Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho3+ modified BiFeO3 multiferroic material

Accepted Manuscript Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho modified BiFeO3 multiferroic

3+

G.L. Song, Y.C. Song, J. Su, X.H. Song, N. Zhang, T.X. Wang, F.G. Chang PII:

S0925-8388(16)33616-7

DOI:

10.1016/j.jallcom.2016.11.155

Reference:

JALCOM 39651

To appear in:

Journal of Alloys and Compounds

Received Date: 9 August 2016 Revised Date:

6 October 2016

Accepted Date: 10 November 2016

Please cite this article as: G.L. Song, Y.C. Song, J. Su, X.H. Song, N. Zhang, T.X. Wang, F.G. Chang, 3+ Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho modified BiFeO3 multiferroic, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.155. 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 proof before it is published in its final 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.

ACCEPTED MANUSCRIPT

Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho3+ modified BiFeO3 multiferroic G. L. Song , Y. C. Song, J. Su , X. H. Song, N. Zhang, T. X. Wang , F. G. Chang (College of Physics and Materials Science, Henan Key Laboratory of Photovoltaic Materials, Affiliated Middle School of Henan Normal University, Henan Normal University, Xinxiang, 453007, China)

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Abstract: Multiferroic Bi1-xHoxFeO3 (x=0, 0.05, 0.1) ceramics have been prepared by rapid liquid phases sintering method. The effect of Ho3+ doping on the crystal structure, dielectric, ferroelectric properties, TN and TM of BiFeO3 ceramics is studied. The result shows that all the peaks for

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Bi1-xHoxFeO3 samples can be indexed according to the crystal structure of pure BiFeO3 by XRD

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and the grain size from 1 to 5um is observed for BiFeO3 samples with Ho3+ doped. The dielectric behavior of Bi1-xHoxFeO3 ceramics varies with frequency and temperature, which might be understood in terms of an oxygen vacancy, the displacement of Fe3+ ions and lattice phase transition. There is a perfect dielectric hysteresis phenomenon in the εr-V curves of Bi1-xHoxFeO3

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samples at bias voltage with 10V. Under the action of the bias voltage, more and more dipoles are frozen in the bias voltage direction and quit polarization gradually, resulting in lower dielectric constant. The unsaturated P-E hysteresis loop of Bi1-xHoxFeO3 (x=0.05, 0.1) samples is obtained

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with a large 2Pr of 3.08µC/cm2. The Pr, Mr of Bi0.9Ho0.1FeO3 sample is nearly 23, 35 times as

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large as that of BiFeO3, respectively. It can be inferred that Ho3+ doping in BiFeO3 ceramics is proved to be an effective way to modulate the ferroelectric and the magnetic properties. It shows that the TN of BiFeO3 changes slightly from 644K to 638K and the TM of Bi1-xHoxFeO3 will reduce from 878K to 860K with increasing Ho3+ content. It can be attributed to the Fe3+–O2+–Fe3+ super-exchange strength and the relative stability of the magnetic structure. Key words: Multiferroic material; ferroelectricity; ferromagnetic; electric hysteresis loops; magnetic hysteresis loops;

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ACCEPTED MANUSCRIPT 1. Introduction Perovskite-based bismuth ferrite, BiFeO3 (BFO), is one of the most promising multiferroic materials and receives special attention in recent years because of its simultaneous ferroelectricity [1-3]

. These

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and antiferromagnetism above room temperature (Tc=1103K and TN=643K)

outstanding properties have aroused extensive research activities on BiFeO3. It is being considered as a superior candidate for next-generation devices including non-volatile memories, spintronics,

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high-density microactuators, piezoelectric sensors, etc [4, 5]. However, it is very difficult to observe

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the ferroelectric loop of a bulk BiFeO3 sample at room temperature due to its low resistivity, and ferromagnetic loop due to spiral spin modulation which is caused by defects and nonstoichiometry in the bulk BiFeO3 sample. This material becomes unsuitable for device applications. Hence, it is essential to improve the ferromagnetic properties of BiFeO3 without much disturbing its

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ferroelectric properties before its actual use in device applications.

Many attempts have been done in order to solve these problems recently. It finds that A-site substitution by lanthanide ions (La, Dy, Ho, Sm, Gd) is known to be an effective way to overcome [6–12]

. On the one hand, the substitution of lanthanides would effectively suppress

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these obstacles

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the formation of impure phases and oxygen vacancies, lower the leakage current, and thus enhance the ferroelectric properties of BiFeO3 [6–9]. On the other hand, the locked magnetization might be released by the suppression of the spiral spin modulation induced by substitution due to the differences in ion radii between lanthanides and Bi3+ ions. Zhang et al reported Bi-site substitution by Ho3+ is effective in controlling the volatility of Bi3+ by suppressing the formation of oxygen vacancies as well as one effective method for reducing the leakage current density

[10-12]

. Tan et al observed the well-saturated P-E hysteresis

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ACCEPTED MANUSCRIPT loops of BHFMO thin film with 2Pr value of 231.4µC/cm2, which can be attributed to the structural transition, reduced leakage current density and increased break down electric field

[13]

.

Kang et al reported Ho3+ substitution for the Bi-site is an effective way to reduce the leakage

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current and enhance the magnetization of BiFeO3 [14]. Yang et al reported the Mr of Bi0.9H0.1FeO3 thin film is nearly 8 times as large as that of BiFeO3 thin film by sol gel method

[15]

. Such

enhanced ferroelectric properties are attributed to ferroelectric distortion derived from the

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structural transformation and microstructure improvement after doping Ho3+ and Mn3+ [16]. Ho3+

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doping is found to enhance ferromagnetic properties, which can be ascribed to the destruction of spin cycloid structure and the emergence of exchange interaction between Ho ion 4f electrons and Fe ion 3d electrons [17]. To our knowledge, there is no report on the dielectric hysteresis loop and magnetic phase transition temperature (TN, TM) of BixHo1-xFeO3 ceramics.

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In this work, the dielectric hysteresis loop of Bi1-xHoxFeO3 for different values of x (x=0.05–0.1) was performed in the range of 0–10V. There is a perfect εr-V curve of Bi1-xHoxFeO3 samples at bias voltage with 10V. The ferroelectric and ferromagnetic properties of BiFeO3 greatly

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enhanced with Ho3+ doping by measuring P-E and M-H. TN of BiFeO3 changes slightly from 644K

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to 638K and the TM of Bi1-xHoxFeO3 will reduce from 878K to 860K with increasing Ho3+ content by M-T and εr-T. It can be attributed to the Fe3+–O2+–Fe3+ super-exchange strength and the relative stability of the magnetic structure. All these results may provide valuable information for further understanding the relationship between structure, electric and magnetic properties of Ho3+ doping BiFeO3. 2. Experimental The Bi1-xHoxFeO3 (x=0, 0.05, 0.1) ceramics were prepared by rapid liquid phase sintering

3

ACCEPTED MANUSCRIPT method

[18]

. The dried materials of Bi2O3, Fe2O3 and Ho2O3 (purity≥99.99%) were carefully

weighted in stoichiometric proportion and thoroughly mixed in an agate mortar ball-milling machine for about 24 hours using high purity isopropyl alcohol as a medium. The mixture was

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dried and pressed into disks with diameter of about 13mm and thickness 1mm. Details of sample preparation processes can be found in Ref [19].

The crystalline structure of Bi1-xHoxFeO3 samples was characterized by x-ray diffraction

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(XRD) using a diffractometer with Cu Ka1 radiation. The microstructure of Bi1-xHoxFeO3 was

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imaged with field emission scanning electron microscope (FE-SEM, ZEISS-German) under operating voltage of 25kV. The dielectric property measurement was performed by a precision impedance analyzer (HP4294A) with the frequency ranging from 40Hz to 110MHz. The electric hysteresis loops were measured using a Multiferroic Ferroelectric Test System at room

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temperature. The magnetic property of Bi1-xHoxFeO3 was measured by a vibrating sample magnetometer integrated in a physical property measurement system (Versa Lab, Quantum Design).

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3. Results and discussions

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Fig.1 presents the x-ray diffraction (XRD) patterns of Bi1-xHoxFeO3 (x=0, 0.05, 0.1) ceramic samples. It is obviously seen in Fig.1 that all the peaks for Bi1-xHoxFeO3 (x=0.05, 0.1) samples agree well with the crystal structure of pure BiFeO3, exhibiting the rhombohedral lattice type under R3c space group. XRD peak intensity ratios observed in above XRD pattern suggests polycrystalline behavior with good crystallinity. Careful analyzation of XRD pattern in Fig.1 revealed that Bi1-xHoxFeO3 samples prepared by rapid liquid phase sintering method are better than that of solid state reaction method [17]. According to Bi2O3–Fe2O3 phase diagram, it mentions

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ACCEPTED MANUSCRIPT the eutectic temperature at 785

, BiFeO3 sample is an incongruently melting compound

[19]

.

Quenching the sample is helpful in keeping the meta-stable BiFeO3 single-phase at room temperature

[19]

. The liquid phase accelerates the synthesizing reaction and probably prevents the

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formation of the second phase (for example: Bi2Fe4O9). Hence, we have not found the impurity phases (Bi2Fe4O9 and Bi46Fe2O72) in our samples as reported in literature has been reported in our previous work

[19]

[12-16, 19]

. The result

. It is noted that for all values of Bi1-xHoxFeO3, XRD

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pattern analysis of the sintered specimen confirmed rhombohedral structure R3c space group

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which strictly obeyed its extinction rule (hhl: l=2n; hkh: k=2n; hkk: h=2n; h00: h=2n; 0k0: k=2n;

(024)

(116) (122)

(006)

(202)

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(300)

(018)

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20

30

(b)

x=0.1 x=0.1

x=0.05

x=0.05

BFO

40

50 2θ(°)

60

(104)

(110)

(a)

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Intensity(a.u)

(012)

(104)

al of reported [5].

(110)

00l: l=2n) for all peaks that are indexed and shown in Fig.1. The resfure is agree with Pradhan et

70

80 30

BFO

32

34

2θ (°)

Fig.1. X-ray diffraction (XRD) patterns of Bi1-xHoxFeO3 samples

It is also observed from X-ray analysis that the characteristic diffraction peaks of BiFeO3 sample became gradually wider and shift to higher angles with the increase of Ho3+. This shift in the diffraction angle might be ascribed to the unit cell contraction or the decrease in lattice constants because the ion radius of Ho3+ (RHo3+=0.0905nm) is smaller than that of Bi3+ ion (RBi3+=0.103nm) [13-14, 19].

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ACCEPTED MANUSCRIPT Fig.2 gives the scanning electron microscopy (SEM) images of Bi1-xHoxFeO3 samples. SEM micrograph revealed microstructures comprising of grains of varying sizes from 1 to 5µm with well-defined boundaries, indicating polycrystalline nature of the material, consistent with that

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reported in other literature [20-21].

(a) BFO

(b) Bi0.95Ho0.05FeO3

(c) Bi0.95Ho0.05FeO3

Fig. 2 SEM micrograph of Bi1-xHoxFeO3 all samples.

SEM images indicated that Ho3+ doping significantly decreased the grain sizes of BiFeO3

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ceramics. For BiFeO3, It might be explained by the appearance of charged defects governed by Fe2+ ions, oxygen vacancies (Vo) and/or bismuth vacancies (VBi) in BiFeO3 sample. According to the following reaction mechanisms [19, 21]:

(1)

2Fe3++O2-→2Fe2++0.5O2↑+2Vo2+

(2)

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2Bi3+ +3O2-→Bi2O3↑+2V3-Bi +3Vo2+

Importantly, the presence of Vo2+ vacancy and V3-Bi as stated in Eqs. (1) and (2) has the

predominant effect on the reduction of the electrical resistivity of the bulk samples, giving birth to high leakage currents in the samples

[19, 21]

. When Bi3+ is partially replaced by Ho3+, which will

suppress Bi3+ volatile because the melting point of Ho2O3 (2367oC) is much higher than Bi2O3 (820oC). Thus, oxygen vacancies of Bi1-xHoxFeO3 become smaller than that of BiFeO3. It is clear that Ho3+ doping can suppress grain growth and lead to small grain sizes from the Fig.2, which is

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ACCEPTED MANUSCRIPT consistent with the report of Tan et al [13]. The decrease of grain size of Ho-doped BiFeO3 ceramics can be interpreted by the suppression of oxygen vacancy concentration, which resulted in slower oxygen ion motion and lower grain growth rate [13, 19-21]. In addition, the rare earth of Ho may also

the nucleation rate greatly

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act as nucleation centers for the perovskite structure, which increases the number of nucleus and [13]

. Therefore, the grain size of BiFeO3 is varied from 1 to 5um with

Ho3+ doping in BiFeO3 samples.

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Fig.3, 4 shows the dielectric constant and the dielectric loss for Bi1-xHoxFeO3 (x=0, 0.05, 0.1)

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samples at room temperature as a function of frequency (40Hz-1MHz). For all the samples, the dielectric constant decrease with increasing frequency, which is consistent with a typical orientation dielectric relaxation process. It is obvious from Fig.3 that the dielectric constant increases dramatically with small amount of Ho3+ substitution (x=0.05, 0.10), especially in low

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frequency. For example, the dielectric constant of Bi0.9Ho0.1FeO3 measured at 100Hz is about an order of magnitude larger than that of pure BiFeO3. However, the dielectric constant of Bi1-xHoxFeO3 samples exhibits strong frequency dependence compared with pure BiFeO3.

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The relatively large dielectric constant of Bi1-xHoxFeO3 samples may be associated with

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orientational polarization of dipoles. BiFeO3 is distorted triangle perovskite structure belonging to R3c space group, whose spontaneous polarization is mainly ascribed to the relative displacement of Fe ion against Fe-O octahedron [19-20]. Doping or substitution always leads to contraction of the unit cell in isotropic ferroelectric ceramics. Because the radius of Ho3+ (RHo3+=0.0905nm) is less than that of Bi3+ (RBi3+=0.103nm), Ho3+ doping causes distortion of oxygen octahedral, and enhances the dipole movement of Fe3+ along the (111) direction, resulting in increase in the dielectric constant of doped samples. However, the dielectric constant of Bi1-xHoxFeO3 samples

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ACCEPTED MANUSCRIPT exhibits strong frequency dependence compared with pure sample, which is mainly due to increase in the loss coefficient of the doped samples (Fig. 4). In addition, the existence of space-charge will also have some contribution to dielectric

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constant. The uneven distribution of electrons injected into the samples from electrodes will form distribution of space-charge in the samples, changing the dielectric constant of samples. Transfer of space charge under the applied electric field will form space-charge limited current, which is [18-19]

. In the dielectric ceramics, since the growth of grain is

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proportional to dielectric constant

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subjected to various constraints, a number of impurities and defects will appear in samples, near which a number of traps are formed. Trapped charge which cannot participate in long-range migration conduction forms space-charge, the transfer of which constitutes the space-charge limited current contributing to electric polarization.

5

10

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4

10

x=0.05 x=0

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Dielectric constant

(a)

3

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10

2

10

2

3

LgF(Hz )

4

5

6

Fig.3 Dielectric constant (εr) vs frequency curves for all samples

A lot of impurities and defects exist in pure BiFeO3, and many oxygen vacancies are formed inside the material because the evaporation of instability and volatile Bi3+ may lead to valence fluctuation of Fe3+ to Fe2+ states in the sintering process [19, 21]. A number of electrons injected into

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ACCEPTED MANUSCRIPT the samples from electrodes are captured by these defects and oxygen vacancies, which cannot participate in long-range migration conduction in low voltage (amplitude of measuring voltage is 500 mV), so the dielectric constant of pure BiFeO3 is relatively small. Because the stability of

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Ho3+ is superior to Bi3+, Ho3+ doping can restrain the volatilization of Bi3+ so that the production of oxygen vacancies is inhibited.

Furthermore, Ho3+ doping can also eliminate the impurities in the samples obtaining fine

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grain structure, so most injected charges are involved in electrical conductivity. Therefore the

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corresponding electric polarization or dielectric constant is relatively large.

2.5 (b)

Dielectric loss

2.0

1.5

x=0.1

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1.0 x=0.05

0.5

x=0

2

3

LgF(Hz )

4

5

6

EP

0.0

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Fig.4 Dielectric loss (tanδ) vs frequency curves for all samples

The dielectric loss of Bi1-xHoxFeO3 samples as a function of frequency is shown in Fig.4. The

dielectric loss of pure BiFeO3 increases with frequency especially in high frequency, which is consistent with a combined response of orientation relaxation of dipoles and the conduction of charge carrier. The dielectric loss of f Bi0.9Ho0.1FeO3 sample behaved in a complicated way with a few Debye peaks, which are higher than that of pure BiFeO3. Relaxation time dispersion theory indicates that the relaxation time of most dielectric material is dispersed in the relaxation process,

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ACCEPTED MANUSCRIPT and the material has energy absorption characteristics in a number of frequencies, so a few Debye peaks are present in the curves [19, 21]. Fig.5 showed the dielectric constant of Bi1-xHoxFeO3 (x=0, 0.05, 0.1) samples at room

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temperature as a function of the bias voltage (εr-V). Dielectric constant of BiFeO3 samples almost does not change under the bias voltage of 10V. However, the dielectric constant of Bi1-xHoxFeO3 decreases with the increasing of the absolute value of bias voltage, and there is a clear dielectric

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hysteresis phenomenon in the εr-V curves of Bi1-xHoxFeO3 samples from the Fig.5

curves of Bi1-xHoxFeO3 samples.

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Two possible explanations may accountable for dielectric hysteresis phenomenon in the εr-V

First of all, the mechanism of external DC bias voltage on the relaxation or ferroelectrics is to change the barrier height of dipole reversal

[22]

, thereby changing the corresponding relaxation

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time. Under the action of the bias voltage, the potential well of the dipole in the bias voltage direction becomes deeper and deeper, the chance to reverse the polarization getting smaller and smaller. So more and more dipoles are “frozen” in the bias voltage direction and quit polarization

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gradually, resulting in lower dielectric constant.

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Secondly, the concept of space-charge limited current is only applicable low voltage [19, 21-23]. With bias voltage increasing, the charge which is trapped by defect or oxygen vacancy and cannot participate in the long-range migration will be stimulated, so samples having more oxygen vacancies or defects will have larger polarization and larger dielectric constant [24-25]. Due to BiFeO3 sample has more defects and oxygen vacancies, resulting in equivalent contribution of the two mechanisms. The dielectric constant of BiFeO3 is almost invariable under bias voltage. Ho3+ doping reduces defects or oxygen vacancies while leading to distortion of

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ACCEPTED MANUSCRIPT oxygen octahedral, so the dipole orientation polarization is enhanced, and large scale dipole is increased. Under bias voltage, these large scale dipoles become slow dipoles, more and more dipoles is directional, leading to major change in dielectric constant with bias voltage. When

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external DC bias voltage being increased to a certain extent, slow dipole is almost completely ‘‘frozen’’ in the bias voltage direction, then dielectric constant is saturated. After bias voltage being removed, directional dipoles do not completely recover to its initial state with the remnant [22]

. It indicates that Ho3+ doping

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polarization, leading to dielectric hysteresis phenomenon

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enhances the remnant polarization and improves effectively ferroelectric properties of BiFeO3 sample. 4

1.8x10

100Hz 4

4

1.2x10

x=0.1

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Dielectric constant

1.5x10

3

9.0x10

3

6.0x10

3

x=0

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3.0x10

x=0.05

-5

0 Bias voltage(V)

5

10

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-10

Fig.5. Dielectric constant (εr) vs bias voltage curves for Bi1-xHoxFeO3 samples at 100 Hz

The polarization-electric field (P–E) hysteresis loops of the Bi1-xHoxFeO3 (x=0, 0.05, 0.1) is

measured using a Multiferroic Ferroelectric Test System with an applied electric field up to 20kV at room temperature, as it is shown in Fig.6. It can be seen from Fig.6 that samples of Bi1-xHoxFeO3 did not give a perfect ferroelectric loop and the entire series show a linear loss and unsaturated loop. It is also well known that well saturate ferroelectric loops in pure BiFeO3 are difficult to obtain due to the high coercive field and high leakage currents. Leakage current is 11

ACCEPTED MANUSCRIPT caused by the existence of the oxygen vacancies and Fe2+ [26]. Recently, oxygen vacancies rather than Fe2+, both of which found to make more contribution towards the high leakage currents, which dramatically decrease the breakdown voltage[26-27]. XPS experiments proved that the Fe

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ions often exist in a mixed-valence state of Fe2+/3+ in the BiFeO3 [28-29]. Hu et al reported that the asymmetric broad peak at 710.0eV observed by XPS for all the prepared samples indicated coexistence of Fe3+ and Fe2+

[28]

. Song et al reported that the ratios of Fe2+/Fe3+ in the BiFeO2.67,

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BiFeO2.95 and BiFeO3.02 samples are calculated as 31/69=0.45, 24/76=0.32 and 17/83=0.20 by

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XPS experiments date, respectively[29].

Obviously, the ferroelectricity of Bi1-xHoxFeO3 (x=0.05, 0.1) samples is significantly improved in compares on with pure BiFeO3 from Fig.6. 2Pr values of the Bi1-xHoxFeO3 (x=0, 0.05, 0.1) samples are 0.13µC/cm2, 1.72µC/cm2, 3.08µC/cm2, and 2Ec values are 1.16 kV/cm, 4.16

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kV/cm, 4.26kV/cm, respectively. Namely, the Pr of Bi1-xHoxFeO3 (x=0.05, 0.1) is 13, 23 times of that of BiFeO3, respectively. It is believed that Ho3+ doping enhanced ferroelectric behavior of the Bi1-xHoxFeO3 ceramics. The significant enhancement in the ferroelectric properties of

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Bi1-xHoxFeO3 may be ascribed to these aspects:

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(i) It is well known that BiFeO3 is distorted triangle perovskite structure belonging to R3c space group, whose spontaneous polarization is mainly ascribed to the relative displacement of the Fe3+ ion against Fe-O octahedron. XRD results confirm the Ho3+ substitution Bi3+ driven distortion of polar R3c phase in the Bi1-xHoxFeO3 samples because the radius of Ho3+ (RHo3+=0.0905nm) is less than that of Bi3+ (RBi3+=0.103nm). Ho3+ doping causes distortion of oxygen octahedral, and enhances the dipole movement of Fe3+ along the (111) direction, resulting in increase in the ferroelectric polarization of doped samples. The structural transition is the primary reason for the

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ACCEPTED MANUSCRIPT large ferroelectric polarization of the Bi1-xHoxFeO3 samples. (ii) The large ferroelectric order and spontaneous polarization of BiFeO3 main results from the stereochemically active 6s2 lone pairs on the Bi3+ ions [8]. The substitution of the smaller Ho3+

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ions for Bi3+ ions will not only change the spacing between Bi3+/Ho3+ ions and Fe-O octahedral, but also alters the long-range ferroelectric order, resulting in increase in the ferroelectric polarization of BiFeO3 samples.

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(iii) The substitution of Ho3+ ions for Bi3+ ions can decrease the oxygen vacancy density and

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minimize the oxygen vacancy effects on ferroelectricity. The leakage current of Bi1-xHoxFeO3 (x=0.05, 0.1) specimen is two orders of magnitude lower than that of BiFeO3 under 4000V/cm in our previous work

[19]

. The increase of grain boundaries due to the reduction of size in the Ho

substituted samples increases the resistivity from Fig.2. It is shown that Ho3+ doping BiFeO3

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enhanced the electrical properties with lower leakage current [30-31].

6

BFO BHFO-5% BHFO-10%

4

x=0.1 x=0.05

x=0

0

-2

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P(µ C/cm2)

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2

-4 -6 -20

-15

-10

-5

0 E(V/cm)

5

10

15

20

Fig.6. Electric field dependence of polarization (P–E) hysteresis loops for Bi1-xHoxFeO3. In general, ferroelectric property is strongly coupled with the crystal structure and the leakage current. Hence, one can infer that such increase of the polar phase may favor the occurrence of

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ACCEPTED MANUSCRIPT pronounced ferroelectricity in the Ho3+ substituted samples. Magnetic hysteresis loops of Bi1-xHoxFeO3 ceramics with an applied field up to 3 T at room temperature is shown in Fig.7. The Ho3+ doping results in the appearance of weak and complete

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magnetic hysteresis loops at room temperature. The structural distortion and super-exchange interaction may be accountable for weak ferromagnetism in Bi1-xHoxFeO3 samples in Ref [19].

1.2

0.8

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BFO BHFO-5% BHFO-10%

0.0

-0.4

-0.8

-1.2

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M(emu/g)

0.4

0 H/Oe

10000

20000

30000

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-30000 -20000 -10000

Fig.7. Magnetization versus magnetic field curves for Bi1−xHoxFeO3 samples at 300 K

Thermo-magnetic (M-T) experiments of B1-xHoxFeO3 (x=0, 0.05, 0.1) ceramics are performed

29]

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under a magnetic field H=0.5T for temperatures in the range of 300–950K in our previous work [19,

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. We concluded that the M-T process is characterized by two evident anomalies. The first one is

approximately 644K, showing the anti-ferromagnetic phase transition temperature (TN=644K) [1-5]. The TN of Bi1-xHoxFeO3 is 638K and that is not larger change with Ho3+ doping from the Fig.7. The shift in antiferromagnetic Néel temperature (TN) can be correlated to the Fe–O–Fe bond angle due to the structural modification by the substitution of Ho3+ ions in Bi3+ ions and antiferromagnetic Néel temperature (TN) is given by the relation [32], TN=JZS(S+1)cosθ

14

3

ACCEPTED MANUSCRIPT where J is the exchange constant S is the spin of Fe3+, Z is the average linkages per Fe3+ ions and θ is the Fe–O–Fe bond angle

[32]

. It shows that TN increases with the increase of cosθ

according to equation (3), the characteristic diffraction peaks of BiFeO3 sample became gradually

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wider and shift to higher angles with Ho3+ doping, and that's what's resulting in the decrease in TN of BiFeO3.

Another anomaly of BiFeO3 happens at 878K, which is defined as magnetic phase transition

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temperature of BiFeO3 (TM=878K). It was obvious that TM of B1-xHoxFeO3 would be reduced from

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878K to 860K with Ho3+ doping. It is proved the fact that the TM of BiFeO3 existences at 878K by measuring the hysteresis loops of different temperatures (for example 850K, 890K and 900K) in our previous work

[19]

, it is three factors that G-type anti-ferromagnetic structure, the structure

distortion and f-d exchange interaction may be accountable for the TN and TM of Bi1-xHoxFeO3

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ceramics changing [19].

In this paper, thermo-dielectric constant (εr-T) experiments are performed at f=100Hz for temperatures in the range of 300–750K. The results are shown in Fig. 8 (a–c). There is a negative

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temperature coefficient (NTC), which the resistivity of BiFeO3 decreased with rise of temperature.

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We cannot test the dielectric properties of high temperature for BiFeO3 because the resistivity of BiFeO3 is relatively small when the testing temperature exceeds 800K. The Curie temperature of 1103K for BiFeO3 isn’t observed in this paper. The εr-T curve of pure BiFeO3 shows a broad peak in Néel temperature (TN=644K), the peaks

of Bi1−xHoxFeO3 (x=0.05, 0.1) samples are relatively narrow and move to lower temperature, the intensity increases significantly compared with pure BiFeO3. This dielectric anomaly is attributed to antiferromagnetic transition of the Bi1−xHoxFeO3 samples. The anomaly in magneto-electrically

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ACCEPTED MANUSCRIPT ordered systems is predicted by Landau-Devonshire theory of phase transition as an influence of vanishing magnetic order on the electric order

[21]

. The narrow and high peaks of Ho3+ doped

samples moving to lower temperature from 644K to 638K, it demonstrates that Ho3+ doping

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reduces the Néel temperature (TN=644K) while enhancing the magnetoelectric effect of BiFeO3 samples. The result is consistent with Perejón et al of reported. Perejón et al reported that the dielectric constant (εr) is found to increase sharply with temperature due to thermally induced

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hopping conduction [33-34].

30000

0.055

BFO

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25000

Delectric constant

TN=644K

20000 15000 10000 5000

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Fig.8. Delectric constant and Magnetization versus temperature curves for Bi1−xHoxFeO3 samples

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In order to verify the fact that the antiferromagnetic phase transition and magnetic phase transition of Bi1-xHoxFeO3 samples occurred at 638K and 860K. We measured the DSC for all samples for temperatures in the range of 300–1300K and DSC results for the Bi1-xHoxFeO3 samples are shown in Fig.8. It can be observed in the Fig.9 that the phase transition of BiFeO3 is

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occurred at 644K, 878K, and 1103K, respectively. Result is consistent with the results reported by

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Perejón et al [34-35].

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Fig.9. DSC curves of Bi1−xHoxFeO3 samples The first peak obtained at 644K in BiFeO3 is attributed to the anti-ferromagnetic phase 17

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rhombohedral to paraelectric cubic phase transition, indicating the Curie temperature TC. Peaks at 644K and 878K are shifted gradually towards the lower temperature in the Bi1−xHoxFeO3 compounds with increasing Ho3+ content and nearly reaches 638K, 860K respectively for x=0.1,

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exhibiting ferroelectric characteristics over a wide temperature range.

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Concerned about the high temperature magnetic phase transition of BiFeO3 samples few reported except our previously literature, there are the only related experimental data to prove this conclusion. In this paper, we find once again that the magnetic phase transition of BiFeO3 is occurred at 878K in a DSC experiment. That is ferromagnetic phase transition of BiFeO3 at 878K

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according to M-T test results. It fully proves that the ferromagnetic phase transition of BiFeO3 materials in the vicinity of 878K. 4. Conclusion

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Polycrystalline Bi1-xHoxFeO3 (x=0, 0.05, 0.1) samples were prepared by rapid liquid phase

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sintering method. The crystalline structure, dielectric, ferroelectric properties and the phase transition temperature of Bi1-xHoxFeO3 ceramics have been investigated in detail. Ho3+ substitution eliminated the impurity phase Bi2Fe4O9 and obtained single phase Bi1-xHoxFeO3 ceramic with all the peaks corresponding to a rhombohedral structure with R3c space group. SEM images indicated that Ho3+ doping significantly decreased the grain sizes of BiFeO3 ceramics. The saturated P-E hysteresis loop of Bi1-xHoxFeO3 (x=0, 0.05, 0.1) sample is obtained with a large remanent polarization (2Pr) of 3.08µC/cm2 and the Pr of Bi0.9Ho0.1FeO3 is twelve times larger than

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doping. All the samples of Bi1-xHoxFeO3 (x=0, 0.05, 0.1) samples exhibits weak ferromagnetic behavior at room temperature. It found that TN of Bi1-xHoxFeO3 decreases from 644K to 638K by measuring εr-T and M-T curves. The change in TN of Bi1-xHoxFeO3 depends mainly on the

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Fe–O–Fe super-exchange strength and the relative stability of the magnetic structure.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Project no: 11504093, U1304518); Basic and Advanced Technology Research Projects in Henan Province, China (Project no: 122300410203, 162300410086);

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Highlights εr-V of Bi1-xHoxFeO3shows dielectric hysteresis phenomenon at bias voltage with 10V.




It is obtained that the unsaturated P-E of Bi0.9Ho0.1FeO3with large 2Pr of 3.08µC/cm2..




TN of BiFeO3 decreases from 644K to 638K and TM from 878K to 860K with doping Ho3+.




TN, TM and TC of BiFeO3 occurred respectively at 644K, 878Kand 1103K by DSC.

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