Magnetodielectric effect in rare earth doped BaTiO3-CoFe2O4 multiferroic composites

Journal of Alloys and Compounds 794 (2019) 402e416

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Magnetodielectric effect in rare earth doped BaTiO3-CoFe2O4 multiferroic composites Mehraj ud Din Rather, Rubiya Samad, Nahida Hassan, Basharat Want* Solid State Research Lab, Department of Physics, University of Kashmir, Srinagar, 190006, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 January 2019 Received in revised form 20 April 2019 Accepted 23 April 2019 Available online 25 April 2019

Magnetically switchable dielectric properties in functional multiferroic composites at room temperature is considered to be a great challenge in order to fabricate magnetoelectric devices. High magnetoelectric effect in such materials is possible provided ferroelectric phase is highly coupled to the magnetic phase. In this report, an attempt has been made to fabricate multiferroic composites (1-x) Ba0.95Ho0.05TiO3 e x CoDy0.1Fe1.9O4 (x ¼ 0.03, 0.06 and 0.09) by solid state route, with tunable magnetodielectric properties. From XRD studies, a decrease in cell volume of CoDy0.1Fe1.9O4 phase is observed with the increase in Ba0.95Ho0.05TiO3 phase, which is responsible for higher magnetodielectric effect. In composites, an appreciable decrease in grain size is one of the main factor that drives the coupling between two ferroic phases. By the incorporation of Ho3þ ions in the BaTiO3 lattice, an increase in ε0 is observed, which resulted in improved magnetodielectric effect in composites. In composites, decrease in hopping length with the increase in ferrite phase, resulted in enhancement in dielectric constant. For all composites, the unsaturated ferroelectric hysteresis loops were observed due to the conducting nature of ferrite phase. The incorporation of Dy3þ ions in the CoFe2O4 expanded its lattice, resulting in strain, which highly affected the magnetic properties. The magnetocrystalline anisotropy of composites with multidomain structure was calculated by using Law of approach to saturation, which is directly related to coupling between two ferroic phases. Magnetodielectric studies revealed that the magnetic ordering of CoDy0.1Fe1.9O4 phase results in increase in number of polar domains of Ba0.95Ho0.05TiO3 phase and thereby improves the magnetodielectric effect. The overall results of this work established the unification of Ba0.95Ho0.05TiO3 and CoDy0.1Fe1.9O4 phases in multiferroic composites, with improved coupling, making them potential candidates for magnetoelectric devices. © 2019 Elsevier B.V. All rights reserved.

Keywords: Multiferroics Rare earths Domain pinning LA approach Magnetodielectrics

1. Introduction The uniqueness of a material increases extremely, if it exhibits various interlinked functionalities. Among various functional materials, the multiferroics unify magnetic, electric and mechanical responses together. In magnetoelectric materials, the electric polarization (P) can be tuned by a magnetic field (H), and conversely magnetization (M) can be controlled by an electric field (E). Such materials in which multiple order parameters (P,M) can be tuned by fields (H,E) respectively are suitable for novel electronic devices such as magnetic field sensors and four state memories [1,2]. In single phase magnetoelectric (ME) materials, the various ferroic orders parameters (P,M) are interlinked, but are weekly coupled. To

* Corresponding author. E-mail address: [email protected] (B. Want). https://doi.org/10.1016/j.jallcom.2019.04.244 0925-8388/© 2019 Elsevier B.V. All rights reserved.

overcome this problem, a realistic approach is to fabricate the artificial composite materials that exhibit high degree of coupling at room temperature. The individual ferroic phases in such composites retain their properties and when bought in contact with each other exhibit multiferroic behavior, where the two ferroic orders are coupled together via a third order parameter. In case of ME composites, the electric (P) and magnetic (M) order parameters are coupled via lattice strain. The basic principle of ME coupling is that when a magnetic field (H) is applied to a ME composite, the magnetic phase changes its shape via magnetostriction. The strain (S) generated in the magnetic phase is then transferred to the ferroelectric phase, resulting in an electric polarization (P). Hence for magnetic phase, we have vS/vH ¼ em and for ferroelectric phase, we can write vP/vS ¼ e, where em and e are the piezomagnetic and piezoelectric coefficients respectively. Thus in ME composite, the coupling cofficient (a) can be written as:

M.D. Rather et al. / Journal of Alloys and Compounds 794 (2019) 402e416

a¼

vP ¼ kc em e vH

(1)

where kc is a coupling cofficient ð0  jkc j  1Þ between the two ferroic phases [3]. Instead of controlling the order parameter (P) by magnetic field (H), an indirect and alternative approach to measure ME effect is the magnetodielectric (MD) phenomenon. Here the change in dielectric constant (ε0 ) is measured as a function of applied magnetic field (H), since in ME composites, the magnetic order is coupled to electric polarization and hence to the dielectric properties. Mathematically, MD effect can be written as: 0

MCð%Þ ¼

0

ε ðHÞ  ε ðH ¼ 0Þ  100 0 ε ðH ¼ 0Þ

(2)

where MC(%) is the percentage magnetocapacitance. ε0 (H) and ε0 (H ¼ 0) are the values of dielectric constant in presence and absence of magnetic field respectively. As the MD effect is extrinsic, so it depends upon microstructure of composite, dielectric constant

Fig. 1. Unit cell of BaTiO3 (a) Cubic phase (b) Tetragonal phase.

403

of ferroelectric phase and magnetostriction of the magnetic phase. Taking this thing into account, it is quite obvious that the MD effect in composites can be improved only provided ferroelectric phase has high value of dielectric constant and magnetic phase has high magnetostriction, so that maximum strain can be transferred from magnetic to ferroelectric phase. Among ferroelectric materials, the BaTiO3 (BT) is a unique candidate due to its fascinating piezoelectric properties, high dielectric constant and is free from toxic lead [4,5]. Below transition temperature (120  C), it is ferroelectric and has low symmetrical tetragonal structure in which central Ti4þ ion is shifted towards one of the apical oxygen as shown in Fig. 1(a). However above transition temperature, it is paraelectric with cubic unit cell having high symmetry as depicted in Fig. 1(b). Similarly from the magnetic oxides, CoFe2O4 (CFO) is a unique candidate due to its high magnetostriction [6]. The crystal structure of CFO is shown in Fig. 2(a), and its front view in Fig. 2(b). The unit cell of CFO contains divalent as well as trivalent cations, so two crystallographic sites are present. One is the octahedral as shown in Fig. 2(c), where A site is surrounded by six oxygen ions; while the other is the tetrahedral, where B site is surrounded by four oxygen ions shown in Fig. 2(d). A vast research on BT-CFO multiferroic composites with various dopants has been carried out till date, wherein the dielectric, magnetic and magnetodielectric properties are extensively studied [7e11]. However, coupling effect between the BT and CFO phase has not been enhanced to a larger extent. An innovative approach to improve the coupling is to enhance the dielectric constant of BT phase and magnetostriction of the magnetic phase, which can be obtained by rare earth doping in these ferroic phases. In this work, the rare earth holmium has been incorporated in BaTiO3 lattice to achieve high value of dielectric constant [12]. The increase in dielectric constant is due to replacement of smaller size Ti4þ ions

Fig. 2. Crystal structure (a) Cubic CoFe2O4 (b) Front view (c) Octahedral site (d) Tetrahedral site.

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M.D. Rather et al. / Journal of Alloys and Compounds 794 (2019) 402e416

(0.74 Ao) by large sized Ho3þ ions (0.89 Ao), which results in the expansion of the BT lattice. As a result of this, the central Ti4þ ion in the octahedra moves towards one of the apical oxygen. This displacement of Ti4þ ion shortens one Ti-O bond and elongates other two Ti-O bonds. This results in polar character and hence increases the dielectric constant of BT phase. To enhance the magnetostriction and magnetic properties of CoFe2O4, the dysprosium is the unique candidate due to its larger ionic radii and magnetic moment (10.6 mB). The partial substitution of smaller sized Fe3þ ions (0.63 Ao) by large sized Dy3þ ions (0.91 Ao) leads to structural distortion, which induces strain and hence significantly modifies the dielectric properties [13]. The various connectivity schemes have been proposed which include 0e3, 2e2, 1e3, etc for the fabrication of ME composites [14]. In the present work, bulk particulate composites in which ferrite particles are uniformly distributed in the ferroelectric matrix were preferred. This is attributed to the fact that ferrite phase is electrically more conductive as compared to the ferroelectric matrix, so its uniform distribution in ferroelectric matrix, by avoiding the percolation conduction can enhance the ME properties. In addition, the bulk composites are easier to synthesize and sophisticated instruments are not needed just like film deposition techniques. The bulk particulate multiferroic composites with general formula (1-x) Ba0.95Ho0.05TiO3 e x CoDy0.1Fe1.9O4 (x ¼ 0.03, 0.06 and 0.09) were prepared by solid state route. To the best of our knowledge, the investigation of multiferroicity has not been carried out in such particulate multiferroic composites. Thus an attempt has been made in this report to discuss the structural, dielectric, ferroelectric, magnetic properties and magnetodielectric properties in detail. 2. Experimental procedure 2.1. Materials For the preparation of Ba0.95Ho0.05TiO3 phase, the high grade BaCO3 (99.95%), TiO2 (99.9%), Ho2O3 (99.9%) and acetone (99.98%) were used as raw materials. The analytical grade chemicals which include Fe(NO3)3.9H2O (99.95%), Co(NO3)2$6H2O (98%), Dy(NO3)2. xH2O (99.9%), C6H8O7 (99.5%), and NH3 (99.95%), were used for the synthesis of CoDy0.1Fe1.9O4 phase. 2.2. Preparation of Ba0.95Ho0.05TiO3 ferroelectric phase For the preparation of Ba0.95Ho0.05TiO3 ferroic phase (H), stoichiometric amounts of BaCO3, TiO2 and Ho2O3 were mixed thoroughly. To attain the homogeneity, the acetone was used as a solvent. The obtained homogenous powders were calcined at 1100  C for 3 h, with the heating rate of 5  C/min and allowed to cool naturally. The resulted powder was then grounded for at least 3 h and sintering was done at 1200  C for 12 h to obtain required Ba0.95Ho0.05TiO3 ferroelectric phase. 2.3. Preparation of CoDy0.1Fe1.9O4 ferrimagnetic phase For the synthesis of CoDy0.1Fe1.9O4 ferrimagnetic phase (D), the metal nitrates which include Fe(NO3)3.9H2O, Co(NO3)2$6H2O and Dy(NO3)2$xH2O were dissolved in de-ionized water separately. Similarly C6H8O7 was dissolved in de-ionized water to obtain homogenous solution. At room temperature, the obtained solutions were mixed together and the PH of resulting solution was maintained by adding ammonia drop by drop. The PH maintained solution was heated at 90  C for 1 h for gel formation and the resulting gel was further heated slowly to ascertain combustion. The ashes left behind were collected and were grounded in a motor and pestle

for about 1 h to obtain homogenous powder. To ascertain the proper phase formation of ferrimagnetic D phase, the resulting powder was sintered at 900  C for 4 h. 2.4. Preparation of H-D particulate multiferroic composites The particulate multiferroic composites [(1-x) H e x D] (x ¼ 0.03, 0.06 and 0.09), equivalently denoted as HD3, HD6 and HD9 respectively, were prepared by using solid state route. The H and D phases by weight in desired ratio were mixed in an agate mortar for 4 h. In order to obtain the high dense structures, the resulting mixture in pellet form having diameter 13 mm were sintered at 1000  C for 4 h in alumina crucible trays. 2.5. Characterization To ascertain the phase formation of ferroelectric phase (H), the ferrimagnetic phase (D), and to confirm the presence of H and D phases in composites (HD3, HD6 and HD9), the structural analysis was carried out by using an X-ray diffractometer with Cu Ka (l ¼ 1.5406 A ) radiation. The surface morphology was revealed by using a Scanning Electron Microscope (Hitachi S-3000H), and the grain size was evaluated by using an Image J software. The room and low temperature (~100 K) dielectric studies were carried out by using LCR meter (Agilent 4284A), where low temperatures were obtained by the help of a microprocessor based furnace fitted with a temperature controller. The ferroelectric behavior was ascertained by tracing P-E hysteresis loops by using a P-E Loop Tracer, Radiant TechnologieseInc. To carry out the dielectric as well as ferroelectric studies, the silver paint was used as an electrode. The room and low temperature magnetic studies were done by tracing M  H hysteresis loops by using a vibrating sample magnetometer (Micro Sense EZ9 VSM, USA). The impact of magnetic field on dielectric properties and coupling interaction between H and D phases was ascertained by a combined setup of VSM (Micro Sense EZ9 VSM, USA) and Wayne Kerr Impedance Analyzer (Model: 6440B). 3. Results and discussions 3.1. Phase identification The XRD patterns of ferroic phases (H and D) and multiferroic composites (HD3, HD6 and HD9) are depicted in Fig. 3(aee). From XRD patterns, it is clear that H and D phases retained their tetragonal (space group P4/mmm) and a cubic inverse spinel structures (space group Fd-3m) respectively. This confirms that no structural change has occurred after the incorporation of rare earth ions in their respective lattices. By the incorporation of Ho3þ ions in the BaTiO3 lattice, a slight decrease in ‘c’ and increase in ‘a’ was observed than theoretical values of pure BaTiO3 [15]. The change in cell parameters is due to replacement of smaller sized Ti4þ ions (0.61 Å) by larger sized Ho3þ ions (0.912 Å), which in turn resulted in expansion of unit cell of BaTiO3 [16]. In a similarly way, although the crystal structure is retained by Dy3þ ion doping in D phase; however, an increase in ‘a’ occurred than pure CoFe2O4 [17]. The reason for this increase in value of ‘a’ is due to the larger ionic radii of Dy3þ ions (0.91 Å) as compared to Fe3þ ions (0.64 Å). This increase in ‘a’ leads to the structural distortion that induces strains, which in turn has significant effect on electric properties [18]. In composites (HD3, HD6 and Hd9), a well defined two set of peaks indexed by ‘C’ and ‘ ’ has been observed which correspond to H and D phases respectively. In addition, the peak intensity of H phase increases with the decrease of D phase, confirming that the two phases coexist in composites. The lattice parameters and unit

M.D. Rather et al. / Journal of Alloys and Compounds 794 (2019) 402e416

D0 ¼

405

0:9l bcosq

(5)

where l is the wavelength of the incident X-rays and q is the Bragg angle of reflection. From the calculated values of D0 , ε and d, it is clear that crystallite size in composites decreases with the increase in D phase. The decrease in D0 resulted in an increase in dislocation density, which enhanced the strain across the interface of H and D phases, resulting in large MD effect (section 3.3).

3.2. Surface morphology and chemical analysis The microstructure of the ferroic phases (H and D) and composites (HD3, HD6 and HD9) was analyzed by using SEM micrographs as shown in Fig. 4(aee) respectively. The SEM micrographs depict high dense monophasic grains for H phase and agglomerated grainy structure for D phase. The grain size (GS) was calculated by Image J Software and the values are tabulated in Table 1. The calculated grain size was found to be larger than the crystallite size, which indicates that every grain is composed of larger number of crystallites. In composites, a decrease in grain size was observed with the increase in D phase, consistent with the crystallite size calculated from XRD data. This decrease in grain size is one of the main factor that drives the coupling between two ferroic phases. This is attributed to the fact that smaller grains reduce the resistivity, thereby enhances the coupling, which is also well supported by the literature [21]. As the grain size is inversely related to porosity, so surface of composites seems more porous from HD3 to HD9. To confirm the same, the percentage porosity (g) was calculated for ferroic phases (H and D) by the following relation:

Fig. 3. XRD pattern (a) H phase (b) D phase (cee) composites (HD3, HD6 and HD9).




cell volume of ferroic phases and that of composites are depicted in Table 1. With the increase in D phase in composites, the unit cell volume of D phase decreases and that of H phase increases. This decrease in cell volume of D phase may be due to compressive strain of H phase and is one of the main important factors responsible for higher MD effect (section 3.6). Since the strain is created by lattice distortion, so it was thought worthwhile to calculate strain (ε) and dislocation density (d). The values of ‘ε’ and ‘d’ were evaluated by the following relations [19]:

gð%Þ ¼ 1 



rB  100 rX

(6)

where rB and rX are the bulk and X-ray densities which are evaluated by the following relations [22]:

rB ¼

M M ¼ 2 V pr d

rX ¼

MZ Vcell NA

(7)

0

d¼

ε¼

1 2 D

(3)

bcosq

where ‘M’ and ‘V’ is mass and volume of pellet respectively, ‘r’ is radius of pellet and ‘d’ is its thickness. In equation (8), M0 is molar weight, NA is avogadro number, Z is number of molecules per unit cell (Z ¼ 1 for H phase and Z ¼ 8 for D phase), Vcell is volume of unit cell (Vcell ¼ a3 for D phase and Vcell ¼ a2c for H phase). The X-ray densities of composites (HD3, HD6 and HD9) tabulated in Table 1 were evaluated by the below relation [22]:

(4)

4

(8)

where b is the full width at half maximum and D0 is the crystallite size, which was calculated by the following relation [20]:

Table 1 Density, porosity and structural parameters of H phase, D phase and composites (HD3, HD6 and HD9). Sample

H D HD3 HD6 HD9

rB (g/cm3)

5.75 4.65 2.51 2.40 2.35

rX (g/cm3)

6.62 5.54 3.09 3.08 3.14

g (%)

13.14 16.06 18.77 22.07 25.16

D' (Ǻ)

0.4331 0.5360 0.9231 0.8763 0.8193

d (Ǻ2)

5.1361 3.4807 1.1736 1.3022 1.4898

ε

0.4269 0.3442 0.1998 0.2105 0.2252

GS (mm)

0.64 0.33 1.97 1.46 1.03

Lattice Parameters and volume of unit cell a(Ǻ)

c(Ǻ)

V(Ǻ3)

a(Ǻ)

V(Ǻ3)

3.997 e 3.9951 3.9928 3.9921

4.031 e 4.0203 4.0297 4.0332

64.39 e 64.17 64.24 64.27

e 8.3791 8.3782 8.3765 8.3747

e 588.29 588.10 587.74 587.36

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M.D. Rather et al. / Journal of Alloys and Compounds 794 (2019) 402e416

Fig. 4. SEM micrographs at a resolution of 10000  (a) H phase (b) D phase (cee) HD3, HD6 and HD9 composites.

rXC ¼

MH þ MD rH rD H X X MH rD X þ MD rX

(9)

where MH and MD is the molar weight of H and D phases in a given D composite, and their corresponding X-ray densities are rH X and rX respectively. The calculated values of ‘g’ show an increasing trend from HD3 to HD9, and hence show an inverse relation with the values of grain size, calculated from SEM micrographs. 3.3. Dielectric formalism In this report, the prepared multiferroic composites (HD3, HD6 and HD9) contain ferroelectric phase (H) as one of its main building block. Since this ferroelectric phase is a special class of dielectric, which retains its polarization after the removal of applied electric

field, so dielectric studies was performed. As the dielectric constant varies with frequency and temperature, so frequency as well as temperature dependent dielectric studies was carried out. The frequency dependent dielectric behavior were carried out to gain information about how the different kinds of polarizations respond to an applied electric field and the temperature dependent dielectric study ascertains the ferroelectric phase transitions in the multiferroic composites. The real (ε0 ) and imaginary part (ε00 ) of the complex dielectric function ε* ¼ ε'þiε00 were calculated from capacitance (C) and dielectric loss (tand) by using the below relations:

0

ε ¼

Cd Aεo

(10)

M.D. Rather et al. / Journal of Alloys and Compounds 794 (2019) 402e416

407

Fig. 5. Variation of ε0 with frequency for composites (HD3, HD6 and HD9), with insets for H phase at different temperatures (a) 300 K (b) 270 K (c) 240 K and (d) 210 K.

00

ε tand ¼ 0 ε

(11)

where C is the capacitance, d is thickness (1 mm) of pellet and A is the area (132 mm2) of the electrode deposited on the pellet. 3.3.1. Frequency dependent dielectric studies The variation of dielectric constant (ε0 ) of composites (HD3, HD6 and HD9) with frequency at different temperatures (300e210 K) is shown in Fig. 5, with insets showing corresponding variations of H phase. A steep decrease in ε0 is observed at lower frequencies and remains constant at higher frequencies, confirming dielectric dispersion. The ε0 of H phase at low frequencies is quite high due to dominance of space charge polarization and polarization due to valence state of cations. However at high frequencies, the ε0 is static as the electric dipoles are unable to follow frequency of the applied electric field. In comparison to H phase, the ε0 of composites (HD3, HD6 and HD9) is small due to incorporation of D phase and the values are tabulated in Table 2. At room temperature, the higher value of ε0 in composites at lower frequencies is attributed to dominance of interfacial polarization created by space charges [23]. However, with the increase in frequency, these charges do not respond to the frequency of the applied electric field. As a result of this, these do not remain confined at the grain boundaries,

resulting in static values of dielectric constant. In composites, an increase in ε0 with the increase in D phase is obvious as the polarization mechanism in ferrites becomes active [24]. The polarity of D phase in composites is due to rotational displacement of dipoles (Fe3þ4Fe2þ and Dy3þ4Dy2þ) visualized by an electron exchange between these ions. This dipole gets aligned in the direction of alternating electric field and hence contributes to the dielectric constant. With the decrease in temperature, the steepness of the curve increases towards lower frequencies, as shown in Fig. 5(bed), which may be due to weak response of space charges to applied field, resulting in drop of ε'. At quite low temperature (~210 K), although space charges may be frozen out, but non zero values of ε0 at low frequency (20 Hz) are observed in all the composites. The finite values of ε0 at such low temperatures is usually associated to hopping of polarons [25,26]. However, beyond the frequency of 1 kHz, the contribution of polarons to ε0 is negligible, resulting in static values of dielectric constant. The confirmation of polaron hopping in multiferroic composites based on BaTiO3 and CoFe2O4 has been confirmed in our previous work [27], where low temperature conductivity data satisfies Mott's Law. In conclusion, in the presented composites (HD3, HD6 and HD9), the polarons contribute to ε0 only at lower frequencies (<1 kHz), and beyond this frequency their contribution to ε0 is negligible. The variation of dielectric loss (tand) with frequency at different temperatures (300e210 K) for composites (HD3, HD6 and HD9) is

Table 2 Dielectric constant (ε0 ) and dielectric loss (tand) at two different frequencies and varying temperatures. Sample T (K) H HD3 HD6 HD9

ε' (20 Hz)

ε' (3 MHz)

tan d (20 Hz)

tan d (3 MHz)

300

270

240

210

300

270

240

210

300

270

240

210

300

270

240

210

4961 1645 2078 2549

2733 929 1683 2070

1824 511 602 1378

1408 214 320 1227

2387 56 615 1086

1325 9 69 12

862 226 163 209

668 46 180 259

10.24 0.27 1.08 1.38

3.13 0.11 0.38 1

2.19 0.23 0.39 0.76

1.09 0.16 0.24 0.81

1.12 0.02 0.01 0.01

0.55 0 0 0

0.07 0 0.02 0

0.04 0 0 0

Note: Temperature is measured in Kelvin denoted by T (K).

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Fig. 6. Variation of tand with frequency for composites (HD3, HD6 and HD9), with insets for D phase at different temperatures (a) 300 K (b) 270 K (c) 240 K and (d) 210 K.

shown in Fig. 6, with insets for D phase. The tand values show a quite similar behavior as that of ε0 with frequency of the applied field. A loss peak is observed in D phase at a frequency of 1.5 kHz, attributed to the fact that frequency of applied field matches with the period of relaxation for a particular polarization process. With the decrease in temperature, this hump of peak is reduced and is shifted to higher frequency side, hence showing dielectric dispersion. Apart from D phase, an increase in tand with the increase in D phase is observed in composites. This is associated to the resistive losses created by the hopping of electrons between Fe3þ4Fe2þ and Dy3þ4Dy2þ ions. However at high frequencies, the contribution of both these effects is negligible, resulting in negligible tand values. In addition, the smaller value of tand at low temperatures may be due to the Mott conductivity of polarons. 3.3.2. Temperature dependent dielectric studies The variation of ε0 with temperature (100e500 K) for composites (HD3, HD6 and HD9) at a selected frequency of 1 kHz is shown in Fig. 7, with inset for H phase. By the incorporation of Ho3þ ions in the BaTiO3 lattice, an increase in ε0 is observed as compared to pure BaTiO3 [28]. This increase in ε0 is one of the prominent reasons for the improved magnetodielectric effect, as the applied magnetic field to composites results in high magnetocapacitance. In composites, with the increase in concentration of D phase, the value of ε0 increases. The increase in ε0 may be due to increasing effect of interfacial polarization and hopping of electrons between Fe3þ4Fe2þ and Dy3þ4Dy2þ ions. In ferroelectric H phase and composites (HD3, HD6 and HD9), two ferroelectric transitions were observed from high symmetric cubic to low symmetric tetragonal phase (TCT), and from tetragonal to orthorhombic phase (TTO) respectively. A drop in transition temperatures (Table 3) is observed in H phase and composites as compared to pure BaTiO3 [29]. This drop in transition temperatures is associated to the replacement of

Fig. 7. Variation of ε0 with temperature for composites (HD3, HD6 and HD9), with insets for H phase and Curie Weiss behavior for H phase and composites (HD3, HD6 and HD9).

smaller sized Ti4þ ions (0.74Ao) by large sized Ho3þ ions (0.93 Ao), which results in expansion of the unit cell of BaTiO3. By the lattice expansion, the interactions between Ho3þ and O2 ions weakens, thereby resulting in drop in transition temperatures. Besides H phase, with the increase of D phase in composites, the values of TCT and TTO are shifted to high temperature. This shift in transition temperatures is due to the coupling between the H phase and D phase, consistent with the literature [30]. Apart from shift in transition temperatures, the value of ε0 increases with the increase in temperature, reaches a maximum value and then starts to decrease with a further increase in temperature. This increase in ε0

M.D. Rather et al. / Journal of Alloys and Compounds 794 (2019) 402e416

409

Table 3 Curie temperature, hopping lengths, bond lengths and diffuseness parameter of H phase and composites (HD3, HD6 and HD9). Sample

ε' (TCT)

ε' (TTO)

TCT (K)

TTO (K)

HT (Ao)

HO (Ao)

BT (Ao)

BO (Ao)

g

H D HD3 HD6 HD9

6026 e 1787 1933 2155

2470 e 722 805 934

385 e 394 403 416

245 e 246 248 250

e 3.6239 3.6236 3.6228 3.6220

e 2.9536 2.9533 2.9527 2.9520

e 1.7537 1.7535 1.7532 1.7528

e 5.1112 5.1107 5.1096 5.1086

0.66 e 1.02 1.20 1.26

with the increase in temperature is attributed to increase in space charge polarization across the interface of H and D phases [31], and the hopping of electrons between Fe3þ4Fe2þ and Dy3þ4Dy2þ ions, resulting in polarization in D phase [32]. Beyond the TCT, the decrease in ε0 is due to the increased vibrations of the electrons and ions [33], and distortion in domain structure [34]. The increased hopping rate in composites with the increase in D phase is well supported by the decrease in hopping lengths and bond lengths (Table 3). Thus smaller the hopping length, greater is the exchange of electron between the octahedral and tetrahedral sites, resulting in enhanced polarization. The hopping lengths between two ions occupying tetrahedral sites and tetrahedral sites are denoted by HT and HO and were calculated by the using following relations:

and D phase. The tand of H phase and composites show a quite similar behavior as that of ε0 , supporting ferroelectric transition. However, in case of D phase, the tand values increases abruptly above room temperature, attributed to increased hopping rate.

pffiffiffi a 3 HT ¼ 4

1 1 ¼ CðT  Tc Þg 0  ε εmax

pffiffiffi a 2 HO ¼ 4

(12)

(13)

where ‘a’ is the lattice constant of the magnetic D phase. Also the tetrahedral and octahedral bond lengths denoted as BT and BO was calculated by using the below equations [35].

 pffiffiffi
1 BT ¼ a 3 u  4 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
 11 43 2 uþ BO ¼ a 3u  4 64

(14)

(15)

where u is oxygen positional parameter (u ¼ 0.381 Ao). The variation of tand with temperature (100e500 K) for composites (HD3, HD6 and HD9) at a selected frequency of 1 kHz is shown in Fig. 8, with insets showing corresponding variations of H

Fig. 8. Variation of tand with temperature for composites (HD3, HD6 and HD9), with insets for H and D phase.

3.3.3. CurieeWeiss behavior From the plots of ε0 versus temperature (Fig. 7), it is clear that a sharp ferroelectric transition in H phase and diffused transitions occurred in composites (HD3, HD6 and HD9). To ascertain the sharp and diffused nature of ferroelectric transitions in H phase and composites respectively, the modified Curie Weiss law of the below form was used:

(16)

where C and g are constants. The constant parameter g provides the degree of diffuseness of the phase transition. For a normal CurieeWeiss law i.e. sharp phase transition g ¼ 1, and for a diffused phase transition 1˂g  2. The dielectric data above the transition temperature (TCT) once fitted to equation (16) as shown in inset of Fig. 7, ascertains a sharp transition for H phase and diffused transitions for composites (HD3, HD6 and HD9). The values of g are tabulated in Table 3. From the obtained values of g, it is clear that degree of disorder increases for higher weight content of D phase in composites. Thus composites obey the rule that the increase in disorder leads to the increase in dielectric constant and is consistent with the literature [36]. 3.4. Ferroelectric studies To determine the multiferroic nature of composites (HD3, HD6 and HD9); firstly, it is necessary to determine their ferroelectric behavior to ascertain the presence of ferroelectric H phase. The ferroelectric behavior of H phase and composites (HD3, HD6 and HD9) was determined by tracing polarization (P) versus electric field (E) hysteresis loops as shown in Fig. 9(aed). A well saturated PE loop with a maximum polarization (Pmax ¼ 29.42 mC/cm2) was obtained for H phase, which is quite large as compared to pure BaTiO3, and other rare earth doped ferroelectrics [28,37e40]. The large polarization generated and high dielectric strength (35.64 kV/ cm) of H phase makes it a proper candidate for magnetoelectric devices. In comparison to H phase, the P-E loops of composites are not well saturated, attributed to conductive nature of ferrimagnetic D phase. Although P-E loops in composites are not saturated; however, they depict high values of maximum polarization (Pmax). These high values are due to the lattice mismatch between H and D phases. As the strain and magnetostriction increases with the increase of D phase in composites, so both these effects result in creation of space charges, thereby enhancing polarization [41]. The coercive field (Ec) of composites is much larger than that of H phase (Table 4). This is attributed to the presence of ferrimagnetic D phase, which hinders and pins the domain wall motion of H phase as shown in Fig. 14(a). In addition, the P-E loop of H phase shows symmetric coercivity distribution with respect to origin. This

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Fig. 9. PeE hysteresis loops (a) H phase (bed) composites (HD3, HD6 and HD9).

Table 4 Ferroelectric and magnetic parameters of H phase, D phase and composites (HD3, HD6 and HD9).

T (K) H D HD3 HD6 HD9

Pmax (mC/cm2)

Ec (kV/cm)

Mr (emu/g)

Ms (emu/g)

Hc (Oe)

RT

RT

RT

RT

RT

243

213

183

29.05 e 5.62 7.13 7.86

6.6 e 8.09 8.52 10.32

e 23.34 0.69 1.63 3.3

e 62.58 2.15 5.4 12.06

e 900 600 610 620

e e 1031 1161 1778

e e 1137 1278 1970

e e 1275 1588 2138

Mr/Ms

B

K (erg/cm3)

153

RT

RT

RT

e e 1621 2138 2445

e 0.37 0.32 0.30 0.27

e 3.59  106 6.16  106 5.13  106 4.85  106

e 4.3  105 1.93  104 4.43  104 9.63  104

Note: Temperature ¼ T, Room temperature ¼ RT & Coercivity (Hc) is noted at different temperatures (from RT to153 K).

symmetry confirms that the internal field developed by bound electrons in H phase is negligible as compared to external applied electric field. Apart from H phase, the P-E loops of composites are unsymmetrical. This asymmetry is highly linked to interaction between the applied field and the internal bias fields, created by conducting nature of D phase [42]. 3.5. Magnetic studies The magnetic study of multiferroic composites is a unique way to confirm the presence of magnetic phase. This can be simply achieved by tracing the magnetization (M) versus magnetic field (H) hysteresis loops at room temperature. The M  H loops of D phase and composites (HD3, HD6 and HD9) are shown in Fig. 10 (a,b). It is clear that the M  H loops of D phase and composites (HD3, HD6 and HD9) get saturated at 5 kOe, confirming an ordered magnetic domain structure. In D phase, a small value of saturation magnetization (Ms ¼ 62.58 emu/g) has been observed than pure CoFe2O4 (Ms ¼ 80 emu/g) [43]. This drop in Ms is due to substitution of Co2þ by Dy3þ ions in the CoFe2O4 lattice [44], which results in weaker A-B exchange interactions. This weakening of exchange interaction is due to decrease in the number of magnetic linkages existing between tetrahedral and octahedral sites. Apart from this,

the Dy3þ4Fe3þ interaction (4fe3d coupling), and the Dy3þ4Dy3þ interaction (indirect 4fe5de4f exchange coupling) also exists, but both are very weak and hence do not contribute to net magnetization [45]. The magnetic behavior of the composites (HD3, HD6 and HD9) is due to the presence of D phase, which is incorporated into the ferroelectric H matrix. In comparison to D phase, the drop in remnant magnetization (Mr) and coercivity (Hc) is observed, attributed to breaking of magnetic linkages between tetrahedral and octahedral cations, by the non magnetic H phase. An increase in Ms and Hc is observed with the increase in D phase in composites. This is due to the rotation of magnetic domain walls in the D phase, which acts as centre of magnetization in multiferroic composites [46]. A slight increase in Hc is observed from HD3 to HD9 as shown in zoomed view of Fig. 10(d), showing an inverse relation with grain size, thereby supporting SEM studies. To ascertain the domain structure of the D phase and composites (HD3, HD6 and HD9), the squareness ratio (Mr/Ms) were calculated (Table 4). From the calculated values, it is clear that both D phase and all composites are multidomain structured (Mr/ Ms < 0.5). For such multidomain structures, reduction in hysteresis is observed with the decrease in D phase, attributed to decrease in magnetocrystalline anisotropy. The confirmation of decrease in

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411

Fig. 10. MeH hysteresis loops (a) D phase (bed) composites (HD3, HD6 and HD9) (c) LA fit (d) Zoomed view of coercivity.

anisotropy was revealed by using ‘Law of Approach (LA)’ method [47]. As per this approach, the magnetization (M) of a sample is related to the applied magnetic field (H) by the below relation:

"

#   02 8 K 1 b  1  M ¼ Ms 1  ¼ M s 105 mo 2 Ms 2 H 2 H2

(17)

where Ms is saturation magnetization and K0 is cubic anisotropy constant. In order to get Ms and b values, the magnetization data above 4 kOe was fitted to the above equation (Fig. 10(c)), and finally the values of magneto-crystalline anisotropy (K) were evaluated by the below equation:

rffiffiffiffiffiffiffiffiffiffiffi 105b K ¼ mo Ms 8

different temperatures (243-153 K) as shown in Fig. 11(aec). A significant increase in Hc is observed with the decrease in temperature, depicted in Fig. 11(d). This is attributed to the fact that at low temperatures, it becomes difficult for the thermal energy to overcome the magnetic energy of blocked moments and hence results in enhanced coercivity [44]. However, as the temperature starts rising towards room temperature, the frozen spin structure start unfreezing, resulting in decrease in Hc values [52]. An attempt was made to fit the Kneller's law [53] to the data of Fig. 11(d), however the fitting showed a large deviation. This supports the fact that Kneller's law is only applicable to single domain systems [52].

3.6. Magnetodielectric studies

(18)

The calculated values of b and K are tabulated in Table 4. The value of K at room temperature for D phase is large as compared to composites. This increase is due to single ion anisotropy of Co2þ ions located at the octahedral sites of lattice [48] and the strong spin-orbit coupling of Dy3þ ions [49]. Apart from D phase, an appreciable decrease in K, with the decrease in D phase is due to decrease in number of Co2þ ions which contribute to the anisotropy and also various magnetic links are broken out by the non magnetic H phase. In multiferroic composites, the anisotropy is directly linked to the coupling between two ferroic phases, which decreases with increase in grain size, consistent with the literature [50]. From room temperature M  H curves, it is quite obvious that no improvement was observed in various magnetic parameters (Ms and Hc) by Dy3þ ion doping in CoFe2O4. This is due to the paramagnetic behavior of dysprosium at room temperature [51]. To check the impact of low temperatures on the magnetic properties, the M  H loops of composites (HD3, HD6 and HD9) were traced at

From ferroelectric and magnetic studies, it is clear that H and D phases coexist in the composites. To establish magnetoelectric (ME) coupling in magnetoelectric multiferroics, one can measure the effect of a magnetic field (H) on ferroelectric polarization (P) or conversely the effect of electric field (E) on magnetization (M). However, in case of poor magnetoelectric insulators, it becomes difficult for them to sustain electric field, which can switch the magnetization [54]. To overcome this problem, a simple and realistic approach is to measure the ε0 as a function of magnetic field (H), since in magnetoelectric multiferroics magnetic order is coupled to electric polarization and hence to the dielectric properties. As the magnetic field alters the magnetic ordering, hence it can indirectly affect the ε0 of magnetoelectric multiferroics, reported in various materials [55e57]. This phenomenon is termed as magnetodielectric (MD)/magnetocapacitance (MC) effect. In this effect, a magnetic field is applied to a sample, which results in magnetostriction in the magnetic phase of the composite. This strain generated is then transferred to the ferroelectric phase, which in turn results change in the capacitance (C) and tand of the

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Fig. 11. Low temperature M  H hysteresis loops for composites (a) HD3 (b) HD6 and (c) HD9 (d) Variation of coercivity with temperature for composites (HD3, HD6 and HD9).

sample. The percentage magnetocapacitance ‘MC(%)’ is evaluated by using equation (2), and percentage magneto-loss ‘ML(%)’ was evaluated by the below relation:

MLð%Þ ¼

tandðHÞ  tandðH ¼ 0Þ  100 tandðH ¼ 0Þ

(19)

where tand(H) and tand(H ¼ 0) are the values of dielectric loss in

presence and absence of magnetic field (H) respectively. As per catalan et al. [58], the existence of MC is not the sufficient condition to predict the existence of ME coupling, unless supported by the frequency dependence of ε0 and tand. So before establishing the ME effect by an indirect means of MC phenomenon, it is necessary to tailor the dielectric parameters (ε0 and tand) by the application of applied magnetic field.

Fig. 12. Variation of ε0 with frequency at different magnetic fields (a) D phase (bed) composites (HD3, HD6 and HD9), with insets showing zoomed view.

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413

Table 5 Percentage magnetocapacitance at different frequencies (1e100 kHz) and values of ε0 at different magnetic fields (0e2T) at selected frequency of 20 Hz and 3 MHz. Sample

D HD3 HD6 HD9

ε' (20 Hz)

MC (%)

ε' (3 MHz)

1 kHz

10 kHz

100 kHz

0T

0.5 T

1T

1.5 T

2T

0T

0.5 T

1T

1.5 T

2T

7.07 2.78 4.63 5.76

5.27 2.06 3.46 4.5

2.83 1.29 2.25 2.47

3220 1664 2210 2480

3248 1674 2223 2508

3278 1690 2235 2530

3318 1714 2246 2551

3368 1749 2276 2573

238 886 486 8

245 893 491 13

251 902 497 17

257 909 506 25

265 916 515 32

The variation of ε0 with frequency at different magnetic fields (0e2T) for D phase and composites (HD3, HD6 and HD9) is shown in Fig. 12(aed). A high value of ε0 is observed at lower frequency and decreases with the increase in frequency. Once the magnetic field is applied in steps of 0.5 T, an appreciable increase is observed in ε0 , the magnitude of which increases at low frequencies and higher fields as compared to higher frequencies (Table 5). This is attributed to the fact that D phase embedded in the H matrix acts as pinning centres for the ferroelectric polar domains of H phase, schematically shown in Fig. 14. In absence of external magnetic field (Fig. 14(a)), the rotation of polar domains of H phase is actually restricted by the D phase, due to weak interface coupling between these two ferroic phases [59], and hence results in smaller values of ε'. However, in presence of the magnetic field (Fig. 14(b)), the magnetic ordering of D phase results in increase in the local magnetic fields around the D phase. This leads to the enhancement in the coupling between D and H phases, resulting in an increase in number of polar domains of H phase and hence in ε' [60]. With the increase in D phase in composites, the local magnetic fields increases, which results enhanced ε0 in HD9 as compared to HD3. The variation of tand with frequency at different magnetic fields (0e2T) for D phase and all composites is shown in Fig. 13(aed). A decrease in tand is observed with increase in frequency and an

increase in tand occurs with the increase of D phase in composites, discussed in detail in section 3.3.1. However, an increase in tand with the increase in applied magnetic field is due to the strong pinning effect of the D phase on the polar domains of H phase, consistent with the literature [61]. From the magnetic field dependent dielectric studies over a wide frequency range, it is clear that ε0 as well as tand are highly affected at low frequencies as compared to higher frequencies. Taking this thing into account, it was though worthwhile to check the degree of coupling between two ferroic phases. This was done by measuring the variation of MC(%) with the applied magnetic field ranging from 20 kOe  H  20 kOe, at different frequencies (1e100 kHz) as shown in Fig. 15(aed) and the values are given in Table 5. The MC(%) produced in composites HD3, HD6 and HD9 at a frequency of 1 kHz is 2.78, 4.63 and 5.76% respectively, which is quite large in comparison to single phase Sc-doped BiFeO3 nanoparticles [62] and lead containing bulk composites [63]. This ascertains high degree of coupling between H and D phases, making these composites important for technological applications. In addition, a favorable increase in MC(%) is observed with the increase in D phase in the composites. This increase is attributed to the increase in dislocation density, which enhanced the strain across the interface of H and D phases, consistent with XRD studies.

Fig. 13. Variation of tand with frequency at different magnetic fields (a) D phase (bed) composites (HD3, HD6 and HD9), with insets showing zoomed view.

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Fig. 14. Schematic illustration of pinning effect of D phase on polar domains of H phase (a) In absence of magnetic field (weak pinning) (b) Presence of magnetic field (Strong pinning).

Fig. 15. Variation of MC (%) with applied magnetic field (a) D phase (bed) composites (HD3, HD6 and HD9).

In addition, the amount of space charges increases with the increase in D phase in composites, since the area of contact between H and D phases increases, which also leads to high magnetocapacitance. In composites, apart from dependence of MC(%) on weight fraction of magnetic D phase, it is also highly dependent upon the frequency of the applied field. At lower frequency (1 kHz), a high value of MC(%) is produced, which increases with the increase in D phase in all the composites. This is attributed to the fact that the interfacial polarization due to space charges across the interface of two ferroic phases responds to only lower frequencies. However, at higher frequency (100 kHz), the contribution of the space charge effect declines, resulting in lower values of MC (%). Just like MC(%), the ML(%) increases with the increase in D phase in composites. Although an appreciable change is observed in MC(%) in all composites; however, the magnetic ML(%) shows negligible variation upon the application of magnetic field as depicted in Fig. 16. In conclusion, the coexistence of H and D phase in composites, with high values of MC(%) and constant values of ML(%) makes such multiferroic composites suitable for ME devices.

4. Conclusions In this work, the particulate multiferroic composites (1-x) Ba0.95Ho0.05TiO3 e x CoDy0.1Fe1.9O4 (x ¼ 0.03, 0.06 and 0.09) were prepared by solid state route. XRD studies revealed the coexistence of two ferroic phases, without formation of any additional phase. The decrease in cell volume of CoDy0.1Fe1.9O4 phase, with the increase in Ba0.95Ho0.05TiO3 phase is one of the key factors for the observed high magnetodielectric effect. The decrease in grain size enhanced the grain connectivity, which in turn enhanced the coupling between two ferroic phases. The substitution of rare earths; Ho3þ ions in the BaTiO3 and Dy3þ ions in CoFe2O4, resulted in expansion of lattices, thereby improving dielectric and magnetic properties. The dielectric constant of composites is found to have inverse relation with the hopping lengths and bond lengths. The coexistence of two ferroic phases in composites was ascertained by tracing respective P-E and M  H loops. The magnetocrystalline anisotropy of composites was calculated by using Law of approach to saturation. From magnetodielectric studies, it was revealed that

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415

Fig. 16. Variation of ML(%) with applied magnetic field (a) D phase (bed) composites (HD3, HD6 and HD9).

the magnetic ordering of CoDy0.1Fe1.9O4 phase resulted in increase in number of polar domains of Ba0.95Ho0.05TiO3 phase, which is a unique way to improve the magnetodielectric effect. In conclusion, the unification of Ba0.95Ho0.05TiO3 and CoDy0.1Fe1.9O4 phases in multiferroic composites was established with improved coupling, proving them novel materials for ME devices. Acknowledgement Mehraj ud Din Rather thanks Dr. K. Asokan (Scientist F), Inter University Accelerator Centre, Aruna Asaf Ali Marg Delhi, for providing facility of low temperature dielectric measurements and Mr. Rizwan Nabi, PhD research scholar, Department of Chemistry, IIT Mumbai for useful discussions related to structural studies. The corresponding author Basharat Want is grateful to authorities of the University of Kashmir for providing facility of the vibrating sample magnetometer facility (Micro Sense EZ9 VSM) for magnetic measurements and to University Grants Commission for providing financial support under Special Assistance Programme Phase-I to the Department of Physics for materials science as a thrust area.

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