Magnetoelectric and magnetodielectric effect in CFMO-PBT nanocomposites

Journal of Physics and Chemistry of Solids 74 (2013) 388–394

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Magnetoelectric and magnetodielectric effect in CFMO-PBT nanocomposites S.M. Salunkhe b, S.R. Jigajeni c, M.M. Sutar a, A.N. Tarale a, P.B. Joshi a,n a b c

Department of Physical Sciences, Solapur University, Solapur 413 255, India Department of Physics, Shivaji University, Kolhapur, India Walchand Institute of Technology, Solapur, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 May 2012 Received in revised form 17 July 2012 Accepted 16 October 2012 Available online 29 November 2012

The present paper reports the dielectric, magnetoelectric (ME) and magnetodielectric (MD) properties of Co1.2Fe1.8  xMnxO4 (CFMO) and Pb0.2Ba0.8TiO3 (PBT) composites. The CFMO is initially studied for variation of electrical resistivity r, saturation magnetization Ms, permeability m, coercive field Hc and coefficient of magnetostriction l to determine a composition suitable to form a ME composite. Here x ¼ 0.1 is observed to be suitable composition therefore the composites are formed using Co1.2Fe1.7Mn0.1O4 (CFMO). The composites y CMPBT¼ (y)CFMOþ (1  y)PBT are synthesized and investigated for their structural, dielectric, magnetoelectric, magnetodielectric properties and variation of magnetoresistance with H. The dielectric constant of composite is observed to exhibit contribution due to interfacial polarization as well as a contribution due to the PBT ferroelectric phase. The variation of linear magnetoelectric coefficient a and quadratic magneto electric coefficient b for various compositions are reported in this paper. The paper also reports the effect of sintering temperature and variation of frequency on the magnitude of a and b. The composite with higher value of b is investigated also for the magnetodielectric property and it is observed that the composite with y ¼0.5 shows useful value of magnetocapacitance (Mc). & 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Magnetoelectric Composites B. CFMO and PBT C. Magnetoelectric Coefficient D. Magnetocapacitance

1. Introduction Magnetoelectric composite is a topic of research interest for nearly 20 years and from the current literature, it could be seen that magnetoelectric composites are still a topic of interest [1–9]. In case of magnetoelectric composites one has to select the piezoelectric phase possessing high value of piezoelectric coupling coefficient d, low values of coercive field Ec and dielectric constant (e) [10,11]. The piezomagnetic or magnetostrictive phase needs to posses high values of coefficient of magnetostriction l, saturation magnetization Ms, permeability m, resistivity r, and low value of coercive field Hc [12–14]. Therefore selection of proper ferroelectric and ferromagnetic phases is a primary requirement for formation of a useful magnetoelectric composite. Since 2005 another multiferroic coupling that is magnetodielectric effect has become a topic of interest [15–20]. In this case also it is observed that the composites of ferroelectric and ferromagnetic materials exhibit a large value of magnetodielectric coupling and the corresponding parameter is magnetocapacitance (Mc). Mc is defined as
M c ¼ eðHÞ eð0Þ =eð0Þ

n

Corresponding author. Tel.: þ91 217 2728825; fax: þ 91 217 27744268. E-mail address: [email protected] (P.B. Joshi).

0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.10.003

where e(H) and e(0) are the magnitudes of dielectric constants with and without applied magnetic field. It is observed that the composites of ferroelectric and CMR materials as well as a few double perovskite systems exhibit a large value of Mc. Thus it would be interesting to investigate a composite which exhibits both the magnetoelectric and the magnetodielectric properties. Owing to the discussion above the present paper reports synthesis of particulate magnetoelectric composites formed using nanocrystalline powders of Ba0.8Pb0.2TiO3 (PBT) and Co1.2Fe1.7Mn0.1O4 (CFMO) [9,21–24]. From the literature it is observed that the PBT possess a useful value of d, saturation polarization Ps, and low value of Ec. Additionally it is known that PBT posses a nearly zero value of coefficient of thermal expansion [25,26]. The magnetostrictive phase (CFMO) is known for its large value of l, Ms, magnetomechanical coupling km and low value of Hc [21–24].To have improved magneto-mechanical coupling, the nanoparticles of both PBT and CFMO are synthesized using the hydroxide co-precipitation route. Using the nanoparticles of PBT and CMFO, the composites yCMPBT ¼ ðyÞCFMO þ ð1yÞPBT,

y ¼ 0:2, 0:3, 0:4, 0:5 and 0:6

are synthesized in the form of pellets of 1.2 cm diameter and are sintered at 1150 1C and 1200 1C. The parent compositions of PBT, CFMO and CMPBT composites are investigated for the dielectric, magnetic and magnetodielectric properties. The observed results are understood in terms of existing theories of magnetoelectric and magnetodielectric systems.

S.M. Salunkhe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 388–394

2.1. Synthesis of CFMO To achieve near atomic level uniformity of the constituent cations, the hydroxide co-precipitation route has been adopted for synthesis of Co1.2Fe1.8  xMnxO4 (CFMO). The Co(No3)2  6H2O, Fe(NO3)3  9H2O, MnCl2 and KMnO4 of AR grade are used as precursors for the hydroxide co-precipitation. The weight proportion of MnCl2 and KMnO4 is selected to be 8:2 so that the valence of the hydroxide residue of Mn is in state þ3. The precursors are dissolved in distilled water to form nearly 40 mM solution of the constituents, while NH4OH is used as a precipitating agent. The precipitates are thoroughly washed in distilled water keeping alkaline medium using NH4OH with pH between 9 and 10 [27]. The dried precipitates are calcinated at 1100 1C for 12 h and final sintering is carried out at 1200 1C for 24 h. The product of final sintering is formed as a powder and also pellets of 1.2 cm diameter. The powder is used for the formation of magnetoelectric composites, while the pellets are used for the determination of resistivity r, Ms, Hc, and l. To form a magnetoelectric composite the requirement is a high value of r, Ms, m, l and a moderate value of Hc. 2.2. Synthesis of PBT Regarding hydroxide co-precipitation of PBT a consideration has to be given to the earlier reports on synthesis of PZT and BaTiO3. It was observed that Pb is soluble in the solutions of KOH as well as NaOH, but insoluble in NH4OH for pH between 9 and 10 [27,28]. Furthermore regarding synthesis of BaTiO3, it is observed that KOH and NaOH are suitable precipitants and KOH or NaOH are used at a molar ratio of 1:1.6 that of Ba and Ti-ions [29]. Therefore for complete precipitation of PBT a two step process is adopted. Initially Pb(OH)2 is precipitated using NH4OH precipitant. For precipitation of Pb(OH)2, PbNO3 is used as a precursor. In step two the stoichiometric amounts of Ba(NO3)2 and K2TiO (C2O4)  2H2O are used as precursors. The co-precipitation of Ba(OH)2 and Ti(OH)2 is achieved using KOH at the required molar ratio. The precipitates of Ba and Ti are prewashed using NH4OH with pH between 9 and 10 to remove the stresses of excess KOH present in the bath. Now the precipitates of Pb, Ba and Ti are mixed together thoroughly and allowed to stabilize for 12 h. The settled precipitates are washed with dilute solution of NH4OH with pH (9–10) for several times. The filtered precipitates are used for further process of synthesis. Here calcination is carried out at 1100 1C for 12 h while final sintering is carried out at 1200 1C for 12 h. The calcinated powder of PBT was pelletized for the investigation of physical and dielectric properties.

sintering temperature on the dielectric, magnetoelectric and magnetodielectric properties of yCMPBT [12]. The dielectric constant and complex impedance spectra were measured using the HP4284A LCR-Q meter. Custom built setups are used for the measurement of the DC resistivity r, the linear magneto-electric coefficient a, quadratic magnetoelectric coefficient b at 850 Hz and variation of a with frequency up to 5 kHz [30]. To understand the crystal structure of the CFMO, PBT and CMPBT composites, Bruker D8 advance XRD spectrometer is used, while the SEM pictures are obtained using JEOL-JSM 6360 SEM.

3. Result and discussion The Fig. 1 shows XRD spectrum of Co1.2Fe1.7Mn0.1O4 (CFMO).The XRD spectra of remaining compositions are similar in nature to that of XRD spectrum for x¼ 0.1. Fig. 1 shows that the ferrites are formed with desired spinel cubic crystal structure. All the peaks could be indexed using JCPDS data of parent cobalt ferrite. Furthermore as discussed earlier the X-ray diffractograms are determined using Bruker D8 advance XRD spectrometer. This machine does not have fluorescent filter in the detector system, which is required for the elimination of the fluorescent background for samples containing Fe-ions. Therefore the base line in the spectrum increases linearly with 2y, however, this background does not affect the positions of the diffraction peaks. The lattice parameter ‘a’ is calculated using XRD spectra and Table 1 shows the variation of ‘a’ with x. From Table 1 it is observed that the calculated values are consistent with the earlier reports [22–24]. The crystallite size of the CFMO powder is estimated using Scherrer formula and it is nearly 100 nm. The crystallite size is also given in Table 1 for other CFMO compositions. The CFMO is initially characterized for determination of its physical parameters and Table 1 shows variation of r, Ms, Hc, m and l as a function of x varying from 0.1 to 0.4. The variations of physical parameters are consistent with the values reported earlier for CFMO system [22,23]. It could be seen that for x ¼0.1, the parameters are optimum for its use as a magnetostrictive phase. And therefore the CFMO with x ¼0.1 is selected as the

(311)

1600 1400 (511)

1200

(400)

(220)

1000

(400)

(422)

800 600

2.3. Formation of composites

20

30

40

50

60

70

80

2θ

The resulting powders of CFMO and PBT are ground thoroughly to form uniform and submicron level particle size. The powders of CFMO and PBT thus formed are used to form the required magnetoelectric (ME) composites using the following formula ðyÞCFMO þ ð1yÞPBT ¼ ðyÞCMPBT,

CFMO (0.1)

1800

Intensity

2. Experimental

389

Fig. 1. XRD spectra of Co1.2Mn0.1Fe1.7O4 (CFMO).

Table 1 Variation of resistivity r, saturation magnetization Ms, permeability m, coercive field coefficient of magnetostriction l, and crystal size for CFMO.

y ¼ 0:2, 0:3, 0:4, 0:5 and 0:6

The composites above are termed as yCMPBT during the course of further discussion. Considering the earlier reports, the composites are formed as pellet shaped samples of 1.2 cm diameter and two separate batches of the composites are formed with sintering temperature Ts ¼ 1150 1C and 1200 1C. Here different sintering temperatures are used to understand the effect of

Co1.2MnxFe1.8  xO4

0.1 0.2 0.3 0.4

Resistivity r (MOm) 1.14 0.91 0.314 0.295

Ms

177 146 167 161

Hc (Oe) 60 52.5 45 46.5

m

l

a ˚ (A)

1287 123 8.2 1427 80 8.6 1662 72 8.27 1930 53 8.13

Crystal size (nm) 41.71 36.55 38.83 39.23

390

S.M. Salunkhe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 388–394

magnetostrictive phase. It is observed that the XRD spectrum of PBT powder (Fig. 2) posses tetragonal crystal structure having ˚ From the XRD lattice parameters a¼ b¼3.98 A˚ and c¼4.04 A. spectrum the crystallite size is observed to be 60 nm. Fig. 3 shows the XRD spectrum of composite 0.CMPBT sintered at 1150 1C, which confirms that both the phases (ferrite and ferroelectric) are distinctly present in the composite. The XRD spectra of remaining compositions are similar to that for y¼0.2. The well defined peaks with specific indices of CFMO and PBT confirm the formation of a pure biphase composite. Fig. 4(a) and (b) shows the XRD spectra for 0.2 and 0.5CMPBT composites for Ts ¼ 1200 1C. It is observed that the peaks corresponding to the PBT and CFMO phases could be separately indexed in the XRD spectra and it is also seen that the relative intensity of (311) peak of CFMO increases with increasing value of y. From Fig. 4(a) and (b) it is also observed that no peak corresponding to any impurity phase is present in the XRD spectra and therefore it could be said that the composites are pure, biphase systems. The other parameters are determined for yCMPBT composites are X-ray density, physical density and percentage density of the composites. The Table 2 shows variation of these parameters as a function y and sintering temperature (Ts). It is observed that for sintering temperature 1150 1C, the density of composites increases slowly as y increases, while on the other hand the density decreases with increasing y for Ts ¼1200 1C. Furthermore the density of CMPBT at 1200 1C is low as compared to the density for Ts ¼1150 1C. The reduction in density for increase in sintering temperature may occur because of increased grain growth at Ts ¼1200 1C. Here increase in the grain size causes increase in the intergrain voids. To confirm this qualitative association, SEM micrographs of yCMPBT composites are determined. Here Fig. 5(a) and (b) shows the SEM micrographs for 0.4CMPBT and PBT

2500

(101)

Fig. 4. (a) XRD spectra of 0.2CMPBT composite sintered at 1200 1C, and (b) XRD spectra of 0.5CMPBT composite sintered at 1200 1C.

INTENSITY

2000 1500 1000

Table 2 Variation of X-ray density, physical density and percent density for yCMPBT composites sintered at 1150 1C and 1200 1C.

(111)

(100)

(211)

(200)

500

(220)

y

0 20

30

40

50

60

70

2θ θ Fig. 2. XRD spectra of Pb0.2Ba0.8TiO3 (PBT).

80

0.2 0.3 0.4 0.5 0.6

d( Xray)

d(Phy) T¼ 1150 1C

d(Phy) T ¼1200 1C

% density T¼ 1150 1C

% density T¼ 1200 1C

6.4292 6.3148 6.2004 6.062 5.9236

6.023 5.93 5.94 5.85 5.70

6.01 5.78 5.52 5.15 4.88

93.68 93.91 95.80 96.50 96.23

93.48 91.53 89.03 84.96 82.38

0.6CMPBT composites sintered at 1150 1C, while Fig. 6(a) and (b) shows SEM micrographs for the same composites sintered at 1200 1C. From SEM micrographs it could be seen that as sintering temperature increases as the grain growth increases which may cause an increase in the intergrain voids. Thus this observation confirms the decrease of density as sintering temperature increases. Furthermore for Ts ¼1200 1C as y increases the grain size is observed to increase, which may lead to decrease in density and y at Ts ¼1150 1C the grain size does not vary significantly with y and the density may remain fairly constant. 3.1. Dielectric properties

Fig. 3. XRD spectra of 0.2CMPBT composite sintered at 1150 1C.

In case of ME composites, it is observed that the dielectric constant of the composites posses two contributions, one due to the parent ferroelectric phase (PBT) and other due to the interfacial polarization occurring at grain–grain interfaces of ferrite

S.M. Salunkhe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 388–394

391

Fig. 5. (a) and (b) SEM images of 0.4CMPBT and 0.6CMPBT composites sintered at 1150 1C.

Fig. 6. (a) and (b) SEM images of 0.4CMPBT and 0.6CMPBT composites sintered at 1200 1C.

Fig. 7. Variation of dielectric constant e with temperature for PBT.

Fig. 8. Variation of e at f¼ 1 kHz for composites sintered at Ts ¼ 1200 1C.

(CFMO) and ferroelectric (PBT) phases [12–14]. Therefore initially the variation of dielectric constant of parent PBT phase is determined. Here Fig. 7 shows the variation of dielectric constant (e) as a function of temperature and frequency for PBT composition. It is observed that the magnitude of e for PBT is fairly low and the e passes through a diffused phase transition for temperature between 110 1C and 190 1C. This observation is similar in nature with the earlier reports on similar PBT compositions [9]. It is seen that for temperatures between 110 1C and 190 1C the e shows varies structures for F ¼1 kHz. Furthermore it is also

observed that the e decreases with increasing frequency (F). This feature is also common for all ferroelectric systems [12–14]. Fig. 8 shows the variation of dielectric constant e represented as relative permittivity at 1 kHz as a function of temperature for yCMPBT sintered at1200 1C for y varying from 0.2 to 0.6. From the Fig. 8 it is observed that all the compositions exhibit diffused phase transition (DPT) in the vicinity at 190 1C, consistent with the earlier reports [9,31,32]. The variation of dielectric constant e as the function of temperature for yCMPBT sintered at 1150 1C is similar in nature as in Fig. 8. It is also observed that the dielectric constant e shows varies structures for temperature between

S.M. Salunkhe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 388–394

2000 1800

0.2 0.3 0.4 0.5 0.6

1600 1400 1200

ε'

110 1C and 190 1C. These structures could be attributed to the structures observed for parent PBT system (Fig. 7). From Fig. 8 it is also observed that the e initially increases for y¼0.2–0.5 and then suddenly decreases for y¼0.6. Furthermore the Tables 3 and 4 show the variation of dielectric constant and quality factor Q for yCMPBT composites sintered at 1150 1C and 1200 1C, respectively. For the composite systems it is known that the dielectric constant possess two contributions one due to parent ferroelectric phase that is PBT and other due to interfacial polarization. The interfacial polarization occurs because of the difference in the resistivity and dielectric constant of PBT and CFMO phases [12]. Furthermore the interfacial polarization could be understood in terms of the Maxwell–Wagner model and Koop’s phenomenological theory [33,34]. To understand the observed behavior of e in perspective of these models, the variation of real part of complex dielectric constant e0 and loss tangent tand as a function of F are determined in the frequency range 500 Hz–1 MHz. Fig. 9(a) and (b) shows the variation of e0 and tand as a function of logF for yCMPBT sintered at 1200 1C, respectively. From Fig. 9(a) it is observed that e0 possess large dispersion at low frequencies as predicted using the Maxwell–Wagner model for the presence of interfacial polarization. From Fig. 9(a) it is also seen that the e0 at low frequencies increases with y up to y ¼0.5 and then decreases for y¼0.6. Furthermore it is known that the number of interfaces between PBT and CFMO would be proportional to the relation yn(1  y). The relation yn(1 y) maximizes for y¼0.5 and therefore e too should become maximum for the same composition that is y¼0.5. Therefore the increasing value of dielectric constant for y¼0.2–0.5 as in Fig. 8 and Fig. 9(a) appears to be mainly due to the contribution of interfacial polarization. The variation of tand as a function of logF is shown in Fig. 9(b). The variation of tand with F is also consistent with the above mentioned theories. Especially for y¼0.4 and 0.5 the variation of tand passes through a resonance peak. The resonance occurs at the frequency where the time required for the charge to transfer across the interface matches with the reciprocal of the frequency. Similar observations are also reported for other titanate systems [35]. Thus it could be said that the dielectric properties of the yCMPBT posses a large contribution of the interfacial polarization for y up to 0.5.

1000 800 600 400 200 0 1

2

3

4

5

6

log F 1.6 1.4

0.2 0.3 0.4 0.5 0.6

1.2 1

tan δ

392

0.8 0.6 0.4 0.2 0 1

2

3

4

5

6

log F Fig. 9. (a) Variation of e0 as a function of logF for composites sintered at Ts ¼ 1200 1C, and (b) variation of tand as a function of logF for composites sintered at Ts ¼ 1200 1C.

25 1150 1200

20

15

α 10

3.2. Magnetoelectric properties The dynamic ME coefficient a and b of the composites yCMPBT are measured at 850 Hz using custom designed measurement

5

0

Table 3 Variation of dielectric constant e and Q at 1 kHz for the composites sintered at 1150 1C. y

e (at room temperature)

Q (at room temperature)

emax

Q

0.2 0.3 0.4 0.5 0.6

50.4 78.3 77.3 13.4 18.5

11.6 5.97 3.37 5.48 15.3

370 1127 1309 1504 78

1.89 0.3 0.39 0.03 0.16

Table 4 Variation of dielectric constant e and Q at 1 kHz for the composites sintered at 1200 1C. y

e (at room temperature)

Q (at room temperature)

emax

Q

0.2 0.3 0.4 0.5 0.6

229 198 264 411 35

9.86 3.15 2.67 1.01 4.13

1529 2828 3582 7419 248

2.15 0.99 0.44 0.16 0.02

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

y Fig. 10. Variation of a as a function of y for composites sintered at Ts ¼ 1150 1C and 1200 1C.

unit [30]. Where a ¼dv/dhnd and b ¼dv/(dHn2h0), Where v is the rms value of voltage developed across the sample in response to an AC magnetic field h, and h0 is also the rms value of applied magnetic field. Furthermore H is the applied DC magnetic field. Figs. 10 and 11 show variation of a and b as a function of y for composites CMPBT sintered at 1150 1C and 1200 1C, respectively. The a is observed to be maximum for y¼0.5. This feature is attributed to the relation yn(1 y) type proportionality of a. The high value of a ¼23 mV/Oe/cm for 0.5CMPBT at Ts ¼1200 1C is an interesting and useful feature of the present observations. From the Fig. 10 it is seen that a increases for increasing sintering temperature Ts and increase in a could be attributed to increasing electromechanical coupling [12]. As required for the ME composites the magnitude of b is fairly small and the b decreases with increasing sintering temperature. This too is also a device related property of ME composites. Fig. 12(a) and (b) shows the variation

S.M. Salunkhe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 388–394 45.0 1200 1150

40.0 35.0 30.0

β

25.0 20.0 15.0 10.0 5.0 0.0 0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

y Fig. 11. Variation of b as a function of y for composites sintered at Ts ¼1150 1C and 1200 1C.

60 0.2 0.3 0.4 0.5 0.6

50

α

40

393

magnetic field would be positive or negative depending on positive or negative value of l. Thus e may decrease with applied magnetic field H for positive value of l while e may increase with H for l being negative. In the present case the CFMO posses a negative value of l therefore it is expected that e increases with increasing H, additionally observation on MnFe2O4 nanoparticles have shown a positive value of Mc and Mc is proportional to square of magnetization (M2). These observations are understood in terms of the Catalans theoretical predictions based on Maxwell–Wagner model [33,36]. Both these observations suggest that yCMPBT composite may exhibit a useful value of Mc because of stress induced effect as well as the Catalan type contribution. The present observations are in support of these predictions. Here the Table 5 shows the magnitude of Mc at various frequencies for the 0.5CMPBT composites sintered at 1150 1C and 1200 1C, whereas the Fig. 13(a) and (b) shows variation of dielectric constant e as a function of frequency and applied magnetic field H for the composite sintered at Ts ¼1150 1C and 1200 1C, respectively. The Table 5 shows that at Ts ¼1150 1C overall magnitude of Mc is large as compared to the magnitude of Mc at Ts ¼ 200 1C. Primarily this feature could be attributed to higher value of Table 5 Variation of magnetodielectric capacitance (Mc) with frequency for composites sintered at 1150 1C and 1200 1C.

30 20 10 0 2.7

2.9

3.1

3.3

3.5

3.7

log F 300

Frequency (Hz)

Mc (1150 1C)

Mc1 (1200 1C)

Mc2 (1200 1C)

500 1000 10,000 100,000 500,000 1,000,000

2.97 6.03 12.73 4.7 4.23 5.82

 5.77  4.45  1.96  1.3  1.23  0.95

6.03 4.67 6.51 2.26 2.15 1.57

0.2 0.3 0.4 0.5 0.6

250

1600 6 kOe

1200 1000

150

ε

α

0 Oe

1400

200

800 600

100

400 50

200 0

0 2.7

2.9

3.1

3.3

3.5

2

3.7

2.5

3

3.5

4

4.5

5

5.5

log F 2000

Fig. 12. (a) Variation of a as a function of logF for yCMPBT sintered at 1150 1C, and (b) variation of a as a function of logF for yCMPBT sintered at 1200 1C.

0 Oe 4 kOe 6 kOe

1800 1600 1400 1200

ε

of magnetoelectric coefficient a as a function of logF for composites yCMPBT sintered at temperature 1150 1C and 1200 1C, respectively. From the basic theory of ME effect a is proportional to (lnkmnd)/e. This means that a is inversely proportional to e. As the e is observed to decrease with frequency, the a is expected to increase with increasing frequency and similar are the present observations.

6

log F

1000 800 600 400 200 0

3.3. Magnetodielectric and magnetoresistance properties In case of PZT–MZF system it is argued that the stress induced on the piezoelectric system causes an increase in polarization and decrease in the dielectric constant e.The stress induced by the

2

2.5

3

3.5

4

4.5

5

log F Fig. 13. (a) Variation of e as a function of logF for composites sintered at Ts ¼1150 1C, and (b) variation of e as a function of logF for composites sintered at Ts ¼1200 1C.

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S.M. Salunkhe et al. / Journal of Physics and Chemistry of Solids 74 (2013) 388–394

quadratic magnetoelectric coefficient b for O.5CMPBT sintered at 1150 1C as compared to b at Ts ¼ 1200 1C. Furthermore it is seen that the Mc at Ts ¼1200 1C shows both negative as well as positive values. For the applied magnetic field Ho0.2 T, the dielectric constant decreases with H and Mc1 is negative. For further increase in H the e increases with H and shows a positive value of Mc2. As discussed above, nanoparticles of MnFe2O4 exhibit a negative value of Mc. This phenomenon is understood in terms of the model proposed by Catalan [36]. In the present case the total dielectric constant of composite is expected to have two contributions, one due to stress induced variation of e and other due to the magnetic field induced variation of dielectric constant of CFMO [18,20]. In the present case the composites are placed in the magnetic field with the magnetic field along the disc axis. Furthermore as discussed earlier l in this field configuration is negative, and therefore contribution due to the induced stress may produce a positive value of Mc. It is reported that nanoparticles of MnFe2O4 or other ferrites possessing spinel crystal structure possess a negative value of Mc. Therefore positive or negative value of total Mc could be interplay between these two phenomenons. To confirm that the CFMO possess a negative value of Mc, CFMO is subjected to the investigation of e as a function of H and F. These observation have shown that the CFMO posses Mc of nearly 1% for frequency 1 kHz. Here the Mc is finite and negative for frequencies even up to 100 kHz. Therefore a positive value of Mc is associated to the stress induced variation of e, while the negative value of Mc would be due to Catalan type contribution of CFMO [36]. As the observations on magnetocapacitance are interesting, the present samples were also subjected to the investigations on magnetoresistance (MR). In the present case the MR is measured at frequency 2.5 kHz, where contribution due to interfacial polarization and the barrier layer effects at electrodes are minimum. The MR is defined as r(H) r(0)/r(0), where r(H) is resistivity with applied magnetic field H and r(0) is the resistivity when H¼0. The variation of MR with H for 0.5CMPBT sintered at 1150 1C and 1200 1C is shown in Fig. 14. It is interesting to note that for Ts ¼1150 1C the MR is negative, while for Ts ¼1200 1C MR is initially positive up to 0.2 T and then becomes negative for further increase in H. Here it could be seen that the Mc is positive where MR is negative, while Mc is negative where MR is positive. For further correlation between Mc and MR one may need to analyze the impedance spectra of these compositions and analyze it interims of the Maxwell–Wagner model [33]. Nevertheless qualitatively the positive value of Mc could be associated with stress induced change in the spontaneous polarization while the negative value of Mc may occur because of the contribution of nanocrystallites of CFMO [36]. 3.4. Conclusions It is observed that the hydroxide co-precipitation route followed by ceramic route of synthesis could be used to form nanocrystalline CFMO and PBT compositions. The dielectric constant of the ME composite is observed to show the contribution due to ferroelectric PBT phase and also a contribution due to interfacial polarization. The CMPBT composites have shown a sufficient large value of linear magnetoelectric coefficient (a) at 850 Hz and also a useful value of magnetocapacitance (Mc). The present observations have also shown that the magnetoresistance shows an interesting behavior. Here the sign of Mc and MR are opposite of each other. For further understanding of these features, it could be interesting to analyze the impedance spectra of these systems.

Fig. 14. Variation of MR as a function of H for yCMPBT composites.

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