- Email: [email protected]
Room temperature colossal magnetodielectric effect in La0.4Gd0.1Ca0.5MnO3
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Room temperature colossal magnetodielectric effect in La0.4Gd0.1Ca0.5MnO3 A. Krichenea,∗, W. Boujelbena, K.N. Rathodb, K. Gadanib, N.A. Shahb, P.S. Solankib a b
Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, Université de Sfax, B.P. 1171, 3000, Sfax, Tunisia Department of Physics, Saurashtra University, Rajkot, 360 005, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Manganite Magnetodielectric effect Charge ordering
We have studied the contribution induced by a low magnetic field (0.6T and 1.2T) on the frequency dependent permittivity, impedance and conductivity recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 polycrystalline compound. Strong magnetodielectric (MD) coupling has been detected at room temperature. Colossal value of negative MD (more than 50%) has been observed under 1.2T field. The MD response is found to be stable in the studied frequency range (up to 2 MHz) suggesting the possibility of using this compound for technological applications at room temperature. The significant MD coupling has been correlated with the presence of charge ordering transition near room temperature.
1. Introduction Multifunctional materials are now the flagship research topic for researchers and scientists. Various perovskite manganites have been well characterized for several physical properties such as magnetocaloric effect [1,2], colossal magnetoresistance [3,4], charge and orbital orderings [1–7], MD coupling [5–7], etc. Half–doped manganites with the equal amounts of Mn3+ and Mn4+ ions may exhibit complex phenomena related to the high correlations between electrons and lattice. During last two decades, few attempts have been successfully made in order to understand the electrical nature of various charge–ordered manganite compounds [7–12]. Recent studies [1–3], carried out for polycrystalline charge–ordered La0.4Gd0.1Ca0.5MnO3 manganite compound, have shown that this compound exhibits a paramagnetic–ferromagnetic (insulator–metal) transition at Curie temperature TC = 100 K. The magnetic structure below TC has been described as a charge–ordered antiferromagnetic matrix in which some ferromagnetic domains exist. Around TC, these ferromagnetic domains induce a considerably large magnetocaloric effect [1,2]. In addition, magnetic phase separation phenomenon has been identified as a responsible cause for the colossal magnetoresistive behavior of this compound [3]. Studies have also revealed the persistence of charge–ordered antiferromagnetic domains above TC with the detection of interesting phenomena such as kinetic arrest and training effect. The search for multifunctional materials has recently triggered the investigations on MD coupling in order to make possible the supplementary physical applications [5–7]. Since, La0.4Gd0.1Ca0.5MnO3
∗
compound has been thoroughly investigated for its structural, magnetic, magnetocaloric, transport and magnetotransport behaviors, it is worth, now, to concentrate on the understanding of the possible MD coupling exhibited by the same compound for its practical physical applications. Any practical application as well as any device performance requires the material with possibly large MD effect under low applied magnetic field at or near the room temperature, since large magnetic field requirement and low temperature condition for obtaining large MD effect are the bottleneck for practical use of the materials. In addition, large leakage or electrical loss also hinders the practical applications of any particular material. In this context, we have studied the effect of an external constant magnetic field on the permittivity, loss, conductivity and impedance behaviors recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. Main aim of this study is to identify any possible strength of this well studied La0.4Gd0.1Ca0.5MnO3 compound for better practical applications. 2. Experimental detail Polycrystalline La0.4Gd0.1Ca0.5MnO3 compound was prepared using conventional solid state reaction method of ceramics, as reported previously [1]. Rietveld refinement of the X–ray diffraction pattern for the studied sample has confirmed that it crystallizes according to orthorhombic Pnma (no. 62) space group with the cell parameters: a = 5.416(5) Å, b = 7.624(2) Å and c = 5.405(3) Å [1]. Magnetic field dependence of magnetization was carried out at 280 and 300 K by using a SQUID vibrating sample magnetometer (VSM; Quantum Design). Temperature dependence of resistivity under zero and 10 T applied
Corresponding author. E-mail address: [email protected] (A. Krichene).
https://doi.org/10.1016/j.ceramint.2019.12.023 Received 11 September 2019; Received in revised form 1 November 2019; Accepted 2 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A. Krichene, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.023
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
52% of negative MD effect is reached under 1.2 T field near 2 MHz. As per our best knowledge, such negative values of MD effect are recorded for the first time for LaCaMnO3 based half doped manganite at room temperature. However, Great positive MD effect was observed at room temperature for YMnO3 [5]. In addition to this, few reports exist on the observation of large MD effect for mixed valent high resistive manganite compounds [6,13–16]. Although, all these compounds either require composite form, low temperature, high magnetic field or provide less than 50% MD effect at sufficiently high temperatures (near to room temperature). The colossal values as well as the stability of the MD response of our sample in the studied frequency range suggest the possibility of using this sample for technological applications at room temperature. Reduction in permittivity with applied magnetic field can be understood as: under applied frequency/ac electric field, dipoles consist of Mn3+ and Mn4+ ions get oriented along the direction of the external field. With increase in frequency, dipoles realize fatigued that is responsible for the reduction in permittivity with increase in frequency (Fig. 1). When magnetic field is applied to the compound understudy, it strives to align the magnetic moment of La0.4Gd0.1Ca0.5MnO3 compound in its direction. In this context, interruption has been generated in the dipole alignment under applied ac electric field/frequency thereby magnetic field (at each applied frequency) reduces the permittivity of the compound understudy. In addition, magnetic field can introduce conduction of eg electrons between Mn3+ and Mn4+ across the La0.4Gd0.1Ca0.5MnO3 manganite lattice thereby enhances the leakage across the same and, hence, permittivity gets suppressed or negative MD effect comes into the picture under the application of external magnetic field, as shown in inset (b) of Fig. 1. Simultaneous existence of (i) conduction across the La0.4Gd0.1Ca0.5MnO3 manganite lattice (through possible oxygen vacancies across the lattice under zero applied field and through active zener double exchange under applied magnetic fields) as well as (ii) dipolar image of different valence states of manganese ions under applied frequency has been confirmed by performing universal dielectric response fittings to the permittivity data, as shown in the inset (a) of Fig. 1. Straight line fits confirm the simultaneous existence of process (i) (conduction across the lattice) and process (ii) (dipole alignment under frequency). In order to understand the effect of low applied external constant magnetic field on the conduction induced leakage processes across the La0.4Gd0.1Ca0.5MnO3 compound lattice, frequency dependent imaginary permittivity was recorded for the same frequency range at room temperature. Fig. 2 shows the variation in dielectric loss (tan δ = ε′′/ε′) with frequency under 0, 0.6 T and 1.2 T applied constant magnetic fields at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. Under all applied constant magnetic fields, dielectric loss initially increases with frequency up to the frequency ~0.2 MHz followed by reduction in its value up to ~0.54 MHz. This non–monotonous variation (below ~ 0.54 MHz) can be ascribed to a presence of relaxation of dipoles at particular frequencies. There is no significant change in the frequency peak (at ~ 0.19 MHz) of loss plots upon the application of magnetic field that confirms no effect of magnetic field in altering the relaxation process of the compound understudy. Above ~0.54 MHz frequency, loss remains almost constant with effective and considerable fluctuations in its values under zero and all applied constant magnetic fields (well above ~ 0.54 MHz frequency). This represents an existence of the complex picture of relaxation processes (i.e. strongly frequency dependent multiple relaxation processes). No considerable changes in the frequency dependent fluctuations in dielectric loss values upon increase in applied external constant magnetic field suggests that the multiple complex relaxation processes do not easily get affected by an external constant applied magnetic field at room temperature. This clearly indicates that observed remarkably high MD effect [inset (b) of Fig. 1] is an extrinsic in nature for the presently studied La0.4Gd0.1Ca0.5MnO3 compound. Overall value of dielectric loss gets enhanced with increase in applied constant magnetic field, without any
Fig. 1. Frequency dependence of the real part of dielectric permittivity recorded at room temperature under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. The solid lines represent the fitting curves according to Cole–Cole Model. The (a) inset shows the volution of log (f ε′) versus log f. The (b) inset shows the frequency dependence of MD effect at room temperature.
magnetic field was determined by standard four–probe technique by using Physical Property Measurement System (PPMS; Quantum Design, 14T model) in the temperature range 200 K–300 K. The frequency dependence of permittivity, loss, impedance and conductivity were measured at room temperature (T = 298 K) via Agilent E4980A precision LCR meter in the frequency range 10 kHz–2 MHz under applied magnetic field values up to 1.2 T. 3. Results and discussion Fig. 1 shows the evolution of the real part of dielectric permittivity (ε′) as a function of frequency (f) at room temperature under 0, 0.6 T and 1.2 T applied magnetic fields for La0.4Gd0.1Ca0.5MnO3 sample. All the curves show the typical evolution, characterized by the decrease of ε′ as f increases throughout the range studied because the dipoles response gets suppressed at higher f values. The decrease in ε′ is more significant at lower frequencies due to Maxwell–Wagner effect. The evolution of ε′ has been found to follow Cole–Cole model given by:
ε = ε∞ +
ε0 − ε∞ 1 + (jfτ )1 − α
(1)
where ε0 is the static permittivity, ε∞ is the high frequency permittivity, f is the frequency, τ is the relaxation time and α exponent is the measure of the distribution of relaxation time. Fitted curves are represented by solid lines in Fig. 1. All the curves follow the universal dielectric response model, which is evident through the linear behavior exhibited by the curves in the inset (a) of Fig. 1. With the application of an external constant magnetic field, ε′ sharply decreases. This fact confirms the presence of strong MD correlation inside the structure of the studied specimen. For 1.2 T magnetic field, ε′ decreases by half indicating a strong field–induced weakening of insulating tendencies. The MD effect can be quantified by using the following relation:
MD (%) =
ε′ (H ) − ε′ (0) × 100% ε′ (0)
(2)
with ε′ (0) and ε′ (H) are the permittivity values recorded under zero and applied magnetic field (H), respectively. The MD response of our sample is shown in the (b) inset of Fig. 1. Colossal values of negative MD effect are recorded near room temperature, i.e. more than 25% for 0.6 T and more than 50% under 1.2 T applied magnetic field. About 2
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
Fig. 2. Frequency dependence of dielectric loss recorded at room temperature under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. The inset shows the frequency dependence of magnetoloss at room temperature.
Fig. 3. Magnetic field dependence of magnetization near room temperature for La0.4Gd0.1Ca0.5MnO3 sample. Lower inset shows the temperature dependence of resistivity above 200 K under 0 and 10 T magnetic fields. The upper inset shows the temperature dependence of (d Lnρ/d(T–1)).
alteration in relaxation processes, up to studied 1.2 T field. This can be attributed to the field induced enhancement in the conduction of charge carriers (i.e. eg electrons) across its lattice thereby reduction in resistance of the compound (i.e. negative MR). Magnetoloss (ML) effect is represented in the inset of Fig. 2 wherein ML has been estimated by using the following formula (3):
ML (%) =
tan δ (H ) − tan δ (0) × 100% tan δ (0)
expect a high charge ordering temperature since TCO of Gd0.5Ca0.5MnO3 is equal to 300 K [19]. Besides, our studied compound is characterized by a great value of cationic mismatch (σA2 = 10.01 × 10–4 Å2). The presence of significant quenched disorder yields important tilting of MnO6 octahedra, resulting in the localization of eg electrons and, hence, the stabilization of charge ordering. This fact indicates that our sample is charge–ordered paramagnetic between 165 K and 292 K. Also, charge–ordered state is not fully destroyed at 298 K due to phase separated nature of the presently studied compound. In other words, between 292 K and 298 K, one can expect coexisting paramagnetic state along with charge ordered state. Such observation can be confirmed through the sharp drop of hopping energy above 298 K (upper inset of Fig. 3). The presence of TCO near room temperature can explain the colossal MD effect obtained in Fig. 1 since charge–ordered manganites possess important MD coupling near to their TCO [6,7]. For the present case, long range charge ordering starts to collapse at 292 K. Negative MD effect can be ascribed to the existence of ferroelectric (i.e. electrically polarizable or dielectric) charge–ordered clusters at room temperature, which are phase separated in nature and highly affected by external applied magnetic field. Applied magnetic field can improve the magnetism inside the compound through weakening of this room temperature poor charge ordering thereby deteriorating the polarization of the clusters. Such deterioration will generate extra paths for charge carriers resulting in a great reduction of the insulating behavior of grains and, hence, negative MD effect has been observed at room temperature for presently studied manganite. Frequency dependent real part of impedance Z′ is shown in Fig. 4. The reduction in Z′ with increase in frequency depicts the capacitive behavior of La0.4Gd0.1Ca0.5MnO3 compound in the studied frequency range. It is noteworthy that Z′ values are very important even at high frequencies where Z′ values are ascribed to the grain contribution (over 10 KΩ). Such high values can be linked to the presence of insulating charge–ordered state. The application of an external magnetic field reduces Z′ values due to the reduction of scattering centers. We can evaluate the magnetoimpedance (MI) of our sample by the following relation (4):
(3)
with tan δ (0) and tan δ (H) are the calculated dielectric loss values under zero and applied magnetic field (H), respectively. Maximum estimated ML are found to be ~25% and ~65% under 0.6 T and 1.2 T applied constant magnetic fields, respectively, at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. These values of ML remains invariant throughout the frequency range studied that again validates the fact that there does not exist any magnetic field induced modifications in the relaxation processes thereby confirms the extrinsic nature of MD effect in this studied compound. Conventionally, it is observed that dielectric loss values are much less than a unity for zero and studied applied magnetic fields, which can be useful aspect of studied material for its possible practical applications. It is worth, here, to investigate the origin of such colossal MD effect in La0.4Gd0.1Ca0.5MnO3 compound for which the magnetic and electrical properties have already been investigated. Fig. 3 shows the evolution of magnetization as a function of magnetic field at fixed temperatures 280 and 300 K. The linear shape exhibited by M (H) curves testifies the paramagnetic behavior of La0.4Gd0.1Ca0.5MnO3 compound near room temperature, down to 280 K. This behavior (paramagnetic nature) of La0.4Gd0.1Ca0.5MnO3 compound cannot explain the observed colossal MD effect. Lower inset of Fig. 3 shows the temperature dependent resistivity recorded in cooling process under zero and 10 T magnetic field. The insulating behavior observed above 200 K is governed by adiabatic small polaron hopping mechanism [3]. The evolution of hopping energy as a function of temperature is given in the upper inset of Fig. 3. It is clear that hopping energy sharply increases while cooling the sample from 300 K to 292 K where it shows a maximum perceptible even at 10 T applied field. Such maximum in hopping energy is generally a signature of charge ordering transition [17,18]. In our previous studies [1–3], we thought that the charge–ordered antiferromagnetic state is annihilated above 165 K. However, it seems that this temperature corresponds to the destruction of orbital ordering (antiferromagnetism) without affecting the charge ordering. We can
MI (%) =
Z ′ (H ) − Z ′ (0) × 100% Z ′ (0)
(4)
The obtained results are depicted in the inset of Fig. 4. One can observe that the MI values are more significant in the higher frequencies, indicating a magnetic field–induced enhancement in the 3
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
values are lower than unity suggesting that correlated barrier hopping (CBH) mechanism [20] is responsible for conduction in the present case. Through s exponent values, one can evaluate the maximum barrier height Wm that eg electrons of Mn3+ ions should overcome in order to reach the nearest site. Wm values can be estimated by following expression (6) [20]:
s=1−
6kT Wm
(6)
where k is the Boltzmann constant and T is the temperature (T = 298 K in our case). The obtained Wm values are 386 meV, 364 meV and 304 meV for an applied magnetic fields of 0, 0.6 T and 1.2 T, respectively. It is clear that increase in the applied magnetic field reduces Wm value due to the field–induced enhancement in double exchange mechanism. Magnetoconductivity (MC) can be evaluated as expression (7): Fig. 4. Frequency dependence of the real part of impedance recorded at room temperature under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. The inset shows the frequency dependence of magnetoimpedance at room temperature.
MC (%) =
(7)
The obtained results are shown in the inset of Fig. 5. Positive values of MC are recorded for our studied sample (inset of Fig. 5) reaching about 7% at 0.6 T field and 9% at 1.2 T field, despite the paramagnetic nature of our sample. The significant MC recorded under a small fraction of magnetic field ~0.6 T can be understood as: at room temperature, there coexists paramagnetic state along with the charge ordered clusters within the La0.4Gd0.1Ca0.5MnO3 compound lattice. Two possibilities can be highlighted: (i) application of a small external constant magnetic field (of 0.6 T) suppresses the magnetic disorder effectively in the lattice. In other words, external constant magnetic field can destroy efficiently the charger ordered clusters within the paramagnetic lattice and convert them into paramagnetic spin state and (ii) a small fraction of external constant magnetic field of 0.6 T can improve slightly the spin orientations of paramagnetic state in direction of applied magnetic field. Both these cases (i) and (ii) can improve the possibility of movements of charge carriers between the manganese ionic sites within the manganite lattice studied, and, hence improve the conductivity under the application of external constant magnetic field. As a result, significant value of MC ~7% under a small fraction of external constant magnetic field of 0.6 T can be observed for the presently studied La0.4Gd0.1Ca0.5MnO3 compound.
capacitive behavior at higher frequencies. Observed negative MI, i.e. magnetic field induced reduction in impedance for La0.4Gd0.1Ca0.5MnO3 compound, can be correlated with and understood as the reported negative MR at room temperature [3] for La0.4Gd0.1Ca0.5MnO3 compound understudy. Interestingly, estimated value of room temperature MI under 1.2 T magnetic field is much higher than that of room temperature MR under the same 1.2 T magnetic field suggests the higher sensitivity of La0.4Gd0.1Ca0.5MnO3 compound for applied ac field. In Fig. 5, we have reported the magnetic field effect on the frequency dependent total conductivity (σ) recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. It is obvious that σ shows a slight increase with increase in frequency. The application of an external magnetic field enhances the conductivity values due to the amelioration of double exchange mechanism induced by magnetic ordering. Jonscher's power law [expression (5)] is found to be perfectly suitable to describe the frequency dependence of conductivity under zero and both applied magnetic fields studied, as shown in Fig. 5:
σ = σDC + σAC = σDC + (A × f s )
σ (H ) − σ (0) × 100% σ (0)
(5)
4. Conclusion
The obtained values of s exponent are 0.6005, 0.5766 and 0.4932 for an applied field of 0, 0.6 T and 1.2 T, respectively. All s exponent
In this communication, we report the effect of low magnetic field on the frequency dependence of the real part of dielectric permittivity, loss, impedance and conductivity recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 polycrystalline compound. The magnetic field application yields a considerable decrease in permittivity, trivial suppression in impedance and an enhancement in conductivity. Characteristic frequency dependent dielectric loss behavior under external constant magnetic fields suggests the presence of multiple complex relaxation processes which are strongly independent to the external constant magnetic field that verifies the extrinsic MD effect in the studied compound. The studied La0.4Gd0.1Ca0.5MnO3 compound is characterized by the presence of charge ordering transition at TCO = 292 K located just below room temperature. We have recorded for the first time more than 50% of stable negative MD effect under 1.2 T applied field at room temperature, indicating that La0.4Gd0.1Ca0.5MnO3 compound is a potential candidate for several technological applications. The colossal MD effect was ascribed to the field induced weakening of charge–ordered clusters. Declaration of competing interest
Fig. 5. Frequency dependence of room temperature conductivity under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. Solid lines show fitted curves according to Joncher's power law. The inset shows the frequency dependence of magnetoconductivity at room temperature.
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant support for this work that could have influenced its outcome. 4
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
References
[10] J.R. Sahu, C.R. Serrao, A. Ghosh, A. Sundaresan, C.N.R. Rao, Solid State Commun. 149 (2009) 49. [11] A. Karmakar, S. Majumdar, A.K. Singh, S. Patnaik, S. Giri, J. Magn. Magn. Mater. 324 (2012) 649. [12] S. Sagar, P.A. Joy, M.R. Anantharaman, Ferroelectrics 392 (2009) 13. [13] K.D. Chandrasekhar, A.K. Das, A. Venimadhav, Appl. Phys. Lett. 98 (2011) 122908. [14] H. Pei, S. Guo, H. Yan, C. Chen, B. Luo, K. Jin, Chin. Phys. B 27 (2018) 097701. [15] U. Chowdhury, S. Goswami, D. Bhattacharya, A. Midya, P. Mandal, P. Das, Y.M. Mukovskii, J. Appl. Phys. 114 (2013) 194104. [16] H.M. Rai, S.K. Saxena, V. Mishra, A. Sagdeo, P. Rajput, R. Kumar, P.S. Sagdeo, J. Mater. Chem. C 4 (2016) 10876. [17] A. Krichene, W. Boujelben, A. Cheikhrouhou, J. Alloy. Comp. 581 (2013) 352. [18] A. Krichene, W. Boujelben, S. Mukherjee, N.A. Shah, P.S. Solanki, Ceram. Int. 45 (2019) 3849. [19] K. Das, T. Paramanik, I. Das, J. Magn. Magn. Mater. 374 (2015) 707. [20] S.R. Elliott, Philos. Mag. B 37 (1978) 553.
[1] A. Krichene, W. Boujelben, A. Cheikhrouhou, J. Alloy. Comp. 550 (2013) 75. [2] A. Krichene, W. Boujelben, J. Supercond. Nov. Magn. 31 (2018) 577. [3] A. Krichene, W. Boujelben, S. Mukherjee, N.A. Shah, P.S. Solanki, Phys. Chem. Chem. Phys. 20 (2018) 12608. [4] A. Krichene, W. Boujelben, S. Mukherjee, P.S. Solanki, N.A. Shah, Acta Mater. 131 (2017) 491. [5] Zalak Joshi, Davit Dhruv, K.N. Rathod, Hetal Boricha, Keval Gadani, D.D. Pandya, A.D. Joshi, P.S. Solanki, N.A. Shah, Prog. Solid State Chem. 49 (2018) 23. [6] T. Zou, F. Wang, Y. Liu, L.Q. Yan, Y. Sun, Appl. Phys. Lett. 97 (2010) 092501. [7] C.R. Serrao, A. Sundaresan, C.N.R. Rao, J. Phys. Condens. Matter 19 (2007) 496217. [8] K. Yoshii, H. Abe, N. Ikeda, J. Solid State Chem. 178 (2005) 3615. [9] M. Sanchez–Andujar, S. Yanez–Vilar, S. Castro–Garcia, M.A. Senaris–Rodriguez, J. Solid State Chem. 181 (2008) 1354.
5
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Room temperature colossal magnetodielectric effect in La0.4Gd0.1Ca0.5MnO3 A. Krichenea,∗, W. Boujelbena, K.N. Rathodb, K. Gadanib, N.A. Shahb, P.S. Solankib a b
Laboratoire de Physique des Matériaux, Faculté des Sciences de Sfax, Université de Sfax, B.P. 1171, 3000, Sfax, Tunisia Department of Physics, Saurashtra University, Rajkot, 360 005, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Manganite Magnetodielectric effect Charge ordering
We have studied the contribution induced by a low magnetic field (0.6T and 1.2T) on the frequency dependent permittivity, impedance and conductivity recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 polycrystalline compound. Strong magnetodielectric (MD) coupling has been detected at room temperature. Colossal value of negative MD (more than 50%) has been observed under 1.2T field. The MD response is found to be stable in the studied frequency range (up to 2 MHz) suggesting the possibility of using this compound for technological applications at room temperature. The significant MD coupling has been correlated with the presence of charge ordering transition near room temperature.
1. Introduction Multifunctional materials are now the flagship research topic for researchers and scientists. Various perovskite manganites have been well characterized for several physical properties such as magnetocaloric effect [1,2], colossal magnetoresistance [3,4], charge and orbital orderings [1–7], MD coupling [5–7], etc. Half–doped manganites with the equal amounts of Mn3+ and Mn4+ ions may exhibit complex phenomena related to the high correlations between electrons and lattice. During last two decades, few attempts have been successfully made in order to understand the electrical nature of various charge–ordered manganite compounds [7–12]. Recent studies [1–3], carried out for polycrystalline charge–ordered La0.4Gd0.1Ca0.5MnO3 manganite compound, have shown that this compound exhibits a paramagnetic–ferromagnetic (insulator–metal) transition at Curie temperature TC = 100 K. The magnetic structure below TC has been described as a charge–ordered antiferromagnetic matrix in which some ferromagnetic domains exist. Around TC, these ferromagnetic domains induce a considerably large magnetocaloric effect [1,2]. In addition, magnetic phase separation phenomenon has been identified as a responsible cause for the colossal magnetoresistive behavior of this compound [3]. Studies have also revealed the persistence of charge–ordered antiferromagnetic domains above TC with the detection of interesting phenomena such as kinetic arrest and training effect. The search for multifunctional materials has recently triggered the investigations on MD coupling in order to make possible the supplementary physical applications [5–7]. Since, La0.4Gd0.1Ca0.5MnO3
∗
compound has been thoroughly investigated for its structural, magnetic, magnetocaloric, transport and magnetotransport behaviors, it is worth, now, to concentrate on the understanding of the possible MD coupling exhibited by the same compound for its practical physical applications. Any practical application as well as any device performance requires the material with possibly large MD effect under low applied magnetic field at or near the room temperature, since large magnetic field requirement and low temperature condition for obtaining large MD effect are the bottleneck for practical use of the materials. In addition, large leakage or electrical loss also hinders the practical applications of any particular material. In this context, we have studied the effect of an external constant magnetic field on the permittivity, loss, conductivity and impedance behaviors recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. Main aim of this study is to identify any possible strength of this well studied La0.4Gd0.1Ca0.5MnO3 compound for better practical applications. 2. Experimental detail Polycrystalline La0.4Gd0.1Ca0.5MnO3 compound was prepared using conventional solid state reaction method of ceramics, as reported previously [1]. Rietveld refinement of the X–ray diffraction pattern for the studied sample has confirmed that it crystallizes according to orthorhombic Pnma (no. 62) space group with the cell parameters: a = 5.416(5) Å, b = 7.624(2) Å and c = 5.405(3) Å [1]. Magnetic field dependence of magnetization was carried out at 280 and 300 K by using a SQUID vibrating sample magnetometer (VSM; Quantum Design). Temperature dependence of resistivity under zero and 10 T applied
Corresponding author. E-mail address: [email protected] (A. Krichene).
https://doi.org/10.1016/j.ceramint.2019.12.023 Received 11 September 2019; Received in revised form 1 November 2019; Accepted 2 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: A. Krichene, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.023
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
52% of negative MD effect is reached under 1.2 T field near 2 MHz. As per our best knowledge, such negative values of MD effect are recorded for the first time for LaCaMnO3 based half doped manganite at room temperature. However, Great positive MD effect was observed at room temperature for YMnO3 [5]. In addition to this, few reports exist on the observation of large MD effect for mixed valent high resistive manganite compounds [6,13–16]. Although, all these compounds either require composite form, low temperature, high magnetic field or provide less than 50% MD effect at sufficiently high temperatures (near to room temperature). The colossal values as well as the stability of the MD response of our sample in the studied frequency range suggest the possibility of using this sample for technological applications at room temperature. Reduction in permittivity with applied magnetic field can be understood as: under applied frequency/ac electric field, dipoles consist of Mn3+ and Mn4+ ions get oriented along the direction of the external field. With increase in frequency, dipoles realize fatigued that is responsible for the reduction in permittivity with increase in frequency (Fig. 1). When magnetic field is applied to the compound understudy, it strives to align the magnetic moment of La0.4Gd0.1Ca0.5MnO3 compound in its direction. In this context, interruption has been generated in the dipole alignment under applied ac electric field/frequency thereby magnetic field (at each applied frequency) reduces the permittivity of the compound understudy. In addition, magnetic field can introduce conduction of eg electrons between Mn3+ and Mn4+ across the La0.4Gd0.1Ca0.5MnO3 manganite lattice thereby enhances the leakage across the same and, hence, permittivity gets suppressed or negative MD effect comes into the picture under the application of external magnetic field, as shown in inset (b) of Fig. 1. Simultaneous existence of (i) conduction across the La0.4Gd0.1Ca0.5MnO3 manganite lattice (through possible oxygen vacancies across the lattice under zero applied field and through active zener double exchange under applied magnetic fields) as well as (ii) dipolar image of different valence states of manganese ions under applied frequency has been confirmed by performing universal dielectric response fittings to the permittivity data, as shown in the inset (a) of Fig. 1. Straight line fits confirm the simultaneous existence of process (i) (conduction across the lattice) and process (ii) (dipole alignment under frequency). In order to understand the effect of low applied external constant magnetic field on the conduction induced leakage processes across the La0.4Gd0.1Ca0.5MnO3 compound lattice, frequency dependent imaginary permittivity was recorded for the same frequency range at room temperature. Fig. 2 shows the variation in dielectric loss (tan δ = ε′′/ε′) with frequency under 0, 0.6 T and 1.2 T applied constant magnetic fields at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. Under all applied constant magnetic fields, dielectric loss initially increases with frequency up to the frequency ~0.2 MHz followed by reduction in its value up to ~0.54 MHz. This non–monotonous variation (below ~ 0.54 MHz) can be ascribed to a presence of relaxation of dipoles at particular frequencies. There is no significant change in the frequency peak (at ~ 0.19 MHz) of loss plots upon the application of magnetic field that confirms no effect of magnetic field in altering the relaxation process of the compound understudy. Above ~0.54 MHz frequency, loss remains almost constant with effective and considerable fluctuations in its values under zero and all applied constant magnetic fields (well above ~ 0.54 MHz frequency). This represents an existence of the complex picture of relaxation processes (i.e. strongly frequency dependent multiple relaxation processes). No considerable changes in the frequency dependent fluctuations in dielectric loss values upon increase in applied external constant magnetic field suggests that the multiple complex relaxation processes do not easily get affected by an external constant applied magnetic field at room temperature. This clearly indicates that observed remarkably high MD effect [inset (b) of Fig. 1] is an extrinsic in nature for the presently studied La0.4Gd0.1Ca0.5MnO3 compound. Overall value of dielectric loss gets enhanced with increase in applied constant magnetic field, without any
Fig. 1. Frequency dependence of the real part of dielectric permittivity recorded at room temperature under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. The solid lines represent the fitting curves according to Cole–Cole Model. The (a) inset shows the volution of log (f ε′) versus log f. The (b) inset shows the frequency dependence of MD effect at room temperature.
magnetic field was determined by standard four–probe technique by using Physical Property Measurement System (PPMS; Quantum Design, 14T model) in the temperature range 200 K–300 K. The frequency dependence of permittivity, loss, impedance and conductivity were measured at room temperature (T = 298 K) via Agilent E4980A precision LCR meter in the frequency range 10 kHz–2 MHz under applied magnetic field values up to 1.2 T. 3. Results and discussion Fig. 1 shows the evolution of the real part of dielectric permittivity (ε′) as a function of frequency (f) at room temperature under 0, 0.6 T and 1.2 T applied magnetic fields for La0.4Gd0.1Ca0.5MnO3 sample. All the curves show the typical evolution, characterized by the decrease of ε′ as f increases throughout the range studied because the dipoles response gets suppressed at higher f values. The decrease in ε′ is more significant at lower frequencies due to Maxwell–Wagner effect. The evolution of ε′ has been found to follow Cole–Cole model given by:
ε = ε∞ +
ε0 − ε∞ 1 + (jfτ )1 − α
(1)
where ε0 is the static permittivity, ε∞ is the high frequency permittivity, f is the frequency, τ is the relaxation time and α exponent is the measure of the distribution of relaxation time. Fitted curves are represented by solid lines in Fig. 1. All the curves follow the universal dielectric response model, which is evident through the linear behavior exhibited by the curves in the inset (a) of Fig. 1. With the application of an external constant magnetic field, ε′ sharply decreases. This fact confirms the presence of strong MD correlation inside the structure of the studied specimen. For 1.2 T magnetic field, ε′ decreases by half indicating a strong field–induced weakening of insulating tendencies. The MD effect can be quantified by using the following relation:
MD (%) =
ε′ (H ) − ε′ (0) × 100% ε′ (0)
(2)
with ε′ (0) and ε′ (H) are the permittivity values recorded under zero and applied magnetic field (H), respectively. The MD response of our sample is shown in the (b) inset of Fig. 1. Colossal values of negative MD effect are recorded near room temperature, i.e. more than 25% for 0.6 T and more than 50% under 1.2 T applied magnetic field. About 2
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
Fig. 2. Frequency dependence of dielectric loss recorded at room temperature under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. The inset shows the frequency dependence of magnetoloss at room temperature.
Fig. 3. Magnetic field dependence of magnetization near room temperature for La0.4Gd0.1Ca0.5MnO3 sample. Lower inset shows the temperature dependence of resistivity above 200 K under 0 and 10 T magnetic fields. The upper inset shows the temperature dependence of (d Lnρ/d(T–1)).
alteration in relaxation processes, up to studied 1.2 T field. This can be attributed to the field induced enhancement in the conduction of charge carriers (i.e. eg electrons) across its lattice thereby reduction in resistance of the compound (i.e. negative MR). Magnetoloss (ML) effect is represented in the inset of Fig. 2 wherein ML has been estimated by using the following formula (3):
ML (%) =
tan δ (H ) − tan δ (0) × 100% tan δ (0)
expect a high charge ordering temperature since TCO of Gd0.5Ca0.5MnO3 is equal to 300 K [19]. Besides, our studied compound is characterized by a great value of cationic mismatch (σA2 = 10.01 × 10–4 Å2). The presence of significant quenched disorder yields important tilting of MnO6 octahedra, resulting in the localization of eg electrons and, hence, the stabilization of charge ordering. This fact indicates that our sample is charge–ordered paramagnetic between 165 K and 292 K. Also, charge–ordered state is not fully destroyed at 298 K due to phase separated nature of the presently studied compound. In other words, between 292 K and 298 K, one can expect coexisting paramagnetic state along with charge ordered state. Such observation can be confirmed through the sharp drop of hopping energy above 298 K (upper inset of Fig. 3). The presence of TCO near room temperature can explain the colossal MD effect obtained in Fig. 1 since charge–ordered manganites possess important MD coupling near to their TCO [6,7]. For the present case, long range charge ordering starts to collapse at 292 K. Negative MD effect can be ascribed to the existence of ferroelectric (i.e. electrically polarizable or dielectric) charge–ordered clusters at room temperature, which are phase separated in nature and highly affected by external applied magnetic field. Applied magnetic field can improve the magnetism inside the compound through weakening of this room temperature poor charge ordering thereby deteriorating the polarization of the clusters. Such deterioration will generate extra paths for charge carriers resulting in a great reduction of the insulating behavior of grains and, hence, negative MD effect has been observed at room temperature for presently studied manganite. Frequency dependent real part of impedance Z′ is shown in Fig. 4. The reduction in Z′ with increase in frequency depicts the capacitive behavior of La0.4Gd0.1Ca0.5MnO3 compound in the studied frequency range. It is noteworthy that Z′ values are very important even at high frequencies where Z′ values are ascribed to the grain contribution (over 10 KΩ). Such high values can be linked to the presence of insulating charge–ordered state. The application of an external magnetic field reduces Z′ values due to the reduction of scattering centers. We can evaluate the magnetoimpedance (MI) of our sample by the following relation (4):
(3)
with tan δ (0) and tan δ (H) are the calculated dielectric loss values under zero and applied magnetic field (H), respectively. Maximum estimated ML are found to be ~25% and ~65% under 0.6 T and 1.2 T applied constant magnetic fields, respectively, at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. These values of ML remains invariant throughout the frequency range studied that again validates the fact that there does not exist any magnetic field induced modifications in the relaxation processes thereby confirms the extrinsic nature of MD effect in this studied compound. Conventionally, it is observed that dielectric loss values are much less than a unity for zero and studied applied magnetic fields, which can be useful aspect of studied material for its possible practical applications. It is worth, here, to investigate the origin of such colossal MD effect in La0.4Gd0.1Ca0.5MnO3 compound for which the magnetic and electrical properties have already been investigated. Fig. 3 shows the evolution of magnetization as a function of magnetic field at fixed temperatures 280 and 300 K. The linear shape exhibited by M (H) curves testifies the paramagnetic behavior of La0.4Gd0.1Ca0.5MnO3 compound near room temperature, down to 280 K. This behavior (paramagnetic nature) of La0.4Gd0.1Ca0.5MnO3 compound cannot explain the observed colossal MD effect. Lower inset of Fig. 3 shows the temperature dependent resistivity recorded in cooling process under zero and 10 T magnetic field. The insulating behavior observed above 200 K is governed by adiabatic small polaron hopping mechanism [3]. The evolution of hopping energy as a function of temperature is given in the upper inset of Fig. 3. It is clear that hopping energy sharply increases while cooling the sample from 300 K to 292 K where it shows a maximum perceptible even at 10 T applied field. Such maximum in hopping energy is generally a signature of charge ordering transition [17,18]. In our previous studies [1–3], we thought that the charge–ordered antiferromagnetic state is annihilated above 165 K. However, it seems that this temperature corresponds to the destruction of orbital ordering (antiferromagnetism) without affecting the charge ordering. We can
MI (%) =
Z ′ (H ) − Z ′ (0) × 100% Z ′ (0)
(4)
The obtained results are depicted in the inset of Fig. 4. One can observe that the MI values are more significant in the higher frequencies, indicating a magnetic field–induced enhancement in the 3
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
values are lower than unity suggesting that correlated barrier hopping (CBH) mechanism [20] is responsible for conduction in the present case. Through s exponent values, one can evaluate the maximum barrier height Wm that eg electrons of Mn3+ ions should overcome in order to reach the nearest site. Wm values can be estimated by following expression (6) [20]:
s=1−
6kT Wm
(6)
where k is the Boltzmann constant and T is the temperature (T = 298 K in our case). The obtained Wm values are 386 meV, 364 meV and 304 meV for an applied magnetic fields of 0, 0.6 T and 1.2 T, respectively. It is clear that increase in the applied magnetic field reduces Wm value due to the field–induced enhancement in double exchange mechanism. Magnetoconductivity (MC) can be evaluated as expression (7): Fig. 4. Frequency dependence of the real part of impedance recorded at room temperature under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. The inset shows the frequency dependence of magnetoimpedance at room temperature.
MC (%) =
(7)
The obtained results are shown in the inset of Fig. 5. Positive values of MC are recorded for our studied sample (inset of Fig. 5) reaching about 7% at 0.6 T field and 9% at 1.2 T field, despite the paramagnetic nature of our sample. The significant MC recorded under a small fraction of magnetic field ~0.6 T can be understood as: at room temperature, there coexists paramagnetic state along with the charge ordered clusters within the La0.4Gd0.1Ca0.5MnO3 compound lattice. Two possibilities can be highlighted: (i) application of a small external constant magnetic field (of 0.6 T) suppresses the magnetic disorder effectively in the lattice. In other words, external constant magnetic field can destroy efficiently the charger ordered clusters within the paramagnetic lattice and convert them into paramagnetic spin state and (ii) a small fraction of external constant magnetic field of 0.6 T can improve slightly the spin orientations of paramagnetic state in direction of applied magnetic field. Both these cases (i) and (ii) can improve the possibility of movements of charge carriers between the manganese ionic sites within the manganite lattice studied, and, hence improve the conductivity under the application of external constant magnetic field. As a result, significant value of MC ~7% under a small fraction of external constant magnetic field of 0.6 T can be observed for the presently studied La0.4Gd0.1Ca0.5MnO3 compound.
capacitive behavior at higher frequencies. Observed negative MI, i.e. magnetic field induced reduction in impedance for La0.4Gd0.1Ca0.5MnO3 compound, can be correlated with and understood as the reported negative MR at room temperature [3] for La0.4Gd0.1Ca0.5MnO3 compound understudy. Interestingly, estimated value of room temperature MI under 1.2 T magnetic field is much higher than that of room temperature MR under the same 1.2 T magnetic field suggests the higher sensitivity of La0.4Gd0.1Ca0.5MnO3 compound for applied ac field. In Fig. 5, we have reported the magnetic field effect on the frequency dependent total conductivity (σ) recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 compound. It is obvious that σ shows a slight increase with increase in frequency. The application of an external magnetic field enhances the conductivity values due to the amelioration of double exchange mechanism induced by magnetic ordering. Jonscher's power law [expression (5)] is found to be perfectly suitable to describe the frequency dependence of conductivity under zero and both applied magnetic fields studied, as shown in Fig. 5:
σ = σDC + σAC = σDC + (A × f s )
σ (H ) − σ (0) × 100% σ (0)
(5)
4. Conclusion
The obtained values of s exponent are 0.6005, 0.5766 and 0.4932 for an applied field of 0, 0.6 T and 1.2 T, respectively. All s exponent
In this communication, we report the effect of low magnetic field on the frequency dependence of the real part of dielectric permittivity, loss, impedance and conductivity recorded at room temperature for La0.4Gd0.1Ca0.5MnO3 polycrystalline compound. The magnetic field application yields a considerable decrease in permittivity, trivial suppression in impedance and an enhancement in conductivity. Characteristic frequency dependent dielectric loss behavior under external constant magnetic fields suggests the presence of multiple complex relaxation processes which are strongly independent to the external constant magnetic field that verifies the extrinsic MD effect in the studied compound. The studied La0.4Gd0.1Ca0.5MnO3 compound is characterized by the presence of charge ordering transition at TCO = 292 K located just below room temperature. We have recorded for the first time more than 50% of stable negative MD effect under 1.2 T applied field at room temperature, indicating that La0.4Gd0.1Ca0.5MnO3 compound is a potential candidate for several technological applications. The colossal MD effect was ascribed to the field induced weakening of charge–ordered clusters. Declaration of competing interest
Fig. 5. Frequency dependence of room temperature conductivity under several values of applied magnetic field for La0.4Gd0.1Ca0.5MnO3 sample. Solid lines show fitted curves according to Joncher's power law. The inset shows the frequency dependence of magnetoconductivity at room temperature.
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant support for this work that could have influenced its outcome. 4
Ceramics International xxx (xxxx) xxx–xxx
A. Krichene, et al.
References
[10] J.R. Sahu, C.R. Serrao, A. Ghosh, A. Sundaresan, C.N.R. Rao, Solid State Commun. 149 (2009) 49. [11] A. Karmakar, S. Majumdar, A.K. Singh, S. Patnaik, S. Giri, J. Magn. Magn. Mater. 324 (2012) 649. [12] S. Sagar, P.A. Joy, M.R. Anantharaman, Ferroelectrics 392 (2009) 13. [13] K.D. Chandrasekhar, A.K. Das, A. Venimadhav, Appl. Phys. Lett. 98 (2011) 122908. [14] H. Pei, S. Guo, H. Yan, C. Chen, B. Luo, K. Jin, Chin. Phys. B 27 (2018) 097701. [15] U. Chowdhury, S. Goswami, D. Bhattacharya, A. Midya, P. Mandal, P. Das, Y.M. Mukovskii, J. Appl. Phys. 114 (2013) 194104. [16] H.M. Rai, S.K. Saxena, V. Mishra, A. Sagdeo, P. Rajput, R. Kumar, P.S. Sagdeo, J. Mater. Chem. C 4 (2016) 10876. [17] A. Krichene, W. Boujelben, A. Cheikhrouhou, J. Alloy. Comp. 581 (2013) 352. [18] A. Krichene, W. Boujelben, S. Mukherjee, N.A. Shah, P.S. Solanki, Ceram. Int. 45 (2019) 3849. [19] K. Das, T. Paramanik, I. Das, J. Magn. Magn. Mater. 374 (2015) 707. [20] S.R. Elliott, Philos. Mag. B 37 (1978) 553.
[1] A. Krichene, W. Boujelben, A. Cheikhrouhou, J. Alloy. Comp. 550 (2013) 75. [2] A. Krichene, W. Boujelben, J. Supercond. Nov. Magn. 31 (2018) 577. [3] A. Krichene, W. Boujelben, S. Mukherjee, N.A. Shah, P.S. Solanki, Phys. Chem. Chem. Phys. 20 (2018) 12608. [4] A. Krichene, W. Boujelben, S. Mukherjee, P.S. Solanki, N.A. Shah, Acta Mater. 131 (2017) 491. [5] Zalak Joshi, Davit Dhruv, K.N. Rathod, Hetal Boricha, Keval Gadani, D.D. Pandya, A.D. Joshi, P.S. Solanki, N.A. Shah, Prog. Solid State Chem. 49 (2018) 23. [6] T. Zou, F. Wang, Y. Liu, L.Q. Yan, Y. Sun, Appl. Phys. Lett. 97 (2010) 092501. [7] C.R. Serrao, A. Sundaresan, C.N.R. Rao, J. Phys. Condens. Matter 19 (2007) 496217. [8] K. Yoshii, H. Abe, N. Ikeda, J. Solid State Chem. 178 (2005) 3615. [9] M. Sanchez–Andujar, S. Yanez–Vilar, S. Castro–Garcia, M.A. Senaris–Rodriguez, J. Solid State Chem. 181 (2008) 1354.
5