Structural, dielectric and magnetic properties of Gd and Dy doped (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 ceramics synthesized by SSR method

Solid State Communications 197 (2014) 1–5

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Solid State Communications journal homepage: www.elsevier.com/locate/ssc

Structural, dielectric and magnetic properties of Gd and Dy doped (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 ceramics synthesized by SSR method Shweta Thakur a, Radheshyam Rai a,n, Ashutosh Tiwari b a b

School of Physics, Shoolini University, Solan 173229, Himachal Pradesh, India Biosensors and Bioelectronics Centre IFM-Linköpings Universitet, 581 83 Linköping, Sweden

art ic l e i nf o

a b s t r a c t

Article history: Received 2 May 2014 Received in revised form 17 July 2014 Accepted 31 July 2014 by R. Merlin Available online 7 August 2014

The multiferroic (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 (where RE is Gd (BGFM) and Dy (BDFM)) has been synthesized by using the solid state reaction (SSR) technique. Effects of Gd and Dy substitutions on the structure, electrical and ferroelectric properties of (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 samples have been studied by performing X-ray diffraction, dielectric measurements and magnetic measurements. The crystal structure of the ceramic samples shows a monoclinic phase. Studies of dielectric properties (dielectric constant (ε) and tangent loss (tan δ)) both as a function of frequency (10 and 100 kHz) and temperatures (20–300 1C) exhibit dielectric anomaly in the range of (225–245 1C) suggesting a possible ferroelectric–paraelectric phase transition in the compounds. The vibrating sample magnetometer (VSM) measurement shows a significant change in the magnetic properties of Gd and Dy doped (Bi0.95RE0.05)(Fe0.95Mn0.05)O3. It is seen that the coercive field (HC) and remanent magnetization (MR) increase for Gd. & 2014 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics C. X-ray diffraction D. Dielectric properties D. Magnetic properties

1. Introduction The magnetoelectric phase BiFeO3 (BFO) was first reported in 1957 [1]. Pervoskite (ABO3 type) BiFeO3 (BFO) is a multiferroic with rich physical properties [2]. For environmental friendly leadfree ferroelectric/piezoelectric [3], doped BFO is being investigated for better properties [4,5]. The single or double doped BFO emulates the performance of lead free ferrolectric/piezoelectric [6,7] at morphotropic phase boundaries (MPBs). Multiferroic compounds are in which two or three ferroic order parameters (i.e., ferromagnetic, ferroelectric, antiferromagnetic) coexist in the same phase [8,9]. The different characteristics of multiferroics promise potential applications in different magnetoelectric and ferroelectric devices. BiFeO3 is the most studied multiferroic material because its synthesis is simple at ambient pressure and its ferroelectric Curie temperature, Tc (1103 K), and antiferromagnetic Néel temperature, TN (643 K), are both well above room temperature [10]. Its ferroelectric state is able to store the electrical data. But few drawbacks such as a high leakage current, high coercive field, small spontaneous polarization (Ps) and remnant polarization (Pr) of BFO are serious problems, which become major obstacles for potentially possible applications in

n

Corresponding author. Tel.: þ 91 8521588056. E-mail address: [email protected] (R. Rai).

http://dx.doi.org/10.1016/j.ssc.2014.07.024 0038-1098/& 2014 Elsevier Ltd. All rights reserved.

devices [11,12]. Enhancements of the ferromagnetic properties are important because the improvement in ferromagnetic properties can help to utilize the compound for different applications. The simplest methods to improve the ferroelectric and ferromagnetic properties are different doping on A-site or B-site. Various doping elements were tried with more or less success, e.g., lanthanides on the Bi (A)-sites [13,14] or Cr3 þ , Mn3 þ , Sc3 þ , Ti4 þ , and Nb5 þ on the Fe (B)-sites [15–17]. Among all these substituted ions at B-site, Mn3 þ is a very good candidate. In case of BiMnO3 ferromagnetic ordering appears at TN  105 K (Néel temperature) and ferroelectric polarization appears at TC  450 K (Curie temperature). At room-temperature, BiMnO3 shows monoclinic phase and ferroelectric transition. Substituted Mn improves the breakdown characteristics of the BFO compound and the leakage currents densities [18], which are promising features for practical implementation. Light rare-earth manganites REMnO3 (RE ¼La–Dy) are orthorhombic and nonferroelectric, whereas, the heavy ones REMnO3 (RE¼Y, Ho–Lu) are hexagonal and ferroelectric. It is noteworthy here that Bi3 þ and RE3 þ ions are very similar both in valence state and ionic radius, which is the primary condition for doping [19]. DyMnO3 has the pervoskite structure and is antiferromagnetic in nature [20]. BDFM exhibit high dielectric losses and are, therefore, candidate microwave absorbing materials. The magnetic and electrical ordering temperatures in the system studied exceed room temperature, that is, these compounds are new hightemperature seignettomagnetic materials [21]. Gd is of interest

S. Thakur et al. / Solid State Communications 197 (2014) 1–5

2. Experimental (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 ceramics were prepared by a high temperature solid state reaction technique using analytical-grade metal oxides powders: Bi2O3 (99.8%), Fe2O3 (99.95%), MnO2 (99.4%), Dy2O3 (99.5%) and Gd2O3 (99.5%). The powders in the stoichiometric ratio of the compositions were weighed and mixed thoroughly in acetone and calcined at 850 1C for 4 h. The calcined powders were pressed into 1–2 mm thick and 10 mm diameter cylindrical disks at a pressure of 300 MPa. The samples were finally sintered at 900 1C for 2 h in air. Silver paste was used as electrodes on the top and bottom surfaces of the samples for the electrical measurement. The crystallite structure of the sintered samples was examined using X-ray diffraction (XRD) techniques with CuKα (l ¼1.5405 Å) radiation (Rigaku Minifiex, Japan) in a wide range of Bragg angles 2θ (201 r2θ r601). The dielectric properties of the ceramics were measured as a function of frequency (10–100 kHz) and temperature (room temperature (RT) to 300 1C) using an impedance analyzer (PSM1735).

(110)

2000

(210)

Intensity(a.u.)

(-201)

(011)

1500

*

4000

BDFM

2000

*

0 20

Fig. 1(a) shows the XRD patterns of (Bi0.95RE0.05)(Fe0.95Mn0.05) O3 (where RE ¼Gd and Dy) ceramics at room temperature. All the compositions have a single phase with monoclinic structure. For both samples the value of lattice parameters, dobs and dcal of all diffraction lines (reflections), are calculated (Table 1) by using the POWD software [22]. The main peak of the samples located at approximately 2θ ¼321 has a hkl value 〈101〉. From the XRD it is clear that with the Gd and Dy doping (Bi1  xREx)(Fe0.95Mn0.05)O3 has a majority phase BiFeO3 crystallization. Because of the kinetics of formation, mixtures of BiFeO3 as a major phase along with other impurity phases are always obtained during synthesis. In all the samples, the impurity phase was observed and it is shown as n in Fig. 1(a). It may be attributed to Bi2Fe4O9. Fig. 1(b) shows the XRD patterns of the ceramics for 2θ ¼45–541. The diffraction peaks shift slightly to a higher angle for Gd doped BFMO. Although Gd and Dy belong to the lanthanide family, the doping behavior of Gd3 þ ions is different from that of the other doped Dy3 þ ions. The diffraction peaks of the compounds are closely observed and the shifts towards the higher angle 2θ position with respect to the doping element suggested that these ions substituted for A sites in compounds. This shift is due to the bigger size of Gd ions as compared to Dy ions and it is attributed to the decrease in the lattice spacing as the Gd3 þ ions are incorporated into the system. This amphoteric behavior is because of the different charge

BGFM (-111)

Intensity(a.u.)

6000

3. Results and discussion

*= Bi2Fe4O9

(101)

8000

The magnetic data were recorded with the help of vibrating sample magnetometer (VSM) (Cryogenic).

(-201)

due to the fact that, owing to its high valence spin state, it may induce some ferromagnetic activity which is more attractive for applications compared to the antiferromagnetism specific for pure BFO. Gd doped BFO has a rhombohedrally distorted perovskite structure. The addition of GdMnO3 to BFO leads to a substantial increase in magnetization [17]. In the present work, we study the effect of Gd and Dy doping on structural, magnetic, and ferroelectric properties. Gd and Dy-doped (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 compounds were synthesized by the solid state reaction method.

(-111)

2

1000 BDFM

500

0

*

30

BGFM

40

50

46

60

48

2θ(deg.)

50 2θ(deg.)

52

54

Fig. 1. (Color online) (a) Room temperature XRD patterns of (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 samples with different (Gd and Dy) rare earth metal doping (b) the broadening of (–111) and (201) peaks.

Table 1 Lattice parameters of Gd and Dy doped (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 ceramics. Sample

Crystal system

Particle size (nm)

BDFM

Monoclinic

 36

Lattice parameter

a¼ 3.9507 b ¼2.7810 c ¼3.9566

BGFM

Monoclinic

 38 a¼ 3.9320 b ¼2.7724 c ¼3.9423

dobs

dcal

hkl

3.9517 2.7826 2.2750 1.9762 1.7755 1.6096 3.931 2.7725 2.2684 1.9721 1.7704 1.606

3.9499 2.7824 2.2752 1.9761 1.7755 1.6102 3.9316 2.7732 2.2684 1.9716 1.7704 1.6058

(100) (101) (011) (–111) (102) (210) (100) (010) (011) (–111) (–201) (012)

S. Thakur et al. / Solid State Communications 197 (2014) 1–5

300

3

25

270 20 240

tan δ

ε'(F/m)

15 210

10

180 5 150 0 120 0

50

100

150

200

250

0

300

50

Temperature (oC)

100

150

200

250

300

Temperature(oC)

Fig. 2. (Color online) (a) Variation of dielectric constant (ε) and (b) loss tangent (tan δ) of BGFM and BDFM with temperature at frequencies 10 KHz and 100 KHz.

compensation mechanisms involved in the tri-valent ion substitution in such type of materials. Also the substitution preferences of these ions are dependent on the firing conditions. The sintering temperature, atmosphere and sintering duration have a strong influence on the solubility amount of these ions [23]. The (101) reflection line in XRD pattern was used for obtaining the average particle size using the Debye–Scherrer equation [24]:

Sample name

Frequency (KHz)

Tmax

εmax

εRT

tan δRT

BGFM

10 100 10 100

240 245 225 240

288.87 168.26 215.06 174.02

147.84 144.76 137.35 133.84

0.03 0.02 0.04 0.02

BDFM

ð1Þ

B ¼ ðB2M  B2S Þ1=2 :

ð2Þ

where t is the diameter of the particle, λ is the X-ray wavelength (0.154 nm), BM and BS are the measured peak broadening and instrumental broadening in radian, respectively, and θB is the Bragg angle of the reflection. The calculated average particle size from Eq. (1) is 35–38 nm. Fig. 2(a) shows the variation of dielectric constant (ε) as a function of temperature of BGFM and BDFM at frequencies 10 KHz and 100 KHz. As in normal ferroelectrics, the dielectric constant of BGFM and BDFM increases gradually with increasing temperature up to the transition temperature and thereafter it decreases with increasing temperature. The observed ferroelectric–paraelectric phase transition temperature shifts towards the higher temperature as we doped Gd in the place of Dy. On the other hand dielectric constant decreases with the increasing frequency (Table 2). Fig. 2(b) shows the variation of dissipation factor (tan δ) as a function of temperature of BGFM and BDFM at frequencies 10 KHz and 100 KHz. Loss tangent decreases with increasing frequency. It increases slowly up to transition temperature and thereafter increases sharply above Curie temperature. It should be noted that the dielectric loss at room temperature is almost same for both frequencies, but at high temperature loss values are higher at low frequency. In this study, the low dielectric loss at higher temperature and higher frequency indicates that the ceramics possess a good thermal stability at high frequencies. Fig. 3 shows the variation of log σac (S/cm) vs 103/T (K  1) of BGFM and BDFM at frequencies 10 KHz and 100 KHz. The value of activation energy is calculated in the paraelectric region from the plot of ln(σ) vs 103/T using the conductivity relation σ ¼ σo exp (  Ea/kBT) [25,26]. The ac electrical conductivity has been calculated from the impedance data collected with an LCR meter and using the formula σ ¼ ωɛɛotan δ, where ɛ is the vacuum dielectric constant, ω is the angular frequency and kB is the Boltzmann constant. At high temperature, the donor cations have a major part to play in the conduction process.

-2

BDFM 10 KHz 100 KHz

BGFM 10 KHz 100 KHz

-4

logσ ac [S/cm]

0:9λ : B cos θB

t¼

Table 2 Details of the physical parameters of Gd and Dy doped (Bi0.95RE0.05)(Fe0.95Mn0.05) O3 ceramics.

-6

BDFM

-8

BGFM -10

1.5

2.0

2.5

3.0

3.5

103/T (K-1) Fig. 3. (Color online) Variation of log σac (S/cm) vs 103/T of BGFM and BDFM at frequencies 10 KHz and 100 KHz.

Fig. 4 shows the variation of magnetization at condition of field cooling (FC) and zero field cooling (ZFC) as a function of temperature. The field-cooled magnetization (MFC) and zero field magnetization (MZFC) were found to decrease continuously with increasing temperature for BDFM. The magnetization (M) vs. temperature (T) curves of BGFM and BDFM samples under the ZFC and FC conditions present a systematic change. It is clear that the magnetization at both conditions ZFC and FC has no magnetic transition in this temperature range (4–00 K). Magnetic measurements in the temperature range 5–300 K indicated that the magnetization of BGFM and BDFM is due to the spontaneous magnetic ordering of the iron-containing sublattice, and the other susceptibility term, attributable to the paramagnetic Gd and Dy sublattice. Gd and Dy are paramagnetic and these align along the applied field.

4

S. Thakur et al. / Solid State Communications 197 (2014) 1–5

0.20

0.14

moment (emu/g)

moment (emu/g)

0.16

0.12

0.10

0.12

0.08

0.04

0.08 0.00

0

50

100

150

200

250

0

300

50

100

150

200

250

300

Tamperature(k)

Temprature (K)

Fig. 4. (Color online) Variation of magnetization (a) FC and (b) ZFC of BGFM and BDFM with temperature (K).

1.5

4

moment (emu/g)

moment (emu/g)

1.0

2

0

-2

0.5 0.0 -0.5 -1.0

-4 -1.5

-10

-5

0

5

10

-10

B (tesla)

-5

0

5

10

B (tesla)

Fig. 5. (Color online) M–H loops of BGFM and BDFM at (a) 5 K and (b) 300 K.

Magnetic hysteresis loops (Fig. 5(a, b)) are traced for both the samples at 5 K and 300 K in a field 1 T. The magnetic properties of ferrite materials depend on the particle size, sintering temperature, additives, microstructure and applied external field [27–29]. There is a canting of antiferromagnetic sublattice in the system which results in a macroscopic magnetization. Superimposed on antiferromagnetic ordering, there is a spiral spin structure which leads to cancellation of macroscopic magnetization. Magnetization is induced in the sample whenever this spiral structure is suppressed [30,31]. The magnetization of BGFM (MS ¼4.88 emu/g at 5 K and 1.54 emu/g at 300 K) sample is higher than BDFM (MS ¼1.21 emu/g at 5 K and 0.94 emu/g at 300 K). This could be due to the structural distortion in the pervoskite, i.e., the canted spin arrangement of unpaired electrons on Fe3 þ ions is caused by incorporating RE3 þ ions to A sites and/or Mn3 þ ions to B sites of the pervoskite structure of BiFeO3. This structural distortion could lead to the enhancement of the magnetization in the system.

4. Conclusion We have developed a simple method to prepare single-phase, Gd and Dy doped (Bi0.95RE0.05)(Fe0.95Mn0.05)O3 ceramics. The ceramics prepared under the optimized conditions show a monoclinic crystal structure. The dielectric constant measured at f ¼10 kHz shows a broad maximum corresponding to FE–PE phase

transition at the temperature of around 225–245 1C. It is observed that the ac conductivity of the material increases with rise in temperature. The magnetization increases for Gd doped samples. A minor loop traced for all these samples indicates an antiferromagnetic nature with weak ferromagnetism.

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