Introducing the magnetic properties in Fe doped ZnO nanoparticles for spintronics application

Proper correlation among the microstructural, optical and magnetic responses of Fe doped ZnO nanoparticles have been established in this work. All the Fe doped ZnO nanoparticles (Zn 1-x Fe x O: x = 0.00, 0.05, 0.10 and 0.15) were prepared using chemical co-precipitation route. Average crystallites size of 18 nm to 28 nm was estimated using Scherrer’s formula. Compressive microstrain was detected in pristine ZnO samples, which moved toward tensile regime upon introducing Fe ions of different weight percentages. Mean crystallites size obtained from Scherrer’s formula was found in almost exact match with the particle size estimated from HRTEM images. Nearly spherical ZnO nanoparticles were seen in HRTEM images and negligible agglomeration among particles was also observed. Direct optical band gaps were found in the range of 2.89 eV to 3.24 eV as estimated from Tauc plots. A decent ferromagnetic signature in non-magnetic ZnO nanoparticles was also introduced at room temperature with the doping of Fe ions.


Introduction
In the last decade, crucial efforts have been dedicated to synthesize and characterize semiconductor materials doped with transition metals. DMS materials show semiconducting as well as ferromagnetic properties at room temperature [1][2][3][4]. Transition metal (TM) doped semiconductors device used in the field of quantum computation, storage and communication devices and logic elements [5][6][7][8]. These materials can be termed as dilute magnetic semiconductor since a small proportion of transition metal can give drift to room temperature ferromagnetism. Theoretically, the presence of stable ferromagnetism is predicted in wide band gap semiconducting material [8][9][10]. The presence of room temperature ferromagnetism in ZnO attracted many researchers' interests in this area. Most of the results are questionable in nature and the magnetic ordering in wide band gap DMS are associated with defects and impurity phases. In a few cases, the absence of magnetic ordering has also been highlighted [11,12]. Synthesis methods play a major role in defining the magnetic ordering. Among these dilute magnetic semiconductor materials ZnO was observed as a potential candidate. ZnO is chemically and thermally stable n-type semiconductor. TM doped ZnO is a promising candidate due to its wide band gap (3.3 eV), large exciton binding energy around 60 meV and high carrier density [12][13][14].
In past few decades, researchers have tried to introduce magnetic signature in various non-magnetic metal oxide semiconductors [15][16][17]. Although partial doping of 3d transition metal ions as well as 4f rare earth ions in non-magnetic nanosized ZnO showed a high possibility of observing magnetic properties at room temperature [12,[16][17][18]. In the development of new generation non-toxic advanced spintronics materials, transition metal (TM) ions doped ZnO has emerged as a promising candidate from the class of oxide based diluted magnetic semiconductors. Zinc oxide has been become the first choice of researchers due to its good piezoelectric effect, biomedical compatibilities and room temperature ferromagnetism [12]. The past decade records the attention of researchers from different fields on this material due its vast technical applications, such as in the field of chemical sensors, UVdetectors, semiconductor lasers of short wavelength, non-linear varistors and in semiconducting-based MEMS/NEMS technology [12,14]. Further enhancement of electrical conductivity for pristine ZnO can be done by doping with selected elements such as Ga [19,20], Mn [17] and Al [21].
In this article, we have presented the synthesis and physical characterizations of ultrafine homogeneous Fe ions doped ZnO nanoparticles. Chemical co-precipitation method has been utilized to prepare all the samples. A decent magnetic signature in non-magnetic ZnO nanoparticles was also recorded at room temperature due to doping of Fe ions. A proper correlation among the structural, optical and magnetic responses of Fe doped ZnO nanoparticles has been established in this work.

Synthesis of nanoparticles
Fe doped ZnO nanoparticles with a generic formula Zn 1-x Fe x O (x = 0.00, 0.05, 0.10 and 0.15) were fabricated using standard chemical co-precipitation method [4,22]. Raw chemicals ZnCl 2 and FeCl 2 were used for synthesizing nanoparticles. All the reagents were purchased from Merck having a purity level of 99.99% for ZnCl 2 and 98% for FeCl 2 . All the chemicals were used without any purification.
At first, all the glassware was washed with nitric acid, distilled water and acetone in that order to ensure that no trace of impurity would contaminate the sample via the glassware. ZnCl 2 and FeCl 2 were dissolved in 200 ml of distilled water in stoichiometric ratio. 500 mg PVP was added into the solution which acts as a binding agent. The solution was kept on a magnetic stirrer and rotated with a rate of 700 rpm for homogeneous mixing. Precipitating agent NaOH solution was mixed in the form of droplets into the solution under continuous stirring. NaOH solution was added continuously with the same rate until the pH of the solution reached 12 to ensure no elements remain unreacted. After that, the precipitate was washed several times using distilled water and ethanol to reduce pH until 7.
The precipitate was dried in an open atmosphere and then ground into the fine powder [23]. All the synthesized samples were indexed as Fe-00, Fe-05, Fe-10 and Fe-15. The prepared samples were kept safely in separate containers and were used for all the characterizations.

Characterizations
Confirmation regarding phase-purity and generation of hexagonal wurtzite structure was verified by diffraction profiles as obtained from Rigaku Ultima IV x-ray diffractometer with copper Kα line Absorption spectra of all samples were recorded between 200 nm to 800 nm at 300 K using Thermo-Scientific Evolution. Magnetic characterization of all Fe doped ZnO nanoparticles were carried out with the help of vibrating sample magnetometer (Quantum Design, VSM) at room temperature.  [2,4]. Rietveld analysis of all the diffraction patterns was performed using the general structure analysis system (GSAS) along with EXPGUI interface. The obtained values of cell parameters and refinement parameters are displayed in table 1. All the diffracted peaks were shaped using the pseudo-Voigt function (superposition of Lorentzian and Gaussian function). Estimated values of reliability factors (W Rp and R p ) below 10% along with decent goodness of fit (χ 2 ) verified a good agreement between obtained and standard experimental results [24]. The observed broadening in diffractograms ensured that the prepared samples were in the nano range. Ratio of lattice constants (c/a) was found close to 1.6 for all the samples which also verified the formation of hexagonal wurtzite structure as noticed in table 1.

Scherrer's formula
The average crystallite size was calculated using Scherrer equation using full width at half maximum (FWHM) as follows [4] 0.94 where λ is wavelength (1.5406 Ȧ) of Cu Kα line, D is the average particle size and β d represents the FWHM value of (101) where is full-width half maxima, represents the geometrical factor (0.9 for spherical shaped particles), signifies wavelength of Cu K α line (= 1.5406 Å), D corresponds to the mean crystallite size, addresses microstrain and θ is the Bragg's angle respectively. Rearranging the equation 3, we get the known form as shown below [25] cos = 0.9 cos + 4 sin Separation of size and microstrain effects from total line width of diffracted peaks was done by plotting cos as a function of 4 sin which is familiar as the Williamson-Hall (W-H) graph as described in equation (4). The slope of this curve represents microstrain present inside the crystal and crystallite size was calculated from intercept on cos axis [24,25]. W-H plot of all the samples is presented in figure 2. The average crystallite size was found in the range of 14 nm -35 nm and negative value of microstrain for pristine ZnO sample also showed its compressive nature [24]. The estimated microstrain was gradually increased and becomes tensile in nature with increasing Fe content as seen from

Halder-Wagner Method
It is well known that the x-ray diffraction peaks are neither pure Gaussian or Lorentzian but rather a convolution of both as the peak region of the pattern matches well with Gaussian function but the tail region fails in matching and similar situation is observed in Lorentzian function in which matching is good at the tail region but not in the peak region. Halder-Wagner method overcomes this problem as it assumes peak broadening to be a symmetric Voigt function which is convolution of Gaussian and lorentzian function [27].
The plot between 2 tan         (y-axis) and tan sin    (x-axis) is a straight line whose slope provides average particle size and intercept provides the value of microstrain [28].

HRTEM image analysis
HRTEM images of Fe-00 and Fe-10 samples are illustrated in figure 4(a) and 4(b) respectively. All the nanoparticles were appeared to be nearly spherical in shape. Excellent homogeneity in both size and shape was achieved as verified by HRTEM micrographs [2,8]. An average particle size of 17±1 nm was obtained for the pristine ZnO (Fe-00) sample, which is a close match with XRD results. A considerable agglomeration among the synthesized nanoparticles was observed which may be attributed to the Van der Waal interactions [25].

UV-vis spectra studies
Room temperature absorption spectra of all Fe doped ZnO nanoparticles were collected within the range of 200 nm to 800 nm using UV-Vis spectroscopy. Bulk ZnO consists of direct optical band gap at higher energy (≈ 3.3 eV) [29]. The direct band gap of prepared samples can be obtained using Tauc relationship as follows [24] ( ) ℎ = ℎ − where is an absorption coefficient, represents an arbitrary constant, corresponds to the optical band gap and denotes an index which attains values of ½ for direct bandgap and 2 for indirect band gap semiconductor. The value of is calculated using this general formula [23,24] = and = log (8) where 'A' is absorbance and 't' indicates the thickness of materials. The absorption coefficient takes the form [24] = 2.303 (9) A graph between ( ℎ ) 2 versus ℎ was plotted to obtain the direct optical band gap of all samples which is well known as Tauc plot [23].  Figure 6 depicts the magnetic hysteresis (M-H) curves registered at room temperature of all the samples. Due to the absence of unpaired 'd' electrons, the pristine ZnO in bulk form exhibits diamagnetic behavior [30]. In nanoscale, the existence of several defect states together with a certain correlation among them induces weak ferromagnetism or paramagnetism in pure ZnO sample [12].

Magnetic studies
Defects driven weak ferromagnetism or paramagnetism in pure ZnO nanoparticles have been reported in many articles [31][32][33]. week ferromagnetic nature at room temperature as verified by hysteresis loops.

Conclusion
In brief, we have successfully fabricated diluted magnetic semiconductor Zn

Author contribution statement
All the authors contributed equally in this work.

Declaration of interests
Authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.