1 Introduction

In the current past, attempts have been introduced in producing multifunctional materials with simultaneous electric and magnetic ordering, which is driven mostly by the growing need for the lowering of consumption of power and additional functionality for next-generation electronic appliances [1]. In this case a strong magnetoelectric coupling is essential for useful applications though the observed strength for intrinsic coupling in such multifunctional materials is generally literally small. It is well established that magnetoelectric coupling exists in a multiferroic composite which consists piezoelectric and piezomagnetic phases with 0–0 and 1–1 connectivity. It is imperative to talk about that such substances are rare as the electronic origins of spontaneous magnetic and electric dipolar ordering are commonly exclusive [2]. Considerable magneto dielectric effect has been detected in spin-spiral systems in recent times such as TbMnO3 [1] and CuO [3] at desire temperatures and even below room temperature, and same effect has also been detected in charge-ordered arrangement like LuFe2O4 [4]. Yang et al. [5]. have mentioned that the magnetodielectric effect in that system is dynamically improved because of the control of magnetic field of the dielectric relaxation.

Owing to interesting properties and broad potential functioning multiferroic materials having coexistence of minimum two ferroic behaviors have been deliberated widely [6]. R2NiMnO6 (where, R = Pr, Nd, Sm, Gd, and Ho) are a particular set of oxides of double perovskite [7] in which a near-ideal \(e_g^2\)–O–\(e_g^0\) electronic exchange leads to ferromagnetic coupling of d-electron spins in accordance to the very well-known established Goodenough–Kanamori rules [8, 9]. It is to be noted that the general formula of double perovskites is A2(BB’)O6, where A stands for an alkali-earth metal and B, B’ are transition metals. These materials have many potential applications because of their substantial behaviors such as sensors, memory devices, radio frequency filters, and phase shifters. The magnetroresistance, colossal magneto-dielectricity and room temperature ferromagnetic insulating properties allow La2NiMnO6 as a potential candidate for alternative magnetoelectric material. In addition, the inherent intrinsic lattice strain of the biphasic composition compounds has attracted considerable interest to the researchers in this research area [10]. The probability of the resulting numerous electronic behavior in this material and expectation on basic understanding of Ni2+–O–Mn4+ electronic interaction would provide new tracks for designing the multiple property materials.

Among the perovskites, multifunctional double perovskite oxides containing rare-earth elements such as La2BBˊO6 (B = Ni; Bˊ = Mn) are the most usually studied materials due to their enriched technological aspects [10]. An ordered double perovskite such as La2NiMnO6 is a ferromagnetic semiconductor having TC = 280 K. Studies on bulk La2NiMnO6 have expressed enormous magnetic field induced variations in the dielectric behaviors and resistivity at temperatures at and above 280 K [10]. This temperature is quite higher than earlier observation for such a coupling of magnetic, dielectric, and electric properties in a ferromagnetic semiconductor. Such properties arising out of possible combination of multiple electronic behavior like ferromagnetism, magnetoresistance, magneto capacitance, and semi-conductivity involving Ni2+–O–Mn4+ electronic interaction in this substance provide a new guideline in designing new multiple property materials.

In this review, crystal structure, electrical, magnetic, and magneto-transport properties of La2NiMnO6 are accumulated from earlier studies. The goal of the current article is to recapitulate the physicochemical behavior of La2NiMnO6, which have been investigated till now. Based on these properties, the possible applications and the direction of future research in this field are furnished.

2 Bulk materials

2.1 Synthesis and crystal structural feature

The double perovskite compound, A2(BBʹ)O6, where A refers to an alkali-earth metal and B, Bʹ are transition metals. It is well known that the cation order–disorder plays a potential role in governing the crystal geometry and behavior of several complex perovskites type oxides. It has been perceived that various physicochemical behavior like superconductivity, ferroelectricity, colossal magnetoresistance, magnetic ordering, multiferroic property, piezoelectricity, ionic or electronic conductivity are dependent strongly on the extent of order of A and/or B sites. Generally, B/Bˊ ordered distribution is stabilized when B and Bˊ cations vary considerably regarding their charge and size [11]. By choosing Ni2+ and Mn4+ and with the help of charge differences the complete B-site ordering may be attained in designed double perovskite La2NiMnO6. In many earlier studies it has been detected that in a well-ordered La2NiMnO6, a rhombohedral or a monoclinic symmetry with Ni2+ and Mn4+ which are alternatively arranged at B sites, whereas in a disordered phase there is an orthorhombic symmetry with a random distribution of Ni3+ and Mn3+ at the sites, B of ABO3 perovskite. Several authors currently worked on La2NiMnO6 and explained the variations of properties on synthetic condition. The summaries of relevant experimental conditions and crystallographic data were shown in Table 1.

Table 1 Reported crystallographic data with synthetic conditions
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La2NiMnO6 is biphasic in nature, which exists at high temperature in rhombohedral phase, whereas it transforms at room temperature to monoclinic or orthorhombic. At low temperature, material’s rhombohedral phase changes to later subjected to the Ni/Mn B-site sub-lattice arrangement [11]. The random arrangement of Mn and Ni over the octahedral sites of perovskite structure is found to happen for rhombohedral R-3c and orthorhombic Pbnm, whereas an ordering of Mn and Ni into a distinguishable site may be arranged in rhombohedral R-3 or R-3m and monoclinic P21/n space groups. Both the rhombohedral and orthorhombic or monoclinic phases of La2NiMnO6 have been observed to co-exist over a wide-ranging of temperature, including room temperature [9]. La2NiMnO6 has a typical double perovskite structure with two different B-sites, as shown in Fig. 1a [12]. In the bulk La2NiMnO6 sample, Mn and Ni are normally ordered in the matter of occupying B-site, as shown in Fig. 1b [12]. The basic structure of La2NiMnO6 showing ordered monoclinic geometry is shown in Fig. 1a [13] and the crystallographic structure is shown by Fig. 1c. However, the boundary atoms of different domains are the same, being either Mn or Ni in the structural domains of the materials. In case two such domains gather, an interface known as anti-site phase boundary is generated [14, 15], containing either Ni–O–Ni or Mn–O–Mn bonds only, as shown in Fig. 1a, b. In the fully ordered La2NiMnO6, ions of Mn and Ni exist as Mn4+ and Ni2+, however, in the fully disordered LaNi0.5Mn0.5O3 (113 phase), they exist as Mn3+ and Ni3+ showing different crystal structures. Previous study has depicted that Ni2+ and Mn4+ are major in the compound while a little proportion of Ni3+ and Mn3+ coexist in La2NiMnO6 depending upon the conditions of sample synthesis [11].

Fig. 1
figure 1

a Schematic illustration of the crystal structure, b the differently ordered antisite phase boundary and domain, and c crystallographic structure of La2NiMnO6 [Reprinted with permisssion from Ref. 12, Copyright Chin. Phys B, 2013, 13]

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The initial report on La2NiMnO6 has revealed that the material crystallizes in orthorhombic crystal symmetry with slight monoclinic distortion prepared at 1100 °C [16]. The compound synthesized at temperature higher than 1300 °C is crystallized as monoclinic structure [11, 17]. The structure of La2NiMnO6 is rhombohedral (R3) at far higher temperature which at low temperature changes to monoclinic (P21/n) and over a wide-ranging of temperature, there is a co-existing of the two structures [18]. Farheen et al. [19] have verified how the variations of annealing temperature and atmosphere affect the properties and crystal structures of La2NiMnO6. Figure 2 displays the XRD patterns of the multiphase compounds annealed in different atmospheres such as nitrogen, oxygen, and air [19]. It is also established that the annealing condition affects the orthorhombic distortion of the cation disordered lattice due to the oxygen non-stoichiometry effect.

Fig. 2
figure 2

PXRD patterns of La2NiMnO6 annealed in different atmospheres of nitrogen (N), oxygen (O), and air (A). (Subscripts m and r are used to mean monoclinic (M) and rhombohedral (R) phase) [Reprinted with permission from Ref. 19, Copyright 2011, Cambride University Press]

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La2NiMnO6 with the usual structure of double ordered perovskites (A2BB′O6) is distorted from the standard double perovskite where the change of temperature affects the magnitude of change of distortion. The preparation technique and post heat treatment of the compound affect the cations ordering or disordering in La2NiMnO6 lattices. The cation ordering in these perovskites is determined by the octahedral cations’ charges and ionic radii differences. However, cation disordering is occurred due to the ionic radii similarity of Mn and Ni, whereas their valence states result in cation ordering in La2NiMnO6 lattice. The concurrent lattice ordering and disordering allow La2NiMnO6 to show different symmetries which in turns to be magnetically complicated. Consequently, it is challenging to obtain double perovskites multiferroic materials. Thus, the magnetic behavior and cationic ordering of bulk La2NiMnO6 is a subject of study in plenty of researches. Various reports have displayed the absence of any cation ordering in La2NiMnO6 lattice, whereas other investigations have expressed lattices with partially or fully cation ordered. Previously reported crystallographic data with the conditions of synthesis are shown in Table 1.

2.2 Theoretical predictions of electronic properties

Energy band structure is important to explore the electronic behavior of a compound. Ullah et al. have calculated [20] the band structure along high symmetry points in reduced zone scheme using GGA + U [21] approximation for La2NiMnO6 (Fig. 3) to show the indirect band behavior of spin down and spin up states each. The minima of conduction band have been found to arise at the Γ symmetry point whereas the maxima of valence band have been found near the Y symmetry point inside the Brillion zone for spin up state. This gives rise to an indirect band gap (Γ-Y) for spin up state with the band gap value of 1.6 eV. Likewise, the minima of the conduction band are observed to be found at Γ symmetry point while the maxima of the valence band recline along Z symmetry point of Brillouin zone for spin down state. The band gap formed along Γ-Z symmetry points is responsible for indirect band nature of the compound in case of spin down state. The band gap energy for La2NiMnO6 has been estimated as 4.4 eV.

Fig. 3
figure 3

Band structure estimation of La2NiMnO6 for both (left panel) spin up and (right panel) spin down states [Reprinted with permission from Ref. 20, Copyright 2015, Elsevier]

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According to the calculation of the density of states (DOS), the estimated total and partial DOS for La2NiMnO6, the contribution in the valence band from −18 eV to Fermi level is partitioned into three regions for spin down and spins up channels each [20]. The predominant contribution in the energy range of −18 eV to −16.5 eV is for the O-2s state, whereas the contribution of the La-5p state is dominant ranging from −13 eV to −12.2 eV. The major contribution in the region just below Fermi level from −5 eV to 0 eV is for the O-2p state. Above Fermi level, the D-states of La, Ni, and Mn contribute predominantly.

2.3 Experimental and theoretical predictions of magnetic properties

Among various rare earth-based double perovskite compounds, La2NiMnO6 is the most widely studied substance for its immense magneto dielectric behavior and transition of ferromagnetic nature at almost ambient temperature (280 K) [10, 11, 22,23,24,25]. The previous studies have clearly demonstrated that La2NiMnO6 is a ferromagnetic semiconductor, whereas the end components LaMnO3 and LaNiO3 are antiferromagnetic and paramagnetic in nature, respectively [26, 27]. Goodenough et al. [23] have depicted that the ferromagnetism in La2NiMnO6 is owing to Mn3+–O–Ni3+ super exchange interactions, whereas Blasse [24] has stated that the ferromagnetism is completely due to Mn4+–O–Ni2+ super exchange interactions. Other studies on La2NiMnO6 have pointed out that the material prepared by solid-state reaction in air has two ferromagnetic (rhombohedral and monoclinic) phases with comparable Tc [11, 18, 28]. Nevertheless, in absence of long-range ordering of Ni and Mn, the most important interactions in La2NiMnO6 resulting from Mn–O–Mn and Ni–O–Ni links are antiferromagnetic. In this case, the assumption is the ferromagnetic interaction in disordered octahedral cations occurs by vibrionic super-exchange coupling [5]. Rogado et al. [10] have determined the magnetization with the variation of temperature keeping the field constant and with the variation of applied magnetic field keeping temperature constant. Figure 4a–d demonstrates the magnetic DC and AC susceptibility of La2NiMnO6 in different applied field and magnetic transition at 280 K indicating the long-range ordering of ferromagnetic nature [10, 17]. The inset of Fig. 4a demonstrates the inverse susceptibility (1/χ) plot from 300 to 320 K and large θ value obtained (+274 K). This large θ value indicates the strong ferromagnetic interaction between Ni2+ and Mn2+ spin. Figure 4b shows the low field ZFC and FC magnetization curves where partially disordered La2NiMnO6 has sample a ferromagnetic transition at about 270 K. sharp frequency independent ferromagnetic transition around 270 K and the details of below 150 K are shown in Fig. 4c, d suggest the presence of dynamical features at low temperatures. Figure 4e shows the dip in the difference ZFC plot at TW for DC memory experiments suggests that La2NiMnO6 exhibits spin-glass-like dynamics at low temperatures. So, La2NiMnO6 behaves like a re-entrant spin glass or reentrant ferromagnet, exhibiting successive transitions from paramagnetic to ferromagnetic, and ferromagnetic to spin-glass states [17].

Fig. 4
figure 4

a Magnetic susceptibility χ(T) at 1 T on ZFC and FC. Insets show a plot of 1/χ versus T fitting to the Curie-Weises law (left) and field dependent isothermal magnetization at 5 K (right) [Reprinted with permission from Ref. 10, Copyright 2005, John Wiley and Sons]. b FC and ZFC, DC magnetization data at 20 Oe. c The imaginary parts of AC susceptibility at H = 4 Oe. d The imaginary parts of AC susceptibility at low-temperature. e ZFC data with and without an intermediate wait (TW) at 25 K for 6000 s, (inset showed memory effect) [Reprinted with permkission from Ref. 17, Copyright 2012, Americal Physical Society]

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By using the first principle DFT-FPAPW method, a ferromagnetic insulating ground state was obtained for La2NiMnO6 [29]. The band gap of La2NiMnO6 was found as 1 eV, which justifies the semiconducting nature of La2NiMnO6. The spin polarized calculations have checked the experimentally observed 5 µB magnetic moment per formula unit. This value fulfills the Hund’s first rule for the moments Mn4+ (3d3): \(t_{2g}^3e_g^0\)and Ni2+(d8): \(t_{2g}^6e_g^2\). This result also validates the prediction of Goodenough Kanamori (GK) rule that the coupling in between the adjacent Mn4+and Ni2+ions are ferromagnetic in nature. The ground states of La2NiMnO6 is ferromagnetic semiconducting with alternative Ni/Mn ordering along the (111) direction. Furthermore, it is found that La2NiMnO6 is half-metal with Ni/Mn ordering along (001) and (110) after considering the effect of electronic correlation [30]. The determined magnetic moment was 4.7 µB per formula unit which is very much close to the bulk magnetic moment.

Depending upon the magnetic properties, La2NiMnO6 may be classified into three categories such as:

(a) A long range ordered Ni/Mn phase, showing a single transition to paramagnetic from ferromagnetic at about 280 K because of ferromagnetic super-exchange interaction of Mn4+–O–Ni2+. (b) A cation disordered phase with random ordering of the Mn/Ni cations, showing a single transition to paramagnetic from ferromagnetic at about 150 K due to the super-exchange interaction of Mn3+–O–Ni3+. (c) An admixture phase, showing two transitions to paramagnetic from ferromagnetic owing to above mentioned exchange interactions [31].

Thus, it is interesting to see whether the fraction of long range order 2+ and 4+cations on B and Bˊ sites could be systematically changed by substitution of other cations such as Fe. Since Fe shows 2+ and 3+ valance states, so the existence of Fe affects the percentage of long range ordered cations. In addition, the substitution of Ba2+ ions at A site can also force Mn to exist in 4+ state. The results of these types of substitutions introduce reduction in ionic radius and change of valence states and magnetic properties with non-distorted crystal structure. The vacant A site i.e., absence of La by substituting Ba2+ also eliminates the probable formation of amorphous La2O3 [11, 32].

By introducing first-principles electronic structure calculation, Zhu et al. [30] examined magnetic and electronic natures of La2NiMnO6 with Ni/Mn ordering in (111), (110), and (001) directions. They observed that below −2 eV, there are mainly O-2p states hybridized with d states of cations, and above 3 eV, there was a narrow peak of La-f states. However, from −2 eV to 3 eV, there are mainly narrow 3d states, in which d states of Ni are lower than those of Mn. The states of d are separated into distinguished eg and t2g states in a strong octahedral crystal field. Further, they slightly split into five singlet states due to additional weak distortions in P21/n monoclinic structure.

Booth et al. [33] have investigated the cation ordered monoclinic R2NiMnO6 (where, R = Pr, Nd, Sm, Gd, and Ho) and reported that the FM-Tc is decreased with the decrease in ionic radii of R3+ ions. La2NiMnO6 exhibits a different polymorphism than the end members (LaMnO3: orthorhombic crystal structure with Pbnm (62) space group; and LaNiO3: trigonal crystal structure with R-3c space group (167) crystal structures which is due to the differences in crystal structures of the end members. Thus, the cation ordering and the magnetic properties of bulk La2NiMnO6 have been a favorable area of research in a many study. Several reports have indicated the absence of cation ordering in the La2NiMnO6 lattice while additional studies have reported partially or fully cation ordered lattices [10, 18, 25, 28, 34]. The presence of Mn4+, Mn3+, Ni2+, and Ni3+ in the ferromagnetic La2NiMnO6 has also been introduced [18, 25, 28]. The neutron diffraction studies revealed the presence of Jahn–Teller distortions in transition metal polyhedra in La2NiMnO6 [11, 18].

Sometimes another magnetic transition is found at low temperature range especially for biphasic sample. The low temperature ferromagnetic transition at Tc = 150 K is due the ordering of Pbnm phase because of super-exchange interaction involving Mn3+–O–Ni3+ [35]. Low temperature spin glass behavior is also reported [17, 36]. The spin glass transition temperature lies around 40 K as found in the temperature-dependent magnetization data [17]. Short range ordering of small ferromagnetic clusters before the actual arrival of TC has been explained by Iliev et al. [37], and S. Zhou et al. [38]. Similar short-range ordering and Griffiths phase like property are found in another prototype of the family, Tb2NiMnO6 [39]. So, Griffiths like phase may be anticipated in La2NiMnO6.

2.4 Magnetoresistance properties

Magnetic field dependent change in resistance of the materials, known as magnetoresistance (MR) effect, has withdrawn a significant attention owing to its enriched physics and technological applications [40,41,42,43,44]. The double perovskite oxide, La2NiMnO6, nearly room-temperature ferromagnetic semiconductor, has stimulated much interest owing to its novel electric and magnetic properties [10]. There has been formulated that its ferromagnetic property arises from the superexchange interactions between the alternately ordered Ni2+ and Mn4+. However, the inevitable Ni/Mn antisite (AS) disorders will result to the antiferromagnetic (AFM) Ni2+-O2--Ni2+ and Mn4+–O2-–Mn4+ configurations [45]. This ferromagnetic material shows antiferromagnetic type interactions. The evaluation of different study predicted that anti-site disorders of bulk La2NiMnO6 might be combine with antiphase boundaries. This phenomenon leads to an antiferromagnetic-coupling among the adjacent ferromagnetic domains [46, 47].

This type of behavior has been obtained in various magnetic oxides, e.g., in the granular perovskite, tunneling magnetoresistance (TMR) and in the manganites, the colossal magnetoresistance (CMR) are observed [40, 41]. The spin-dependent tunneling among the ferromagnetic grains originates the TMR effect, which shows an improved low-field response in comparison with the intrinsic TMR effect. Among the tunneling-type MR materials, the introduction of room-temperature low-field MR in double perovskite oxide Sr2FeMoO6 has opened a new hope for prospective spintronic devices [42,43,44]. The research with B-site ordered Sr2FeMoO6 have found the magnetic domains of Fe–O–Mo ferromagnetic ordering segregated due to the formation of Fe–O–Fe antiferromagnetic interaction while Fe/Mo anti-site disorder arises. The low field magneto-resistive behavior of Sr2FeMoO6 could be attributed to spin dependent tunneling of barriers initiated from the existence of AFM areas [48] While, anti-site disorders are natural growth defect and usually noticed in the family members of double perovskite [34, 49] and attracted attention in other ferromagnetic double perovskites for TMR effect.

Considering the similarity in magnetic structure with those of Sr2FeMoO6 and nearly room-temperature FM transition, La2NiMnO6 could be a potent candidate of the materials with a nearly room-temperature TMR effect. It has been evidenced that La2NiMnO6 exhibits a clear MR effect nearly room-temperature (Fig. 5) and magnetic field dependent MR may be categorized into two sectors as low-field and high-field MR [50]. The experimental results of combine properties of electric and magnetic measurements demonstrated that magnetoresistance at low field can be attributed to tunneling effect between the neighboring ferromagnetic domains where magnetoresistance at high filed could be accredited to restraint of scattering from spin defects due to Ni/Mn AS-disorders.

Fig. 5
figure 5

a Temperature dependent resistivity ρ under zero and 200 Oe field at 200 K ≤ T ≤ 400 K. b Plot of absolute value of magnetoresisrance versus T. c Plots of ln ρ against (1/T)1/4. d Plot of temperature dependent square of the normalized magnetization (M/M200K)2. e MR against external magnetic field H at 250 K. f Magnetization M versus magnetic field H at 250 K [50]

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2.5 Colossal magnetodielectricity

Magnetodielectric materials (Fig. 6) exhibiting dielectric properties at room temperature depending upon the applied magnetic field are very much encouraging for future device applications [17, 51,52,53]. Choudhury et al. have shown [17] which partly disordered La2NiMnO6 has an array of remarkable behavior like a disordered ferromagnetism at higher temperatures and a reentrant spin-glass transition at lower temperatures, a relaxer-type dielectric behavior, which is a rare example of an intrinsic multi glass system in contrast to Mn-doped SrTiO3 [54, 55]. This is an advanced insulator having colossal magnetodielectric coupling (up to 20%) acting on a wide range of temperature and it is no considerable magnetoresistance property. From this study it has been obtained that the mechanism of asymmetric hopping contributes positively to the dielectric constant. Thus, magnetodielectric coupling may be employed in these types of materials for probing the asymmetric hopping on the relative-spin orientations of adjacent sites. This introduces an innovative path to materialize magnetodielectric materials, which are independent of any type of magnetostriction or electrostriction results.

Fig. 6
figure 6

a Dielectric constants at 10 kHz under zero and external field, respectively. b Temperature dependent magnetization (left axis, measured with H = 20 Oe) and magnetodielectricity (right axis) [Reprinted with permissions from Ref. 17, Copyright 2012, American Physical Society]

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3 La2NiMnO6 thin film

In the recent past, little reports have been prepared to elaborate structural and functional behaviors of thin films of La2NiMnO6 unlike the bulk phase of the same material [37, 45, 56,57,58,59,60]. Singh et al. [56, 57] have concluded that at room temperature the films showing multiple magnetic transitions have disordered and ordered phases together and the second magnetic phase transition below 200 K was linked with the disordered parts of films [56, 57]. In other study, Subramanian et al. have established that the optimization of the growth parameters may result a single magnetic transition [61] which has also been reported by Kitamura et al. [60]. Further, these films exhibit a strong spin-lattice coupling and magnetodielectric effect far below the magnetic Curie temperature [10, 37, 62, 63].

The magnetic behaviors of bulk La2NiMnO6 have been investigated extensively for last few years to achieve an elaborate knowledge on the behavior of magnetic exchange interactions. Strong magnetodielectric effect in bulk La2NiMnO6 [10] and thin films of La2CoMnO6 [62] near relevant magnetic transition nearly room temperature expresses the far-reaching technological potential of these materials. However, such strong effect found very near to the FM-TC of these materials disputes with the magnetodielectric effect of La2NiMnO6 thin films [63] found far below the FM-Tc. The absence of long-range structural order probably is a reason for such type of behavior because ordered films show a saturation magnetization close to 4.8 μB/f.u. and FM-Tc around 270 K, while the disordered ones have saturation magnetization, 3.7 μB/f.u. and FM-Tc around 138 K [64]. In such bulk La2NiMnO6 and thin films of La2CoMnO6, Ni (Co) and Mn ions exchange along precise crystallographic directions over very long distances [65]. It has been stated that this kind of structural order is restricted only to small domain sizes of about 50–100 nm in the thin films of La2NiMnO6 [56, 57]. Such studies have established that B-site ordering can play a key role in finding the physicochemical behaviors of double perovskites, especially the coincidence of strong magnetoelectric response and magnetic transition.

The studies on thin films have evidently depicted that there is either a short-range-ordered phase or coexistence of multiple phases like the polycrystalline bulk samples in thin films of La2NiMnO6 [37, 45, 56, 57, 59,60,61]. In fact, the long-range-ordered La2NiMnO6 and fully disordered LaNi0.5Mn0.5O3 phases have yet to be stabilized independently in thin-film forms. Such condition also obstructs us to find and analyze their respective functional behavior. Singh et al. [64] have compared the physicochemical behavior of long-range-ordered and completely disordered La2NiMnO6 thin films, which were prepared by pulsed-laser deposition. They have detected that ordered and the disordered La2NiMnO6 films have different magnetic and electronic behavior, e.g., the magnetic-transition temperatures, low-temperature saturation magnetizations, and phonon-dispersion relations are different for different types of the materials. Wu et al recently epitaxially grown the multiferroic thin films of La2NiMnO6 on SrTiO3 (STO), KTaO3 (KTO), LaAlO3 (LAO), and DyScO3 (DSO) using pulsed laser deposition in investigated by advanced electron microscopy and reported as monoclinic and rhombohedral structure of La2NiMnO6 films under tensile strain compressive strain, respectively [66]. Thus, they tuned the long-range ordering of B-site cations in La2NiMnO6 films in monoclinic and rhombohedral phases (Fig. 7).

Fig. 7
figure 7

a Model representation of B-site ordered La2NiMnO6, (b–d) Atomic-resolution HAADF image and EDS maps. e–h HAADF picture of the La2NiMnO6/LaAlO3 interface and EDS maps of Al, Mn and Ni. i–l HAADF picture of the La2NiMnO6/SrTiO3 interface and EDS maps of Ti, Mn and Ni. m Epitaxial-strain-induced phase transition in La2NiMnO6 films [66]

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4 La2NiMnO6 nanoparticle

Two or more simple oxides are present in a perovskite oxide, such high temperature is required to synthesis in single phase which can reduce the surface area. Catalytic properties are highly sensitive to the surface area of perovskite oxides. Thus, material scientist are trying to synthesize in alternative way at low temperature. Zhao et al. [67] have prepared La2NiMnO6 nanoparticles with sizes lying in the range of 16–66 nm. They have found that with the decrease of particle size the ratio of disordered phase increases, which may be realized from the changes of Curie temperature and the occurrence of glass like properties. The experimental findings have clearly shown that structural and magnetic properties of such material are responsive to the particle sizes (Table 2), which can lead charge disproportion of Mn and Ni of mix valence. In this study it has also been concluded that particle sizes of this materials play a crucial role in determining the ordered and disordered phases [67].

Table 2 Annealing temperature Tann, the corresponding particle sizes D obtained from TEM, values of freezing temperature TP, Curie temperature Tc1 and Tc2 from the magnetic curves [67]
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Nanomaterials bear a wide range of potential technological applications [68]. Nanoparticles (NPs) with various structures [68], especially perovskites [69] exhibit a quite dissimilar property as compared with the bulk counterpart. However, bulk La2NiMnO6 shows numerous fascinating properties such as colossal magneto-dielectric (~16%) behavior with high ferromagnetic Curie temperature (~280 K) [10]. La2NiMnO6 NPs also show numerous interesting properties where nanoscale inhomogeneity or disorder is considered to play the main role in showing ferroelectric and ultimate multiferroic behaviors. In double perovskites, the disorder induced polar property in relaxor ferroelectric or ferroelectric form was determined by Singh et al. [70]. Ferroelectric (relaxor) property is found in La2NiMnO6 NPs; whereas bulk counterpart does not show this property [71]. Moreover, La2NiMnO6 NPs show a broad relaxor dielectric peak temperature (~220 K), closer to the ferromagnetic transition temperature (FM-Tc ~ 220 K), under a low magnetic field (0.5 T) with a significant improvement of magneto capacitive effect (MC ~ 30%) [71], where the nano-dimension effect plays the vital role by persuading disorder or inhomogeneity. The chemical and valence mixing relating to the surface and anti-site disorders is responsible for the inhomogeneity in ferromagnetic La2NiMnO6 NPs. Ferromagnetic transition (~150 K) with low saturation magnetization is obtained by Chandrsekhar et al. [72] from ~27 nm La2NiMnO6 NPs due to the existence of anti-site effects. The authors have also reported the frequency-dependent AC susceptibility and exchange bias effects, which conforms the competing interactions, leading to glassy behavior in nanoparticles. A high temperatures paramagnetic insulating phase, low temperatures ferromagnetic insulating phase, and intermediate temperature range Griffiths-like phase in nano crystalline La2NiMnO6 was reported by Chakraborty et al. [73]. It has been obtained that the nonlinear AC conductivity is a sensitive probe to recognize various magnetic phases in the system pointing to a significant correlation between electric and magnetic degrees of freedom [74].

5 Applications

Owning to its magnetic and electric properties, La2NiMnO6 double perovskites is a promising candidate for novel applications in electronics, energy, environment, and health.

5.1 Applications in solar cell

Hybrid organic inorganic perovskites solar cells have gained a prompt progress after introducing the perovskite solar cell in the place of organic dyes in 2009 [75]. The high-power conversion efficacy of perovskite solar cells (~21%) made it as a potential alternative of conventional silicon solar cells [75,76,77]. The proper band gap of the perovskites made the hybrid perovskite solar cells appreciable for remarkable light to electricity conversion efficiency. For instance, a system namely methyl ammonium lead iodide (CH3NH3PbI3) with band gap of 1.5 eV show high absorption coefficient and carrier mobility and long electron-hole diffusion length up to 175 mm [77], and bipolar carrier transport ability [76, 77]. However, lead based materials possess some detrimental effects, such as the instability of the perovskite materials may result in weak long-term stability of the solar cells and more importantly its toxicity to environment and health [78, 79]. Consequently, stable and less toxic perovskite materials are required in order to overcome these limitations. It is reported that multiferroic inorganic perovskite semiconductors may be an encouraging substitute for solar cell applications [79].

La2NiMnO6 double perovskites showing 1.5 eV band gap has been reported by Kitamura et al. [60]. It is easier to synthesize La2NiMnO6 double perovskites by simple chemical method than the synthesis of Bi2CrFeO6 and [KNbO3]1−x[BaNi1/2Nb1/2O3-d]x, where in most of the cases complicated method like vapor deposition method is used [80]. Dass et al. [11] and Sun et al. [80]. have characterized the different crystal structures of La2NiMnO6 synthesized under different conditions. In such reports it has been obtained that the crystal structures of La2NiMnO6 have a profound affect upon their physical properties such as dielectric and magnetic properties [11, 80]. However, there is still quite a lack of research on electronic properties of different structured La2NiMnO6, such as B-site and phase affecting band gap and electronic structures, which is a key factor for solar cell application.

The solar cell application of La2NiMnO6 double perovskite considering their electronic structure and band gap was evaluated by Lan et al. [81] theoretically and experimentally. They have synthesized the samples in two different conditions, one sample annealed at 600 °C for 10 h in air (LNM600) and the other at 900 °C keeping the annealing time same in a tube furnace at oxygen atmosphere (LNM900). An energy diagram is shown in Fig. 8 drawn based on band gap and valence states. The conduction bands for both the samples LNM600 and LNM900 are found to be greater than that of ITO and SrRuO3 thin films. The crystal structure has been found to affect the energy level where the monoclinic LNM900 and rhombohedral LNM600 showed energy differences to those of SrRuO3 as 0.8 and 0.7 eV, respectively. Consequently, the photon-excited electron in La2NiMnO6 could be injected into SrRuO3 layer, considering SrRuO3 as an appropriate electron conductor [82], which can form a Schottky junction. In case of Bi2CrFeO6, the simple ITO/ Bi2CrFeO6/SrRuO3 layer-structured Schottky solar cells achieved high-power conversion efficiency, even though there were many defects in Bi2CrFeO6. Hence, the band gap and energy level matching aspects clearly demonstrate that La2NiMnO6 can function like Bi2CrFeO6 in order to fabricate a lead-free perovskite solar cell. The results clearly revealed that monoclinic La2NiMnO6 perovskite could be a better alternative than rhombohedral one for light harvesting.

Fig. 8
figure 8

Energy band diagrams of LNM600, LNM900, and related materials reported [Reprinted with permission from Ref. 81, Copyright 2016, Elsevier, 128]

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5.2 Biological applications

Magnetic NPs possess remarkable biological and medical applications [83] like bio-separation [84], antibiotic application [85], quantitative immunoassay [86], and hyperthermia [87]. In recent past, nano-perovskite have drawn a considerable interest to the researchers for their various technological applications [66, 68, 88, 89]. MnFe2O4, Fe2O3, Fe3O4, CoFe2O4, and Fe NPs having magnetic property are extensively used for biomedical applications [90,91,92,93,94]. For the use of La2NiMnO6 NPs in the aforementioned fields, the NPs have to be monodispersed in order to interact with the biomolecules like protein. Proteins generally show a tendency to aggregate at the interface between the aqueous solution and solid surfaces [95,96,97,98,99]. Proteins have to be adsorbed on the surface for their applications such as biotechnology, environmental science, and bio-medical engineering. It is noteworthy to mention here that, the adsorbing capability of La2NiMnO6 NPs on the bovine serum albumin was reported to be affected by the annealing temperature of La2NiMnO6 NPs [100]. The authors reported that bovine serum albumin exhibited a remarkable room temperature adsorbing tendency on magnetic La2NiMnO6 NPs. However, the highest adsorbing ability of 219.6 mg/g was obtained for the La2NiMnO6 NPs sintered at 850 °C. The adsorbing ability of La2NiMnO6 NPs are summarized in Table 3 [100].

Table 3 Average grain size and magnetic and BSA adsorption properties of La2NiMnO6 nanoparticles [15]
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5.3 Applications in electric tunable devices

The dielectric tunable materials have drawn a remarkable interest to the researchers because of potential applications of this type of electric devices in this capacitors, phase shifters, microwave communication devices, filters, and oscillators [101, 102]. The high dielectric tunability of conventional ferroelectric substances generally allow them to function under a large DC electric field and to follow temperature dependency around their Curie temperature [103]. Tang et al. [104] have depicted that bulk La2NiMnO6 show giant dielectric tunability, which is induced by a comparatively low electric field (40 V/cm) to nearly room temperature. Notable dielectric tunability has also been obtained in porous La2NiMnO6 ceramic synthesized by gel method using PVA [105]. Its large tunability is also complemented by the high permittivity and loss like ferroelectrics [106]. In addition, modifications of La2NiMnO6 have been executed to tuning its behavior. Substitution of La by Sr in La2NiMnO6 results a transition to half-metallic phase from ferromagnetic insulating phase [107]. The ferromagnetic La2NiMn1−xTixO6 is found to be transformed into its antiferromagnetic phase when Mn is substituted by Ti. The dilution effect suppress the dielectric constant and relaxor-like properties [108]. The mixed monoclinic and rhombohedral phases of La2NiMnO6 is transformed to only monoclinic phase after Sr–Sb co-doping. Room temperature high-dielectric permittivity and dielectric tunability of Sr–Sb co-doped La2NiMnO6 is resulted from the electrode effect [109]. The high dielectric constant and relaxation property has been explained by the local polarization from the charge ordering of Mn4+ and Ni2+ [104, 110, 111]. However, the intrinsic origin for high dielectric constant of La2NiMnO6 was enquired and extrinsic Maxwell–Wagner (M–W) polarization was suggested to report for its irregular dielectric behavior [112]. The presence of internal boundary layers or external depletion layers formation at the interface of the sample/electrode caused by the Schottky barrier effect in the inhomogeneity of microstructure and composition result in M–W polarization of polycrystalline ceramic [113, 114]. However, its applications may be hampered by the external contribution from the depletion layers [114]. For practical application, it is essential to repress its dielectric loss, however, there is a controversy on the source of the high dielectric constant [104, 115]. Non-ferroelectric material such as MgO is added to Ba1−xSrxTiO3 to balance the dielectric constant and tenability as a fruitful step [116].

DC voltage dependence of normalized dielectric permittivity at selected frequencies for La2NiMnO6 is shown in Fig. 9 [117] and dielectric tunability detected at the low frequency range (<10 kHz). The increasing frequency continuously suppressed the dielectric tunability and disappear nearly at a critical frequency (200 kHz). In an earlier study, Chen et al. [117] have reported the effect of addition of MgO on the microstructure, electrical, and dielectric behavior of La2NiMnO6 ceramics. The impedance of La2NiMnO6−xMgO ceramics is dominated by electrode response nearly room temperature. The high dielectric constant of La2NiMnO6–xMgO ceramics is resulted from two dielectric relaxations arising out of electrode and grain boundary effects. The dielectric constant is decreased by the addition of MgO. At high temperature and low frequency range, the sizable dielectric tunability is originated from the electrode effect. The electrode response of La2NiMnO6–xMgO is increased by the addition of MgO to nearly room temperature, enhancing the tunability, and this property is applicable for uses of electric-tunable devices.

Fig. 9
figure 9

The DCB voltage dependence of relative tunability (nr) at selected frequency for La2NiMnO6 ceramics [Reprinted with permission from Ref. 117, Copyright 2014, Elsevier]

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5.4 Applications in gas sensing

Biswal et al. [118] have introduced a new direction of use of La2NiMnO6 as a sensor of gas using impedance measurements technique. Impedance measurement was done by Lock–in Amplifier. The electrical conduction has been reported to be sensitive to gaseous environments and concentration. The gases are adsorbed at the interfaces of electrodes and grain boundaries constituting space charge. The space charge affects the electrical resistance of the sensor enormously [119]. The investigation on La2NiMnO6 at different gaseous (nitrogen, oxygen, argon) atmosphere with different concentrations of the gases at different stable temperatures as shown in Fig. 10 have suggested strong dependency of the resistance value with the concentration of gases. In addition, at critical gas pressure (Pc), resistance increases with sharp change in slope as concentration of gas is increased. It has been found that below Pc the sensitivity (slope change) is more and among above mentioned three gases it is maximum for argon. At elevated temperatures resistance values show two dissimilar activated regimes of distinct activation energies. At 1 Tesla magnetic field nitrogen shows nearly zero, oxygen shows a positive and argon shows a negative value of magneto impedance. The maximum magnetoimpedance value of ~ 5% (in magnitude) is found under 1 T magnetic field. Thus, in near future, the sensing device can be fabricated by making thin films of La2NiMnO6 and the gas sensing can be elaborated to gas concentrations at ppm level or for various toxic gases.

Fig. 10
figure 10

Resistance versus gas concentration for the three gases. The arrow marks the critical pressure (Pc) at which the slope changes distinctly. The red lines show the linear fit [Reuse the data from the Ref. 118]

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6 Conclusion and future prospects

La2NiMnO6 is a multifunctional oxide with a double perovskite structure showing simultaneous electric and magnetic ordering which have been studied for a last few years as next-generation electronic appliances [120,121,122,123,124,125,126,127]. The charge ordering involves at B-site due to the presence of Ni2+–O–Mn4+ electronic interaction which is responsible for the ferromagnetism, magnetoresistance, magneto capacitance, and semi-conductivity. Characteristic features of La2NiMnO6 such as crystal structure, electrical, magnetic, and magneto-transport properties are discussed and correlated to establish the mechanism of its action. The emphasis has been given how temperature and magnetic field alters magnetoelectric and electronic behavior of such material. Earlier studies have revealed that the morphology of this material in the form of bulk phase, thin layer, and nanoparticles affect its physical characteristics considerably [124]. The shell surface corelated with grain size, which leads to relaxation of superexchange at the surface with reducing of particle size. The transition metal-oxygen bond angle in La2NiMnO6 decreases with the reduction of particle size, which is mainly corelated with magnetic and electrical properties. The present review clearly depicts an idea why the application of La2NiMnO6 substituting conventional substances in solar cells, electric tunable devices, biomolecular and gas sensing technologies is advantageous. The focus of this article is to gather all such physical behavior of La2NiMnO6 to find its possible beneficial applications in new arena of material research, which may also be directive of future research in double perovskite materials.