Weak ferromagnetism and temperature dependent ...

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Weak ferromagnetism and temperature dependent dielectric properties of Zn 0.9 Ni 0.1 O diluted magnetic semiconductor Article in Materials Research Bulletin · March 2015 Impact Factor: 2.29 · DOI: 10.1016/j.materresbull.2014.11.039

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Materials Research Bulletin 63 (2015) 32–40

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Weak ferromagnetism and temperature dependent dielectric properties of Zn0.9Ni0.1O diluted magnetic semiconductor Raju Ahmed a,b , A.S.M. Moslehuddin b , Zahid Hasan Mahmood b , A.K.M. Akther Hossain c, * a b c

Department of Electrical and Electronic Engineering, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh Department of Applied Physics, Electronics and Communication Engineering, University of Dhaka, Dhaka 1000, Bangladesh Department of Physics, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 May 2014 Received in revised form 16 November 2014 Accepted 18 November 2014 Available online 20 November 2014

In this study the room temperature ferromagnetic behaviour and dielectric properties of ZnO based diluted magnetic semiconductor (DMS) have been investigated using nominal chemical composition Zn0.9Ni0.1O. The X-ray diffraction analysis confirmed formation of single phase hexagonal wurtzite structure. An increase in grain size with increasing sintering temperature was observed from scanning electron microscopy. Field dependent DC magnetization values indicated dominant paramagnetic ordering along with a slight ferromagnetic behaviour at room temperature. Frequency dependent complex initial permeability showed some positive values around 12 at room temperature. In dielectric measurement, an increasing trend of complex permittivity, loss tangent and ac conductivity with increasing temperature were observed. The temperature dependent dispersion curves of dielectric properties revealed clear relaxation at higher temperature. Frequency dependent ac conductivity was found to increase with frequency whereas complex permittivity and loss tangent showed an opposite trend. ã 2014 Published by Elsevier Ltd.

Keywords: B. Magnetic properties C. X-ray diffraction D. Dielectric properties

1. Introduction Diluted magnetic semiconductor (DMS) materials have attracted most of the recent research interests because of their possible application in spintronics [1] where the spin of electron has also been deployed along with charge. Typically, ferromagnetism is induced in DMS by incorporation of transition metal (TM) ions into substitutional sites of semiconductors keeping the concentration of the magnetic substituent very low (1–10%) [2]. A number of studies have been carried out on ZnO based DMS systems after the theoretical prediction by Dietl [3], that ZnO would show ferromagnetism at a temperature higher than 300 K. In TM doped ZnO based DMSs, due to substitution of TM ions for Zn2+ the localized d and f electrons of TM atoms couple with the extended electrons in the semiconducting band [4] and this coupling results in a ferromagnetic behaviour. Apart from the effect of TM ions, Hsu et al. [5] reported effect of oxygen vacancies as a source of ferromagnetism because of the notable changes it causes in semiconducting band structure of ZnO. However, the magnetic properties of DMSs are controversial [6]. For Ni doped

* Corresponding author. Tel.: +880 29665650x7582; fax: +880 28613026. E-mail address: [email protected] (A.K.M. A. Hossain). http://dx.doi.org/10.1016/j.materresbull.2014.11.039 0025-5408/ ã 2014 Published by Elsevier Ltd.

ZnO, conflicting observations were reported by different groups [7]. Therefore, the controversy can be resolved by careful structural and magnetic investigations with carefully fabricated bulk samples. After the theoretical prediction of ferromagnetism in Ni doped ZnO above room temperature by Sato and Katayama [6], mainly structural, optical and magnetic properties of Ni doped ZnO have been extensively studied by many researchers [8–10]. On the other hand, the electrical transport properties like ac conductivity and dielectric constant have been studied by very few researchers for this material system [11]. The dielectric properties of ZnO are dependent upon several factors namely, chemical composition, method of preparation, grain size etc. Earlier, Ghosh et al. [12] reported the effect of Co doping on the static dielectric constant of ZnO. Ansari et al. [13] studied the temperature and frequency dependence of ac conductivity and dielectric behaviour of Co doped ZnO and they reported an increase in both of the properties with increasing temperature. One of the most relevant findings was claimed by Saleem et al. [14], who reported a decreasing trend in resistivity with increasing temperature for Ni doped ZnO. As ZnO is of great interest as a suitable material for high temperature, high frequency electronic devices either as the active material or as suitable substrate for the epitaxial growth, DMS composition Zn0.9Ni0.1O, can be a good choice for all those applications in spintronic devices. Thus, the main aim of the present work is to

R. Ahmed et al. / Materials Research Bulletin 63 (2015) 32–40

investigate the origin of room temperature ferromagnetic behaviour of DMS composition Zn0.9Ni0.1O in bulk form and the effect of temperature and frequency on its dielectric properties. 2. Experimental details The polycrystalline sample of Zn0.9Ni0.1O was synthesized by the standard solid-state reaction method. Appropriate amounts of ZnO and NiO powders were mixed and ground. Then the powder was calcined at 550 C for 5 h in air, and thereafter they were furnace cooled. After cooling, the resulting material was reground. The fine powder was pressed into different shapes (disc and toroid) and finally the samples were sintered at three different temperatures (TS) 900, 1000 and 1100 C in air for 12 h. The Zn0.9Ni0.1O samples, prepared as explained above, were first characterized by X-ray diffraction (XRD) (Phillips PW3040X’Pert Pro with wavelength of 1.54 Å in the wide angle region from 25 to 75 on 2u scale). The surface morphologies of samples sintered at various temperatures were studied using a scanning electron microscope (SEM) (JEOL JED-2300). Complex initial permeability of toroidal shaped samples was measured as a function of frequency using a precision impedance analyzer (Wayne Kerr, 6500B) both in room temperature (301 K) and in liquid nitrogen environment (78 K). Measurements of field dependent magnetization at room temperature were carried out using a vibration sample magnetometer (Micro Sense, EV9). Temperature dependence of dielectric properties were measured in frequency range of 20 Hz–1 MHz and temperature range from 28 C (room temperature) to 275 C using a computer controlled combined set up of the impedance analyzer and a furnace (Eurotherm C-1000).

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partial substitution of Zn by Ni, the diffraction patterns for all sintering temperatures showed almost the same peaks, which indicates that the wurtzite structure was not disturbed by the substitutions. The change in ZnO lattice parameters due to Ni doping and different sintering temperatures are shown in Table 1. The small difference between the ionic radii of the tetrahedrally coordinated Zn2+ ion (0.60 Å), and Ni2+ ion (0.55 Å) [16] allows incorporation of Ni2+ into ZnO with only slight change in the lattice parameters a and c [17]. It is interesting to note that with increasing sintering temperature, the value of a decrease whereas the value of c increases. However, the c/a values were found to increase gradually and approach toward the ideal value for hexagonal close packed group P63mc. This behaviour may be due to the random distribution of Ni ions or local defects affected by changing the sintering temperature. This lattice distortion seems to be playing a crucial role in controlling the structural properties and the diffusion behaviour of randomly substituted Ni ions thereby influencing the distribution of local defects such as oxygen vacancies [18]. 3.1.2. SEM analysis The scanning electron micrographs (SEM) of Zn0.9Ni0.1O for various sintering temperatures are shown in Fig. 2(a)–(c), respectively. The samples sintered at lower temperature were found to contain larger pores and rough surface. A larger grain size and better smoothness were achieved at higher sintering temperature. The average grain sizes for various sintering temperatures were calculated using the lineal intercept method [19] and they are listed in Table 1. 3.2. Magnetic characterization

3. Results and discussion 3.1. Structural and morphological analysis 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of Zn0.9Ni0.1O sintered at various temperatures. The XRD spectra can be indexed to the peaks of ZnO wurtzite structure (JCPDS 36-1451) [15] having space group P63mc along with some extra peaks corresponding to NiO. The NiO peaks, marked as ‘*’, were found to show a decrease in intensity with increasing sintering temperature which indicates higher solubility of Ni into ZnO at higher sintering temperature. On the whole, after

Fig. 1. X-ray diffraction patterns for Zn0.9Ni0.1O sintered at (a) 900, (b) 1000 and (c) 1100 C.

3.2.1. Measurement of DC magnetization The DC magnetization (M) of Zn0.9Ni0.1O samples sintered at various temperatures were measured in A/m by employing a vibrating sample magnetometer (maximum field of 20 kOe or 1.59 106 A/m). Fig. 3 shows the M versus magnetic field (H) curves, measured at 300 K. The shapes of M–H curves indicate paramagnetic ordering with a weak ferromagnetic behaviour at lower field. As pure ZnO is diamagnetic [20], the paramagnetic ordering of M–H curve indicates some exchange interaction between nonmagnetic semiconductor and transitional metal ions. Interestingly, at lower field, the shape of the M–H curve exhibits some non-linearity for a short range of applied field which in turns indicates weak ferromagnetic behaviour. However, the ferromagnetic ordering has been saturated at very low field, within 5 104 A/m (amplified form shown in the inset of Fig. 3) and thereby no definite coercivity (Hc) and remanence (Mr) was observed. Many researchers reported clear room temperature ferromagnetism for Ni (5%) – doped ZnO [18,21–23], where the ferromagnetism were found to be extremely dependent on crystallite size and fabrication process. In most of those studies, room temperature ferromagnetism was achieved for nano particle. Meanwhile, in our study, the fabrication process was solid state reaction technique and sintering temperatures were in the range of 1000 C which resulted in a grain size of few hundred nanometers (Table 1). Thus, same result could not be obtained. However, the small non linearity in M–H curve at lower field indicates substitution of Ni2+ in the Zn sites at a very lower extent. This slight ferromagnetic behaviour of fabricated sample could arise from a number of possible sources. Phase segregated NiO are an unlikely source of this ferromagnetism because of the very low Curie temperatures for NiO nanocrystals (e.g., TC < 5 K) [24]. Metallic nickel precipitants are also unlikely candidates because the synthesis of the DMS composition was performed under oxidizing conditions in which metallic nickel is unable to form [21]. Thus, it can be said that some

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Table 1 Variation of structural and magnetic properties of Zn0.9Ni0.1O with sintering temperatures. Ts ( C)

a (Å)

c (Å)

c/a

Average grain size (mm)

Permeability at 120 MHz

Magnetization at 1.59 106 A/m

900 1000 1100

3.2547 3.2533 3.2524

5.1952 5.1957 5.1967

1.5962 1.5970 1.5978

0.49 0.74 1.55

14 12 11

172 116 71

Fig. 2. SEM micrograph of Zn0.9Ni0.1O sintered at (a) 900, (b) 1000 and (c) 1100 C.

exchange interaction has taken place between wide band gap semiconductor ZnO and Ni2+ ions. Besides, it is interesting to note that with increasing sintering temperature, the M–H curves were found to show a lower slope and magnetization values were found to follow a decreasing trend (shown in Table 1). This might be due to the decrease in participated ferromagnetic exchange couplings between the bound magnetic polarons and the ferromagnetic exchange interactions [18]. It is worth mentioning that some of the researchers [14,25] claimed room temperature ferromagnetism for Ni doped ZnO on the basis of field dependent magnetization. However, the magnetic behaviour observed in the present sample was further explained on the basis of complex initial permeability.

Fig. 3. Field dependence of magnetization of Zn0.9Ni0.1O sintered at various temperatures.

3.2.2. Complex initial permeability The complex initial permeability measurements on the toroidshaped samples were carried out at room temperature (301 K) and at liquid nitrogen environment (78 K) in the frequency range 100 Hz–120 MHz. Measurements of initial permeability usually involve the measurements of the change in self-inductance of a coil in presence of the magnetic core. The behaviour of a self-inductance can be elaborated assuming ideal loss less air coil of inductance Lo. On insertion of a magnetic core with permeability ‘m’ the inductance will be ‘mLo’. The complex impedance (Z) of this


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Table 2 Temperature dependence of ac magnetic susceptibility for Zn0.9Ni0.1O sintered at various temperatures.

Fig. 4. The ac frequency dependence of the real part of complex initial permeability of Zn0.9Ni0.1O (measurements taken at room temperature, 301 K and at liquid nitrogen environment, 78 K).

coil [26] can be expressed as Z = R + JX = JvmLo = Jv Lo (m0 m00 ). The RF permeability can be derived from the complex impedance Z, of a coil. The core was taken as toroidal to avoid demagnetizing effects. The real part (mi0 ) and imaginary part (mi00 ) of the complex initial permeability mi were determined using the relations mi0 = Ls/Lo and mi00 = mi0 tand respectively. Where, Ls is the self-inductance of sample core, Lo = moN2S/p d is derived from geometrical relations

Measurement temperature (K)

AC susceptibility (xm) at 100 MHz Ts = 900 C Ts = 1000 C Ts = 1100 C

78 301

29 12

24 10

21 9

showing the inductance of the winding of the coil without the sample core, N is the number of turns of the coil (in our case N = 4), S is the area of cross-section and d is the mean diameter of the toroidal sample. The relative quality factor is determined from the ratio mi0 /tand .The real part of the permeability depends on the imaginary impedance, and conversely, the imaginary part of the permeability is determined by the real impedance. Furthermore, the permeability measurement is also an effective way to determine the magnetic susceptibility (xm). The magnetic susceptibility (xm) is directly related to permeability by the formula, m = 1 + xm. The real part of the permeability in Fig. 4. showed a plateau followed by a decrease, i.e., a relaxation toward a smaller value at lower frequency (around 2.5 kHz), and then remained almost same for higher frequencies. This behaviour, with this dispersions is well characterized and allows a simple interpretation: at low frequencies, the active magnetization processes are domain wall bulging (domain walls are pinned to the surface and defects) and spin rotation. As frequency increases, domain walls become unable to follow the ac excitation field and show a relaxation. The values of real permeability at highest frequency (120 MHz) for all sintering temperatures are listed in

Fig. 5. Real part of dielectric constant of Zn0.9Ni0.1O sintered at (a) 900, (b) 1000 and (c) 1100 C.

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Fig. 6. Imaginary part of dielectric constant of Zn0.9Ni0.1O sintered at (a) 900, (b) 1000 and (c) 1100 C.

Table 1. The real permeability values were found to remain same (a value around 12) for wide range of frequency and no ferromagnetic relaxation was observed. The imaginary part of complex initial permeability showed a maxima at lower frequency and decreased exponentially as frequency increased and the result is an analogy to the result obtained in case of its real part. It is interesting to note that complex initial permeability and corresponding magnetic susceptibility was found to be extremely dependent on temperature. The ac susceptibility has been calculated from complex initial permeability. The temperature dependence of ac susceptibility has been summarized in Table 2. The samples exhibited a larger value of susceptibility when they were measured at liquid nitrogen environment (78 K). This decrease in magnetic susceptibility at higher temperature can be explained on the basis of thermal vibration. At higher temperature, thermal vibrations become strong enough to overcome Zeeman interactions and initiate randomization of the magnetic moments, which in turn decreases in the magnetization [18]. Furthermore, the real part of complex initial permeability were found to be dependent on sintering temperature. The sample sintered at lower temperature exhibited a larger permeability and this result is analogous to the result obtained from VSM. It is worth mentioning that the decrease in magnetic properties i.e., permeability and susceptibility with increasing measurement temperature exhibits analogy with the temperature dependent magnetization measurements of Ni doped ZnO performed by Srinivas et al. [18], Schwartz et al. [21] and Pei et al. [22]. Likewise, the experimental results exhibited by the present sample indicate very weak ferromagnetic ordering at room temperature. However, in case of Ni doped ZnO, the origin of ferromagnetism is very complex. Although different mechanisms might be responsible,

some researchers explained the phenomenon using the bound magnetic polarons (BMP) model [18,27]. According BMP model, the localized spins of the dopant ion interact with the charge carriers such as oxygen vacancies, resulting in a magnetic polarization of the surrounding local moments. In our present investigation, due to the random substitution of Ni2+ a number of free charge carriers and oxygen vacancies might have been introduced to maintain the charge neutrality leading to the formation of bound magnetic polarons (BMP). Thus, the observed ferromagnetism can be a result of the exchange interactions between these BMPs, which are coupled with the randomly distributed neighboring Ni2+ ions. Furthermore, the decrease of specific magnetization with increasing sintering temperature may be attributed to the less number of BMPs and their overlapping behaviour by decreasing the available oxygen vacancies surrounded by Ni2+ ions. Besides, many isolated polarons may not contribute to the ferromagnetic exchange interactions [18]. However, the mechanism of exchange coupling induced by oxygen vacancies or due to inhomogeneity in dopant distributions which are responsible for the observed slight ferromagnetic behaviour in room temperature is still not clear and requires further studies. 3.3. Dielectric characterization 3.3.1. Dielectric constant The response of fabricated samples to ac fields was characterized by a complex permittivity or dielectric constant which can be represented by, er = e0 –je00 where, er is total permittivity or dielectric constant, e0 is its real part which describes the energy stored in and e0 is its imaginary part which describes the dissipated energy. Using the capacitance value of any sample, it is possible to calculate the


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Fig. 7. Dielectric loss factor of Zn0.9Ni0.1O sintered at (a) 900, (b) 1000 and (c) 1100 C.

real and imaginary dielectric constant. The real part of dielectric constant is calculated using formula e0 = Cd/eoA where, eo is the permittivity of free space which is equal to 8.85 1012 F/m, A is the area of disc shaped sample, d is the thickness and C is the value of capacitance measured in farad. The imaginary part of dielectric constant can be written as e00 = e0 tand, where, tand is the dielectric loss factor. Figs. 5 and 6 display the variations in e0 and e00 with temperature at different frequencies for all sintering temperatures. It is obvious from all these figures that dielectric constant is less affected at lower temperature. At higher temperature, all samples exhibited a dielectric relaxation, where the dielectric constants (e0 and e00 ) decreased as the frequency increased. The rate of change in dielectric constant was huge for lower frequency. For the frequency above 1 kHz, the dispersions followed a similar trend i.e., no sharp transition and the value of dielectric constants were found to be smaller than lower frequency. However, smaller values of dielectric constants at higher frequencies are illustrated in inset for convenience. The observed behaviour of increasing dielectric constant with increasing temperature can be explained on the basis of Koop’s theory based on Maxwell–Wagner’s two layer model [13,28]. According to the model, any material system with a heterogeneous structure can be imagined as the system containing high conducting grains (with e1,s1 and thickness d1) separated by highly resistive thin grain boundaries (with e2, s 2 and thickness d2) [13]. The assumptions made by Koop’s were x = d2/d1