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Magnetoelectric and magnetodielectric effect in Ba1−xSrxTiO3 and Co0.9Ni0.1Fe2−xMnxO4 composites
Solid State Sciences 14 (2012) 1064e1070
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Magnetoelectric and magnetodielectric effect in Ba1xSrxTiO3 and Co0.9Ni0.1Fe2xMnxO4 composites M.M. Sutar a, A.N. Tarale a, S.R. Jigajeni a, S.B. Kulkarni b, V.R. Reddy c, P.B. Joshi a, * a
Solapur University, 413 255 Solapur (MS), India Institute of Science, Mumbai (MS), India c UGC-DAE-CSR, Indore (MP), India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2011 Received in revised form 9 April 2012 Accepted 20 May 2012 Available online 26 May 2012
Ba1xSrxTiO3 (BST) is a ferroelectric material known to possess ferroelectric transition temperature Tc in the vicinity of room temperature w30 C for x ¼ 0.3. As the BST with x in the vicinity of 0.3 is elastically soft at 30 C, the compositions with x ¼ 0.2, 0.25 and 0.3 are chosen to investigate their possible magnetoelectric (ME) and magnetodielectric (MD) applications. The (CoNi) FeMn2O4 (CNFMO) is selected to be a piezo-magnetic phase. Here hydroxide co-precipitation route is adopted so as to synthesize BST and CNFMO of nearly 100 nm crystallite size. Starting with the BST and CNFMO powders, the composites yCNBST ¼ yCNFMO þ (1 y) BST are synthesized for y ¼ 0.3 and 0.4. The parent compositions of BST as well as the CNFMO are characterized for dielectric and magnetic properties to confirm the formation of the desire ferroelectric and magnetostrictive phases. The composites are investigated for the crystal structure, dielectric, magnetoelectric and magnetodielectric properties. The results show that the composite Ba0.75Sr0.25TiO3eCo0.9Ni0.1Fe1.7Mn0.3O4 exhibit excellent ME and MD properties simultaneously. The results on the ME and MD properties are understood in terms of the stress induced variations in polarization and dielectric constant 3 respectively. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: BST CNFM Magneto-electric Magneto dielectric properties Magneto-capacitance
1. Introduction Ba1xSrxTiO3 is a well-known ferroelectric material where the transition temperature Tc could be reduced from 120 C down to 231 C by varying x between 0 and 1 [1]. The compounds with x in the vicinity of 0.3 are of special interest owing to its Tc ¼ 30 C (zRT) and very high value of dielectric constant 3 in the vicinity of Tc in both ferroelectric and paraelectric regions [1e4]. Here the dielectric constant refers to relative permittivity to free space. BST with x ¼ 0.3 is known to possess useful value of electrical tunability {3 (E) 3 (0)}/3 (0). Therefore BST finds its applications in tunable filters, tunable dielectric resonators, multilayer capacitance devices etc. [5e8]. As far as the magnetoelectric (ME) and magnetodielectric (MD) properties are concerned, it is expected that ME and MD coefficients will be high in the ferroelectric region and in the vicinity of Tc. It is seen that magnitudes of d3 /dT i.e. rate of change of permittivity with temperature and dPr/dT i.e. rate of change of polarization with temperature are maximum in this region of
* Corresponding author. Fax: þ91 217 2744768. E-mail address: [email protected] (P.B. Joshi). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.05.016
temperature and reduces as the temperature is decreased below Tc. Further the dPr/dT is negative, while d3 /dT is positive in the vicinity of Tc. As the ME phenomenon is proportional to the change of polarization as a function of applied stress, the ME, MD properties are expected to be sensitive to the compositional variations of BST for x varying from 0.2 to 0.3. Here the Tc for x ¼ 0.3 is at 30 C and increases to nearly 58 C for x ¼ 0.2. Therefore d3 /dT and jdPr/dTj for BST at x ¼ 0.25 and x ¼ 0.3 will be large as compared to their values for x ¼ 0.2. Owing to the discussion above we have selected x ¼ 0.2, 0.25 and 0.3 compositions for ferroelectric phase of ME/MD composites. Here Co0.9Ni0.1Fe1.7Mn0.3O4 (CNFMO) ferrite is selected to be a magnetostrictive phase considering its high value of coefficient of magnetostriction l. Simultaneously to our efforts, considering the importance of BST as discussed above very recently the physical properties of BST synthesized via Pechini method are reported [2]. Therefore to avoid the repetition of discussion of physical properties of BST is kept limited to comparison of the previously reported results and present observations. Further ME properties of the composites are maximum for composition in the vicinity of y ¼ 0.5, as the ME properties are governed by the relation (1 y), where y is the fraction of the magnetostrictive phase in the composite. Thus the present paper
M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070
reports the magnetic, dielectric, ME and MD properties of the CNFMOeBST composites. To form nano particles of individual phases, both CNFMO and BST powders are synthesized via hydroxide co-precipitation route. The paper reports the synthesis and characterization of different CNFMO and BST compositions and composites. The crystal structure and crystallite size is determined using XRD spectra. The physical properties of CNFMO like dc resistivity rdc, initial permeability m, saturation magnetization Ms etc. are determined to obtain optimum composition with high values of rdc, Ms, l and low value of Hc. Further the paper also reports the dielectric, ME and MD properties of the CNBST composites as a function of frequency F and temperature T. 2. Experimental 2.1. Synthesis of Co0.9Ni0.1Fe2xMnxO4 (CNFMO) The CNFMO was synthesized by employing hydroxide coprecipitation route followed by ceramic processing approach for x ¼ 0, 0.1, 0.2, 0.3 and 0.4. The AR-grade Fe(NO3)3$9H2O, Co(NO3)2$6H2O, Ni(NO3)2$6H2O, MnCl2$4H2O and KMnO4 were used as precursors, while a mixture of NH4OH and KOH were used as precipitating agents. The details of the co-precipitation route are similar as reported earlier [9,10]. The precipitates formed were washed thoroughly and calcined at 1100 C for 12 h to achieve complete ferrite phase formation. After calcination the calcined product was pressed into pellets and sintered at 1200 C for 10 h for characterization of bulk properties like rdc, Ms, m and l. The remaining powder was calcined again at 1200 C for 10 h. The calcined powder was used as a starting material for the formation of composites. For complete characterization of these compositions, rdc was measured using potential divider arrangement. The physical density dBulk was measured using the liquid displacement method, while saturation magnetization Ms was measured using Hysteresis loop tracer from Ms. Arun Electronics, Mumbai (India). The permeability m was measured using a LVDT arrangement and l is measured using magnetostriction setup. 2.2. Synthesis of Ba1xSrxTiO3 (BST) The Ba1xSrxTiO3 powders for x ¼ 0.2, 0.25 and 0.3 were synthesized using similar process as described above. High purity (>99.9%) Barium acetate Ba(COOCH3)2, Strontium nitrate Sr(NO3)2, Potassium Titanium Oxalate [K2TiO(C2O4)2$2H2O] were used as precursors. For complete precipitation of Ba(OH)2 and TiO(OH)2, the molar ratio of KOH to (BaTi) of 1.6 has been used, based on the earlier report [11]. It has been observed that the Ba(OH)2 and Sr(OH)2 is fractionally soluble in water but insoluble in alkaline medium. Therefore the precipitates were washed in dilute NH4OH solution with pH w10 [12]. The remaining procedure of coprecipitation was similar to that of CNFMO. The powders were calcined at 1000 C for 10 h and after calcination; part of calcined product was pressed in the form of pellets and sintered at 1100 C for 12 h. The remaining powder was calcined again at 1100 C for 12 h. The sintered pellets were used for characterization of bulk properties. The calcined powder was used as a starting material for the formation of composites. For further discussion the samples are denoted as BSTx for x ¼ 0.2, 0.25 and 0.3.
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Where x ¼ 0.2, 0.25 and 0.3 for y ¼ 0.3 and 0.4. The calcined powders of CNFMO and BST were grounded together thoroughly using ethanol as a medium. The pellets of diameter 1.2 cm were formed as discussed above. The pellets were sintered at 1200 C to form desired ME/MD composites. The parent compositions CNFMO and BST as well as their composites (CNBST) were investigated for the structural properties using X-ray powder diffractometer (Rigaku Miniflex). SEM images of calcined powders of parent compositions were obtained using a Jeol-JSM6360SEM scanning electron microscope. For dielectric measurements LCR-Q meter (HP4284A) was used in the frequency range from 100 Hz to 1 MHz for temperature T between room temperature to 250 C for measurement of Cp, tan d as a function of frequency F, temperature T and magnetic field H for dielectric and MD characterization of BST and composites. The linear and quadratic magnetoelectric coefficients a and b were determined using a custom designed instrument as reported earlier [13,14]. The P-E measurements on BST samples at 50 Hz were carried out using P-E loop tracer by Ms. Radient Technology, USA. 3. Results and discussion The XRD spectrum for calcined CNFMO powder for x ¼ 0.3 is shown in Fig. 1(a). It is seen that the observed reflections could be associated with the corresponding (hkl) planes using standard JCPDS data on CoFe2O4 (JCPDS card no. 22-1086). Using XRD data, it is observed that CNFMO ferrite possesses the spinal cubic crystal structure with lattice parameter ‘a’ as shown in Table 1. It is observed that the lattice parameter ‘a’ increases gradually with increasing ‘x’ owing to larger ionic radius of Mn as compared to Fe cations. Further the crystallite size D is calculated using Scherrer formula and is also shown in Table 1. From XRD spectra it is observed that all the compositions possess the crystallite size in the range of 100e140 nm. Further the Table 1 also shows the magnitude of dc resistivity rdc, physical density dBulk, crystalline density dx-ray, and % porosity p. It is observed that variation of rdc could be understood based on the earlier report on Ni substituted CoFe2O4 [15]. It is observed that rdc is very high at x ¼ 0 and decreases slowly with increasing Mn content. The presence of Fe2þ and Fe3þ ions on B-sites is known to cause polaronic conduction in case of ferrites. At very low concentration the Mn and Ni reduce the percentage of fractional Fe2þ ions formed during the process of synthesis. It is
2.3. Formation of composites The CNFMO and BST composites are formed bearing the formula
yðCNFMO BSTxÞ ¼ yCNFMO þ ð1 yÞBSTx
Fig. 1. XRD spectra of calcined (a) CNFMO powder for x ¼ 0.3 and (b) BST0.25 powder.
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Table 1 a: Lattice parameter, D: crystallite size, rdc: dc resistivity, dBulk: bulk density, dx-ray: X-ray density and p: porosity. CNFMO
x x x x x
¼ ¼ ¼ ¼ ¼
0.0 0.1 0.2 0.3 0.4
‘a’ Å
Crystallite size ‘D’ (nm)
‘rdc’ 106 U-met
dBulk gm/cm3
dx-ray gm/cm3
R (%)
8.18 8.23 8.25 8.28 8.31
134 119 114 121 129
67.1 12.92 10.76 2.236 0.145
5.38 5.29 5.03 5.01 5.00
5.69 5.58 5.54 5.46 5.54
5.37 5.24 9.17 8.10 9.75
important to note that the ionic radii of Mn3þ, Fe3þ, Mn2þ, Ni2þ and are known to be 0.72, 0.63, 0.80 and 0.69 Å respectively [16]. The Asites in ferrite are known to possess tetrahedral coordination, while the B-sites are coordinated in the octahedral form. Therefore Fe3þ or Mn3þ ions may prefer the smaller size A-sites as compared to the B-sites. Nevertheless the distribution of ions amongst A-sites or Bsites is not solely decided by the ionic radii only. As per as the earlier report, substitution of Mn/Ni below 0.04 atom percentage causes an increase in resistivity [15]. For further increase in Mn/Ni concentration rdc decreases slowly with increasing x. The present observations are in agreement with the earlier report [17]. The magnitude of porosity is as shown in Table 1. It is observed that the samples are dense and porosity is less than 10% comparable with the porosities reported earlier for similar sintering conditions [18]. Fig. 1(b) shows the XRD spectrum of calcined BST0.25 powder. The observations of XRD spectra for x ¼ 0.2 and 0.3 are similar to the XRD shown in Fig. 1(b). It is observed that the degree of tetragonality c/a reduces from 1.0083 to 1.0022 as the x is varied from x ¼ 0.2 to 0.3 as shown in Table 2. These results are in confirmation with the earlier report [2]. Thus as the x increases, the tetragonality decreases and sample becomes nearly cubic for x > 0.30. Using Scherrer formula the crystallite size is determined to be between 100 and 120 nm (Table 2). Further, no peak corresponding to any impurity phase is observed in XRD spectrum and formation of a pure single-phase composition is confirmed from the XRD spectrum. Fig. 2(aeb) shows the XRD spectra of CNBST0.25 for y ¼ 0.3 and CNBST0.3 for y ¼ 0.4 composites. The peak corresponding to the reflections of BST and CNFMO could be separately indexed in Fig. 2(aeb). No peak corresponding to any impurity phase is recorded in XRD spectra. Further from Fig. 2(aeb) it is seen that relative intensity of the (220) peak of CNFMO increases from 33% to 46% as the concentration of ferrite increase from x ¼ 0.3 to x ¼ 0.4. These observations are sufficient to say that composites are formed as pure biphasic system. The SEM images of calcined powders of CNFMO for x ¼ 0.3, BST0.2, BST0.25, and BST0.3 are shown in Fig. 3(aed). The SEM images are obtained for as calcined powder without any addition of dispersing agent. Therefore, the images show a distribution of particles not well separated from each other. Nevertheless, the large numbers of CNFMO particles are of 1 mm, while BST particles are of nearly 400 nm. Thus as the starting powders of CNFMO or BST fairly small in size, the resulting composites may possess the useful values of electromechanical coupling coefficient.
Fig. 2. XRD spectra of (a) CNBST0.25 (y ¼ 0.3) and (b) CNBST0.30 (y ¼ 0.4) composites (B corresponds to reflections of BST, while C corresponds to reflections of CNFMO).
Fig. 4 shows the variation of l with applied magnetic field H for CNFMO series. It is observed that, the l increases and appears to saturate for H > 4 kOe. The highest value of l for H ¼ 4.5 kOe is termed as lsat. Table 3 shows the observed magnetic properties of the CNMFO series. From Table 3 it is observed that the saturation magnetization Ms increases with increasing Mn content as expected for Mn substituted cobalt ferrite [19]. As suggested, a fraction of Mn3þ may reduce to Mn2þ and Ms may increase with increasing Mn2þ. The presence of Mn2þ on B-sites causes an increase in magnetization per unit cell. Further increase in doping level, may decrease the exchange interaction and Ms reduces for further increase in x [19]. This suggests that the Mn ions may fractionally occupy A as well as B sites. The actual distribution of Mn ions on A and B sites may depend on the sintering conditions and/or presence of other cations. An observation in this support is the earlier report on Co1.2Fe1.8xMnxO4 [20]. In this case it was observed that for substitution of Mn in Co1.2Fe1.8xMnxO4, the Ms has increase for x up to 0.1 only and decreases for further increase in Mn concentration. In case of this report for Co1.2Fe1.8xMnxO4, for x ¼ 0, 0.1, 0.2 and 0.3 the reported values of Ms are nearly 68, 72, 63 and 62 emu/g respectively [20]. On the other hand the present observations show that Ms increases monotonically for Mn even up to x ¼ 0.4 (Table 3). This may occur because of presence of Ni on B sites. Further, Hc for all the compositions are closer to 100 Oe and this observation too is in confirmation with the earlier reports [19,20]. Table 3 shows the variation of m as a function of x. The m is expected to be proportional to Ms2 and the present observations are concurrent with this prediction [21]. The earlier reports on the Mn substituted cobalt ferrite predict that the presence of Mn ions on Asites causes an increase in l and magneto mechanical coupling of CNFMO [22]. Table 3 shows variation of lsat as a function of x. With small decrease, the l increases with x for x ¼ 0.2 and 0.3 and then reduces slightly for x ¼ 0.4. Though l is observed to follow Ms, that
Table 2 D: Crystallite size, Tc: ferroelectric transition temperature, PMax: maximum polarization, Pr: remanent polarization. Composition
D (nm)
Tc C
‘a’ (Å)
‘c’ (Å)
c/a
Pmax (mC/cm2)#
Pr (mC/cm2)#
Ba0.80Sr0.20TiO3(BST0.20) Ba0.75Sr0.25TiO3(BST0.25) Ba0.70Sr0.30TiO3(BST0.30)
104 120 118
58 42 30
3.920 3.824 3.809
3.9525 3.8415 3.8173
1.0083 1.0046 1.0022
0.9789 1.270 1.052
0.1414 0.1805 0.0622
# At V ¼ 1000 V and T ¼ 20 ms.
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Fig. 3. Calcined powder SEM images of (a) CNFMO (x ¼ 0.3), (b) BST0.2, (c) BST0.25 and (d) BST0.3.
is occupancy of Mn on A-sites, up to x ¼ 0.3 for higher values of x, the l does not follow this relation. This could be because of the fractional occupancy of Mn on B-sites as x increases. Fig. 5 shows the variation of dielectric constant 3 as a function of T for F ¼ 1 kHz for BST0.2, BST0.25 and BST0.3. Here the dielectric constant refers to permittivity with respect to free space. From Fig. 5 it is seen that 3 passes through maximum at Tc. The observed Tc for BST0.2, BST0.25 and BST0.3 are shown in Table 2. Further it is observed that 3 increases with T again in the paraelectric region above Tc. This feature is predicted to occur because of grainegrain boundary effect [2]. To understand these features in details the 3 is determined as a function of F also and Fig. 6(a) shows the variation of 3 as a function of T for F varying between 100 Hz and 1 MHz for BST0.25, while Fig. 6(b) shows the variation of tan d as a function of T for F ¼ 100 Hz, 1 kHz, 10 kHz and 100 kHz respectively. Here it is
observed that the increases in 3 with T occur for frequencies below 100 kHz only and d3 /dT decreases with increase in F. These features occur due to the presence of interfacial polarization at grainegrain boundary interface. The grainegrain boundary interface occurs probably due to fractional Ti3þ acceptor state in the bulk of grain [23,24]. Similar features are also reported by Iansculescu et al. [2]. A dielectric relaxation originated from defects extrinsically formed during the preparation process (particularly O-vacancies) is a common phenomenon in Ti-perovskite at high temperatures of 400e700 C. By creating more defects (vacancies, free electrons and substituted ions), the substitution heterogeneity and characteristic of the BST solid solutions determines the lowering of the activation energy of such relaxations. The observed behavior is a competitive phenomenon between the dielectric relaxation and the electrical conduction of the relaxing species. Its relationship with composition and microstructures has to be further analyzed in detail. Now Table 4 shows the variation of dynamic ME coefficients a and b respectively for CNBST composites. Here a represents linear, while b represents quadratic ME coefficient defined by the relations as below.
a ¼ dE=dH ¼ ðDV0 Þ=G*t*ðDhÞ and b ¼ ðDV0 Þ=2*d*h*ðDHÞ Where DV0 is the r.m.s output voltage developed across the sample in mV, G is the gain of the amplifier, t is the effective thickness of the sample, h is the r.m.s. value of the AC field at frequency 850 Hz and H is the imposed dc magnetic field. From Table 4, it is observed that Table 3 Ms: Saturation magnetization, Hc: coeresive field, m: permeability, lsat: saturation magnetostriction. CNFMO
Fig. 4. Variation of magnetostriction coefficient l with applied magnetic field H.
x x x x x
¼ ¼ ¼ ¼ ¼
0 0.1 0.2 0.3 0.4
Ms emu/g
Hc Oe
m
lsat 106
141 199 222 252 269
100 112 112 93 81
255 502 636 777 911
115 90 160 167 142
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M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070 Table 4 a and b: linear and quadratic ME coefficients. Composite
CNBST0.2 CNBST0.25 CNBST0.3
Fig. 5. Variation of
3
with T at F ¼ 1 kHz for BST0.2, BST0.25 and BST0.3.
a is maximum for CNBST0.25 and y ¼ 0.4. Similar are the features for variation of b. Here the change in b is insignificant for x ¼ 0.25
and x ¼ 0.3 and y ¼ 0.3 and 0.4. From the basic theory of ME effect in composites a and b are proportional to y(1 y), where y is the percentage of the piezo-magnetic phase in the composites. Further the a is proportional to piezoelectric coefficient d, magnetostriction coefficient l magneto-mechanical coupling coefficient km and inversely proportional to 3 . It is already reported that the maximum polarization Pmax and remanent polarization Pr of BST composites at RT decreases with increasing Sr content and become very low for x ¼ 0.3. Additionally the magnitude of Pmax and Pr may increase with improved crystallinity in case of polycrystalline materials. It is previously reported and present observations the crystallinity is maximum for BST0.25 as compared to other BST compositions. These features are discussed in the next paragraph. At present it is observed that in case of piezoelectric materials, ‘d’ becomes maximum on the higher polarization side of Tc or morphological phase boundary (MPB) [25]. Here it is also known that material become elastically soft at T ¼ Tc or at MPB [26]. Therefore it is expected that ‘d’ and hence a becomes maximum probably for BST0.25 as ‘d’ becomes maximum. The previous report as well as present observations shows that the BST0.25 possesses higher crystallinity in comparison with the other BST ceramics [2].
b (103 mV/cm-Oe2)
a (mV/cm-Oe) y ¼ 0.3
y ¼ 0.4
y ¼ 0.3
y ¼ 0.4
4.99 5.57 3.46
5.04 5.57 3.46
0.44 0.43 0.12
0.45 0.43 0.20
Therefore, these observations also favor a becoming maximum for BST0.25. Fig. 7(aeb) shows the variation of polarization PE as a function of electric field E. Fig. 7(a) shows P-E hysteresis loops for BST0.2, BST0.25 and BST0.3, while Fig. 7(b) shows P-E behavior for ME composites with y ¼ 0.3. The magnitudes of Pmax and Pr for various compositions studied are shown in Table 2 for pure BST and Table 5 for CNBST composites. The present observations for the BST compositions are on the similar lines as reported earlier [2]. In this case, the tendency to reduce the remanent polarization, while increasing the Sr content is related to the reducing tetragonality i.e. with approaching the ferroepara phase transition at room temperature. The magnitude of Pmax is lower as compared to the earlier report, probably because of the lower crystallite size obtained in the present case. The Pmax and Pr for BST0.25 is observed to be slightly more as compared to BST0.2, probably because of improved crystallinity for x ¼ 0.25. Essentially the Pmax is observed to show decreasing trend as x increases. From Fig. 7(b) and Table 5 it is observed that in case of composites also the Pmax and Pr show a decreasing trend for increasing x from 0.2 to 0.3. Nevertheless because of the presence of CNFMO at y ¼ 0.3, hysteresis loss increases for the ME composites as compared to the pure BST composites. Here the presence of CNFMO causes leakage of the charge of polarization providing the conducting path to the polarization charges. Fig. 8(aeb) shows the variation of dielectric constant 3 as a function of log F with and without applied magnetic field in the frequency range from 100 Hz to 1 MHz of CNBST0.2 and CNBST0.3 for y ¼ 0.3 and 0.4 respectively. It is observed that 3 passes through a maximum at F ¼ 500 kHz. This frequency appears to be equal to the radial mode of oscillations of the ME material of the disc shaped sample with diameter equal to 1.2 cm. Therefore, F is observed independent of the composition. Further it is observed that the 3 decreases with increasing applied magnetic field almost linearly and rate of change of 3 with magnetic field is maximum in the vicinity of electromechanical resonance (EMR) frequency. The
Fig. 6. (a). Variation of 3 with T for F ¼ 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz for BST0.25. (b). Variation of tan d with T for F ¼ 100 Hz, 1 kHz, 10 kHz and 100 kHz for BST0.25.
M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070
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Fig. 7. (a). Variation of PE as a function of E for BST0.2, BST0.25 and BST0.3. (b). Variation of PE as a function of E for CNBST0.2, CNBST0.25 and CNBST0.3 composites.
Table 5 D3 /3 0 : Magneto-capacitance, Pmax: maximum polarization, Pr: remanent polarization. Composite
CNBST0.2 CNBST0.25 CNBST0.3
D3 /3 0 At F ¼ 300 kHz
D3 /3 0 At F ¼ 600 kHz
Pmax (mC/cm2)
Pr (mC/cm2)
y ¼ 0.3
y ¼ 0.4
y ¼ 0.3
y ¼ 0.4
y ¼ 0.3*
y ¼ 0.4**
y ¼ 0.3*
y ¼ 0.4**
3.05 4.04 1.85
2.46 4.23 4.09
3.83 3.04 2.35
6.06 6.17 5.30
0.8672 0.9280 0.5833
0.3306 0.5311 0.2170
0.6511 0.8513 0.3758
0.4351 0.4498 0.2209
*For V ¼ 500 V and T ¼ 20 ms, **For V ¼ 200 V and T ¼ 20 ms.
magnitude of magneto-capacitance Mc is defined as (3 (H) 3 (0))/ is also shown in Table 5. It is observed that the Mc becomes maximum for CNBST0.25 and y ¼ 0.4. The Mc in this case was predicted to occur because of strain induced change in polarization with applied magnetic field. It is known that as the magnetic field is increased the stress on the BST particles also increases. Because of the applied stress polarization may also increase, reducing the corresponding value of 3 . This phenomenon is explained on the basis of two layer ME composite structure by Gridnev et al. [27]. As the electromechanical coupling factor Km is maximum at the resonant frequency, the Mc too is maximum at EMR frequency 500 kHz.
3 (0)
The tan d variation with log F for CNBST0.2 for y ¼ 0.3 and 0.4 with and without magnetic field are shown in Fig. 9(aeb). As expected from the basic theory, here the tan d also passes through a maximum at EMR frequency of 500 kHz. 4. Conclusions The BST composites with x in the vicinity of x ¼ 0.3 show interesting ferroelectric properties, variation of relative dielectric permittivity, maximum value of polarization and remanent polarization. The present study shows that the CNFMOeBST composite shows both ME and MD behavior. Further the magnitudes of the ME
Fig. 8. (a). Variation of 3 as a function of log F with and without magnetic field for F varying between 100 Hz and 1 MHz for CNBST0.2, y ¼ 0.3 composite. (b). Variation of a function of log F with and without magnetic field for F varying between 100 Hz and 1 MHz for CNBST0.3, y ¼ 0.4 composite.
3
as
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M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070
Fig. 9. (a). Variation of tan d with log F for CNBST0.2, y ¼ 0.3 composite. (b). Variation of tan d with log F for CNBST0.2, y ¼ 0.4 composite.
coefficients are useful and a is observed maximum at 5.57 mV/cm-Oe for x ¼ 0.25, y ¼ 0.4. The magnitude of magneto-capacitance is also high nearly 6% at EMR frequency 600 kHz and possesses a useful value of above 4% for F between 100 kHz and 800 kHz. The behavior of magneto-capacitance could be understood in terms of the landau thermodynamic theory as detailed in earlier reports [27,28]. These observations suggest that the compositions studied are interesting in terms of both physics as well as commercial applications. Acknowledgements The author M.M. Sutar would like to thanks the University Grant Commission, New Delhi, India, for a Teacher Research Fellowship award under FIP, XIth Plan 2007-12. References [1] A. Mark McCormick, Roeder, Elliott B. Slamovich, J. Mater. Res 16 (4) (2001) 1200. [2] A. Iansculescu, D. Berger, L. Mitoseriu, L.P. Curecheriu, N. Dragan, D. Crisan, E. Vasile, Ferroelectrics 369 (1) (2008) 22. [3] F. zimmerman, M. Voiats, W. Menesklou, E. Ivezs-Tiffee, J. Eur. Ceram. Soc. 24 (2004) 1729. [4] A.T. Chien, X. Xu, J.H. Kim, J. Sachleben, J.S. Speck, F.F. Lange, J. Mater. Res. 14 (1999) 3330. [5] J.H. Haeni, P. Irvin, W. Chang, R. Ucker, et al., Nature 140 (2004) 758e760. [6] A.K. Tagantsev, V.O. Sherman, K.F. Astafiev, J. Venkatesh, N. Setter, J. Electroceram. 11 (2003) 5. [7] D. fuck, S. Dorman, Phys. Rev. B 73 (2006) 104116. [8] M. Jain, S.B. Mujumdar, R.S. Katiyar, A.S. Bhalla, Mater. Lett. 57 (2003) 4232.
[9] S.R. Jigajeni, S.R. Kulkarni, Y.D. Kolekar, S.B. Kulkarni, P.B. Joshi, J. Alloys Compd. 492 (2010) 402. [10] S.S. Veer, D.J. Salunkhe, S.V. Kulkarni, S.B. Kulkarni, P.B. Joshi, Indian J. Eng. Mater. Sci. 15 (2008) 121. [11] S.K. Lee, T.J. Park, G.J. Choi, K.K. Koo, Sang Woo Kim, Mater. Chem. Phys. 82 (2003) 742. [12] S.V. Kulkarni, S.S. Veer, D.J. Salunkhe, S.B. Kulkarni, P.B. Joshi, Mate. Sci. Res. India 6 (2) (2009) 521. [13] Giap V. Duong, R. Groessinger, M. Schoenhart, D. Bueno-Basques, J. Magn. Magn. Mater. 316 (2007) 390. [14] D.J. Salunkhe, S.S. Veer, S.V. Kulkarni, S.B. Kulkarni, P.B. Joshi, J. Instrum. Soc. India 38 (4) (2006) 294. [15] L.G. Van Uitert, J. Chem. Phys. 23 (1955) 1883. [16] F.Albert Cotton, Geoffrey Wilinkinson, Carlos A. Murillo, Manfred Bochmann, Avanced Inorganic Chemistry, sixth ed. A Wiley-interscience Publication, John Wiley and Sons, Inc, USA, 1999, pp. 1301e1304. [17] Chang Hyun Kim, Yoon myung, Yang Jae Cho, Han Sung Kim, Seong-Hun Park, et al., J. Phys. Chem. C 113 (2008) 7885. [18] B.S. Chauhan, R. Kumar, K.M. Jadhav, M. Singh, J. Magn. Magn. Mater. 283 (2004) 71. [19] Ovidu Caltun, G.S.N. Rao, K.H. Rao, B. Parvatheeswara rao, Cheol Gi Kim, et al., J. Magn. Magn. Mater. 316 (2007) e618. [20] Shekhar D. Bhame, P.A. Joy, J. Appl. Phys. 99 (2006) 1. [21] J. Smith, Wijn, H.P.J., Ferrites, John Wiley and Sons, Inc., New York, pp. 251e256. [22] S.D. Bhame, P.A. Joy, J. Appl. Phys. D 40 (2007) 3263. [23] O. Bidaut, P. Goux, M.K. Chikech, M. Belkaoumi, M. Manglion, Phys. Rev. B 49 (1994) 7868. [24] S.K. Choi, B.S. Kang, Y.W. Cho, Y.M. Vysochanskii, Diffused dielectric anamoly in ferroelectric materials, J. Electroceram. 13 (2004) 493. [25] T. Takenaka, H. Nagata, Y. Hiruma, IEEE Trans. Ultrason. Feroelectr. Freq. Control 56 (8) (2009) 1595. [26] Dezhen Xue, Yumei Zhou, Huixin Bao, Chao Zhou, Jinghui Gao, Xiaobing Pen, J. Appl. Phys. 109 (2011) U54110(1e6). [27] S.A. Gridnev, A.V. Kalgin, V.A. Chernykh, Integr. Ferroelectr. 109 (2009) 70. [28] C.G. Zhong, Q. Jiang, J. Phys. D: Appl. Phys. (2008) 115002 (6 pp).
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Magnetoelectric and magnetodielectric effect in Ba1xSrxTiO3 and Co0.9Ni0.1Fe2xMnxO4 composites M.M. Sutar a, A.N. Tarale a, S.R. Jigajeni a, S.B. Kulkarni b, V.R. Reddy c, P.B. Joshi a, * a
Solapur University, 413 255 Solapur (MS), India Institute of Science, Mumbai (MS), India c UGC-DAE-CSR, Indore (MP), India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2011 Received in revised form 9 April 2012 Accepted 20 May 2012 Available online 26 May 2012
Ba1xSrxTiO3 (BST) is a ferroelectric material known to possess ferroelectric transition temperature Tc in the vicinity of room temperature w30 C for x ¼ 0.3. As the BST with x in the vicinity of 0.3 is elastically soft at 30 C, the compositions with x ¼ 0.2, 0.25 and 0.3 are chosen to investigate their possible magnetoelectric (ME) and magnetodielectric (MD) applications. The (CoNi) FeMn2O4 (CNFMO) is selected to be a piezo-magnetic phase. Here hydroxide co-precipitation route is adopted so as to synthesize BST and CNFMO of nearly 100 nm crystallite size. Starting with the BST and CNFMO powders, the composites yCNBST ¼ yCNFMO þ (1 y) BST are synthesized for y ¼ 0.3 and 0.4. The parent compositions of BST as well as the CNFMO are characterized for dielectric and magnetic properties to confirm the formation of the desire ferroelectric and magnetostrictive phases. The composites are investigated for the crystal structure, dielectric, magnetoelectric and magnetodielectric properties. The results show that the composite Ba0.75Sr0.25TiO3eCo0.9Ni0.1Fe1.7Mn0.3O4 exhibit excellent ME and MD properties simultaneously. The results on the ME and MD properties are understood in terms of the stress induced variations in polarization and dielectric constant 3 respectively. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: BST CNFM Magneto-electric Magneto dielectric properties Magneto-capacitance
1. Introduction Ba1xSrxTiO3 is a well-known ferroelectric material where the transition temperature Tc could be reduced from 120 C down to 231 C by varying x between 0 and 1 [1]. The compounds with x in the vicinity of 0.3 are of special interest owing to its Tc ¼ 30 C (zRT) and very high value of dielectric constant 3 in the vicinity of Tc in both ferroelectric and paraelectric regions [1e4]. Here the dielectric constant refers to relative permittivity to free space. BST with x ¼ 0.3 is known to possess useful value of electrical tunability {3 (E) 3 (0)}/3 (0). Therefore BST finds its applications in tunable filters, tunable dielectric resonators, multilayer capacitance devices etc. [5e8]. As far as the magnetoelectric (ME) and magnetodielectric (MD) properties are concerned, it is expected that ME and MD coefficients will be high in the ferroelectric region and in the vicinity of Tc. It is seen that magnitudes of d3 /dT i.e. rate of change of permittivity with temperature and dPr/dT i.e. rate of change of polarization with temperature are maximum in this region of
* Corresponding author. Fax: þ91 217 2744768. E-mail address: [email protected] (P.B. Joshi). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2012.05.016
temperature and reduces as the temperature is decreased below Tc. Further the dPr/dT is negative, while d3 /dT is positive in the vicinity of Tc. As the ME phenomenon is proportional to the change of polarization as a function of applied stress, the ME, MD properties are expected to be sensitive to the compositional variations of BST for x varying from 0.2 to 0.3. Here the Tc for x ¼ 0.3 is at 30 C and increases to nearly 58 C for x ¼ 0.2. Therefore d3 /dT and jdPr/dTj for BST at x ¼ 0.25 and x ¼ 0.3 will be large as compared to their values for x ¼ 0.2. Owing to the discussion above we have selected x ¼ 0.2, 0.25 and 0.3 compositions for ferroelectric phase of ME/MD composites. Here Co0.9Ni0.1Fe1.7Mn0.3O4 (CNFMO) ferrite is selected to be a magnetostrictive phase considering its high value of coefficient of magnetostriction l. Simultaneously to our efforts, considering the importance of BST as discussed above very recently the physical properties of BST synthesized via Pechini method are reported [2]. Therefore to avoid the repetition of discussion of physical properties of BST is kept limited to comparison of the previously reported results and present observations. Further ME properties of the composites are maximum for composition in the vicinity of y ¼ 0.5, as the ME properties are governed by the relation (1 y), where y is the fraction of the magnetostrictive phase in the composite. Thus the present paper
M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070
reports the magnetic, dielectric, ME and MD properties of the CNFMOeBST composites. To form nano particles of individual phases, both CNFMO and BST powders are synthesized via hydroxide co-precipitation route. The paper reports the synthesis and characterization of different CNFMO and BST compositions and composites. The crystal structure and crystallite size is determined using XRD spectra. The physical properties of CNFMO like dc resistivity rdc, initial permeability m, saturation magnetization Ms etc. are determined to obtain optimum composition with high values of rdc, Ms, l and low value of Hc. Further the paper also reports the dielectric, ME and MD properties of the CNBST composites as a function of frequency F and temperature T. 2. Experimental 2.1. Synthesis of Co0.9Ni0.1Fe2xMnxO4 (CNFMO) The CNFMO was synthesized by employing hydroxide coprecipitation route followed by ceramic processing approach for x ¼ 0, 0.1, 0.2, 0.3 and 0.4. The AR-grade Fe(NO3)3$9H2O, Co(NO3)2$6H2O, Ni(NO3)2$6H2O, MnCl2$4H2O and KMnO4 were used as precursors, while a mixture of NH4OH and KOH were used as precipitating agents. The details of the co-precipitation route are similar as reported earlier [9,10]. The precipitates formed were washed thoroughly and calcined at 1100 C for 12 h to achieve complete ferrite phase formation. After calcination the calcined product was pressed into pellets and sintered at 1200 C for 10 h for characterization of bulk properties like rdc, Ms, m and l. The remaining powder was calcined again at 1200 C for 10 h. The calcined powder was used as a starting material for the formation of composites. For complete characterization of these compositions, rdc was measured using potential divider arrangement. The physical density dBulk was measured using the liquid displacement method, while saturation magnetization Ms was measured using Hysteresis loop tracer from Ms. Arun Electronics, Mumbai (India). The permeability m was measured using a LVDT arrangement and l is measured using magnetostriction setup. 2.2. Synthesis of Ba1xSrxTiO3 (BST) The Ba1xSrxTiO3 powders for x ¼ 0.2, 0.25 and 0.3 were synthesized using similar process as described above. High purity (>99.9%) Barium acetate Ba(COOCH3)2, Strontium nitrate Sr(NO3)2, Potassium Titanium Oxalate [K2TiO(C2O4)2$2H2O] were used as precursors. For complete precipitation of Ba(OH)2 and TiO(OH)2, the molar ratio of KOH to (BaTi) of 1.6 has been used, based on the earlier report [11]. It has been observed that the Ba(OH)2 and Sr(OH)2 is fractionally soluble in water but insoluble in alkaline medium. Therefore the precipitates were washed in dilute NH4OH solution with pH w10 [12]. The remaining procedure of coprecipitation was similar to that of CNFMO. The powders were calcined at 1000 C for 10 h and after calcination; part of calcined product was pressed in the form of pellets and sintered at 1100 C for 12 h. The remaining powder was calcined again at 1100 C for 12 h. The sintered pellets were used for characterization of bulk properties. The calcined powder was used as a starting material for the formation of composites. For further discussion the samples are denoted as BSTx for x ¼ 0.2, 0.25 and 0.3.
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Where x ¼ 0.2, 0.25 and 0.3 for y ¼ 0.3 and 0.4. The calcined powders of CNFMO and BST were grounded together thoroughly using ethanol as a medium. The pellets of diameter 1.2 cm were formed as discussed above. The pellets were sintered at 1200 C to form desired ME/MD composites. The parent compositions CNFMO and BST as well as their composites (CNBST) were investigated for the structural properties using X-ray powder diffractometer (Rigaku Miniflex). SEM images of calcined powders of parent compositions were obtained using a Jeol-JSM6360SEM scanning electron microscope. For dielectric measurements LCR-Q meter (HP4284A) was used in the frequency range from 100 Hz to 1 MHz for temperature T between room temperature to 250 C for measurement of Cp, tan d as a function of frequency F, temperature T and magnetic field H for dielectric and MD characterization of BST and composites. The linear and quadratic magnetoelectric coefficients a and b were determined using a custom designed instrument as reported earlier [13,14]. The P-E measurements on BST samples at 50 Hz were carried out using P-E loop tracer by Ms. Radient Technology, USA. 3. Results and discussion The XRD spectrum for calcined CNFMO powder for x ¼ 0.3 is shown in Fig. 1(a). It is seen that the observed reflections could be associated with the corresponding (hkl) planes using standard JCPDS data on CoFe2O4 (JCPDS card no. 22-1086). Using XRD data, it is observed that CNFMO ferrite possesses the spinal cubic crystal structure with lattice parameter ‘a’ as shown in Table 1. It is observed that the lattice parameter ‘a’ increases gradually with increasing ‘x’ owing to larger ionic radius of Mn as compared to Fe cations. Further the crystallite size D is calculated using Scherrer formula and is also shown in Table 1. From XRD spectra it is observed that all the compositions possess the crystallite size in the range of 100e140 nm. Further the Table 1 also shows the magnitude of dc resistivity rdc, physical density dBulk, crystalline density dx-ray, and % porosity p. It is observed that variation of rdc could be understood based on the earlier report on Ni substituted CoFe2O4 [15]. It is observed that rdc is very high at x ¼ 0 and decreases slowly with increasing Mn content. The presence of Fe2þ and Fe3þ ions on B-sites is known to cause polaronic conduction in case of ferrites. At very low concentration the Mn and Ni reduce the percentage of fractional Fe2þ ions formed during the process of synthesis. It is
2.3. Formation of composites The CNFMO and BST composites are formed bearing the formula
yðCNFMO BSTxÞ ¼ yCNFMO þ ð1 yÞBSTx
Fig. 1. XRD spectra of calcined (a) CNFMO powder for x ¼ 0.3 and (b) BST0.25 powder.
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Table 1 a: Lattice parameter, D: crystallite size, rdc: dc resistivity, dBulk: bulk density, dx-ray: X-ray density and p: porosity. CNFMO
x x x x x
¼ ¼ ¼ ¼ ¼
0.0 0.1 0.2 0.3 0.4
‘a’ Å
Crystallite size ‘D’ (nm)
‘rdc’ 106 U-met
dBulk gm/cm3
dx-ray gm/cm3
R (%)
8.18 8.23 8.25 8.28 8.31
134 119 114 121 129
67.1 12.92 10.76 2.236 0.145
5.38 5.29 5.03 5.01 5.00
5.69 5.58 5.54 5.46 5.54
5.37 5.24 9.17 8.10 9.75
important to note that the ionic radii of Mn3þ, Fe3þ, Mn2þ, Ni2þ and are known to be 0.72, 0.63, 0.80 and 0.69 Å respectively [16]. The Asites in ferrite are known to possess tetrahedral coordination, while the B-sites are coordinated in the octahedral form. Therefore Fe3þ or Mn3þ ions may prefer the smaller size A-sites as compared to the B-sites. Nevertheless the distribution of ions amongst A-sites or Bsites is not solely decided by the ionic radii only. As per as the earlier report, substitution of Mn/Ni below 0.04 atom percentage causes an increase in resistivity [15]. For further increase in Mn/Ni concentration rdc decreases slowly with increasing x. The present observations are in agreement with the earlier report [17]. The magnitude of porosity is as shown in Table 1. It is observed that the samples are dense and porosity is less than 10% comparable with the porosities reported earlier for similar sintering conditions [18]. Fig. 1(b) shows the XRD spectrum of calcined BST0.25 powder. The observations of XRD spectra for x ¼ 0.2 and 0.3 are similar to the XRD shown in Fig. 1(b). It is observed that the degree of tetragonality c/a reduces from 1.0083 to 1.0022 as the x is varied from x ¼ 0.2 to 0.3 as shown in Table 2. These results are in confirmation with the earlier report [2]. Thus as the x increases, the tetragonality decreases and sample becomes nearly cubic for x > 0.30. Using Scherrer formula the crystallite size is determined to be between 100 and 120 nm (Table 2). Further, no peak corresponding to any impurity phase is observed in XRD spectrum and formation of a pure single-phase composition is confirmed from the XRD spectrum. Fig. 2(aeb) shows the XRD spectra of CNBST0.25 for y ¼ 0.3 and CNBST0.3 for y ¼ 0.4 composites. The peak corresponding to the reflections of BST and CNFMO could be separately indexed in Fig. 2(aeb). No peak corresponding to any impurity phase is recorded in XRD spectra. Further from Fig. 2(aeb) it is seen that relative intensity of the (220) peak of CNFMO increases from 33% to 46% as the concentration of ferrite increase from x ¼ 0.3 to x ¼ 0.4. These observations are sufficient to say that composites are formed as pure biphasic system. The SEM images of calcined powders of CNFMO for x ¼ 0.3, BST0.2, BST0.25, and BST0.3 are shown in Fig. 3(aed). The SEM images are obtained for as calcined powder without any addition of dispersing agent. Therefore, the images show a distribution of particles not well separated from each other. Nevertheless, the large numbers of CNFMO particles are of 1 mm, while BST particles are of nearly 400 nm. Thus as the starting powders of CNFMO or BST fairly small in size, the resulting composites may possess the useful values of electromechanical coupling coefficient.
Fig. 2. XRD spectra of (a) CNBST0.25 (y ¼ 0.3) and (b) CNBST0.30 (y ¼ 0.4) composites (B corresponds to reflections of BST, while C corresponds to reflections of CNFMO).
Fig. 4 shows the variation of l with applied magnetic field H for CNFMO series. It is observed that, the l increases and appears to saturate for H > 4 kOe. The highest value of l for H ¼ 4.5 kOe is termed as lsat. Table 3 shows the observed magnetic properties of the CNMFO series. From Table 3 it is observed that the saturation magnetization Ms increases with increasing Mn content as expected for Mn substituted cobalt ferrite [19]. As suggested, a fraction of Mn3þ may reduce to Mn2þ and Ms may increase with increasing Mn2þ. The presence of Mn2þ on B-sites causes an increase in magnetization per unit cell. Further increase in doping level, may decrease the exchange interaction and Ms reduces for further increase in x [19]. This suggests that the Mn ions may fractionally occupy A as well as B sites. The actual distribution of Mn ions on A and B sites may depend on the sintering conditions and/or presence of other cations. An observation in this support is the earlier report on Co1.2Fe1.8xMnxO4 [20]. In this case it was observed that for substitution of Mn in Co1.2Fe1.8xMnxO4, the Ms has increase for x up to 0.1 only and decreases for further increase in Mn concentration. In case of this report for Co1.2Fe1.8xMnxO4, for x ¼ 0, 0.1, 0.2 and 0.3 the reported values of Ms are nearly 68, 72, 63 and 62 emu/g respectively [20]. On the other hand the present observations show that Ms increases monotonically for Mn even up to x ¼ 0.4 (Table 3). This may occur because of presence of Ni on B sites. Further, Hc for all the compositions are closer to 100 Oe and this observation too is in confirmation with the earlier reports [19,20]. Table 3 shows the variation of m as a function of x. The m is expected to be proportional to Ms2 and the present observations are concurrent with this prediction [21]. The earlier reports on the Mn substituted cobalt ferrite predict that the presence of Mn ions on Asites causes an increase in l and magneto mechanical coupling of CNFMO [22]. Table 3 shows variation of lsat as a function of x. With small decrease, the l increases with x for x ¼ 0.2 and 0.3 and then reduces slightly for x ¼ 0.4. Though l is observed to follow Ms, that
Table 2 D: Crystallite size, Tc: ferroelectric transition temperature, PMax: maximum polarization, Pr: remanent polarization. Composition
D (nm)
Tc C
‘a’ (Å)
‘c’ (Å)
c/a
Pmax (mC/cm2)#
Pr (mC/cm2)#
Ba0.80Sr0.20TiO3(BST0.20) Ba0.75Sr0.25TiO3(BST0.25) Ba0.70Sr0.30TiO3(BST0.30)
104 120 118
58 42 30
3.920 3.824 3.809
3.9525 3.8415 3.8173
1.0083 1.0046 1.0022
0.9789 1.270 1.052
0.1414 0.1805 0.0622
# At V ¼ 1000 V and T ¼ 20 ms.
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Fig. 3. Calcined powder SEM images of (a) CNFMO (x ¼ 0.3), (b) BST0.2, (c) BST0.25 and (d) BST0.3.
is occupancy of Mn on A-sites, up to x ¼ 0.3 for higher values of x, the l does not follow this relation. This could be because of the fractional occupancy of Mn on B-sites as x increases. Fig. 5 shows the variation of dielectric constant 3 as a function of T for F ¼ 1 kHz for BST0.2, BST0.25 and BST0.3. Here the dielectric constant refers to permittivity with respect to free space. From Fig. 5 it is seen that 3 passes through maximum at Tc. The observed Tc for BST0.2, BST0.25 and BST0.3 are shown in Table 2. Further it is observed that 3 increases with T again in the paraelectric region above Tc. This feature is predicted to occur because of grainegrain boundary effect [2]. To understand these features in details the 3 is determined as a function of F also and Fig. 6(a) shows the variation of 3 as a function of T for F varying between 100 Hz and 1 MHz for BST0.25, while Fig. 6(b) shows the variation of tan d as a function of T for F ¼ 100 Hz, 1 kHz, 10 kHz and 100 kHz respectively. Here it is
observed that the increases in 3 with T occur for frequencies below 100 kHz only and d3 /dT decreases with increase in F. These features occur due to the presence of interfacial polarization at grainegrain boundary interface. The grainegrain boundary interface occurs probably due to fractional Ti3þ acceptor state in the bulk of grain [23,24]. Similar features are also reported by Iansculescu et al. [2]. A dielectric relaxation originated from defects extrinsically formed during the preparation process (particularly O-vacancies) is a common phenomenon in Ti-perovskite at high temperatures of 400e700 C. By creating more defects (vacancies, free electrons and substituted ions), the substitution heterogeneity and characteristic of the BST solid solutions determines the lowering of the activation energy of such relaxations. The observed behavior is a competitive phenomenon between the dielectric relaxation and the electrical conduction of the relaxing species. Its relationship with composition and microstructures has to be further analyzed in detail. Now Table 4 shows the variation of dynamic ME coefficients a and b respectively for CNBST composites. Here a represents linear, while b represents quadratic ME coefficient defined by the relations as below.
a ¼ dE=dH ¼ ðDV0 Þ=G*t*ðDhÞ and b ¼ ðDV0 Þ=2*d*h*ðDHÞ Where DV0 is the r.m.s output voltage developed across the sample in mV, G is the gain of the amplifier, t is the effective thickness of the sample, h is the r.m.s. value of the AC field at frequency 850 Hz and H is the imposed dc magnetic field. From Table 4, it is observed that Table 3 Ms: Saturation magnetization, Hc: coeresive field, m: permeability, lsat: saturation magnetostriction. CNFMO
Fig. 4. Variation of magnetostriction coefficient l with applied magnetic field H.
x x x x x
¼ ¼ ¼ ¼ ¼
0 0.1 0.2 0.3 0.4
Ms emu/g
Hc Oe
m
lsat 106
141 199 222 252 269
100 112 112 93 81
255 502 636 777 911
115 90 160 167 142
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M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070 Table 4 a and b: linear and quadratic ME coefficients. Composite
CNBST0.2 CNBST0.25 CNBST0.3
Fig. 5. Variation of
3
with T at F ¼ 1 kHz for BST0.2, BST0.25 and BST0.3.
a is maximum for CNBST0.25 and y ¼ 0.4. Similar are the features for variation of b. Here the change in b is insignificant for x ¼ 0.25
and x ¼ 0.3 and y ¼ 0.3 and 0.4. From the basic theory of ME effect in composites a and b are proportional to y(1 y), where y is the percentage of the piezo-magnetic phase in the composites. Further the a is proportional to piezoelectric coefficient d, magnetostriction coefficient l magneto-mechanical coupling coefficient km and inversely proportional to 3 . It is already reported that the maximum polarization Pmax and remanent polarization Pr of BST composites at RT decreases with increasing Sr content and become very low for x ¼ 0.3. Additionally the magnitude of Pmax and Pr may increase with improved crystallinity in case of polycrystalline materials. It is previously reported and present observations the crystallinity is maximum for BST0.25 as compared to other BST compositions. These features are discussed in the next paragraph. At present it is observed that in case of piezoelectric materials, ‘d’ becomes maximum on the higher polarization side of Tc or morphological phase boundary (MPB) [25]. Here it is also known that material become elastically soft at T ¼ Tc or at MPB [26]. Therefore it is expected that ‘d’ and hence a becomes maximum probably for BST0.25 as ‘d’ becomes maximum. The previous report as well as present observations shows that the BST0.25 possesses higher crystallinity in comparison with the other BST ceramics [2].
b (103 mV/cm-Oe2)
a (mV/cm-Oe) y ¼ 0.3
y ¼ 0.4
y ¼ 0.3
y ¼ 0.4
4.99 5.57 3.46
5.04 5.57 3.46
0.44 0.43 0.12
0.45 0.43 0.20
Therefore, these observations also favor a becoming maximum for BST0.25. Fig. 7(aeb) shows the variation of polarization PE as a function of electric field E. Fig. 7(a) shows P-E hysteresis loops for BST0.2, BST0.25 and BST0.3, while Fig. 7(b) shows P-E behavior for ME composites with y ¼ 0.3. The magnitudes of Pmax and Pr for various compositions studied are shown in Table 2 for pure BST and Table 5 for CNBST composites. The present observations for the BST compositions are on the similar lines as reported earlier [2]. In this case, the tendency to reduce the remanent polarization, while increasing the Sr content is related to the reducing tetragonality i.e. with approaching the ferroepara phase transition at room temperature. The magnitude of Pmax is lower as compared to the earlier report, probably because of the lower crystallite size obtained in the present case. The Pmax and Pr for BST0.25 is observed to be slightly more as compared to BST0.2, probably because of improved crystallinity for x ¼ 0.25. Essentially the Pmax is observed to show decreasing trend as x increases. From Fig. 7(b) and Table 5 it is observed that in case of composites also the Pmax and Pr show a decreasing trend for increasing x from 0.2 to 0.3. Nevertheless because of the presence of CNFMO at y ¼ 0.3, hysteresis loss increases for the ME composites as compared to the pure BST composites. Here the presence of CNFMO causes leakage of the charge of polarization providing the conducting path to the polarization charges. Fig. 8(aeb) shows the variation of dielectric constant 3 as a function of log F with and without applied magnetic field in the frequency range from 100 Hz to 1 MHz of CNBST0.2 and CNBST0.3 for y ¼ 0.3 and 0.4 respectively. It is observed that 3 passes through a maximum at F ¼ 500 kHz. This frequency appears to be equal to the radial mode of oscillations of the ME material of the disc shaped sample with diameter equal to 1.2 cm. Therefore, F is observed independent of the composition. Further it is observed that the 3 decreases with increasing applied magnetic field almost linearly and rate of change of 3 with magnetic field is maximum in the vicinity of electromechanical resonance (EMR) frequency. The
Fig. 6. (a). Variation of 3 with T for F ¼ 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz for BST0.25. (b). Variation of tan d with T for F ¼ 100 Hz, 1 kHz, 10 kHz and 100 kHz for BST0.25.
M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070
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Fig. 7. (a). Variation of PE as a function of E for BST0.2, BST0.25 and BST0.3. (b). Variation of PE as a function of E for CNBST0.2, CNBST0.25 and CNBST0.3 composites.
Table 5 D3 /3 0 : Magneto-capacitance, Pmax: maximum polarization, Pr: remanent polarization. Composite
CNBST0.2 CNBST0.25 CNBST0.3
D3 /3 0 At F ¼ 300 kHz
D3 /3 0 At F ¼ 600 kHz
Pmax (mC/cm2)
Pr (mC/cm2)
y ¼ 0.3
y ¼ 0.4
y ¼ 0.3
y ¼ 0.4
y ¼ 0.3*
y ¼ 0.4**
y ¼ 0.3*
y ¼ 0.4**
3.05 4.04 1.85
2.46 4.23 4.09
3.83 3.04 2.35
6.06 6.17 5.30
0.8672 0.9280 0.5833
0.3306 0.5311 0.2170
0.6511 0.8513 0.3758
0.4351 0.4498 0.2209
*For V ¼ 500 V and T ¼ 20 ms, **For V ¼ 200 V and T ¼ 20 ms.
magnitude of magneto-capacitance Mc is defined as (3 (H) 3 (0))/ is also shown in Table 5. It is observed that the Mc becomes maximum for CNBST0.25 and y ¼ 0.4. The Mc in this case was predicted to occur because of strain induced change in polarization with applied magnetic field. It is known that as the magnetic field is increased the stress on the BST particles also increases. Because of the applied stress polarization may also increase, reducing the corresponding value of 3 . This phenomenon is explained on the basis of two layer ME composite structure by Gridnev et al. [27]. As the electromechanical coupling factor Km is maximum at the resonant frequency, the Mc too is maximum at EMR frequency 500 kHz.
3 (0)
The tan d variation with log F for CNBST0.2 for y ¼ 0.3 and 0.4 with and without magnetic field are shown in Fig. 9(aeb). As expected from the basic theory, here the tan d also passes through a maximum at EMR frequency of 500 kHz. 4. Conclusions The BST composites with x in the vicinity of x ¼ 0.3 show interesting ferroelectric properties, variation of relative dielectric permittivity, maximum value of polarization and remanent polarization. The present study shows that the CNFMOeBST composite shows both ME and MD behavior. Further the magnitudes of the ME
Fig. 8. (a). Variation of 3 as a function of log F with and without magnetic field for F varying between 100 Hz and 1 MHz for CNBST0.2, y ¼ 0.3 composite. (b). Variation of a function of log F with and without magnetic field for F varying between 100 Hz and 1 MHz for CNBST0.3, y ¼ 0.4 composite.
3
as
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M.M. Sutar et al. / Solid State Sciences 14 (2012) 1064e1070
Fig. 9. (a). Variation of tan d with log F for CNBST0.2, y ¼ 0.3 composite. (b). Variation of tan d with log F for CNBST0.2, y ¼ 0.4 composite.
coefficients are useful and a is observed maximum at 5.57 mV/cm-Oe for x ¼ 0.25, y ¼ 0.4. The magnitude of magneto-capacitance is also high nearly 6% at EMR frequency 600 kHz and possesses a useful value of above 4% for F between 100 kHz and 800 kHz. The behavior of magneto-capacitance could be understood in terms of the landau thermodynamic theory as detailed in earlier reports [27,28]. These observations suggest that the compositions studied are interesting in terms of both physics as well as commercial applications. Acknowledgements The author M.M. Sutar would like to thanks the University Grant Commission, New Delhi, India, for a Teacher Research Fellowship award under FIP, XIth Plan 2007-12. References [1] A. Mark McCormick, Roeder, Elliott B. Slamovich, J. Mater. Res 16 (4) (2001) 1200. [2] A. Iansculescu, D. Berger, L. Mitoseriu, L.P. Curecheriu, N. Dragan, D. Crisan, E. Vasile, Ferroelectrics 369 (1) (2008) 22. [3] F. zimmerman, M. Voiats, W. Menesklou, E. Ivezs-Tiffee, J. Eur. Ceram. Soc. 24 (2004) 1729. [4] A.T. Chien, X. Xu, J.H. Kim, J. Sachleben, J.S. Speck, F.F. Lange, J. Mater. Res. 14 (1999) 3330. [5] J.H. Haeni, P. Irvin, W. Chang, R. Ucker, et al., Nature 140 (2004) 758e760. [6] A.K. Tagantsev, V.O. Sherman, K.F. Astafiev, J. Venkatesh, N. Setter, J. Electroceram. 11 (2003) 5. [7] D. fuck, S. Dorman, Phys. Rev. B 73 (2006) 104116. [8] M. Jain, S.B. Mujumdar, R.S. Katiyar, A.S. Bhalla, Mater. Lett. 57 (2003) 4232.
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