- Email: [email protected]
Dielectric and ferroelectric properties of Nd3Fe5O12
Solid State Communications 305 (2020) 113766
Contents lists available at ScienceDirect
Solid State Communications journal homepage: http://www.elsevier.com/locate/ssc
Communication
Dielectric and ferroelectric properties of Nd3Fe5O12 Tingsong Zhang a, Chenyang Zhang b, Hongming Yuan c, **, Mingze Xu a, * a
International Joint Research Center for Nanophotonics and Biophotonics, School of Science, Changchun University of Science and Technology, Changchun, 130022, People’s Republic of China b College of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang, 453003, People’s Republic of China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, 130012, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Communicated by T. Kimura
Multiferroics have attracted research interest for decades due to their potential applications. Multiferroics are relatively scarce due to the mutual exclusivity between magnetic and ferroelectric properties. New roomtemperature multiferroics are looking forward to be exploited. Room-temperature multiferroic properties have been observed in a new rare-earth iron garnet (REIG), Nd3Fe5O12 (NdIG). The dielectric properties of NdIG over a wide temperature range (300 K–650 K) at different frequencies (100 Hz–1 MHz) and in the temperature range from 70 K to 300 K at 100 Hz are investigated herein. The results show that this material has two dielectric re laxations in the temperature range from 290 K to 525 K and a dielectric anomaly around the magnetic ordering temperature (565 K). The first dielectric relaxation in the temperature range from 290 K to 500 K is attributed to the conduction with an activation energy of 0.75 eV. The other dielectric relaxation in an elevated temperature range has an activation energy of 0.87 eV, which originates from the inhomogeneous structure. Furthermore, the ferroelectric polarization of approximately 28 μC/m2 at 77 K is determined by the positive-up-negative-down method and induced by the effective magnetic field created by the iron sublattice magnetic ordering and electric-dipole moments of the neodymium ions.
Keywords: Garnet Dielectric properties Multiferroics
1. Introduction Multiferroics, which exhibit magnetic and ferroelectric ordering simultaneously, have been attracting research interest because of their promising applications in spintronics and data storage [1,2]. One of the most appealing aspects of multiferroics is their so-called magnetoelec tric (ME) coupling. In magnetoelectric materials, ferroelectricity (mag netic ordering) could be tuned by the magnetic field (electric field). However, ferroelectricity and magnetic ordering have different occu pation rules for d orbital electrons that are not compatible with each other and thus lead to a lack of multiferroic materials [3]. Currently, two main methods are widely applied to fabricate multiferroics [4]. The first approach is that the magnetic ordering and ferroelectric ordering orig inate from different subsystems, such as for BiFeO3, where the antifer romagnetic ordering (Neel temperature TN ¼ 643 K) and ferroelectric ordering (Curie temperature TC ¼ 1103 K) are derived from iron ions and bismuth ions, respectively [5]. Although this kind of multiferroics has a high polarization and critical temperature, the ME coupling effect is weak, which is because the ferroelectricity and magnetism are derived
from different units. The second is an approach in which the ferroelec tricity is induced by breaking the magnetic ordering symmetry; for example, in YMn2O5, electric polarization is induced by collinear spin orders in frustrated magnets [6]. In TbMnO3, the noncollinear spiral magnetic structure is responsible for the generation of spontaneous polarization at temperatures below the corresponding magnetic tem perature (TN ¼ 27 K) [7]. The advantage of this kind of multiferroic is that it has a strong ME coupling effect but weak ferroelectricity and a low transition temperature. Therefore, the development of room-temperature multiferroics with a strong magnetoelectric coupling effect is still an urgent issue that remains to be solved. Recently, rare-earth iron garnets (REIG) have attracted widespread attention as they were found to show a strong ME coupling effect at room temperature and under low magnetic fields. Magnetodielectric (MD) properties of Lu3Fe5O12 have been investigated, and an MD coupling (Dε/ε~18%) was observed at room temperature and under low magnetic fields (H~2 kOe) [8]. The ME and dielectric properties of Ho3Fe5O12 have been investigated over a wide temperature range, a dielectric anomaly was observed near the Neel temperature, and ME
* Corresponding author. Tel.: þ86-431-85582739. ** Corresponding author. Tel.: þ86-431-85168318 E-mail addresses: [email protected] (H. Yuan), [email protected] (M. Xu). https://doi.org/10.1016/j.ssc.2019.113766 Received 30 August 2019; Received in revised form 4 October 2019; Accepted 15 October 2019 Available online 16 October 2019 0038-1098/© 2019 Elsevier Ltd. All rights reserved.
T. Zhang et al.
Solid State Communications 305 (2020) 113766
coupling was observed at room temperature and under low magnetic fields [9,10]. After additional research, the multiferroic properties of REIGs were also discussed. P. Manimuthu et al. reported that Lu3Fe5O12 has a dielectric anomaly near 625 K and attributed it to a paraelectric to ferroelectric transition [11]. A. I. Popov et al. reported that REIGs have an antiferroelectric structure due to the effective magnetic field created by the iron sublattice magnetic ordering and electric-dipole moments of the rare-earth ions, and calculated the REIG should have a polarization of 10–100 μC/m2, which is induced by the effective field in the garnets [12,13]. Although the reports mentioned above confirmed that REIGs are room-temperature multiferroics, the ferroelectric hysteresis loops were not given. Nd3Fe5O12 (NdIG), which was not synthesized because the Nd3þ lattice parameter is larger than 12.540 Å [14], has recently been prepared with a hydrothermal method. The NdIG shows ferri magnetic behavior, and the ferrimagnetic Curie temperature, which has been inferred from thermomagnetization curves, was 565 K [15]. As a new and important member of the REIG family, there are only a few reports about its magnetic properties [15,16]. However, the electrical properties of NdIG have not yet been studied. In this work, the dielectric and ferroelectric properties of NdIG were investigated in detail above room temperature. Dielectric and ferroelectric analyses reveal that NdIG exhibits ferroelectricity below 565 K. Therefore, the results presented here indicate multiferroicity above room temperature in the sample material.
Fig. 1. XRD pattern of the sintered Nd3Fe5O12 at room temperature.
2. Experimental The NdIG powders were prepared by the hydrothermal method, as we reported previously [15]. The prepared crystals were ground and pressed into pellets, and were then annealed at 673 K in an O2 atmo sphere for 24 h. Electrodes were attached by using silver paste applied on the two faces of each pellet. The X-ray diffraction data for the sintered powder was collected on Rigaku D/Max 2500 V/Pc X-ray diffractometer with Cu kα radiation (λ ¼ 1.5418 Å) at 50 kV and 200 mA at room tem perature. The temperature dependence of dielectric constants was car ried out in Sigma furnace by using the Agilent Precision (LCR) meter. The dielectric measurements in the temperature range from 70 K to 300 K at 100 Hz were performed in Physical Property Measurement System (Quantum Design). We used electro chemical interface SI 1287 (solartron) and impedance/gain-phase analyzer SI1260 (solartron) to detect electric conduction properties of NdIG. The polarization-electric field (P-E) hysteresis loop was measured using a Radiant Precision II with the positive-up-negative-down (PUND) method. To avoid any contamination of moisture during the dielectric and electrical conduc tion experiments, the sample was preheated to 473 K with an annealing time of 2 h, and then cooled to room temperature prior to the measurements.
Fig. 2. (a) Temperature dependence of ε and tan δ between 300 K and 645 K at selected frequencies in the heating cycles, (b) variation of (tanδ)max with fre quency versus 1000/T for the LTDR, (c) variation of (tanδ)max with frequency versus 1000/T for the HTDR. The symbols are the experimental points, and the solid line is the Arrhenius fit.
relaxation frequency at an infinite temperature, Ea is the activation energy, kB is the Boltzmann constant and T is the absolute temperature. Fig. 2(b) shows the Arrhenius plot of the log of the frequency versus 1/T, and fitting parameters Ea ¼ 0.732 eV are obtained. We refer to this relaxation behavior as low-temperature dielectric relaxation (LTDR). The dielectric properties of NdIG as a function of temperature from 70 K to 300 K at 100 Hz are shown in Fig. 3. The dielectric constant shows an obvious increase, and the loss tangent (tan δ) exhibits a peak at approximately 290 K, which is in good agreement with the data in Fig. 2 (a). The loss tangent peak of 100 Hz comes from the LTDR phenomenon in the sample. In the temperature range from 500 K to 525 K, the dielectric constant shows a step-like increase, and the loss tangent peak shifts its position a low temperature to an elevated temperature with increasing frequency. The relaxation behavior occurring in the loss tangent can be seen in the low frequency range from 100 Hz to 10 kHz; however, it disappears at the high frequency. To distinguish this relaxation behavior from the LTDR, we refer to this relaxation behavior as high-temperature
3. Results and discussion 3.1. Dielectric property The XRD patterns of the sintered NdIG powder are shown in Fig. 1. The X-ray diffraction pattern agrees well with the reported data [15], suggesting that our sample is pure. The dielectric properties above room temperature in the warming process are shown in Fig. 2(a). In the temperature range below 500 K, the dielectric constant shows an obvious increase and then reaches a plateau, whereas there is a differ ence in the inflection temperature for ε, which shifts toward high tem peratures as the frequency increases. In the corresponding temperature range, the presence of a peak in the tanδ curve is observed, and the peak position also shifts to high temperatures with increasing frequency, but the loss tangent peak of 100 Hz is out of the temperature measurement range (inside of Fig. 2(a)). This indicates that a thermally driven relaxation process exists in the sample, which can be approximately described by the Arrhenius relation, f ¼ f0 exp(Ea/kBT), where f0 is the 2
T. Zhang et al.
Solid State Communications 305 (2020) 113766
Fig. 4. P-E hysteresis loop measured by the PUND method at 77 K.
21] and demonstrates the intrinsic coupling between the magnetic ordering and dielectric ordering of the NdIG. The maximum remanent polarization value of NdIG is comparable with that of related magnetic multiferroics and in agreement with the reported values for the pre dicted range [13]. Therefore, it is possible that the polarization in the NdIG below 565 K could mainly be derived from the effective magnetic field created by the iron sublattice magnetic ordering and electric-dipole moments of the neodymium ions. The neodymium ions acquire electric-dipole moments due to the action of a crystal field with a low symmetry. The effective magnetic field is created by the iron sublattice magnetic ordering and electric-dipole moments of the neodymium ions and leads to the formation of an antiferroelectric structure. In the presence of magnetic domain walls, the effective magnetic field becomes inhomogeneous, and as a consequence, the antiferroelectric structure rearranges and a nonzero polarization appears [12,13].
Fig. 3. Temperature dependence of ε and tan δ between 70 K and 300 K at 0.1 kHz during the heating cycles.
dielectric relaxation (HTDR). The variation of (tanδ)max for the fre quency versus 1000/T is shown in Fig. 2(c). This variation also follows the Arrhenius law, f ¼ f0 exp(Ea/kBT), where the fitting parameters Ea ¼ 0.87 eV are obtained for the HTDR. The activation energy is close to the reported values for inhomogeneous structures, such as grain boundaries [17]. It is clear that both the dielectric constant ε and the loss tangent tan δ exhibit anomalous peaks at approximately 565 K for all the applied frequencies (Fig. 2(a)). The positions of these peaks do not vary in terms of the frequency, indicating that the 565 K transition is not related to a ferroelectric relaxor-like behavior [18]. Therefore, it is very likely that these unusual dielectric features are the characteristics of a ferroelectric - paraelectric phase transition.
3.3. Conductivity property Fig. 5(a) shows the variation in the ac conductivity (σac) with the frequency at selected temperatures. It is obvious that σac monotonously decreases with decreasing frequency and becomes independent of the frequency after a certain value. Extrapolating these curves toward a low frequency gives the dc conductivity σdc. The resulting σdc is plotted as a function of reciprocal temperature in Fig. 5(b) and obeys the Arrhenius law, σ ¼ σ0 exp (Econd/kB T), where σ0 is the preexponential term, Econd is the activation energy, and kB is the Boltzmann constant. The fitting Econd is 0.75 eV, which is close to the activation energy of the LTDR. This suggests that the conductivity may play a very important role in the LTDR.
3.2. Ferroelectric property
4. Conclusions
The essential feature of a ferroelectric phase is that the polarization can be reversed through the application of an external electric field. Therefore, we performed P-E hysteresis loop measurements of NdIG. Because the annealing temperature is low and the sample may exhibit high leakage currents, we measured the electrical hysteresis loops using the PUND method at 77 K. The PUND method can subtract nonferro electric components, such as leakage currents and space charge. This means that the true ferroelectric component can be obtained using this method [19,20]. As shown in Fig. 4, a saturated P-E loop is observed at 77 K under an applied electric field of 25 kV/cm, giving a saturation polarization of 28 μC/m2 and a coercive field of 12.18 kV/cm. There fore, the above results reveal the ferroelectricity of the sample. The dielectric anomaly occurs at 565 K, which agrees with the Curie tem perature [15]. The change in magnetic ordering clearly affects the dielectric constant value, which is also observed in other REIGs [9,10,
Two dielectric relaxations, which are attributed to the conduction and homogeneous structure, are identified in the temperature range from 290 K to 525 K in NdIG. The dielectric anomaly around 565 K is due to the coupling between the magnetic ordering and dielectric ordering of the NdIG and indicates that a ferroelectric phase transition occurred in this sample. Furthermore, the maximum value of the remanent polari zation at 77 K is approximately 28 μC/m2, which confirms the existence of a ferroelectric phase transition; thus, our analysis results indicate multiferroicity above room temperature in the sample. The emerging ferroelectricity is attributed to the effective field in NdIG. These obser vations suggest that NdIG is an attractive and new room-temperature multiferroic.
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Solid State Communications 305 (2020) 113766
Fig. 5. (a) Frequency dependence of ac conductivity for NdIG at various temperatures, (b) variation of σdc versus 1000/T for NdIG. The symbols are the experimental points, and the solid line is the Arrhenius fit.
Credit author statement
[6] S.W. Cheong, M. Mostovoy, Nat. Mater. 6 (2007) 13. [7] T. Kimura, Annu. Rev. Mater. Res. 37 (2007) 387. [8] X. Wu, X. Wang, Y. Liu, W. Cai, S. Peng, F. Huang, X. Lu, F. Yan, J. Zhu, Appl. Phys. Lett. 95 (2009) 182903. [9] M.M. Selvi, D. Chakraborty, C. Venkateswaran, J. Magn. Magn. Mater. 423 (2017) 39. [10] J. Su, X. Lu, C. Zhang, J. Zhang, H. Sun, C. Ju, Z. Wang, K. Min, F. Huang, J. Zhu, Physica B 407 (2012) 485. [11] P. Manimuthu, M. Manikandan, M. Malar Selvi, C. Venkateswaran, AIP. Conf. Proc. 1447 (2012) 1205. [12] A.I. Popov, D.I. Plokhov, A.K. Zvezdin, Phys. Rev. B. 90 (2014) 214427. [13] A.I. Popov, D.I. Plokhov, A.K. Zvezdin, Phys. Rev. B. 92 (2015) 144420. [14] N. Kimizuka, A. Yamamoto, H. Ohashi, T. Sugihara, T. Sekine, J. Solid State Chem. 49 (1983) 65. [15] L. Guo, K.K. Huang, Y. Chen, G.H. Li, L. Yuan, W. Peng, H.M. Yuan, S.H. Feng, J. Solid State Chem. 184 (2011) 1048. [16] L. Guo, L. Yuan, K.K. Huang, M.Y. Shang, W. Peng, H.M. Yuan, S.H. Feng, J. Appl. Phys. 110 (2011), 083921. [17] S.F. Shao, J.L. Zhang, P. Zheng, W.L. Zhong, C.L. Wang, J. Appl. Phys. 99 (2006), 084106. [18] Z. Kutnjak, J. Petzelt, R. Blinc, Nature 441 (2006) 956. [19] M. Fukunaga, Y. Noda, J. Phys. Soc. Jpn. 77 (2008), 064706. [20] S.M. Feng, Y.S. Chai, J.L. Zhu, N. Manivannan, Y.S. Oh, L.J. Wang, Y.S. Yang, C. Q. Jin, K. Hoon Kim, New J. Phys. 12 (2010), 073006. [21] S.K. Patri, R.N.P. Choudhary, B.K. Samantaray, Solid State Commun. 144 (2007) 441.
Tingsong Zhang: Investigation, Writing – Original Draft. Chenyang Zhang: Validation, Funding Acquisition. Hongming Yuan: Formal Analysis, Resources. Mingze Xu: Conceptualization, Writing – Review & Editing, Funding Acquisition. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51801171) and Science Foundation for Young Scientists of Changchun University of science and technology (XQNJJ-2018-05). References [1] [2] [3] [4] [5]
J.F. Scott, Nat. Mater. 256 (2007) 6. W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. N.A. Spaldin, S.W. Cheong, R. Ramesh, Phys. Today 63 (2010) 38. D. Khomskii, Phys 2 (2009) 20. J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 5613.
4
Contents lists available at ScienceDirect
Solid State Communications journal homepage: http://www.elsevier.com/locate/ssc
Communication
Dielectric and ferroelectric properties of Nd3Fe5O12 Tingsong Zhang a, Chenyang Zhang b, Hongming Yuan c, **, Mingze Xu a, * a
International Joint Research Center for Nanophotonics and Biophotonics, School of Science, Changchun University of Science and Technology, Changchun, 130022, People’s Republic of China b College of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang, 453003, People’s Republic of China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, 130012, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Communicated by T. Kimura
Multiferroics have attracted research interest for decades due to their potential applications. Multiferroics are relatively scarce due to the mutual exclusivity between magnetic and ferroelectric properties. New roomtemperature multiferroics are looking forward to be exploited. Room-temperature multiferroic properties have been observed in a new rare-earth iron garnet (REIG), Nd3Fe5O12 (NdIG). The dielectric properties of NdIG over a wide temperature range (300 K–650 K) at different frequencies (100 Hz–1 MHz) and in the temperature range from 70 K to 300 K at 100 Hz are investigated herein. The results show that this material has two dielectric re laxations in the temperature range from 290 K to 525 K and a dielectric anomaly around the magnetic ordering temperature (565 K). The first dielectric relaxation in the temperature range from 290 K to 500 K is attributed to the conduction with an activation energy of 0.75 eV. The other dielectric relaxation in an elevated temperature range has an activation energy of 0.87 eV, which originates from the inhomogeneous structure. Furthermore, the ferroelectric polarization of approximately 28 μC/m2 at 77 K is determined by the positive-up-negative-down method and induced by the effective magnetic field created by the iron sublattice magnetic ordering and electric-dipole moments of the neodymium ions.
Keywords: Garnet Dielectric properties Multiferroics
1. Introduction Multiferroics, which exhibit magnetic and ferroelectric ordering simultaneously, have been attracting research interest because of their promising applications in spintronics and data storage [1,2]. One of the most appealing aspects of multiferroics is their so-called magnetoelec tric (ME) coupling. In magnetoelectric materials, ferroelectricity (mag netic ordering) could be tuned by the magnetic field (electric field). However, ferroelectricity and magnetic ordering have different occu pation rules for d orbital electrons that are not compatible with each other and thus lead to a lack of multiferroic materials [3]. Currently, two main methods are widely applied to fabricate multiferroics [4]. The first approach is that the magnetic ordering and ferroelectric ordering orig inate from different subsystems, such as for BiFeO3, where the antifer romagnetic ordering (Neel temperature TN ¼ 643 K) and ferroelectric ordering (Curie temperature TC ¼ 1103 K) are derived from iron ions and bismuth ions, respectively [5]. Although this kind of multiferroics has a high polarization and critical temperature, the ME coupling effect is weak, which is because the ferroelectricity and magnetism are derived
from different units. The second is an approach in which the ferroelec tricity is induced by breaking the magnetic ordering symmetry; for example, in YMn2O5, electric polarization is induced by collinear spin orders in frustrated magnets [6]. In TbMnO3, the noncollinear spiral magnetic structure is responsible for the generation of spontaneous polarization at temperatures below the corresponding magnetic tem perature (TN ¼ 27 K) [7]. The advantage of this kind of multiferroic is that it has a strong ME coupling effect but weak ferroelectricity and a low transition temperature. Therefore, the development of room-temperature multiferroics with a strong magnetoelectric coupling effect is still an urgent issue that remains to be solved. Recently, rare-earth iron garnets (REIG) have attracted widespread attention as they were found to show a strong ME coupling effect at room temperature and under low magnetic fields. Magnetodielectric (MD) properties of Lu3Fe5O12 have been investigated, and an MD coupling (Dε/ε~18%) was observed at room temperature and under low magnetic fields (H~2 kOe) [8]. The ME and dielectric properties of Ho3Fe5O12 have been investigated over a wide temperature range, a dielectric anomaly was observed near the Neel temperature, and ME
* Corresponding author. Tel.: þ86-431-85582739. ** Corresponding author. Tel.: þ86-431-85168318 E-mail addresses: [email protected] (H. Yuan), [email protected] (M. Xu). https://doi.org/10.1016/j.ssc.2019.113766 Received 30 August 2019; Received in revised form 4 October 2019; Accepted 15 October 2019 Available online 16 October 2019 0038-1098/© 2019 Elsevier Ltd. All rights reserved.
T. Zhang et al.
Solid State Communications 305 (2020) 113766
coupling was observed at room temperature and under low magnetic fields [9,10]. After additional research, the multiferroic properties of REIGs were also discussed. P. Manimuthu et al. reported that Lu3Fe5O12 has a dielectric anomaly near 625 K and attributed it to a paraelectric to ferroelectric transition [11]. A. I. Popov et al. reported that REIGs have an antiferroelectric structure due to the effective magnetic field created by the iron sublattice magnetic ordering and electric-dipole moments of the rare-earth ions, and calculated the REIG should have a polarization of 10–100 μC/m2, which is induced by the effective field in the garnets [12,13]. Although the reports mentioned above confirmed that REIGs are room-temperature multiferroics, the ferroelectric hysteresis loops were not given. Nd3Fe5O12 (NdIG), which was not synthesized because the Nd3þ lattice parameter is larger than 12.540 Å [14], has recently been prepared with a hydrothermal method. The NdIG shows ferri magnetic behavior, and the ferrimagnetic Curie temperature, which has been inferred from thermomagnetization curves, was 565 K [15]. As a new and important member of the REIG family, there are only a few reports about its magnetic properties [15,16]. However, the electrical properties of NdIG have not yet been studied. In this work, the dielectric and ferroelectric properties of NdIG were investigated in detail above room temperature. Dielectric and ferroelectric analyses reveal that NdIG exhibits ferroelectricity below 565 K. Therefore, the results presented here indicate multiferroicity above room temperature in the sample material.
Fig. 1. XRD pattern of the sintered Nd3Fe5O12 at room temperature.
2. Experimental The NdIG powders were prepared by the hydrothermal method, as we reported previously [15]. The prepared crystals were ground and pressed into pellets, and were then annealed at 673 K in an O2 atmo sphere for 24 h. Electrodes were attached by using silver paste applied on the two faces of each pellet. The X-ray diffraction data for the sintered powder was collected on Rigaku D/Max 2500 V/Pc X-ray diffractometer with Cu kα radiation (λ ¼ 1.5418 Å) at 50 kV and 200 mA at room tem perature. The temperature dependence of dielectric constants was car ried out in Sigma furnace by using the Agilent Precision (LCR) meter. The dielectric measurements in the temperature range from 70 K to 300 K at 100 Hz were performed in Physical Property Measurement System (Quantum Design). We used electro chemical interface SI 1287 (solartron) and impedance/gain-phase analyzer SI1260 (solartron) to detect electric conduction properties of NdIG. The polarization-electric field (P-E) hysteresis loop was measured using a Radiant Precision II with the positive-up-negative-down (PUND) method. To avoid any contamination of moisture during the dielectric and electrical conduc tion experiments, the sample was preheated to 473 K with an annealing time of 2 h, and then cooled to room temperature prior to the measurements.
Fig. 2. (a) Temperature dependence of ε and tan δ between 300 K and 645 K at selected frequencies in the heating cycles, (b) variation of (tanδ)max with fre quency versus 1000/T for the LTDR, (c) variation of (tanδ)max with frequency versus 1000/T for the HTDR. The symbols are the experimental points, and the solid line is the Arrhenius fit.
relaxation frequency at an infinite temperature, Ea is the activation energy, kB is the Boltzmann constant and T is the absolute temperature. Fig. 2(b) shows the Arrhenius plot of the log of the frequency versus 1/T, and fitting parameters Ea ¼ 0.732 eV are obtained. We refer to this relaxation behavior as low-temperature dielectric relaxation (LTDR). The dielectric properties of NdIG as a function of temperature from 70 K to 300 K at 100 Hz are shown in Fig. 3. The dielectric constant shows an obvious increase, and the loss tangent (tan δ) exhibits a peak at approximately 290 K, which is in good agreement with the data in Fig. 2 (a). The loss tangent peak of 100 Hz comes from the LTDR phenomenon in the sample. In the temperature range from 500 K to 525 K, the dielectric constant shows a step-like increase, and the loss tangent peak shifts its position a low temperature to an elevated temperature with increasing frequency. The relaxation behavior occurring in the loss tangent can be seen in the low frequency range from 100 Hz to 10 kHz; however, it disappears at the high frequency. To distinguish this relaxation behavior from the LTDR, we refer to this relaxation behavior as high-temperature
3. Results and discussion 3.1. Dielectric property The XRD patterns of the sintered NdIG powder are shown in Fig. 1. The X-ray diffraction pattern agrees well with the reported data [15], suggesting that our sample is pure. The dielectric properties above room temperature in the warming process are shown in Fig. 2(a). In the temperature range below 500 K, the dielectric constant shows an obvious increase and then reaches a plateau, whereas there is a differ ence in the inflection temperature for ε, which shifts toward high tem peratures as the frequency increases. In the corresponding temperature range, the presence of a peak in the tanδ curve is observed, and the peak position also shifts to high temperatures with increasing frequency, but the loss tangent peak of 100 Hz is out of the temperature measurement range (inside of Fig. 2(a)). This indicates that a thermally driven relaxation process exists in the sample, which can be approximately described by the Arrhenius relation, f ¼ f0 exp(Ea/kBT), where f0 is the 2
T. Zhang et al.
Solid State Communications 305 (2020) 113766
Fig. 4. P-E hysteresis loop measured by the PUND method at 77 K.
21] and demonstrates the intrinsic coupling between the magnetic ordering and dielectric ordering of the NdIG. The maximum remanent polarization value of NdIG is comparable with that of related magnetic multiferroics and in agreement with the reported values for the pre dicted range [13]. Therefore, it is possible that the polarization in the NdIG below 565 K could mainly be derived from the effective magnetic field created by the iron sublattice magnetic ordering and electric-dipole moments of the neodymium ions. The neodymium ions acquire electric-dipole moments due to the action of a crystal field with a low symmetry. The effective magnetic field is created by the iron sublattice magnetic ordering and electric-dipole moments of the neodymium ions and leads to the formation of an antiferroelectric structure. In the presence of magnetic domain walls, the effective magnetic field becomes inhomogeneous, and as a consequence, the antiferroelectric structure rearranges and a nonzero polarization appears [12,13].
Fig. 3. Temperature dependence of ε and tan δ between 70 K and 300 K at 0.1 kHz during the heating cycles.
dielectric relaxation (HTDR). The variation of (tanδ)max for the fre quency versus 1000/T is shown in Fig. 2(c). This variation also follows the Arrhenius law, f ¼ f0 exp(Ea/kBT), where the fitting parameters Ea ¼ 0.87 eV are obtained for the HTDR. The activation energy is close to the reported values for inhomogeneous structures, such as grain boundaries [17]. It is clear that both the dielectric constant ε and the loss tangent tan δ exhibit anomalous peaks at approximately 565 K for all the applied frequencies (Fig. 2(a)). The positions of these peaks do not vary in terms of the frequency, indicating that the 565 K transition is not related to a ferroelectric relaxor-like behavior [18]. Therefore, it is very likely that these unusual dielectric features are the characteristics of a ferroelectric - paraelectric phase transition.
3.3. Conductivity property Fig. 5(a) shows the variation in the ac conductivity (σac) with the frequency at selected temperatures. It is obvious that σac monotonously decreases with decreasing frequency and becomes independent of the frequency after a certain value. Extrapolating these curves toward a low frequency gives the dc conductivity σdc. The resulting σdc is plotted as a function of reciprocal temperature in Fig. 5(b) and obeys the Arrhenius law, σ ¼ σ0 exp (Econd/kB T), where σ0 is the preexponential term, Econd is the activation energy, and kB is the Boltzmann constant. The fitting Econd is 0.75 eV, which is close to the activation energy of the LTDR. This suggests that the conductivity may play a very important role in the LTDR.
3.2. Ferroelectric property
4. Conclusions
The essential feature of a ferroelectric phase is that the polarization can be reversed through the application of an external electric field. Therefore, we performed P-E hysteresis loop measurements of NdIG. Because the annealing temperature is low and the sample may exhibit high leakage currents, we measured the electrical hysteresis loops using the PUND method at 77 K. The PUND method can subtract nonferro electric components, such as leakage currents and space charge. This means that the true ferroelectric component can be obtained using this method [19,20]. As shown in Fig. 4, a saturated P-E loop is observed at 77 K under an applied electric field of 25 kV/cm, giving a saturation polarization of 28 μC/m2 and a coercive field of 12.18 kV/cm. There fore, the above results reveal the ferroelectricity of the sample. The dielectric anomaly occurs at 565 K, which agrees with the Curie tem perature [15]. The change in magnetic ordering clearly affects the dielectric constant value, which is also observed in other REIGs [9,10,
Two dielectric relaxations, which are attributed to the conduction and homogeneous structure, are identified in the temperature range from 290 K to 525 K in NdIG. The dielectric anomaly around 565 K is due to the coupling between the magnetic ordering and dielectric ordering of the NdIG and indicates that a ferroelectric phase transition occurred in this sample. Furthermore, the maximum value of the remanent polari zation at 77 K is approximately 28 μC/m2, which confirms the existence of a ferroelectric phase transition; thus, our analysis results indicate multiferroicity above room temperature in the sample. The emerging ferroelectricity is attributed to the effective field in NdIG. These obser vations suggest that NdIG is an attractive and new room-temperature multiferroic.
3
T. Zhang et al.
Solid State Communications 305 (2020) 113766
Fig. 5. (a) Frequency dependence of ac conductivity for NdIG at various temperatures, (b) variation of σdc versus 1000/T for NdIG. The symbols are the experimental points, and the solid line is the Arrhenius fit.
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Tingsong Zhang: Investigation, Writing – Original Draft. Chenyang Zhang: Validation, Funding Acquisition. Hongming Yuan: Formal Analysis, Resources. Mingze Xu: Conceptualization, Writing – Review & Editing, Funding Acquisition. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51801171) and Science Foundation for Young Scientists of Changchun University of science and technology (XQNJJ-2018-05). References [1] [2] [3] [4] [5]
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