**3.1. Crystal structure of La2MMnO6 (M = Ni, Co) ceramics**

The phase structure of the microwave sintered LNMO and LCMO nanoparticles was charac‐ terized by X-ray diffraction (XRD). As shown in Figure 4, no extra reflection peaks other than those of pure perovskite phase are detected, indicating the high purity of nanoparticles can be obtained in 10 min by this microwave sintering approach, which confirms the formation of single phase compositions of LNMO and LCMO double perovskites [30].

The crystallite size was calculated from XRD patterns using the Debye–Scherrer formula [7], described by the Eq. (7):

$$D = \frac{0.94 \times \lambda}{\beta\_{\text{Y}} \times \cos \theta} \tag{7}$$

where *D* = crystallite size, *λ* = radiation wavelength (1.5405 Å), *β*1/2 = half-width of diffraction profile, and *θ* = diffraction angle.

The average crystal size was found to be 23 nm for LNMO and 28 nm for LCMO, which are 2–3 times smaller than the particle/grain sizes measured by TEM as shown in below section.

Raman spectroscopy is one of the most important tools to attain the information about the structure of the samples. The Raman spectra of microwave sintered LNMO and LCMO ceramics are shown in Figure 5. The Raman spectra display two characteristics peaks at around 514, 653 cm–1 for LNMO and 488, 670 cm–1 for LCMO ceramics, corresponding to the welldocumented A band and B band, respectively. Martín–Carron et al. have assign the two peaks

where: D = crystallite size, λ = radiation wavelength (1.5405 Å), β1/2 = half-width of

2–3 times smaller than the particle/grain sizes measured by TEM as shown in below section.

**Figure 4.** XRD patterns of the microwave sintered (a) LNMO and (b) LCMO ceramics.

nm line of an argon laser under ambient conditions. The composition, morphology, and microstructures of the products were characterized by transmission electron microscope (TEM FEI Tecnai F20 microscope, Japan) and field emission scanning electron microscope (FESEM, Hitachi S-4800, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS). Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet 5700 spectrometer in the wave number range of 400–4000 cm–1. The spectroscopic grade KBr pellets were used for collecting the spectra with a resolution of 4 cm–1 performing 32 scans. X-ray photoelectron spectroscopy (XPS) was performed on an ESCA-UK XPS system with an Mg K<sup>α</sup> excitation source (hν = 1486.6 eV), where the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. The magnetic properties were measured using a physical property measurement system (PPMS-9, Quantum Design, Inc., San Diego, CA, USA) at room temperature under a maximum field of 20 kOe. Silver paste was applied on both sides of the pellet for the electrical measurements. The variation of dielectric constant and dielectric loss as a function of frequency at room temperature and as a function of temperature at different frequencies was measured using computer interfaced HIOKI 3532-50 LCR-HITESTER.

**3. Microwave-sintering of engineered double perovskite ceramic materials**

The phase structure of the microwave sintered LNMO and LCMO nanoparticles was charac‐ terized by X-ray diffraction (XRD). As shown in Figure 4, no extra reflection peaks other than those of pure perovskite phase are detected, indicating the high purity of nanoparticles can be obtained in 10 min by this microwave sintering approach, which confirms the formation of

The crystallite size was calculated from XRD patterns using the Debye–Scherrer formula [7],

cos

where *D* = crystallite size, *λ* = radiation wavelength (1.5405 Å), *β*1/2 = half-width of diffraction

The average crystal size was found to be 23 nm for LNMO and 28 nm for LCMO, which are 2–3 times smaller than the particle/grain sizes measured by TEM as shown in below section. Raman spectroscopy is one of the most important tools to attain the information about the structure of the samples. The Raman spectra of microwave sintered LNMO and LCMO ceramics are shown in Figure 5. The Raman spectra display two characteristics peaks at around 514, 653 cm–1 for LNMO and 488, 670 cm–1 for LCMO ceramics, corresponding to the welldocumented A band and B band, respectively. Martín–Carron et al. have assign the two peaks

l

 q

´ <sup>=</sup> ´ (7)

1 2

b

0.94

**3.1. Crystal structure of La2MMnO6 (M = Ni, Co) ceramics**

described by the Eq. (7):

10 Advanced Ceramic Processing

profile, and *θ* = diffraction angle.

single phase compositions of LNMO and LCMO double perovskites [30].

*D*

to the *Ag* antisymmetric stretching (or Jahn–Teller stretching mode) and *Bg* symmetric stretching vibrations of the MnO6 octahedra, respectively [34–36, 52–54]. It is well known that the *Ag* band is usually assigned to antisymmetric stretching (or Jahn–Teller stretching mode), while the *Bg* band distributed to symmetric stretching vibrations. A noticeable difference is seen between our LNMO/LCMO ceramics and the bulk sample: the *Ag* and *Bg* peaks for the nanoparticles shift to higher binding energy, 13 and 25 cm–1, respectively, when compared to the bulk crystal. The shifting may occur due to surface strain of the crystal structure [55]. Raman spectroscopy is one of the most important tools to attain the information about the structure of the samples. The Raman spectra of microwave sintered LNMO and LCMO ceramics are shown in Figure 5. The Raman spectra display two characteristics peaks at around 514,653 cm–1 for LNMO and 488, 670 cm–1 for LCMO ceramics, corresponding to the well-documented A band and B band, respectively. Martín–Carron et al. have assign the two peaks to the *Ag* antisymmetric stretching (or Jahn–Teller stretching mode) and

Figure 4. XRD patterns of the microwave sintered (a) LNMO and (b) LCMO ceramics.

The microstructure and morphology of microwave sintered LNMO and LCMO ceramics were investigated by FESEM and TEM techniques. Typical SEM images of La2MMnO6 (M = Ni, Co) nanoparticles are shown in Figure 6, the average grain size is about 52 nm and 58 nm for La2NiMnO6 and La2CoMnO6, respectively. The grain size of La2CoMnO6 is bigger than that of La2NiMnO6, which obeys the rule that relatively large ionic radius id benefit to the diffusion in the microwave sintering process. 13 *Bg* symmetric stretching vibrations of the MnO6 octahedra, respectively [52–54].34–36 It is well

From the morphologies of both samples, the grains seem to be homogeneous and form a group of cluster phenomenon. The perovskite material has better microwave absorption capability [5, 51] and leads to fine grain growth during the sintering process.

The EDX spectra (inset of 6a and 6b) and their corresponding tables confirm the presence of the constituent elements (La, Ni, Co, Mn, and O), the composition being nearly the same as that of stoichiometric La2NiMnO6 and La2CoMnO6, respectively.

As shown in Figures 7a and 7b, transmission electron microscopy (TEM) was applied for all samples to determine particle size and confirmed that the particle sizes are about 53 ± 12 and 60 ± 15 nm for LNMO and LCMO, respectively, which agrees good agreement with the SEM **Figure 5.** Raman of the microwave sintered (a) LNMO and (b) LCMO ceramics.

Figure 5. Raman<\$%&?>of<\$%&?>the<\$%&?>microwave<\$%&?>sintered<\$%&?>(a)<\$%&?>LNMO<\$%&?>and<\$%&?>(b)<\$%&?>LCM

%&?>Ni,<\$%&?>Co)<\$%&?>nanoparticles<\$%&?>are<\$%&?>shown<\$%&?>in<\$%&?>Figure<\$%&?>6,<\$%&?>the<\$%&? >average<\$%&?>grain<\$%&?>size<\$%&?>is<\$%&?>about<\$%&?>52<\$%&?>nm<\$%&?>and<\$%&?>58<\$%&?>nm<\$%&?> for<\$%&?>La2NiMnO6<\$%&?>and<\$%&?>La2CoMnO6,<\$%&?>respectively.<\$%&?>The<\$%&?>grain<\$%&?>size<\$%&? >of<\$%&?>La2CoMnO6<\$%&?>is<\$%&?>bigger<\$%&?>than<\$%&?>that<\$%&?>of<\$%&?>La2NiMnO6,<\$%&?>which<\$% &?>obeys<\$%&?>the<\$%&?>rule<\$%&?>that<\$%&?>relatively<\$%&?>large<\$%&?>ionic<\$%&?>radius<\$%&?>id<\$%&?> benefit<\$%&?>to<\$%&?>the<\$%&?>diffusion<\$%&?>in<\$%&?>the<\$%&?>microwave<\$%&?>sintering<\$%&?>process.

30 40 50 60 70 80 90

Diameter (nm)

7 60 nm

\$%&?>and<\$%&?>LCMO<\$%&?>ceramics<\$%&?>were<\$%&?>investigated<\$%&?>by<\$%&?>FESEM<\$%&?>and<\$%&? >TEM<\$%&?>techniques.<\$%&?>Typical<\$%&?>SEM<\$%&?>images<\$%&?>of<\$%&?>La2MMnO6<\$%&?>(M<\$%&?>=<\$ Figure 6. FESEM images, EDX spectra (inset) and elemental data of the microwave sintered (a) LNMO and (b) LCMO ceramics. **Figure 6.** FESEM images, EDX spectra (inset) and elemental data of the microwave sintered (a) LNMO and (b) LCMO ceramics.

10

and (b) LCMO ceramics.

(a) (b)

No. of particles

20 30 40 50 60 70 80

Diameter (nm)

Figure 7. TEM images and particle size distributions of the microwave sintered (a) LNMO

No. of particles

55 nm

LNMO LCMO

a group of cluster phenomenon. The perovskite material has better microwave absorption

The EDX spectra (inset of 6a and 6b) and their corresponding tables confirm the presence of the constituent elements (La, Ni, Co, Mn and O), the composition being nearly

As shown in Figures 7a and 7b, transmission electron microscopy (TEM) was applied for all samples to determine particle size, and confirmed that the particle sizes are about 53±12 and 60±15 nm for LNMO and LCMO, respectively, which agrees good agreement with the SEM results. From the TEM micrograph, nano-sized grains with quasi spherical shape

capability [5, 51] and leads to fine grain growth during the sintering process.

the same as that of stoichiometric La2NiMnO6 and La2CoMnO6, respectively.

can be observed.

**300 400 500 600 700 800**

**Raman Shift (cm-1**

**)**

Figure 5. Raman<\$%&?>of<\$%&?>the<\$%&?>microwave<\$%&?>sintered<\$%&?>(a)<\$%&?>LNMO<\$%&?>and<\$%&?>(b)<\$%&?>LCM

Figure 6. FESEM<\$%&?>images,<\$%&?>EDX<\$%&?>spectra<\$%&?>(inset)<\$%&?>and<\$%&?>elemental<\$%&?>data<\$%&?>of<\$%&?>t

0 200 400 600 800 1000 1200 LaLa

Energy eV

Mn

Mn

Co

Co

Figure 7. TEM<\$%&?>images<\$%&?>and<\$%&?>particle<\$%&?>size<\$%&?>distributions<\$%&?>of<\$%&?>the<\$%&?>microwave<\$%&

Element Wt% At% OK 25.15 55.55 LaL 04.25 01.08 CoK 46.75 28.03 MnK 23.85 15.34 Matrix Correction ZAF

The<\$%&?>microstructure<\$%&?>and<\$%&?>morphology<\$%&?>of<\$%&?>microwave<\$%&?>sintered<\$%&?>LNMO< \$%&?>and<\$%&?>LCMO<\$%&?>ceramics<\$%&?>were<\$%&?>investigated<\$%&?>by<\$%&?>FESEM<\$%&?>and<\$%&? >TEM<\$%&?>techniques.<\$%&?>Typical<\$%&?>SEM<\$%&?>images<\$%&?>of<\$%&?>La2MMnO6<\$%&?>(M<\$%&?>=<\$ %&?>Ni,<\$%&?>Co)<\$%&?>nanoparticles<\$%&?>are<\$%&?>shown<\$%&?>in<\$%&?>Figure<\$%&?>6,<\$%&?>the<\$%&? >average<\$%&?>grain<\$%&?>size<\$%&?>is<\$%&?>about<\$%&?>52<\$%&?>nm<\$%&?>and<\$%&?>58<\$%&?>nm<\$%&?> for<\$%&?>La2NiMnO6<\$%&?>and<\$%&?>La2CoMnO6,<\$%&?>respectively.<\$%&?>The<\$%&?>grain<\$%&?>size<\$%&? >of<\$%&?>La2CoMnO6<\$%&?>is<\$%&?>bigger<\$%&?>than<\$%&?>that<\$%&?>of<\$%&?>La2NiMnO6,<\$%&?>which<\$% &?>obeys<\$%&?>the<\$%&?>rule<\$%&?>that<\$%&?>relatively<\$%&?>large<\$%&?>ionic<\$%&?>radius<\$%&?>id<\$%&?> benefit<\$%&?>to<\$%&?>the<\$%&?>diffusion<\$%&?>in<\$%&?>the<\$%&?>microwave<\$%&?>sintering<\$%&?>process.

he<\$%&?>microwave<\$%&?>sintered<\$%&?>(a)<\$%&?>LNMO<\$%&?>and<\$%&?>(b)<\$%&?>LCMO<\$%&?>ceramics.

LNMO LCMO

O

Co

Counts

Mn

?>sintered<\$%&?>(a)<\$%&?>LNMO<\$%&?>and<\$%&?>(b)<\$%&?>LCMO<\$%&?>ceramics.

Figure 6. FESEM images, EDX spectra (inset) and elemental data of the microwave

**Figure 6.** FESEM images, EDX spectra (inset) and elemental data of the microwave sintered (a) LNMO and (b) LCMO

10

Figure 7. TEM images and particle size distributions of the microwave sintered (a) LNMO

No. of particles

55 nm

30 40 50 60 70 80 90

Diameter (nm)

7 60 nm

LNMO LCMO

**514 cm-1**

**A g**

**488 cm-1**

**LNMO**

**LCMO**

**670 cm-1**

**653 cm-1**

**B g**

**(b)**

**(a)**

Element Wt% At% OK 23.45 57.12 LaL 04.12 01.23 NiK 45.95 27.01 MnK 23.67 15.63 Matrix Correction ZAF

(a) (b)

and (b) LCMO ceramics.

No. of particles

20 30 40 50 60 70 80

Diameter (nm)

La La <sup>C</sup>

0 200 400 600 800 1000 1200

M n

Energy eV

Ni M n

**Figure 5.** Raman of the microwave sintered (a) LNMO and (b) LCMO ceramics.

(a) (b)

sintered (a) LNMO and (b) LCMO ceramics.

**Intensity (a.u.)**

12 Advanced Ceramic Processing

O<\$%&?>ceramics.

Ni

O

Counts

ceramics.

16 **Figure 7.** TEM images and particle size distributions of the microwave sintered (a) LNMO and (b) LCMO ceramics.

results. From the TEM micrograph, nanosized grains with quasi spherical shape can be observed.

The formation mechanism of the perovskite type structure in the microwave sintered LNMO and LCMO ceramics is further supported by FT-IR spectrum shown in Figure 8. The FTIR spectrum is used to characterize the phase composition and purity of the prepared samples. The intense peak around 3423 cm–1 is referring to the stretching vibration of hydroxyl group. In addition, the bands at about 1629 cm–1 can be ascribed to the asymmetric COO stretching vibrations. The bands at 1450 and 1357 cm–1 attributed to the asymmetric stretching of *CO*<sup>3</sup> 2− . The intensive absorption band observed at 597cm–1 can be assigned to Fe–O stretching vibrations formed by the octahedral MnO6 group.

The chemical states of elements of Ni, Mn in LNMO, and Co, Mn in LCMO ceramics was further investigated by X-ray photoelectron spectroscopy. The XPS core level spectra of Ni 2p, Co 2p, and Mn 2p of La2MMnO6 (M = Ni, Co) are presented in Figure 9. A Ni 2P3/2 signal was observed at 851.3 eV along with a satellite peak at 858.5 eV. Another peak was noticed at 869.5 eV and

Figure 8. FTIR of the microwave sintered (a) LNMO and (b) LCMO ceramics. **Figure 8.** FTIR of the microwave sintered (a) LNMO and (b) LCMO ceramics.

ascribed to the Ni 2P1/2 level. Auger electron peak of Ni of [Figure 9a] explains the presence of +2 oxidation state of the nickel in LNMO ceramics. The Mn 2p3/2 peak of LNMO is at 638.4 eV, while the same Mn 2p3/2 peak is at 641.5 eV for Mn2O3 [56]. In the spectrum of Co2p (Figure 6c), the peaks of Co 2p3/2 and Co 2p1/2 states were located at 777.3 and 782.7 eV, respectively [57]. The Mn 2p3/2 peak of LCMO shown in Figures 6 b and 6d is found at 637.6 eV, close to that in Mn2O3 [56]. further investigated by X-ray photoelectron spectroscopy. The XPS core level spectra of Ni 2p, Co 2p and Mn 2p of La2MMnO6 (M = Ni, Co) are presented in Figure 9. A Ni 2p3/2 signal observed at 851.3 eV, with a satellite peak at 858.5 eV, and the peak at 869.5 eV was ascribed to the Ni 2p1/2 level. The Auger electron peaks of Ni are shown in Figure 9(a), which further explains that the nickel is in the +2 oxidation state. The Mn 2p3/2 peak of LNMO is at 638.4 eV, while the same Mn 2p3/2 peak is at 641.5 eV for Mn2O3 [56]. In the spectrum of

Co2p (Figure 6c), the peaks of Co 2p3/2 and Co 2p1/2 states were located at 777.3 and 782.7

The chemical states of elements of Ni, Mn in LNMO and Co, Mn in LCMO ceramics was

#### *3.1.1. Magnetic analysis of La2MMnO6 (M = Ni, Co) ceramics* eV respectively [57]. The Mn 2p3/2 peak of LCMO shown in Figure 6 (b&d) is found at 637.6

The magnetization characteristics were measure both as a function of applied magnetic field at fixed temperatures and as a function of temperature at fixed fields. The room temperature hysteresis loops of the microwave sintered La2MMnO6 (M = Ni, Co) ceramics were measured using physical property measurement system (PPMS). The magnetization curves, as shown in Figures 10a and 10c, display relatively high saturation magnetization. The magnetic saturation (*Ms*) values of LNMO and LCMO are 42.9 and 65.4 emu/g, respectively, which is lower their theoretical values of 47.5 and 71.21 emu/g reported in the literature [40]. One can note that MWS products saturation magnetization was higher than for the conventionally sintering products [36], indicating that MWS method is efficient to fabricated high quality double perovskite material. 16 eV, close to that in Mn2O3 [56].

The frequency dependence of saturation magnetization hysteresis curves was recorded at room temperature for the LNMO ceramics as shown in Figure 10a. A hysteresis loop has been observed at 5 K with a coercive field of ~282 Oe and remnant magnetization of ~7.7 emu/g, which show that the LNMO sample exhibit typical ferromagnetic behavior. Figure 10c shows the variation of magnetization as a function of magnetic field for LCMO ceramics. A hysteresis

Figure 9. High-resolution XPS spectra's of (a) Ni 2p peaks, (b) Mn 2p peaks for LNMO and (c) **Figure 9.** High-resolution XPS spectra's of (a) Ni 2p peaks, (b) Mn 2p peaks for LNMO and (c) Co 2p peaks, (d) Mn 2p peaks for LCMO ceramics.

ascribed to the Ni 2P1/2 level. Auger electron peak of Ni of [Figure 9a] explains the presence of +2 oxidation state of the nickel in LNMO ceramics. The Mn 2p3/2 peak of LNMO is at 638.4 eV, while the same Mn 2p3/2 peak is at 641.5 eV for Mn2O3 [56]. In the spectrum of Co2p (Figure 6c), the peaks of Co 2p3/2 and Co 2p1/2 states were located at 777.3 and 782.7 eV, respectively [57]. The Mn 2p3/2 peak of LCMO shown in Figures 6 b and 6d is found at 637.6 eV, close to

Figure 8. FTIR of the microwave sintered (a) LNMO and (b) LCMO ceramics.

**4000 3500 3000 2500 2000 1500 1000 500**

**Wavenumber cm-1**

The chemical states of elements of Ni, Mn in LNMO and Co, Mn in LCMO ceramics was further investigated by X-ray photoelectron spectroscopy. The XPS core level spectra of Ni 2p, Co 2p and Mn 2p of La2MMnO6 (M = Ni, Co) are presented in Figure 9. A Ni 2p3/2 signal observed at 851.3 eV, with a satellite peak at 858.5 eV, and the peak at 869.5 eV was ascribed to the Ni 2p1/2 level. The Auger electron peaks of Ni are shown in Figure 9(a), which further explains that the nickel is in the +2 oxidation state. The Mn 2p3/2 peak of LNMO is at 638.4 eV, while the same Mn 2p3/2 peak is at 641.5 eV for Mn2O3 [56]. In the spectrum of Co2p (Figure 6c), the peaks of Co 2p3/2 and Co 2p1/2 states were located at 777.3 and 782.7 eV respectively [57]. The Mn 2p3/2 peak of LCMO shown in Figure 6 (b&d) is found at 637.6

**1629**

**1623**

**1452**

**<sup>1357</sup> <sup>1450</sup>**

**518**

**424**

**597**

**619**

**863**

The magnetization characteristics were measure both as a function of applied magnetic field at fixed temperatures and as a function of temperature at fixed fields. The room temperature hysteresis loops of the microwave sintered La2MMnO6 (M = Ni, Co) ceramics were measured using physical property measurement system (PPMS). The magnetization curves, as shown in Figures 10a and 10c, display relatively high saturation magnetization. The magnetic saturation (*Ms*) values of LNMO and LCMO are 42.9 and 65.4 emu/g, respectively, which is lower their theoretical values of 47.5 and 71.21 emu/g reported in the literature [40]. One can note that MWS products saturation magnetization was higher than for the conventionally sintering products [36], indicating that MWS method is efficient to fabricated high quality double

The frequency dependence of saturation magnetization hysteresis curves was recorded at room temperature for the LNMO ceramics as shown in Figure 10a. A hysteresis loop has been observed at 5 K with a coercive field of ~282 Oe and remnant magnetization of ~7.7 emu/g, which show that the LNMO sample exhibit typical ferromagnetic behavior. Figure 10c shows the variation of magnetization as a function of magnetic field for LCMO ceramics. A hysteresis

16

that in Mn2O3 [56].

perovskite material.

*3.1.1. Magnetic analysis of La2MMnO6 (M = Ni, Co) ceramics*

eV, close to that in Mn2O3 [56].

**(b)**

**LCMO**

**3426**

**3423**

**Figure 8.** FTIR of the microwave sintered (a) LNMO and (b) LCMO ceramics.

**(a)**

**Tranmittance (a.u.)**

14 Advanced Ceramic Processing

**LNMO**

loop has been observed at 5 K with a coercive field of ~972 Oe and remnant magnetization of ~8.14 emu/g, which show that the LCMO sample exhibit typical super paramagnetic behavior. Apart from the magnetic characteristics presented here, detailed examination is in progress and the extensive and expected results will be reported elsewhere shortly. Co 2p peaks, (d) Mn 2p peaks for LCMO ceramics. *Magnetic analysis of La2MMnO6 (M = Ni, Co) ceramics*  The magnetization characteristics were measure both as a function of applied magnetic field at fixed temperatures and as a function of temperature at fixed fields. The

Figures 10b and 10d show the temperature dependence of magnetization measurements for LNMO and LCMO under an applied field was carried out by field-cooled (FC) and zero-fieldcooled (ZFC) processes at an applied magnetic field of 100 Oe in the temperature range of 5– 400 K. For the LNMO, It could be observed from ZFC as well as FC magnetization that the material shows two ferromagnetic transitions around 270 K and 240 K, which is reliable with the presence of two phases as showed by the X-ray diffraction studies. As the ferromagnetic transition temperature in the pure monoclinic phase is found to be near 255 K, we attribute the ferromagnetic transition at 240 K to the rhombohedral phase. FC magnetization reaches a maximum value of ~3.2 emu/g at 5 K. The magnetic transition at ~255 K indicates the onset of FE long-range ordering, very close to the magnetic transition temperature (*T*c = ~280 K) reported earlier in the literature [41]. It is pertinent to maintain that there is a divergence between ZFC and FC magnetization curves below 220 K for the LCMO. Noticeable difference has also been observed in the case of low field ZFC and FC magnetization for LNMO particles. These LCMO nanoparticles possess a single magnetic transition at about 225 K under 100 Oe 18 room-temperature hysteresis loops of the microwave sintered La2MMnO6 (M = Ni, Co) ceramics were measured using physical property measurement system (PPMS). The

fabricated high quality double perovskite material.

room-temperature hysteresis loops of the microwave sintered La2MMnO6 (M = Ni, Co) ceramics were measured using physical property measurement system (PPMS). The magnetization curves, as shown in Figure 10 (a & c), display relatively high saturation

magnetization. The magnetic saturation ( *Ms* ) values of LNMO and LCMO are 42.9 and 65.4

emu/g, respectively, which is lower their theoretical values of 47.5 and 71.21emu/g reported

**Figure 10.** Magnetic field dependent magnetization data at 5 K (a and c), zZero-Ffield-Ccooled (ZFC) and Ffield-Ccooled (FC) magnetization as a function of temperature (b and d) the microwave sintered LNMO and LCMO ceram‐ ics.

field. This observation is very close to the behavior of bulk LCMO ceramics [58]. The maximum FC magnetization is noticed about 4.8 emu/g at 5 K. Figure 10. Magnetic field dependent magnetization data at 5 K (a and c), Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetization as a function of temperature (b and d) the

#### **3.2. Dielectric properties of La2MMnO6 (M = Ni, Co) ceramics** 19

The temperature variation of dielectric constant (*ε* ' ) and loss tangent (tan *δ*) at different frequencies ranging from 1 kHz to 1 MHz for the microwave sintered La2MMnO6 (M = Ni, Co) ceramics is shown in Figures 11a–11d. Noticeably, the dielectric constant (*ε* ' ) decreases significantly with increasing frequency. An interesting Maxwell–Wager relaxation [59]-type dielectric behavior (at high dielectric constant) has been noticed around 450 K in these materials and also strong dispersion in the relaxation spectra. The dielectric constant is gradually increased first along with the increase in temperature and attains significant growth at a critical temperature. The critical temperature value shifts toward higher side as and when the measuring frequency increases. These features indicate the thermally activated process [59]. This phenomenon has been most widely described in various earlier reports [24, 59–61].

) decreases

Such dielectric performance could be attributed to the cationic disorder prompted by the exchange of B sites [62]. In the present systems, Ni3+/Co3+ and Mn4+ ions instantly exist in B sites, which results in two kinds of BO6 octahedra in the structure of La2MMnO6. Therefore, the ion disorder in the unit cell should be one of the causes for this behavior. reports [24, 59–61]. Such dielectric performance could be attributed to the cationic disorder prompted by the exchange of B sites [62]. In the present systems, Ni3+/Co3+ and Mn4+ ions instantly exist in B sites, which results in two kinds of BO6 octahedra in the structure of La2MMnO6. Therefore,

the ion disorder in the unit cell should be one of the causes for this behavior

frequencies ranging from 1 kHz to 1 MHz for the microwave sintered La2MMnO6 (M = Ni, Co)

significantly with increasing frequency. An interesting Maxwell–Wagner relaxor-like [59] ielectric behavior with a high dielectric constant is observed around 450 K in these nanoparticles, and strong frequency dispersion is found in the relaxation spectra. The dielectric constant gradually increased first with increasing temperature first, and then significant growth is observed at a critical temperature. The critical temperature shifts towards

ceramics is shown in Figure 11 (a-d). Noticeably, the dielectric constant ( '

21 **Figure 11.** Temperature dependence of dielectric constant and dielectric loss of the microwave sintered (a & b) LNMO and (c & d) LCMO ceramics at various frequencies.

field. This observation is very close to the behavior of bulk LCMO ceramics [58]. The maximum

**Figure 10.** Magnetic field dependent magnetization data at 5 K (a and c), zZero-Ffield-Ccooled (ZFC) and Ffield-Ccooled (FC) magnetization as a function of temperature (b and d) the microwave sintered LNMO and LCMO ceram‐

Figure 10. Magnetic field dependent magnetization data at 5 K (a and c), Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetization as a function of temperature (b and d) the

**1**

ZFC

**2**

**3**

**M (emu/g)**

**4**

**5**

**1.0**

**1.5**

**2.0**

**M (emu/g)**

ZFC

FC

**2.5**

**3.0**

**3.5**

room-temperature hysteresis loops of the microwave sintered La2MMnO6 (M = Ni, Co) ceramics were measured using physical property measurement system (PPMS). The magnetization curves, as shown in Figure 10 (a & c), display relatively high saturation

magnetization. The magnetic saturation ( *Ms* ) values of LNMO and LCMO are 42.9 and 65.4

emu/g, respectively, which is lower their theoretical values of 47.5 and 71.21emu/g reported in the literature [40]. One can note that MWS products saturation magnetization was higher than for the conventionally sintering products [36], indicating that MWS method is efficient to

frequencies ranging from 1 kHz to 1 MHz for the microwave sintered La2MMnO6 (M = Ni, Co)

19

significantly with increasing frequency. An interesting Maxwell–Wager relaxation [59]-type dielectric behavior (at high dielectric constant) has been noticed around 450 K in these materials and also strong dispersion in the relaxation spectra. The dielectric constant is gradually increased first along with the increase in temperature and attains significant growth at a critical temperature. The critical temperature value shifts toward higher side as and when the measuring frequency increases. These features indicate the thermally activated process [59]. This phenomenon has been most widely described in various earlier reports [24, 59–61].

ceramics is shown in Figures 11a–11d. Noticeably, the dielectric constant (*ε* '

) and loss tangent (tan *δ*) at different

**100 200 300 400**

**Temperature (K)**

**H= 100 Oe** FC

**Temperature (K)**

**50 100 150 200 250 300 350 400**

) decreases

**(d)**

**(b)**

**LNMO H= 100 Oe**

**LCMO**

FC magnetization is noticed about 4.8 emu/g at 5 K.

**-20,000-15,000-10,000-5,000 0 5,000 10,000 15,000 20,000**

Magnetic Field (Oe)

**-20000-15000-10000 -5000 0 5000 10000 15000 20000**

fabricated high quality double perovskite material.

Magnetic Field (Oe)

**-50 -40 -30 -20 -10 0 10 20 30 40 50**

**(c)**

**LCMO**

**-80 -60 -40 -20 0 20 40 60 80**

Magnetic Moment (emu/g)

ics.

Magnetic Moment (emu/g) **LNMO**

16 Advanced Ceramic Processing

**(a)**

The temperature variation of dielectric constant (*ε* '

**3.2. Dielectric properties of La2MMnO6 (M = Ni, Co) ceramics**

Figures 12a and 12b show the dielectric constant (*ε* ' ) as a function of frequency of LNMO and LCMO ceramics at different temperatures. It can be observed that the dielectric constant of both ceramics decreases as frequency increases. The decrease in the dielectric constant with increase in frequency can be explained by the behavior on the basis of electron happing from Fe2+ to Fe3+ ions or on basis of decrease in polarization with the increase in frequency. Polari‐ zation of a dielectric material is the quantity of the contributions of ionic, electronic, dipolar, and interfacial polarizations [63]. At low frequencies, polarization mechanism is keenly observed at low frequencies to the time varying electric fields. As the frequency of the electric field increases, different polarization contributions are filter out under leads to the decrement in net polarization under dielectric constant. Similar behavior has also been reported by different investigators earlier in the literature [60, 64]. The physical, magnetic, and dielectric properties of LMNO and LCMO are summarized in Table 1.


**Table 1.** Physical and multiferroic characteristics of the microwave sintered LNMO and LCMO nanoparticles
