**1.4. Brief introduction of multiferroics**

Multiferroic materials exhibits both ferroelectric and magnetic in nature and have much attracted research interest due to their potential application in multistate data storage and electric field controlled spintronics. Among all the studies related to the materials, transition metal oxides with perovskite structure are noteworthy [22, 26].

Multiferroic materials with double-perovskite structure (*AA*' *BB* ' *O*6) are solid solutions of two perovskites: (*ABO*3) and (*A*' *B* ' *O*3). In (*AA*' *BB* ' *O*6), A and A' represent alkaline rare earth cations (La, Y, and Ce), while B and B' are transition metal cations (Ni and Co). If A and A′ represent the same chemical element, the double perovskite has the general formula (*A*2*BB* ' *O*6) and the crystal structure of *A*2*BB* ' *O*6 -type perovskite, as shown in Figure 2. Alkali-earth and lanthanide (smaller ion) ions are alone usually occupied in the A site [27, 28]. If the A ion is too small, the common expected distortions are cation displacement with BO6 and octahedral ones [29].

The most representative (*A*2*BB* ' *O*6) ferromagnetic double perovskites are La2NiMnO6 [30–33], La2CoMnO6 [5, 34, 35], La2BMnO6 [36–48], and La2FeMnO6 [41, 42].

**Figure 2.** Crystal structure of A2BB'O6 type perovskite. The spheres at A and A′-site are for La and at B′-site are for Ni, Co. The network of corner-sharing BO6 octahedra isare shown where oxygen atoms are in the corner of octahedra.

La2NiMnO6 (LNMO) has gained more attention as a rare example of a single-material platform with multiple functions, such as ferromagnetic (FM) semiconducting properties up to room temperature, magnetocapacitance, and magnetoresistance effects. The spin lattice coupling characteristics of LNMO exhibits a larger magnetodielectric (MD) effect close to room tem‐ perature. It has been well documented that the spins, electric charge, and dielectric functions in LNMO are turned by magnetic or electric fields. LNMO is considered as an FM semicon‐ ductor and shows Curie transition temperature (*T*c) very close to room temperature. This property in LNMO makes the Ni2+ and Mn4 ions ordered and occupied the centers of BO6 (corner shared) and B′O6 structures respectively. This arrangement leads to the distribution of ideal double perovskite.

**•** Controllable electric field distribution

6 Advanced Ceramic Processing

**1.4. Brief introduction of multiferroics**

perovskites: (*ABO*3) and (*A*'

crystal structure of *A*2*BB* '

(La, Y, and Ce), while B and B'

The most representative (*A*2*BB* '

**1.3. Applications of microwave sintering of ceramic materials**

metal oxides with perovskite structure are noteworthy [22, 26].

*O*3). In (*AA*'

La2CoMnO6 [5, 34, 35], La2BMnO6 [36–48], and La2FeMnO6 [41, 42].

*BB* '

the same chemical element, the double perovskite has the general formula (*A*2*BB* '

Multiferroic materials with double-perovskite structure (*AA*'

*B* '

processing than the conventional ones (takes hours).

Now microwave processing has been found that this technique can also be applied as effi‐ ciently and effectively to powdered metals as to many ceramics. Finally, The MWS operational expenses are less than 50–80% to the conventional sintering techniques. The MWS technique works 20 times faster than the conventional sintering method and takes only few minutes for

Multiferroic materials exhibits both ferroelectric and magnetic in nature and have much attracted research interest due to their potential application in multistate data storage and electric field controlled spintronics. Among all the studies related to the materials, transition

(smaller ion) ions are alone usually occupied in the A site [27, 28]. If the A ion is too small, the common expected distortions are cation displacement with BO6 and octahedral ones [29].

**Figure 2.** Crystal structure of A2BB'O6 type perovskite. The spheres at A and A′-site are for La and at B′-site are for Ni, Co. The network of corner-sharing BO6 octahedra isare shown where oxygen atoms are in the corner of octahedra.

*BB* '

are transition metal cations (Ni and Co). If A and A′ represent

*O*6) ferromagnetic double perovskites are La2NiMnO6 [30–33],

*O*6 -type perovskite, as shown in Figure 2. Alkali-earth and lanthanide

*O*6), A and A' represent alkaline rare earth cations

*O*6) are solid solutions of two

*O*6) and the

LNMO's structural system, La2CoMnO6 (LCMO), possesses an FM *T*<sup>c</sup> ~225 K with an insulating behavior. The magnetic properties of the LCMO are strongly depending on the cation ordering, valences, defects, and synthetic conditions.

Among them, double perovskite La2NiMnO6 (LNMO) and La2CoMnO6 (LCMO) ceramics are attractive due to their impressive properties and potential on industrial applications [30–33, 42, 43]. LNMO is a ferromagnetic semiconductor with high critical temperature of *T*c~280 K, which may be used in commercial solid-state thermoelectric (Peltier) coolers [42]. LCMO is also a ferromagnetic semiconductor with critical temperature of *T*<sup>c</sup> ~230 K [35–37]. Several crystal structures have been identified, and it is confirmed that the ferromagnetic semicon‐ ductors LNMO and LCMO with high *T*c are *P*1/*<sup>n</sup>* 2 monoclinic structure, in which octahedra with Ni (or Co) and Mn centers alternately stacking along (111). Recent reports indicate LNMO and LCMO have considerable magnetodielectric effects at room temperature, which is very useful for future electronic device [29, 35, 44, 45].

The double perovskites La2MMnO6 (M = Co and Ni) are one of the most commonly occurring and important in all of materials science because they can exhibit novel magnetic, electric, and optical properties [28–44]. La2MMnO6 crystallizes in a double perovskite structure with rock salt configuration of MO6 and MnO6 octahedra. The ordering of M2+ and Mn4+ gives rise to 180° super exchange interactions based on Goodenough–Kanamori rules and consequently high ferromagnetic Curie transition temperature [43].

It is familiar that the properties of double perovskite compounds are strongly influenced by the materials composition and microstructure, which are sensitive to the preparation technique employed for their synthesis [46]. Various synthesis techniques such as sol–gel [30, 32, 35], coprecipitation [31], solid-state reaction method [33, 34], microwave sintering process [5], molten-salt synthetic process [26, 27] sol-gel autocombustion [41], and chemical solution deposition method [47] have been reported in the preparation of double perovskite com‐ pounds. Each of the techniques has its own merits and limitations. For instance, conventional sintering is a simple and relatively cheap method with a long holding time (several hours), formation of lots of undesirable intermetallic compounds, and nonhomogeneous pore-size distribution. In the recent years, microwave sintering has emerged as a new sintering method for ceramics, semiconductors, metals, and composites.

Microwave sintering (MWS) technique has gained a lot of significance in recent times for materials (metals, composites, ceramics/nanoparticles) synthesis and sintering mainly because of its intrinsic advantages [5] such as rapid heating rates, reduced processing times, substantial energy savings novel and improved properties, finer microstructures, and being environmen‐ tally more clean. Therefore, it is viewed as one of the most advanced sintering techniques in material processing [5, 48] and improved physical and mechanical properties [7]. It has been shown that microwave sintering technique may provide enhanced densification in sintering of metal, oxides and non-oxide ceramics [5, 48, 49, 50].

However, to the best of our knowledge in the open literature, there have been only a few reports so far on the fabrication of double perovskite nanoparticles by microwave sintering approach [5, 51]. The purpose of the current chapter will focus on fabrication of the double perovskite La2MMnO6 (M = Ni, Co) ceramics and in order to further improve their magnetic and dielectric properties for practical spintronic applications through microwave sintering approach.
