Evolution of Microstructure and Exchange Bias Characteristics of NiFe2O4/NiO nanocomposite

Among the different well studied ferrite/metal oxide nanocomposites, NiFe2O4/NiO system has attracted considerable attention because of its excellent catalytic [15] and exchange bias (EB) characteristics [16-23]. The existence of EB coupling in this system demands its potentiality for designing permanent magnet [24-26] as well as other various applications like data storage (M-RAM), microelectronics (spin and tunneling valves), magnetic sensors (read/write heads), and medicine (drug delivery) [27, 28].


Introduction
In recent years, NiFe 2 O 4 has attracted considerable research attentions due to its diverse possible technological application possibilities in microwave devices [1], magnetic drug delivery [2], magnetic high-density data storage [3], catalysis [4,5], radio-frequency range [6], hyperthermia [7], gas sensing [8], humidity sensing [9] etc.The reduction of the size from bulk to nanoscale optimizes the material properties regarding certain technological purposes in many of these applications as new phenomena arise due to enhancement of surface effect in the nanoscale regime [10].The study of the properties of magnetic nanoparticles is not only important from basic but also form technological point of view [11,12].In addition, the transition from single component to composite systems in the nanoscale regime offers great possibilities for tailoring the performance of the resultant composite [13].The magnetic nanocomposites have attracted much attention in different fields due to their synergistic effect and magnetic exchange interactions.These nanocomposites opened up a new opportunity to develop the excellent multifunctional materials with their excellent magnetic properties [14].strongly influenced by the synthesis conditions.For example: (i) by changing the precursor or solvent in a similar procedure, (ii) by changing the synthesis method using the same precursor.Other parameters like sintering temperature, sintering time etc. also play the crucial role in controlling the microstructure of the NiFe 2 O 4 .The evolution of microstructure of NiFe 2 O 4 under different synthesis conditions are discussed in the followings.

Effect of Synthesis Method
Zhao et al. [19] synthesized NiFe 2 O 4 /NiO nanocomposite from a nickel iron layered double hydroxide (NiFe-LDH) precursor and by the chemical co-precipitation method.These authors have seen that the calcination of NiFe-LDH precursor at 500 0 C led to the development of NiFe 2 O 4 is ~ 6 nm in the NiFe 2 O 4 /NiO nanocomposite.These authors also reported the formation of 20 nm size crystallite of NiFe 2 O 4 by the calcination of the precursor synthesized by the chemical co-precipitation method at 500 °C.The crystallite size of the NiFe 2 O 4 obtained by co-precipitation method is about three times that of the material prepared using the LDH precursor method.Ma et al. [32]

Effect of Initial Precursor
It has been seen that the changing the initial precursor for the synthesis in a particular synthesis procedure influences the microstructure like crystallite size of the material.Even with the same initial precursor with a slightly modification in the synthesis condition also affect the crystallite size of the material.Tian et al. [16] and He et al. [18] have synthesized NiFe 2 O 4 /NiO nanocomposite by co-precipitation method using different initial precursor and these authors have obtained different crystallite size of NiFe 2 O 4 (Table 1).Gong et al. [21] have used same initial precursor and adopted same synthesis method as used by Tian et al. [16] but due to slight modification in both their synthesis condition led to the formation of different size NiFe 2 O 4 (Table 1).[17] and subjected the Fe doped NiO product to high temperature sintering [22].These authors have studied the effect of sintering temperature and time on the microstructural property of the material [23].The evolution of crystallite size of NiO and NiFe 2 O 4 nanoparticles with sintering temperature is presented in Table 2.As indicated from the table (2) that crystallites size of NiFe (1)

Effect of Calcination/Sintering Temperature and Time
The size of the crystallites of NiFe 2 O 4 sintered at 600 0 C also shown to increase from 3 nm to 8 nm with increasing sintering time from 1 to 3 hours (Table 2).

Effect of Molar Ratio
He et al. [18] synthesized Ni 1 -x Fe x O (x = 0.15, 0.2, 0.3 and 0.5) bulk samples obtained using co-precipitation method  [21] have synthesized (100-x)NiO/(x)NiFe 2 O 4 nanocomposites with x= 0, 2.5, 5, 8.3, 12.5, 25 were prepared by a chemical co-precipitation method followed by sintering at 600 0 C for 3h.They have reported that all the samples show an average particle sizes of 12 nm with the formation of NiFe 2 O 4 / NiO nanocomposite for x>2.5.

Evolution of Magnetic Properties of NiFe 2 O 4 present in NiFe 2 O 4 /NiO
The occurrence of different types of magnetic ordering of NiFe 2 O 4 present in NiFe 2 O 4 /NiO is discussed in the followings.

Ferromagnetic property
He et al. [18] obtained NiFe 2 O 4 /NiO nanocomposite by annealing Ni 1 -x Fe x O (x = 0.15, 0.2, 0.3 and 0.5) synthesized by co-precipitation method.Their temperature dependent of field cooled magnetization measured at a field of 1 kOe indicated the occurrence of ferromagnetic type magnetization in all samples.Their study further indicated that the existence of the effective exchange coupling interface between NiO and NiFe 2 O 4 .In another work, Zhang et al. [34] also have reported the occurrence of ferromagnetic behaviour in NiFe 2 O 4 /NiO nanocomposite synthesized by using NiFe-LDH precursor.

Superparamagnetic property
Zhao et al. [19] reported the the occurrence of superparamagnetism NiFe 2 O 4 /NiO nanocomposite synthesized by using NiFe-LDH precursor and co-precipitation method.Their zero field cool (ZFC) and field cool (FC) magnetization of the samples measured at an applied field of 100 Oe indicated the sample prepared using NiFe-LDH precursor having 6 nm size NiFe 2 O 4 nanoparticles show the blocking temperature (T B ) of 380 K whereas the T B is ~280 K for the sample prepared using co-precipitation method having 20 nm size NiFe 2 O 4 nanoparticles.Their study indicated: (i) the occurrence of higher value of T B than the single phase Ni-Fe 2 O 4 nanoparticles with size < 10 nm [35,36], (ii) the coupling of NiO and NiFe 2 O 4 could be a source of enhanced magnetization stability as a result of which T B increased.

Spin glass Property
Tian et al. [16,22] studied the effect of annealing on magnetic behaviour of NiFe 2 O 4 /NiO nanocomposite.Their magnetization characterization indicated that the saturation magnetization of all the samples increase with increasing annealing temperature.The coercivity on the other hand follows a nonmonotonic size dependencies and show the maximum value for the sample with 15 nm size NiFe 2 O 4 nanoparticles.These authors pointed out that the nonmonotonic behaviour of coercivity could be due to the variation of total anisotropy arise from the contribution of NiFe 2 O 4 cores and the interfacial exchange coupling energy [37,38].Their ZFC and FC magnetization indicated that the ZFC and FC magnetization curves diverge below certain temperature called irreversibility temperature.The divergences become smaller with increasing the size of NiFe 2 O 4 nanoparticles and both the curve fully coincides for sample with size of NiFe 2 O 4 nanoparticles is above 32 nm.These authors also have seen that the peak of ZFC magnetization curves shifts to lower temperatures, correlated with the reduction of magnetic disorder at interfaces [22].
In order to understand the low temperature magnetic behaviour of the samples, these authors [22], undertaken temperature dependent ZFC and FC magnetization at various magnetic fieldfor sample with 8 nm size NiFe 2 O 4 nanoparticles.Their study indicated that the occurrence of spinglass phase due to the existence of intermediate structural disordered layers at NiFe 2 O 4 /Ni 1-x Fe x O interfaces [16,22].

Exchange bias (EB) effect in NiFe 2 O 4 /NiO
In 1956, the phenomenon of exchange bias (EB) was discovered by Meiklejohn and Bean [39] in ferromagnetic (FM) Co particles coated antiferromagnetic (AFM) CoO which referred to the shift of a hysteresis loop along the magnetic field axis.In order to generate EB effect, there must be the co-existence of two phases i.e. one with net magnetic moment whose magnetic moment can be reversed and other phase which can exert a torque action on the spins of the former during the process of magnetization reversal [22,28,40].Since its discovery, EB effect has also been studied in other systems like ferrimagnetic (FiM) coupled with other (AFM or FM) [41,42] or spinglass (SG) coupled with other (FM, AFM orFiM) [28,43,44].In recent years, EB in nanoparticle systems has attracted much attention because of the unique properties of nanoparticles and their potential technological applications [45].In contrast to bilayer systems, the inclusion of FM cluster in AFM matrix is interesting from fundamental point of view [22].
The phenomenon of EB in the system is quantified by calculating EB field, H EB as and the coercivity, H C as

Evolution of Microstructure and Exchange Bias Characteristics of NiFe 2 O 4 / NiO nanocomposite
Copyright: © 2016 Mallick (3) where H are the left and right coercive field, respectively [46,47].The vertical magnetization shift ( Shift

M
) is also calculated by taking the difference of saturation magnetization between the measured hysteresis loop and the loopcentered in the M axis [22].
In this section, we discuss the EB effect of NiFe 2 O 4 nanoparticles embedded in NiO matrix.There are several effects like field cycle, molar ratio, size, temperature etc. which affects the EB in the system are discussed in the followings.

Effect of Cooling Field
Tian et al. studied the effect of cooling field on the EB characteristics of NiFe 2 O 4 nanoparticles embedded in NiO matrix [16].Their study indicated that the H EB increases with increasing cooling field from 0 to 12 kOe and the same saturates thereafter.These authors argued that only a part of spins at the Ferri/AFM interface are pinned along the cooling field direction for smaller fields and more spins are pinned along the field direction which causes the enhancement of exchange interaction with increasing cooling field.As a result of which H EB increases with increasing cooling field.For higher cooling field (>12 kOe), the pinned spins along the field saturates which resulted into the saturation of H EB .

Effect of Field Cycle
Tian et al. [17] studied the effect of field cycle on the EB characteristics of NiFe 2 O 4 nanoparticles embedded in NiO matrix.Their study indicated that both H EB and Shift M decrease with the increasing field cycling [17].

Effect of Cycle Dependent Cooling Field
In another work, Tian et al. studied the effect of cycle dependent cooling field on the EB characteristics of NiFe 2 O 4 nanoparticles embedded in NiO matrix [20].Their study indicated that the H EB increases with increasing cooling field for a particular cycle and the relative change in H EB i.e.H EB decreased by ~ 71% from its initial value for cooling field at 0Oe whereas the same decreased by 54% for cooling field at 40 kOewith increasing loop cycle to n = 9 [20].These authors argued that decrease of H EB with increasing loop cycle reflects the configurational rearrangements of the spin structure and can be expressed by the following discretized Landau-Khalatnikov equation [48,49]: H and γ are fitting parameters which describe H EB vs. n in the limit of infinite loops and the characteristic decay rate of the training behavior, respectively.The monotonic decrease of γ with increasing cooling filed in case of Tian et al. [20] also confirm the reduction of training effect for high cooling field.The authors assumed the coexistence of the two possible mechanism on training effect [50] to their case i.e. (i) FM magnetization is in a multi-domain state for H EB = 0 Oe, which reverses via a symmetric mechanism, and can lead to the suppression of training effect, (ii) higher field cooling results in a single-domain state, asymmetric reversal process via domain wall and nucleation process take place which led to larger training effect.

Effect of Size Dependent Field Cycle
In order to see the size dependent field cycle effect on EB characteristics, Tian et al. [23] [51,52].The reduction in H EB could be due to the weakening of interfacial exchange coupling energy due to the depinning of frozen AFM spins and the loss of interfacial coupling energy between FM/ AFM system after each field cycle.Further, the slower change in H EB for larger particle size could be due to enhancement of the interfacial coupling stiffness for larger AFM particles which leads to relatively smaller interfacial energy losses after each field cycle [23].

Effect of Molar Ratio
Effect of NiFe 2 O 4 ratio: Gong et al. [21] synthesized (100-x) NiO/(x)NiFe 2 O 4 nanocomposites by varying x(x= 0, 2.5, 5, 8.3, 12.5, 25) by a chemical coprecipitation method.These authors pointed out that NiFe 2 O 4 has not been detected by XRD for the composite sample with x=2.5.For x concentration > 2.5, both antiferromagnetic NiO and ferrimagnetic NiFe 2 O 4 coexist in the nanocomposites.These authors calculated the average particle size as 12 nm for all the samples from XRD line width using Scherrer formula [53] which agree with their TEM observation.Their study indicated that the H EB for pure NiO (i.e. with x=0) is 1.1 kOe which increases with increasing x concentration i.e. concentration of NiFe 2 O 4 up to x = 2.5 and the same decreases with further increasing x concentration.The observation of H EB for pure NiO is attributed to the exchange coupling between the spins of the AFM core and the uncompensated surface moments of NiO nanoparticles [54][55][56].The large loop shift and coercivity observed in AFM nanoparticles is a consequence of the transition from two-sublattice to multisublattice of the AFM NiO nanoparticles [44,57].It has been reported that theH EB and C H decrease with increasing temperature in AFM/FM or AFM/FI systems because the excess thermal energy decreases the exchange coupling between AFM/FM and AFM/FI [58].The strong interfacial exchange coupling between NiO and NiFe 2 O 4 is the cause for the increase of H EB with increasing NiFe 2 O 4 concentration up to x = 2.5 [16].Hence, the authors suggested that nanocomposite with x=0.25 is the percolation threshold of the nanocomposites which further confirms the observation of He et al. [18] that the presence of more NiFe 2 O 4 phase in NiO or the formation of isolated NiFe 2 O 4 in the composite for x>2.5 reduces the interface coupling between NiO and NiFe 2 O 4 as a result of which H EB decreases in the samples.Similar type of
Effect of Fe concentration in Ni 1-x Fe x O: He et al. [18] synthesized Ni 1-x Fe x O with different Fe concentration by coprecipitation method and finally annealed the samples at 650 0 C for 12 hours.In order to study the effect of Fe concentration of the EB characteristics of the samples, the authors calculated H EB as a function of temperature for the samples with different Fe concentrations from M-H curve of the samples taken by cooling the sample at an applied field of 1kOe from 300K.These authors reported that the value of H EB decreases with increasing the Fe content x at the same temperature.These authors argued that the enhancement of ferromagnetic type interactions among NiFe 2 O 4 particles with increasing Fe content could be the cause of suppression of H EB .

Effect of Sintering Temperature
In order to understand the suppression of H EB with increasing Fe content as discussed above, He et al. [18] also undertaken the effect of sintering temperature which resulted into larger size NiFe 2 O 4 particles in NiFe 2 O 4 /NiO with increasing sintering temperature.Their study indicated that the H EB of the sample decreases with increasing sintering temperature and the H EB nearly vanishes for the sample sintered at 850 0 C for 12 hours.With increasing sintering temperature, the size of particles become larger and the NiFe 2 O 4 particles may form ferromagnetic type clusters in NiO matrix as result of which effective coupling among the NiFe 2 O 4 and NiO decreases.Similar types of observations have also been reported [19,33,[60][61][62].Hence it is clear from the study that relatively larger size NiFe 2 O 4 particles or more ferromagnetic NiFe 2 O 4 phase in the system is not the good choice for the occurrence of EB in this system.
In another work, Tian et al. [22] sintered their sample at various temperatures to investigate the NiFe 2 O 4 size dependent H EB of NiFe 2 O 4 /NiO.The authors have seen that the size of NiFe 2 O 4 increasedfrom 0 to 55 nm with increasing sintering temperature from 550 to 1000 0 C (Figure 1).The authors also sintered the same sample at 600 0  for the samples with size of NiFe 2 O 4 particles ≤ 8 nm indicated the correlation among them.The existence of linear dependency hence EB could be due to the coupling between the ferrimagnetic phase and the spin glass like phase formed due to structural disorder at the interface.The authors also pointed out that the open and unsaturation of field cooled loop measured at 10 K for sample with 8 nm size NiFe 2 O 4 [16] further confirms the existence of spin glass like phase [60,63].The linear relationship between H EB and Shift M deviates for the sample with size of NiFe 2 O 4 particles is ~ 12 nm and the same show two linear dependency for the sample with size of NiFe 2 O 4 particles is ~ 22 nm.These authors [22] stated that the deviation of linear dependency could be due to coexistence of two interfacial exchange coupling: one is the exchange coupling between ferromagnetic NiFe 2 O 4 core and the spinglass like layer related to the structural disorder region in Ni 1-x Fe x O nanoparticles and the other arise from the coupling of the NiFe 2 O 4 phase and antiferromagnetic NiO phases.

Conclusion
Effect of various parameters on the evolution of microstructure and exchange bias characteristics of NiFe 2 O 4 / NiO nanocomposite is reviewed.The microstructure of NiFe 2 O 4 present in NiFe 2 O 4 /NiO nanocomposite is strongly influenced by the synthesis methods as well as the synthesis conditions.The effects of various parameters like field cycle, molar ratio, size, temperature etc. have been shown to affect the EB characteristics in the system.It is suggested through this study that relatively larger size NiFe 2 O 4 particles or more ferromagnetic NiFe 2 O 4 phase is not the good choice for the occurrence of EB in this system.

Evolution of Microstructure and Exchange Bias Characteristics of NiFe 2 O 4 / NiO nanocomposite
Copyright: © 2016 Mallick synthesized NiFe 2 O 4 / NiO composite by thermal oxidation of electroplated Ni-Fe alloy.These authors have reported the formation of homogeneously distributed dense and flat structure of NiFe 2 O 4 in the matrix of NiO.Pebley et al. [33] have synthesized nanostructured NiFe 2 O 4 / NiO films using high-pressure microplasma-based deposition by varying Ni molar compositions (x Ni ).These authors reported that the presence of both NiFe 2 O 4 and NiO in the samples prepared with x Ni = 0.49−0.74.From the elemental mapping of Fe and Ni, these authors suggested that 50−100 nm size NiFe 2 O 4 crystals are dispersed in a matrix of larger ∼100−200 nm NiO grains.It is clear from the above comparison that the size of NiFe 2 O 4 is being influenced by changing the synthesis method.
2 O 4 increases 0 to 55 nm with increasing sintering temperature from 550 to 1000 0 C (Figure.1).Fitting the variation of crystallite size ( D ) of NiFe 2 O 4 with sintering temperature (T ) to linear fit (Figure. 1) indicated that the size of NiFe 2 O 4 increases linearly with the sintering temperature following the relation:

Figure 1 :
Figure 1:Variation of crystallite size of NiFe 2 O 4 in Ni 1−x Fe x O (x = 0.09) sintered at different temperaturefor 3 hours (Data taken from the ref.[23]).Line represents the linear fit to the data points.
Zhao et al. [19] also synthesized NiFe 2 O 4 /NiO nanocomposite using NiFe-LDH precursor with Ni/Fe molar ratio ~ 3.These authors calcined the NiFe-LDH precursor at 500 0 C and seen that the crystallite size of NiFe 2 O 4 is ~ 6 nm.In an another work, Zhang et al. [34] also synthesized NiFe 2 O 4 /NiO nanocomposite NiFe-LDH precursor and obtained 44.73 nm size crystallites of NiFe 2 O 4 in NiFe 2 O 4 /NiO nanocomposite by calcined the NiFe-LDH precursor (with a Ni/Fe molar ratio ~ 3) at 900 0 C for 4 hours.Tian et al. synthesised Ni 1−x Fe x O (x = 0.09) by co-precipitation method

Table 1 :
NiFe 2 O 4 nanoparticles with different size present in NiFe 2 O 4 /

of Microstructure and Exchange Bias Characteristics of NiFe 2 O 4 / NiO nanocomposite Copyright: © 2016 Mallick and
annealed at 650 0 C for 12h in order to get NiFe 2 O 4 /NiO nanocomposite.These authors have reported the formation of 20 nm size NiFe 2 O 4 nanoparticles in NiFe 2 O 4 /NiO nanocomposite.Gong et al.
studied the variation of normalized H EB and normalized coercivity enhancement ( NiFe 2 O 4 nanoparticles embedded in NiO matrix.Their result indicated that both the parameter decrease with increasing size of NiFe 2 O 4 with a minima for the sample with 22 nm size NiFe 2 O 4 nanoparticles.The suppression of normalized H EB and

/Fe molar ratios:
Zhao et al. [19] synthesized different NiFe 2 O 4 /NiO nanocomposites by calcining NiFe-LDH-LDH precursors with different Ni II /Fe III molar ratios at 500 0 C.These authors have seen that the value of H EB increased from 53.6 Oe to 330.4 Oe with increasing Ni II /Fe III molar ratios from 2:1 to 4:1.This study further confirms the suppression of H EB with increasing size or fraction of NiFe 2 O 4 phase in NiFe 2 O 4 /NiO as was seen by He et al. [18].In another work, Pebley et al., [33] synthesized NiFe 2 O 4 / NiO nanocomposites by varying Ni molar fraction and studied the EB characteristics of the samples with Ni molar fraction.These authors reported that both C H and H EB under FC condition increase with increasing Ni molar fraction in the sample.This is in accordance with the previous observations that the interfacial density between the NiFe 2 O 4 and NiO phases increases with increasing Ni content.
C for 1 hour to get the sample with ~ 3 nm size NiFe 2 O 4 particles.Their study of temperature dependent H EB and Shift M indicated that the EB field decreases with increasing temperature and vanishes above 250 K for all samples (i.e.irrespective of crystallite size of NiFe 2 O 4 ) and the same decreases rapidly for the sample with smaller size ( ≤ 12 nm) NiFe 2 O 4 .The EB for smaller size NiFe 2 O 4 particles.However, no similar variation in both the parameters are shown for samples with size NiFe 2 O 4 particles > 22 nm.In order to understand this behaviour, the authors plotted H EB vs. NiFe 2 O 4 particles.Their results indicated that the existence of linear relationship between H EB and Shift M