shown by red and green lines in Fig. 4(b) and (c). The various fitted phonon modes
Fig. 5. Temperature dependence of the modes Ag(4), Ag(3), Ag(1) and B2g(1). The solid lines are the fitted curves as described in the text. 41
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Fig. 6. Mössbauer spectra collected at different temperatures above and below TN1.
phonon frequency shows a significant softening of around 3–4 cm−1 for most of the modes, indicating strong spin-phonon coupling in this system. It is worth noting that in the regime of softening, the mode position as a function of temperature showed a distinct deviation at TN1 and TN2 for all the modes (highlighted by dotted lines). This showed that the Raman mode frequency is sensitive to the antiferromagnetic order. Thus, we can conclude that most of the phonons show softening well above the magnetic ordering temperature. The significant phonon softening seen in the present experiment thus represents strong spinphonon coupling which is related to the magnetic correlations present in the compound. 3.3. Mössbauer study The Mössbauer spectra were recorded at different temperatures in the range 25–300 K and are shown in Fig. 6. The observed Mössbauer spectrum, which is almost a symmetric line, is similar to that observed by M. Lantieri et al. [32], but with relatively broader lines (Fig. 6). One may note that with temperature, there is a considerable change in the width of the lines. However, in view of the relatively lower statistics in the present work, we have adopted a simple method for analyzing the data by fitting with a Lorentzian line and by keeping the width and centre as free parameters. The variation in width and centre can be envisaged as the hyperfine field and centre shift variation, respectively. It is to be noted that as Y and Eu are expected to be non-magnetic, the observed hyperfine field at Eu site is essentially the transferred field from the magnetic ordering of Mn ions in the studied compound. The obtained variation of centre shift (CS) and the width (Γ) are plotted in Fig. 7(b) and (c), respectively, and for better comparison, the temperature dependence of the DC magnetic susceptibility is also plotted in the same figure (Fig. 7(a)). The observed CS values confirm the presence of Eu3+ in the studied sample, as expected. Usually, the measured centre shift is a sum of isomer shift and second order Doppler shift (δ ≡ δIS + δSOD). The observed CS variation is fitted with a straight line from the high temperature data and one can clearly see the anomalous variation of CS between 30 and 100 K. The CS is found to increase with decrease of temperature below 100 K, reaching a maximum at about 70 K with further decrease till the lowest studied
Fig. 7. (a) DC-χ measured under 100 Oe magnetic field as a function of temperature, (b) Temperature dependence of Centre Shift deduced from Mössbauer spectra, (c) temperature dependence of the line width (hyperfine field).
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temperature of 25 K. This observation is inferred as change in the electronic configuration of Eu3+ as temperature approaches that of TN1. It is worth noting here that the temperature where the peak width showed a maximum value is concomitant with the phonon softening temperature shown by the B2g mode. The further change in the peak width or the upturn below 40 K again matches with the change in slope seen in the phonon frequency vs temperature behaviour. The increase of the hyperfine field (taken as width of the spectrum, Γ) below 47 K due to the onset of antiferromagnetic ordering below TN1 is consistent with the phase diagram of the studied compound. As the Mössbauer spectra were measured only till 25 K, we could not see the signatures of TN2 in the present work. However, the observed hyperfine field shows some remarkable changes between the temperatures of 50–200 K. Below 200 K, the hyperfine field increases sharply, reaching a maximum at about 130 K, and then decreasing at a slower rate till 47 K, and again increasing below TN1. The sharp increase of hyperfine field below 200 K matches with the bifurcation between ZFC and FC magnetization data (TG ∼ 188 K) which showed signatures of Griffiths phase between TG and TN1. Further, the Raman spectra showed softening of phonon modes below 100 K where the hyperfine field shows a decrease. Both these observations clearly show that the spin-phonon coupling observed in present case has origin in magnetic states of the compound.
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4. Conclusion Detail investigations were carried out using magnetization (χ vs T, χ−1 vs T and M(H)), Mossbauer spectroscopy and Raman spectroscopy to determine the possible origin of spin-phonon interaction in Eu0.75Y0.25MnO3. The magnetization and Mossbauer analysis showed that the long range magnetic order (antiferromagnetic) sets in below 47 K and 22 K. However, a short range Griffiths like ferromagnetic correlation concomitant with magnetic frustration sets in well above TN1. These short range correlations are responsible for the phonon frequency renormalization observed well above TN1. Acknowledgements One of the authors (SG) would like to acknowledge UGC-DAE CSR for financial assistance. References [1] I.A. Sergienko, E. Dagotto, Phys. Rev. B 73 (2006) 094434.
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