**3.3 Magnetic study**

The magnetization (M) versus magnetic field (H) loop for pure BTFO and composite BLB523 samples are recorded at RT. **Figure 3(a)** and **(b)** display the linear and slightly non-linear M-H loop of pure cum composite samples. This linear behavior of the M-H loop indicates the paramagnetic (PM) behavior of the pure sample. With the addition of magnetic LSMO and BFO phase, the M-H loop slightly changes to the non-linear behavior with a small opening in the field range of ±1 kOe, as shown lower inset of **Figure 3(b)**. This behavior signifies the weak ferromagnetic (FM) nature of composite. In composite BLB523, saturation magnetization is not achieved even in a high magnetic field of 15 kOe. It indicates the presence of canted spins in the composite. Since the composite sample consists of the three magnetic phases, i.e*.,* PM BTFO, FM LSMO, and AFM BFO phases [17, 18]. The overall magnetic behavior of the composite shows the FM behavior. As the pure sample exhibit the paramagnetic behavior, with the addition of magnetic Mn ion (LSMO) and Fe ion (BFO) to the BTFO phase. The magnetic moment of the composite sample is enhanced. The different magnetic parameters such as maximum magnetization (Mmax), coercive field (Hc), and remanent magnetization (Mr) are extracted from the fitting of M-H loop [19] and listed in **Table 2**. It is observed that the magnetic parameters (Mmax, Hc, and Mr) increase by adding an extra magnetic phase to the pure sample. Since the maximum magnetization is an intrinsic property of the sample, which depends upon the spin configuration and spin–spin interaction between the magnetic ions. Whereas, the coercive field and remanent magnetization is affected by extrinsic factors such as particle morphology, domain structure, disorder, defects, etc. [20].

In the present discussion, the coercivity value of the BLB523 composite is increased to 145 Oe (nearly three times) than the pure sample. It could be due to the addition of manganite (LSMO) and ferrite (BFO) phase leading to the hindrance of domain interaction and resulting in the pinning effect in the composite. A similar trend of coercivity is also observed in many composites [21]. The enhanced magnetization in the composite sample is addressed due to adding extra magnetic ions, i.e*.,* Mn and Fe ions of LSMO and BFO phase, respectively. Additionally, the enhanced

**Figure 3.** *Magnetic hysteresis loops (M-H) of (a) BTFO and (b) BLB523 at RT.*


### **Table 2.**

*Magnetic parameters are estimated from the M-H hysteresis loop.*

magnetization is supplemented by inherent magnetization arising due to *d-d*, *f-d*, and *f-f* exchange interaction between the Fe-Fe, Fe-Mn, and Mn-Mn ions [22]. The squareness ratio is estimated from the ratio of remanent and saturation magnetization of the samples and is given in **Table 2**. Usually, the ratio of (*M*r/*M*s) ≥ 0.5 indicates the single-domain structure, whereas (*M*r/*M*s) < 0.5 signifies the multi-domain nature of the sample [20]. In the present composite, the squareness ratio lower than 0.5 exhibits the multi-domain nature of the sample.

### **3.4 Dielectric study**

The frequency variation of dielectric permittivity (*ε*′) at different external magnetic fields displays indirect access to define the magnetoelectric coupling of the material. There are the various techniques to observe the magnetodielectric effect of the sample, such as (i) the relative change of capacitance/impedance with the application of an external magnetic field and (ii) the appearance of magnetic/dielectric transition temperature in the temperature variation of dielectric/magnetic studies. In the present pure cum composite system, we have preferred the first technique to analyze the MD effect. The RT frequency dependence dielectric permittivity at a fixed magnetic field (0 T to 1.3 T with a difference of 0.2 T) is illustrated in **Figure 4(a)** and **(b)**. Both the sample shows the appreciable change in capacitance/dielectric under the application of field up to 1.3 T. A close view of the change in dielectric permittivity is plotted in the upper inset of the pure (BTFO) and composite (BLB523) samples. The reduction of ε′ with the change in a magnetic field signifies the presence of negative MD coupling in

#### **Figure 4.**

*The frequency dependence of dielectric permittivity at the different magnetic fields of 0 T to 1.3 T (a) pure BTFO and (b) composite BLB523.*

*Structural, Magnetic, and Magnetodielectric Properties of Bi-Based Modified Ceramic Composites DOI: http://dx.doi.org/10.5772/intechopen.106569*

the sample. The appearance of the positive or negative sign of the MD effect depends on the neighboring spin pair correlation and the coupling constant [23]. The dielectric constant of both the samples decreases with the increase of frequency, suggesting the usual dielectric characteristics of the sample. The moderate change of dielectric constant under the different magnetic fields in the low-frequency regime is attributed to interfacial polarization i.e*.,* Maxwell-Wagner polarization, space charge polarization, magnetoresistance, etc. [24]. Whereas, in the high-frequency region, the dielectric value decreases due to the intrinsic dipolar contribution and suppression of extrinsic effects. The maximum strength of the ε′ is found to be ~105 and 375 at 100 kHz for BTFO and BLB523 samples, respectively. It is observed that the strength of the ε′ increases nearly three times in the composite sample. The strength of ε′ is reduced towards the high-frequency side by the suppression of extrinsic effects. Hence, the frequency plays a vital role in observing the sample's dielectric properties even in the presence of the magnetic field. To suppress the extrinsic contribution towards the dielectric and magnetodielectric properties of the sample. It is pivotal to study in the high-frequency region of ≥50 kHz. The aforementioned discussion gives a signature of the existence of MD coupling in the sample. The upcoming section has demonstrated a clear view of the field variation MD effect and its possible source of origin.

### **3.5 Magnetic field-dependent MD analysis**

To realize the influence of magnetic field on dielectric properties of the pure and composite samples, field variation of MD measurement is recorded at RT. The MD effect can be extracted experimentally by recording the field variation dielectric data. While for the magnetoloss (ML) effect, dielectric loss data is taken as a function of a magnetic field. Both the MD and ML percentage is estimated using the following mathematical expression [19]:

$$\mathbf{MD}(\%) = \left[ \frac{\varepsilon'(H, T)}{\varepsilon'(\mathbf{0}, T)} - \mathbf{1} \right] \times \mathbf{100} \tag{3}$$

$$\mathbf{ML}(\%) = \left[ \frac{\tan \delta(H, T)}{\tan \delta(\mathbf{0}, T)} - \mathbf{1} \right] \times \mathbf{100} \tag{4}$$

here ε′( *H T*, ) , ε′(**0**,*T*) , tan , δ( *H T*) , and tan , δ(**0** *T*) are dielectric permittivity and loss with magnetic field and zero magnetic field, respectively. **Figure 5(a)**–**(d)** illustrates the magnetic field variation of MD% and ML% of pure cum composite samples at a constant frequency of 50 kHz. The chosen high frequency plays a vital role in exploring the extrinsic and intrinsic origin of the MD effect. The extrinsic origin arises due to the Maxwell-Wagner polarization and magnetoresistance of the sample. At the same time, the intrinsic contribution arises due to the dipolar polarization. To exclude the unwanted extrinsic contribution from the observed MD effect, the observed MD data is taken at a high-frequency region (50 kHz). The pure and composite samples show a completely different MD loop with the maximum field sweep of ±13 kOe. The BTFO pure sample shows the linear nature of the MD effect with field variation. It is because of the dominating contribution attributed by the space charge polarization. Interestingly, with the addition of the LSMO and BFO

**Figure 5.**

*(a) and (b) Magnetic field variation of magnetodielectric (MD%) at a frequency of 50 kHz. (c) and (d) Magnetoloss (ML%) vs. H at 300 K for pure and composite samples.*

magnetic phase, the behavior of the MD hysteresis loop is improved. The composite sample exhibits the symmetric hysteresis loop in both positive and negative magnetic field sweeps. This MD loop is termed the inverted butterfly loop. The maximum strength of the MD% for the pure sample is ~0.01% at 13 kOe of field. With the addition of LSMO and BFO samples, the MD% of the composite sample increases to ~0.12%, which is nearly 12 times more than the pure sample. In composite, the change of MD% slope is significant around the ±8 kOe field. After that, the change of MD% tends to saturate towards the higher field of ±13 kOe. This may be due to the addition of magnetostrictive LSMO phase arising from the Mn ions spin reorientation, which makes a good mechanical coupling with the different phases. As a result, MD coupling increases up to a certain magnetic field after that, the value of magnetostriction becomes saturated with a further increase of the magnetic field. A similar kind of observation is also reported in another magnetostrictive compound [25]. According to the G. Catalan formalism, it is crucial to consider the ML effect for demonstrating the origin of the MD effect [9]. The composite sample shows the contrasting nature of ML% to that of MD%. The maximum strength of ML% (~1.08%) at 13 kOe for the pure sample is decreased to ~0.08% in the composite sample. It indicates a slight decrease in loss is observed in the composite sample. The microscopic source of the MD effect can be explained in this way: (i) the magnetostrictive LSMO phase generates the strain at the interface of the ferroelectric phase. This strain is transferred to

*Structural, Magnetic, and Magnetodielectric Properties of Bi-Based Modified Ceramic Composites DOI: http://dx.doi.org/10.5772/intechopen.106569*

the ferroelectric phase, which results in the capacitance of the sample [26]. (ii) the addition of the AFM BFO phase can enhance the magnetic moment due to the presence of canted spins. The non-collinear alignment of magnetic spin results in modifying the electric polarization or capacitance of the sample via inverse Dzyaloshinskii-Moriya interaction (IDM) [27]. This IDM interaction is the fundamental source behind the MD effect in the non-collinear spin structure. However, the exact source of MD coupling in the composite is still to be observed via different experimental and theoretical investigations. Hence, the symmetric switching MD hysteresis loop with good coupling strength (~0.12%) in the composite sample might be used in the charge storage devices.
