Crystal structure, leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics

Author’s Accepted Manuscript Crystal structure, leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics Jie Wei, Yang Liu, Xiaofei Bai, Chen Li, Yalong Liu, Zuo Xu, Pascale Gemeiner, Raphael Haumont, Ingrid C. Infante, Brahim Dkhil www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)30716-7 http://dx.doi.org/10.1016/j.ceramint.2016.05.106 CERI12911

To appear in: Ceramics International Received date: 10 November 2015 Revised date: 16 May 2016 Accepted date: 17 May 2016 Cite this article as: Jie Wei, Yang Liu, Xiaofei Bai, Chen Li, Yalong Liu, Zuo Xu, Pascale Gemeiner, Raphael Haumont, Ingrid C. Infante and Brahim Dkhil, Crystal structure, leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO 3 ceramics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.05.106 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Crystal structure, leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics Jie Wei, 1, 2,* Yang Liu, 2 Xiaofei Bai, 2 Chen Li, 1 Yalong Liu, 1 Zuo Xu, 1 Pascale Gemeiner, 2 Raphael Haumont, 3 Ingrid C. Infante, 2 and Brahim Dkhil 2 1

Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education & International Center for

Dielectric Research, Xi'an Jiaotong University, Xi'an 710049, P. R. China 2

Laboratoire Structures, Propriétés et Modélisation des Solides, Centralessupélec, CNRS-UMR8580, Université

Paris-Saclay, Grande Voie des Vignes, 92295 Chatenay-Malabry Cedex, France 3

Laboratoire de Physico-Chimie de l'Etat Solide, ICMMO, CNRS-UMR 8182, Bâtiment 410 -Université Paris-Sud

XI, 15 rue Georges Clémenceau 91405 Orsay Cedex, France

ABSTRACT: Ceramics of pure phase Yttrium (Y) doped BiFeO3 prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO3. Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO3 to a Poole–Frenkel emission behavior in Bi0.90Y0.10FeO3. A Fowler-Nordheim tunneling mechanism in Bi0.95Y0.05FeO3 ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi0.90Y0.10FeO3 ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications.

KEYWORDS: BiFeO3;

Dopant;

Multiferroic;

Leakage mechanism;

Raman spectra

1

1. INTRODUCTION Over the past few years, multiferroic materials have attracted tremendous interest due to their considerable application potentials as well as their fascinating fundamental physics

1,2

. Amongst

these multiferroics, BiFeO3 has been intensely studied as the prototypical multiferroic oxide

3-10

,

since it can exhibit both ferroelectricity and antiferromagnetism above room temperature (the ferroelectric Curie temperature TC ~ 820℃ and the antiferromagnetic Néel temperature TN ~ 370℃). At ambient conditions, BiFeO3 phase is described as a rhombohedrically distorted perovskite structure belonging to the R3c space group, which allows antiphase octahedral tilting and ionic displacements from the centrosymmetric positions about and along, respectively, a same <111> cubic-like direction. 11 Since a very large ferroelectric polarization (remanent polarization Pr ~ 60 μC/cm2) was achieved in a [001]-oriented BiFeO3 thin film epitaxially grown on SrRuO3/SrTiO3,3 significantly increasing

studies

12-15

have been stimulated in synthesis of BiFeO3 thin films leading to

polarizations ranging from 50 to 150 μC/cm2. Theoretical work has also addressed the extraordinarily large ferroelectric polarization of BiFeO3.4 Unfortunately, BiFeO3 ceramics still exhibit very weak ferroelectric behavior, in contrast to thin films, mainly due to its high electrical conductivity and appearance of secondary phases, which further hinders its practical application. The origin of this high leakage current may be attributed to the presence of oxygen vacancies or the coexistence of Fe3+/Fe2+

16

. Moreover, in addition to the G-type antiferromagnetic spin ordering in

BiFeO3, a cycloid-type spatial spin modulation that prevents the observation of an intrinsic and weak ferromagnetic moment driven by Dzyaloshinsky–Moriya (DM) interactions occurs below the Néel temperature TN. Therefore, it is still more challenging to directly obtain the room temperature magneto-electric coupling interaction in bulk BiFeO3. In order to overcome these problems, the use of different synthesis methods, 17, 18 the addition of different dopants,5, 8 or the formation of solid solutions with other perovskites,19,20 have been thus intensively investigated. Among these different possible strategies, typical A(Bi)- or B(Fe)-site substitution in BiFeO3 by rare earth elements or transition metal elements has been proposed as a promising approach to 2

suppress the formation of secondary phases,8 improve ferroelectric properties by reducing leakage current,21,22 and induce ferromagnetism23,24 by destroying the spatially modulated cycloid spin structures.25,26 Although great efforts have been made to investigate the effect of rare-earth dopants (La, Sm, Dy, etc.)

22, 27-29

or divalent metal dopants (Ca, Ba, Pb, etc.)

30,31

on the multiferroic

properties of BiFeO3 ceramics, studies on the leakage conduction mechanism and its evolution in these compounds are still very scarce. In the present study, we thus investigated the leakage mechanism evolution and multiferroic properties of Yttrium (Y) doped BiFeO3 ceramics. It is found that the leakage current density of BiFeO3 ceramic is greatly reduced by Y doping, leading to improved ferroelectric properties in Y-doped BiFeO3 ceramics. The leakage conduction mechanism also changes from space-charge-limited conduction type in pure BiFeO3, into a Poole–Frenkel emission behavior in Bi0.90Y0.10FeO3, and Fowler-Nordheim tunneling type in Bi0.95Y0.05FeO3 ceramic under high electric fields. In addition, a weak ferromagnetism is obtained since Y-doping destroys the spatially modulated cycloid spin structure of BiFeO3. Especially, an intriguing switching behavior in magnetization under low magnetic fields is observed in Bi0.90Y0.10FeO3 ceramic, suggesting promising magneto-electric applications together with its improved ferroelectric properties.

2. EXPERIMENTAL PROCEDURE Bi1-xYxFeO3 ceramics (x=0, 0.05, 0.10, 0.15) were prepared by a conventional solid-state sintering procedure. High-purity oxides of Bi2O3, Fe2O3, and Y2O3 were used as the starting materials. After weighing, ball milling and drying, the mixed powders were pressed into small cylindrical pellets with a diameter of ~8 mm and thickness of ~0.6 mm. Then, the samples were sintered at 810℃ for 3h in air. Phase purity and crystal structure were characterized by X-ray diffraction (XRD) on a Rigaku D/MAX-2400 X-ray diffractometer using Cu Kα radiation (λ=1.5406 Å). Rietveld refinements were performed using XND analysis program. Room-temperature polarized Raman spectra were recorded in a back-scattering geometry using a LABRAM Jobin–Yvon spectrometer (He–Ne laser, 632.8 nm). In this regard, it was verified that the used laser power did not produce any significant heating or damage of the samples. The high-temperature Raman measurements were carried out by using a commercial LINKAM heating stage placed under the Raman microscope. The magnetization 3

hysteresis loops (M-H) were obtained using a superconducting quantum interference device magnetometer (SQUID, Quantum Design). Polarization hysteresis loops (P-E) and leakage currents were measured at room temperature using a ferroelectric test system (TF Analyzer 2000, aixACCT). Before the ferroelectric and leakage current measurements, the sintered disk samples were polished down with the thickness being of 0.3mm and then sputtered with Au electrodes for the corresponding electrical measurements.

3. RESULTS AND DISCUSSION A. Crystal structure Figure 1 shows typical XRD patterns of pure BiFeO3 and Y-doped BiFeO3 ceramics (Bi0.95Y0.05FeO3 and Bi0.90Y0.10FeO3). The XRD patterns clearly indicate a pure phase for all sintered ceramics without any secondary phases. All the diffraction peaks can be successfully indexed as a rhombohedrally distorted perovskite structure with a space group of R3c (ICDD File Card No. 86-1518), showing that the crystal structure of Bi1-xYxFeO3 even with 10% Y doping remains rhombohedral R3c. However, a small amount of secondary phases can be detected with higher doping like in the sample of Bi0.85Y0.15FeO3 (not shown here). It is not possible to achieve a pure phase of Bi0.85Y0.15FeO3 even though the sintering temperature is increased upon 880℃ using a rapid liquid sintering technology.

17

This could be an indication of the saturation level of Y

substitution in BiFeO3 system. It should be mentioned that Y-doped BiFeO3 ceramics (Bi0.95Y0.05FeO3 and Bi0.90Y0.10FeO3) have the same crystalline structure as that of the parent compound, implying that the ferroelectric features of Y-doped BiFeO3 ceramics should be close to that of BiFeO3.

[Fig.1 here.]

For each pattern, we performed a Rietveld refinement by using XND software. Tables inset of Fig.1 show the variation of unit cell parameters versus the content of Y in Bi1-xYxFeO3 (x=0, 0.05, and 0.10) ceramics. With increasing Y content, both a and c parameters decrease, leading to the contraction of the unit cell volume, because of the smaller ionic radius of Y3+ (1.02 Å) with respect 4

to Bi3+ (1.17 Å). Contraction in the lattice should lead to the variation in the inter-atomic bond distances, bond angles and octahedral tilts, which further affect the multiferroic properties of BiFeO3. Interestingly, our refinements reveal a decrease in the average Fe-O-Fe bond angle which changes from 157.760 in BiFeO3 to 155.980 in Bi0.90Y0.10FeO3. As a result, Y doping tends to buckle FeO6 octahedra, which is due to the lattice contraction. Consequently, the magnetic properties would change because Fe ions are more close to each other implying an enhancement in the oxygen-related super-exchange interaction. 8

B. Raman scattering spectroscopy Raman scattering spectroscopy, that measures phonons at the Brillouin zone centre, possesses strong selection rules and is known to be a powerful technique for the investigation of even subtle structural distortions within a space group (via band shifts) or due to a phase transition (via band splitting and/or soft modes etc.). At room temperature BiFeO3 crystallizes in a highly rhombohedrally distorted perovskite, which belongs to the space group R3c. With respect to the cubic Pm-3m structure, the rhombohedral structure can be represented by an anti-phase tilt of the adjacent FeO6 octahedra and a displacement of both Fe3+ and Bi3+ cations from their centrosymmetric position along the pseudo-cubic [111]pc direction. The 10 atoms in the unit cell of its rhombohedral R3c (C3v) structure give rise to 13 Raman phonon modes in the zone centre (k ~ 0) 32

Raman, R 3c  4 A1  9E

(1)

Figure 2 shows the measured Raman spectra of BiFeO3, Bi0.95Y0.05FeO3 and Bi0.90Y0.10FeO3 at room temperature. Through fitting the measured spectra and decomposing the fitted curves into individual Lorentzian components, 13 peak positions corresponding to each Raman active mode, were obtained in all the three samples. The corresponding results are summarized in Table I. As expected by theory

33

, all the Raman modes (4A1+9E) associated with R3c structure of BiFeO3 are

observed. Any other extra Raman mode possibly arising from impurity phases cannot be found in our samples. It further confirms that Y doping does not change the crystalline structure of parent BiFeO3, as revealed by the XRD data (see Fig.1).

5

[Fig.2 here.]

Table I Raman modes of Bi1-xYx FeO3 (x=0, 0.05, and 0.10). Raman modes A1-1 A1-2 A1-3 A1-4 E E E E E E E E E

Raman shift (cm-1) Bi0.90Y0.10FeO3 Bi0.95Y0.05FeO3 142 141 168 171 230 227 426 425 72 73 108 110 258 260 272 273 336 338 366 368 471 470 522 520 616 616

BiFeO3 140 173 225 426 75 110 261 277 347 370 472 522 617

A closer inspection of the Raman spectra shown in Fig.2 reveals that the spectral features of Y-doped BiFeO3 ceramics show a drastic intensity reduction of A1-2 mode (173 cm-1) , and slight broadening or slow shifting of some A1 and E modes, which suggest apparent changes in phonon behavior. It is believed that these changes should be attributed to a change in the local atomic structure due to the incorporation of Y in the BiFeO3 lattice. Based on first-principles calculations, Hermet et al

33

reported that Bi atoms only participate to low-frequency modes up to 167 cm-1 and

the motion of oxygen atoms dominates modes above 262 cm-1, whereas Fe atoms are mainly involved in modes between 152 and 262 cm-1 with possible contribution to higher frequency modes. Accordingly, the two characteristic modes such as A1-1 mode (~140 cm-1) and E mode at ~75 cm−1 should be governed by Bi-O covalent bonds, which control the dielectric constant and the ferroelectric phase of BiFeO3 33, 34. The above Raman spectra results reflect that the Bi-O covalent bonds and crystal structure remain relatively stable in Y-doped BiFeO3 ceramics, because the aforementioned two characteristic modes do not present any change although their line widths show slight broadening due to the disorder induced in A-site by Y substitution. It suggests that Y doping 6

does not disturb the ferroelectric nature of BiFeO3. However, the A1-2 mode (~173 cm-1) and A1-3 mode (~ 225 cm-1) associated with Fe atoms

33

responsible for magnetism exhibit drastic changes

due to Y doping. As the Y content increases, the intensity of A1-2 mode strongly decreases, while the intensity of A1-3 mode gradually increases. It implies that the magnetic properties of BiFeO3 might be affected by Y doping because Fe are the atoms responsible for ferromagnetism. In recent years, the spin-phonon coupling in multiferroic BiFeO3 has gained much attention, because understanding the correlation between structure and spin–phonon coupling could provide a good understanding of magnetoelectric properties in BiFeO3. Haumont et al

35

observed strong

phonon anomalies in the Raman spectra of BiFeO3 crystal across the Neel temperature (TN), which have be associated to the manifestation of the spin phonon coupling. M. K. Singh et al 36 and D. Rout et al

37

also reported phonon anomalies near magnetic ordering temperature TN owing to the

spin–phonon coupling in BiFeO3 thin film and ceramic, respectively. In order to study the potential effect of Y dopants on the magnetic properties or Neel temperature of BiFeO3, we performed temperature-dependent Raman spectra studies on Y-doped BiFeO3 ceramics. Figure 3 shows temperature-dependent Raman spectra and temperature-dependent evolution of some spectral features for Y-doped BiFeO3 ceramics. As seen in Fig.3a and 3c, some changes can be evidenced in the Raman spectra of all the samples with increasing temperature: (1) gradual reduction in intensity of all major peaks, (2) most peaks shift continuously towards lower frequency at lower temperatures, and (3) the high frequency peaks severely broaden and merge into a broad peak, which later disappear completely as the temperature rises towards the Neel temperature TN. The above observations such as red shift in band positions and broadening in the bandwidth can be explained by thermal broadening and thermal disorder that are so-called anharmonic effects of the lattice 35. However, the phonon anomalies in the vicinity of TN, for example, 600K~680K for Bi0.95Y0.05FeO3 and 540~650K for Bi0.90Y0.10FeO3 ceramic, could not be attributed to the anharmonic effects. To clearly display the phonon anomalies in the vicinity of TN, the band position and full width at half maximum (FWHM) of two representative A1 modes namely at 140 and 173  cm-1 for Y-doped BiFeO3 ceramics have been plotted as a function of increasing temperature, as shown in Fig.3b and 3d. The accurate phonon frequencies were evaluated by fitting the spectra using multiple peaks fitting with a Lorentzian line 7

shape function. In accordance with above analysis, the phonon modes softening behavior (such as the band position shifts to low frequency and FWHM increases with increasing temperature ) at low temperature should be attributed to the anharmonic effects of the lattice. Nevertheless, apparent step-like anomalies were observed in both band position and FWHM of the two representative A1 modes at temperatures of about 640K for Bi0.95Y0.05FeO3 and about 630k for Bi0.90Y0.10FeO3 ceramic. No extra mode is induced through the entire temperature range, which confirms the absence of any structural transition around this temperature in these ceramics. By comparison with the earlier reports 35-37

, this phonon anomaly behavior is believed to be due to the manifestation of the spin phonon

coupling and it should be linked to structural instability originated by gradual rotation of FeO6 octahedra near TN. Accordingly, the temperatures of 640K and 630K where the phonon anomalies occurred should correspond to the Neel temperature for Bi0.95Y0.05FeO3 and Bi0.90Y0.10FeO3 ceramic, respectively. In brief, temperature-dependent Raman spectra of Y-doped BiFeO3 ceramics reveal that Y doping does not disturb the antiferromagnetic temperature of BiFeO3, due to the strong spin-phonon interactions existing in these ceramics.

[Fig.3 here.]

C. Magnetic properties It is known that, BiFeO3 has a G-type antiferromagnetic (AFM) spin ordering that is modulated by a cycloid-type spatial spin structure with a long periodicity of 62 nm

16

. As a result, this

modulation prevents the observation of an intrinsic and weak ferromagnetic moment driven by Dzyaloshinsky–Moriya (DM) interactions below TN. Nevertheless, it is possible to perturb this spatial spin modulation by doping the crystal structure so as to allow the appearance of a net magnetization through the canting of the spins.

[Fig.4 here.]

Figure 4a shows the magnetization hysteresis loops of pure BiFeO3 and Y-doped BiFeO3 8

ceramics measured at room temperature. It can be seen that, pure BiFeO3 ceramic exhibits an intrinsic antiferromagnetic behavior because of its almost linear M-H curve (see the enlarged M-H curves in the inset of Fig.4a) and a negligible remanent magnetization (Mr, shown in Table II). However, a weak ferromagnetism with the remanent magnetization of Mr ~ 0.025emu/g is observed in Bi0.95Y0.05FeO3 ceramic. The remanent magnetization increases as Y content increases. Therefore, the largest Mmax and Mr of 1.227 emu/g (corresponding to 0.069μB/Fe) and 0.086 emu/g are respectively achieved in Bi0.90Y0.10FeO3 ceramic (see Table II). More surprisingly, a quick jump of the magnetization is obviously seen in case of Bi0.90Y0.10FeO3 ceramic. We will come later on this observation.

Table II magnetic parameters of Bi1-xYx FeO3 (x=0, 0.05, and 0.10) Magnetic properties Samples

Mmax at 50kOe (emu/g) (μB/Fe)

Mishra et al.

38

Mr (emu/g) HC (kOe)

BiFeO3

0.384

0.022

0.003

0.2

Bi0.95Y0.05FeO3

0.445

0.024

0.025

2.0

Bi0.90Y0.10FeO3

1.227

0.069

0.086

0.1

investigated the magnetic property of Y-doped BiFeO3 nanoparticles, and the

enhanced magnetization was partially attributed to the reduction in particle size to the range of the modulation length of the spin cycloid (∼62 nm). Obviously, this argument is not valid for Y-doped BiFeO3 ceramics. Commonly, magnetic secondary phases such as Fe3O4, γ-Fe2O3 are speculated as the possible contribution to the enhanced magnetization in Y-doped BiFeO3 ceramics, because the mere presence of Y2O3 might induce formation of these magnetic secondary phases. In order to investigate the origin of the induced weak ferromagnetism in Y-doped BiFeO3 ceramics and its stability above room temperature, the high temperature magnetic measurements (M-T curves) were carried out in the temperature range of 300K~800K under a applied field of 2 kOe by using vibrating sample magnetometer (VSM, LakeShore 730)

39

. As shown in Fig.4b, the high temperature M-T

curves show anomalies at temperatures of about 640K for Bi0.95Y0.05FeO3 and about 630k for Bi0.90Y0.10FeO3 ceramic, which correspond to the magnetic phase transition temperatures TN of Y 9

doped BiFeO3 ceramics. The high temperature magnetic behaviors (M-T curves) of our samples further confirm that Néel temperatures TN of Y doped BiFeO3 ceramics are close to the one of parent BiFeO3, which is in good agreement with our Raman measurement results. There is no trace of the magnetic ordering temperatures of possible magnetic secondary phases such as Fe3O4, γ-Fe2O3 that should occur beyond 700K as the magnetization remains rather flat until 800K. Hence, the contribution from magnetic secondary phases to enhanced magnetization in Y-doped BiFeO3 ceramics can be excluded. The origin of the enhancement in the magnetization of Y-doped BiFeO3 ceramics should be probably explained according to the following two aspects, (i) Y doping perturbs the spatial spin modulation and partially destroys the spiral structure; (ii) Contraction in the lattice due to Y doping makes Fe ions close to each other, thus leads to an enhancement in the oxygen-related super-exchange interaction, which is supported by our foregoing XRD and Raman spectra results. More interestingly, we observed an unusual switching behavior in the magnetic hysteresis loop of Bi0.90Y0.10FeO3 ceramic. To be more specific, magnetization shows a sudden and sharp increase in low magnetic fields while it increases almost linearly at higher magnetic fields, indicating a switchable behavior (see the corresponding M-H curve in Fig.4a). Such a noticeable change in the M–H loops from a straight line for BiFeO3 to a sizable and quickly switchable loop for Bi0.90Y0.10FeO3 ceramic may imply a transformation of Fe sub-lattice from antiferromagnetism to weak ferromagnetism. Definitely, the substitution of Y3+ ions being non-magnetically active cannot contribute directly to the enhancement of magnetization, which may rather arise from Y-doping that induces the collapse of space modulated cycloidal spin structure. However, this unique switching behavior in Bi0.90Y0.10FeO3 ceramic is quite different from other A-site or B-site doped BiFeO3 ceramics

40, 41

, and the mechanism needs to be further studied combining with theory investigation

such as first-principles calculations.

C. Electrical properties In the previous studies, the coexistence of weak ferromagnetism and ferroelectricity has been suggested in ions substituted BiFeO3 ceramics, 28-31 but mostly supported by the indirect ferroelectric characteristics. Nevertheless, the most indisputable method confirming the ferroelectric properties of one material is to measure its polarization hysteresis loop (P-E). However, there have only a few reports on saturated ferroelectric hysteresis loops for BiFeO3-based ceramics at room temperature 10

until now

42, 43

. Furthermore, a severe discrepancy regarding the effect of rare-earth doping on the

ferroelectric properties of BiFeO3 still exists in previous literature. For example, Yan et al.

22

and

Zhang et al. 43 reported an increase of Pr with increasing doping concentration of Yb up to 15% and Dy up to 10 % in BiFeO3 ceramics, respectively. On the other hand, Khomchenko et al. 44 reported a gradual decrease of polarizable phases with increasing concentration of Gd. Similarly, Jeon et al

45

found that the ferroelectric property was degraded by rare-earth Ho doping and Pr of Ho doped BiFeO3 was lower than that of the undoped one under high electric fields.

[Fig.5 here.]

Figure 5 shows the polarization hysteresis loops (P-E) of pure and Y-doped BiFeO3 ceramics. The P–E loops for all the samples were measured at a frequency of 30 Hz. As expected, pure BiFeO 3 ceramic shows a severe leaky characteristic observed from its "inflated" polarization hysteresis loop. Unfortunately, it was not possible to achieve well saturated polarization loops in Y-doped BiFeO3 ceramics even though the applied field was beyond the breakdown field.

Nonetheless, it can be

seen that the leaky characteristics are greatly improved by Y-doping (see Fig.6). Thus, one could anticipate to obtain saturated polarization loops in high quality epitaxial thin films of Y-doped BiFeO3. Furthermore, we studied the leakage behavior of both undoped and Y doped BiFeO3 ceramics at room temperature, and then analyzed the evolution of leakage mechanism in these samples. It is believed that leakage current is the key factor affecting the ferroelectric properties of BiFeO3. Typical leakage data I-V as a function of applied voltage with both positive and negative biases for all samples measured at room temperature are shown in Fig.6. Obviously, Y doping greatly reduces the leakage current of BiFeO3 ceramic, and the smallest leakage current density is obtained in the sample of Bi0.90Y0.10FeO3. For example, under the applied electric field of 5kV/cm, the leakage current density of Bi0.90Y0.10FeO3 ceramic is lowered by two orders of magnitude in comparison with pure BiFeO3.

[Fig. 6 here.] 11

In order to well understand the origin of this reduction in leakage current induced by Y doping, we further made discussions about the leakage conduction mechanism of both undoped and doped BiFeO3 ceramics. According to the previous literature, there exist several theoretical models that may be used to describe the possible leakage current mechanisms for BiFeO3 and other similar ferroelectric perovskite oxides. 46, 47 These mechanisms can be divided into two categories, bulk limited and interface-limited conduction mechanisms. Recent studies have suggested four possible leakage mechanisms for BiFeO3 thin films, such as the bulk-limited space-charge -limited conduction (SCLC) mechanism, bulk limited Poole–Frenkel conduction (PF) mechanism, interface-limited Schottky emission, and interface-limited Fowler-Nordheim tunneling (FN) mechanism. 48-50 SCLC is expressed as: 48

9 E2 J SCLC   r 0  ( ) 8 d

(2)

where  r is the relative dielectric constant of the sample,  0 is the permittivity of free space,  is the charge carrier mobility,

E is the electric field and d is the thickness of the sample.

The mechanism of SCLC is similar to the transport conduction of electrons in a vacuum diode. At low bias voltage, the current is limited by thermionic emission from the cathode of a vacuum diode. With the voltage increasing, the charges injected from the cathode increase and gradually fill with the space between the cathode and anode. Therefore, it is the current limiting effect at high current range. Generally, this regime is depicted as the space-charge-limited-current, in which the current density J follows an E2/d dependence rather than an Ohmic relationship. Fig.7a shows the plots of log(J) vs log(E) for undoped and doped BiFeO3 ceramics at a positive bias at room temperature. The plot for pure BiFeO3 ceramic shows a linear behavior with a slope of around 2, which is in good agreement with SCLC mechanism. Similarly, Ramachandran et al. 51 also found a SCLC conduction mechanism in BiFeO3 ceramic. As for Bi0.95Y0.05FeO3 ceramic, the plot also shows a linear behavior with a slope of around 2 under low electric fields less than 251 kV/m, revealing a dominating role of SCLC mechanism in this region. At higher fields, the plot shows a 12

nonlinear behavior, thus SCLC can be ruled out for the leakage mechanism in this field region. Indeed, this result implies that there exists more than one leakage mechanism in Bi0.95Y0.05FeO3 ceramic. Concerning Bi0.90Y0.10FeO3 ceramic, the plot shows a linear behavior with a slope of around 1 under low electric fields less than 160 kV/m, indicating an Ohmic conduction behavior. At higher fields, the plot shows an exponential trend, implying that PF emission may be the dominant mechanism in this region.

[Fig. 7 here.]

As for PF mechanism, PF emission is very similar to Schottky emission. It involves a process where charge carriers trapped in defect centers emit into the conduction band of the dielectric and contribute to the conduction process

52

. Therefore, PF emission is sometimes called the “internal

Schottky emission”. The trap centers could be distributed in the forbidden region between the valence band and the conduction band of the material. The carriers in the traps could be activated either thermally or electrically. Under an applied electric field, at a given temperature, the ionization of traps induces the emission of charge carriers and gives rise to conduction. The current density due to the bulk limited PF emission is given by 50

  (t  e eE /  r 0 )  J PF  AE exp   k BT   where A is a constant,

(3)

t is the trap ionization energy, e is the electron charge,  r is the relative

dielectric constant of the sample,

 0 is the permittivity of free space, k B is Boltzmann’s constant,

and T is the temperature. Since PF emission is owing to the thermal activation under an electric field, this conduction mechanism is often observed at high temperature and high electric fields. Fig.7b shows the plots of ln(J/E) vs. E0.5 at a positive bias measured under room temperature. Based on our analysis of Eq. (3), the plot should present a linear relation between ln(J/E) and E0.5, in case PF emission is dominant. As expected, the semilog plot of J/E vs E0.5 presents a linear behavior, implying a dominant PF mechanism for Bi0.90Y0.10FeO3. Importantly, the dielectric constant (  r ) and 13

the refractive index (n) can be calculated from the slope of ln(J/E) vs E0.5, where  r =n2. The refraction index of BiFeO3 was reported to be n=2.5 (Ref. 16). Therefore, a dielectric constant close to 6.25 is expected in Bi0.90Y0.10FeO3 ceramic. The plots of ln(J/E) vs E0.5 yield to an experimentally extracted dielectric constant of ~7.13 in Bi0.90Y0.10FeO3 ceramic, which is very close to that expected. It therefore further supports that PF emission is the dominant conduction mechanism in Bi0.90Y0.10FeO3 ceramic. Similarly, Yuan et al.21 also proposed a dominant PF conduction mechanism in Nd-doped BiFeO3 ceramics. In case of Bi0.95Y0.05FeO3 ceramic, there is not any linear behavior observed, implying that PF mechanism for such composition can be ruled out. The injection of charge carriers into a ferroelectric layer from electrodes may take place by tunneling through an interfacial energy barrier, which is called as FN tunneling and can be described by: 50

J FN

 Ci  BE exp( E 2

3/ 2

(4)

)

where B and C are constants and i is the potential barrier height, respectively. Fig.7c shows plots of ln(J/E2) vs. (1/E) at different temperatures of 300, 310, 320, 330 and 340 K. A linear relation is obtained at all temperatures under high electric fields, suggesting an FN tunneling behavior in Bi0.95Y0.05FeO3 ceramic under high electric fields. The onset electric field Et of FN tunneling decreases with increasing temperature, as shown in the inset table (Fig. 7c), suggesting a decrease in the potential barrier height. According to the previous literature, the trap centers in BiFeO3 thin films are likely either oxygen vacancies or Fe2+ ions.

48, 49

For example, Pabst et al.

48

reported a PF emission behavior in

BiFeO3 thin films with zero-field trap ionization energy of 0.65–0.8 eV and claimed that Fe2+ ions are the likely trap centers in BiFeO3. In contrast, Khan et al 49 reported that (1−x) BiFeO3– x PbTiO3 (x=0.4 and 0.5) thin films exhibited the PF conduction mechanism with zero-field trap ionization energy of 1.08~1.27 eV. Thus, they concluded that the oxygen vacancies are the likely trap centers in these films. To determine the origin of trap centers in Bi0.90Y0.10FeO3, we measured leakage current density as a function of temperature in Bi0.90Y0.10FeO3 ceramic. Fig.7d shows the plots of ln(J/E) vs 1000/T under different applied fields. Based on the analysis of Eq. (4), the trap ionization energy

t

can be deduced from the slope of ln(J/E) vs 1000/T at a fixed electric field. As seen in the inset of 14

Fig.7d, an extrapolation gives zero-field trap ionization energy of 0.935 eV, which is expected for oxygen vacancies mobility. This result implies that oxygen vacancies are the likely trap centers in Bi0.90Y0.10FeO3 ceramic, rather than Fe2+ ions. Therefore, we may conclude that Y substitution can reduce oxygen vacancies and suppress the transformation of Fe3+ into Fe2+, while reducing the leakage current and enhancing the ferroelectric properties of bulk BiFeO3.

4. CONCLUSIONS In summary, pure and single phase Y-doped BiFeO3 ceramics (x=0, 0.05 and 0.10) were synthesized by conventional solid-state sintering route and characterized by X-ray diffraction, Raman spectroscopy, the magnetic and electrical measurements. Both XRD results and Raman spectra suggested that Y-doping does not disturb the ferroelectric nature of BiFeO3 but does affect its magnetic property.

A weak ferromagnetism and an unexpected switching behavior in the

magnetization were observed in Bi0.90Y0.10FeO3 ceramic. Enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. Leakage current is greatly reduced by Y-doping and leakage conduction mechanism is observed to change from SCLC mechanism in pure BiFeO3 into PF conduction mechanism in Bi0.90Y0.10FeO3. A FN tunneling mechanism in Bi0.95Y0.05FeO3 ceramic under high electric fields is also evidenced. Therefore, the weak ferromagnetism and improved ferroelectric property in Y-doped BiFeO3 ceramics reveal a promising potential using such materials for developing a new generation of electric devices to resolve the power consumption and variability issues in today’s microelectronics industry. ACKNOWLEDGEMENTS This work has been supported by the National Science Foundation of China (NSFC No. 51272204), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20110201120003) and the French ANR program NOMILOPS (ANR-11-BS10-016-02) project. J. Wei, Y. Liu and X.F. Bai also wish to thank the China Scholarship Council (CSC) for funding their stay in France.

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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ;

[email protected] (J. Wei)

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Captions of Figures Fig.1

XRD patterns and structural refinements (observed and calculated patterns) of Bi1-xYx FeO3

(a) x=0, (b) x= 0.05, and (c) x=0.10. Table inset of each pattern shows Perovskite unit cell parameters.

Fig.2 Raman spectra of Bi1-xYx FeO3 (x=0, 0.05, and 0.10) ceramics.

Fig.3

Temperature-dependent Raman spectra of (a)Bi0.95Y0.05 FeO3 and (c) Bi0.90Y0.10FeO3 ceramics;

Temperature-dependent evolution of band position and FWHM of A1-1 and A1-2 modes for (b) Bi0.95Y0.05 FeO3 and (d) Bi0.90Y0.10 FeO3 ceramics.

Fig.4

(a) Magnetization hysteresis loops (M-H) of Bi1-xYx FeO3 (x=0, 0.05, and 0.10), inset :

Zoom-in of M-H curves to clearly display the remanent magnetization and coercive field; (b) Temperature dependence of the magnetization for the Y doped BiFeO3 ceramics in a high temperature range.

Fig.5

Ferroelectric polarization loops of Bi1-xYxFeO3 ceramics (a) x=0, (b) x= 0.05, and (c) x=0.10.

Fig.6

J-E characteristics of both undoped and doped BiFeO3 ceramics for both negative and

positive bias fields measured at room temperature.

Fig.7

(a) log(J) vs log(E) plots at the positive bias; (b) plots of ln(J/E) vs E0.5 at the positive bias; (c)

ln (J/E2) vs (1/E) at the positive bias. Et is the onset electric field. (d) Temperature dependence of conductivity with linear fits (dotted lines) used to extract trap ionization energies; inset shows the plot of

t vs E0.5 in order to extrapolate the zero-field trap ionization energy.

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