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Modifications in room temperature ferromagnetism by dense electronic excitations in Zn0.9Mg0.1O thin films
Accepted Manuscript Modifications in room temperature ferromagnetism by dense electronic excitations in Zn0.9Mg0.1O thin films Parmod Kumar, Hitendra K. Malik, S. Gautam, K.H. Chae, K. Asokan, D. Kanjilal PII:
S0925-8388(17)31106-4
DOI:
10.1016/j.jallcom.2017.03.304
Reference:
JALCOM 41342
To appear in:
Journal of Alloys and Compounds
Received Date: 14 February 2017 Revised Date:
23 March 2017
Accepted Date: 26 March 2017
Please cite this article as: P. Kumar, H.K. Malik, S. Gautam, K.H. Chae, K. Asokan, D. Kanjilal, Modifications in room temperature ferromagnetism by dense electronic excitations in Zn0.9Mg0.1O thin films, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.304. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Modifications in Room Temperature Ferromagnetism by Dense Electronic Excitations in Zn0.9Mg0.1O Thin Films Parmod Kumar1,2*, Hitendra K. Malik1, S. Gautam3$, K. H. Chae3 and K. Asokan2, D. Kanjilal2 1
Department of Physics, Indian Institute of Technology Delhi, New Delhi - 110016, India Materials Science Division, Inter University Accelerator Centre, New Delhi - 110067, India 3 Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136791, South Korea
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Abstract
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Present study shows that dense electronic excitations caused by ion beam interaction in Zn0.9Mg0.1O films lead to significant changes in room temperature ferromagnetism. It is found that defects are playing dominant role in the modification of various physical properties including ferromagnetic behaviour. Presence of defects were investigated through photoluminescence spectroscopy along with local electronic structure study using X-ray absorption spectroscopy. This novel observation is relevant to get further insight into ferromagnetic origin and maybe applied to tune magnetic properties for spintronic applications. A strong correlation exists between the ferromagnetic properties and defects produced during high energy irradiation.
Keywords: Swift heavy ion irradiation, defects induced magnetism, photoluminescence spectroscopy and local electronic structure
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Corresponding author: Parmod Kumar *Email:[email protected] $ Presently at: Dr. SSB UICET, Panjab University, Chandigarh−160014, India.
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1. Introduction Defect induced ferromagntism has been a topic of current interest in the scientific community due to lack of a well established theory. These reports created great excitement and threw a wider debate on the origin and understanding of magnetism in the oxide semiconductors, as it is believed that partially filled 3d and 4f-shell electrons are mainly
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responsible for the ferromagnetism [1]. This unconventional ferromagnetism have been found in various undoped and doped oxides systems, such as ZnO, HfO2, TiO2, SnO2 and In2O3 [2, 3] and origin of magnetism is still not well understood. Among various oxides, ZnO has been choosen as it is extensively preferred for optoelectronic device applications due to its wide
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bandgap (3.37 eV) and large exciton binding energy (60 meV) at room temperature [4-5]. The observation of room temperature ferromagnetism adds another dimension towards the multifunctionality of this material.
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Preliminary studies on transition metal (TM) / rare earth (RE) ions doped ZnO depicts that it is not clear whether ferromagnetism is originating from the uniformly distributed TM / RE ions in the host matrix or it is due to the extrinsic magnetization arising from precipitation of secondary phases such as magnetic cluster formation [6-9]. After these complications, researchers put efforts to understand the origin of magnetization in pure and
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non-magntic ions doped ZnO. Mg proves to be an ideal dopant in ZnO host matrix since it enhances optical bandgap and also exhibits unusual ferromagnetic behaviour. In addition, similar ionic radii of the Mg2+ (0.57 Å) and Zn2+ (0.60 Å) ions makes the incorporation of Mg2+ ions into ZnO lattice quite feasible. The origin of magnetism in pure and non-magnetic
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dopants is supposed to be strongly dependent on defects such as O vacancies (VO), Zn interstitials (Zni) and other surface effects [10-11]. A report by Hong et al. [12] showed that
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defects could tune ferromagnetic (FM) behaviour and a perfect crystallinity might destroy the FM, or presence of more oxygen could degrade the magnetic ordering. On the other hand, Lorite et al. [13] found that Zn related defects are mainly responsible for the magnetic ordering in ZnO. Additionally, the induced strain by doping and other factors also play dominant role. Few reports also depicts that the presence of threshold value of defects and the excitation of spin state to higher state in effects of thermal energy is required for the inducing magnetism [14]. Therefore, control and engineering of defects become a very interesting and challenging issue. It would be very useful if we are able to tune the defects as per the requirement for the enhancement of magnetic ordering in these materials [15]. It has been realized from recent observations that ion beam irradiation is very effective approach for
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ACCEPTED MANUSCRIPT creating and engineering the defects in controlled fashion. Swift heavy ions (SHI) irradation induces point/cluster/columnar defects and structural disorder depending upon the extent of electronic energy loss mechanism in the system [16]. There are a limited number of reports on magnetism observed in ZnO by Mg doping and scarcely any reports available on SHI introduced defects in ZnMgO. In our earlier study, we focused on magnetic behaviour
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introduced by varying Mg content in host ZnO lattice for bulk samples. The present investigation aims to understand the role of defects on magnetic properties of Mg doped ZnO thin films. In this study, SHI (200 MeV, Ag beam) has been used as a tool for the creation of controlled defects in Zn0.9Mg0.1O thin films. X-ray absorption sectroscopy (XAS) in
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combination with photoluminescence (PL) have been used to understand the underlying mechanism and corroborate it with defects and hence magnetism in these samples.
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2. Experimental
Zn0.9Mg0.1O thin films were deposited on a silicon (Si) substrate using RF sputtering technique. Sputtering was done in Ar gas environment at a substrate temperature of 500 °C and 150 W RF power for 30 min. The grown films (having thickness ~ 150 nm) were irradiated with 200 MeV Ag15+ ions using the 15UD Pelletron Accelerator at IUAC, New Delhi at increased fluences varying from 1×1012 ions/cm2 to 1×1013 ions/cm2. The pristine
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and irradiated films were then studied for their structural, magnetic, optical and electronic properties. The crystal structure and topography were examined by X-ray diffraction (Bruker D8 X-ray diffractometer) and atomic force microscope (Nanoscope-IIIa). The room
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temperature ferromagnetic behaviour of the films was studied using SQUID magnetometer. Further, XAS measurements were performed at PAL 10D XAS-KIST beam line.
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3. Results and Discussion 3.1 X-ray Diffraction
XRD pattern show the growth of highly c-axis (002) oriented Zn0.9Mg0.1O thin films
(Figure 1). A visual inspection indicates that crystalline phase of the films is maintained even at the highest fluence. It can be seen from the figure that the (002) peak shifts systematically from 34.15° for pristine sample to the higher angle side 34.36° for the fluence of 1×1013 ions/cm2. This continuous shift towards higher angle is related with the reduction in c-axis lattice constant, which is calculated using the relation c=λ/sin θ [5] and found to vary from 5.245 Å to 5.213 Å with the increase in the ion fluence. The passage of high energy beam through the material creates defects due to large electronic energy transfer [17]. This creates a 3
ACCEPTED MANUSCRIPT high density of defects which induces the stress in the films and responsible for the systematic shift in the peak or the decrease in lattice constant. The induced stress in the films is estimated using the relation [5]. σ = − 453.6
GPa
The irraditation induced stress is found to be tensile in nature and increases
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monotonically with the increase in ion fluences (Table 1). Vijayalakshmi et al. [18] reported that the compressive stress may be attributed to the presence of Zni, and the tensile stress is likely to be associated with the presence of VO. Further investigations on the crystal structure shows that the variation in broadening of (002) peak results in the change in crystallite size,
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which is estimated using Scherrer’s formula D = 0.9λ/β cosθ [19]. Here D is the crystallite size, λ is the wavelength and β is the FWHM. Initially, there is a reduction in the crystallite
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size up to the fluence of 5×1012 ions/cm2. However, further increase in fluence results in small increase of crystallite size. The origin of such kind of variation in crystallite size is
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discussed in the next section.
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Figure 1 : XRD pattern of SHI irradiated Zn0.9Mg0.1O thin films. Inset shows the enlarged view of (002) peak.
Table 1:Various parameters observed from XRD pattern of SHI irradiated Zn0.9Mg0.1O thin films.
Ion-fluence (ions/cm2)
Crystallite Size (nm)
c-axis lattice parameter (Å)
Strain
Stress (GPa)
Pristine
12.9
5.245
------
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1×1012
10.9
5.243
-0.343E-3
0.16
5×1012
10.6
5.229
-3.050E-3
1.38
1×1013
13.2
5.213
-6.100E-3
2.78
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3.2 Atomic Force Microscopy The detailed investigation on modifications in the topological nature of Zn0.9Mg0.1O thin films after ion beam irradiation has been carried out by atomic force microscopy. Figure 2 shows the AFM images of pristine and SHI irradited Zn0.9Mg0.1O thin films. It is observed that pristine film exhibits larger grain size and there is a reduction in the grain size with an
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increase in ion fluence. However, for highest fluence (for 1×1013 ions/cm2), the grain size again increases. Singh et al. [20] also observed similar kind of behaviour of grain size in MgO thin films and explained on the basis of total energy deposited by electronic excitations/ionizations (i.e. Se×Φ) in the films by energetic ions based on thermal spike and
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Coulomb’s explosion models. The incoming SHI imparts sufficient energy to the surface of the grains resulting in the fragmentation and leads to the reduction in grain size of Zn0.9Mg0.1O thin films. However, grain growth occurs (increase in grain size) after a
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particular ion fluence. Kaoumi et al. [21] have proposed that grain growth occurs under the effect of ion irradiation after a threshold value due to the increase in local temperature of the grains. It is worth mentioning here that SHI irradiation induced ordering and disordering has already been reported in ZnO thin films [22]. The estimated grain size from AFM micrographs is found to be larger as compared to the one calculated using XRD pattern.
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Usually the XRD peaks are broadened due to the presence of internal stress and defects that cause the reduction in size. It is also known that AFM measures the particles that constitute of several nanograins. Hence, the mean grain size estimated from Scherrer’s formula is
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normally smaller than that measured from AFM micrographs [23].
Figure 2: AFM micrographs of Pristine, 5×1012 and 1×1013 ions/cm2 irradited Zn0.9Mg0.1O thin films.
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visual inspection of the magnetic data shows that the saturation magnetization value decreases initally with the increase in ion fluence (5×1012 ions/cm2) and then again increases
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for the maximum ion fluence (1×1013 ions/cm2).
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Figure 3: Magnetization curves of Pristine, 5×1012 and 1×1013 ions/cm2 irradiated Zn0.9Mg0.1O thin films. Inset shows the magnified view of hysteresis curve of sample irradiated with ion fluence of 5×1012 ions/cm2 .
There are reports mentioning that the origin of ferromagnetism can either be extrinsic or intrinsic in nature. Therefore, one must be very careful while assigning the origin of ferromagnetism in these systems. Since there is no reason to attribute the introduction of
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ferromagnetism to dopant (Mg) as it is non-magnetic in nature. ZnO doesnot contain any vacant 3d electrons and hence no interaction originates due to 3d orbitals. Moreover, the
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absence of any impurity phase from the XRD pattern rules out the existence of ferromangetism due to any extrinsic origin. [25]. Thus, one must reconsider the possibility that was previously assumed for undoped ferromagnetic oxides. Many authors proposed that uncontrolled formation of lattice defects can generate carriers that can mediate ferromagnetic ordering. There are mainly two kinds of lattice defects present in the system i.e. Zn interstitials and O vacancies, which contribute towards the magnetization. Krishnamurti et al. [26] pointed out the origin of magnetism on the basis of oxygen vacancies probed by XAS. To understand the local electronic structure and find out the type of defects that are contributing towards magnetization, XAS and PL measurements were carried out.
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3.4 X-ray Absorption & Photoluminescence Spectroscopy XAS is well known technique for gathering information related to the local electronic structure and the defects present in the system [27]. Figure 4 (a) represents the normalized room temperature O K-edge spectra of the pristine and irradiated Zn0.9Mg0.1O thin films. The O K-edge spectra consist of spectral features at ~ 530, 535, 538, 541 and 552eV. On the basis
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of band structure calculations and existing reports on ZnO, the peaks originating in the energy range 530−536 eV (peaks A and B) arise due to the hybridization of O 2p electrons with highly dispersive Zn 4s states [26, 27]. The spectral features in the energy range 537−548eV (peaks C and D) are due to the hybridization between the O 2p states with the Zn
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4p states and above 548 eV, the features arises due to O 2p states that extend to Zn higher orbitals. The observed O K-edge spectra of Zn0.9Mg0.1O thin films are similar to that of the
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earlier reports [28] however, there is change in the peak intensities with varying fluences. A visual inspection over the XAS spectra shows that there is an increase in the peak intensity with the increase in fluence value up to 5×1012 ions/cm2 irradiated sample; however, there is a decrease in the peak intensity for the highest fluence (1×1013 ions/cm2). As evident, the variation in peak intensities of O K-edge XAS spectra is associated with the change in number of unoccupied O 2p-derived states and oxygen content. Therefore, the decrease in
and vice-versa.
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overall peak intensities infers to the reduction in number of unoccupied O 2p-derived states
In order to get better insight of the defects present in the system contributing towards magnetism, the O K-edge (Figure 4 (a)) and Zn L-edge XAS spectra (Inset of Figure 4 (a))
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were analysed in detail. Zn L-edge spectra depict that the spectra of all the samples are similar. This indicates that SHI irradiation does not affect the electronic states of Zn, and the
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presence of Zn interstitials is neglected [29]. Further, it has been reported that peaks at 538− 541eV (peak C and D) in O K-edge spectra corresponds to the defects (O vacancies). Presence of these peaks in all the samples confirms that VO are the prominent defects, which results the change in magnetization with SHI irradiation. The variation in peak intensities is also consistent with the XRD and AFM results.
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Figure 4:(a). O K-edge XAS spectra of Pristine, 5×1012 and 1×1013 ions/cm2 irradiated Zn0.9Mg0.1O thin films and the inset shows the Zn L-edge for pristine and irradiated (1×1013 ions/cm2), (b) room temperature photoluminescence spectra of Pristine, 5×1012 and 1×1013 ions/cm2 irradiated Zn0.9Mg0.1O films.
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Photoluminescence spectroscopy has been employed to elucidate the origin of room temperature ferromagnetism in ZnO based systems. Figure 4(b) shows the room temperature photoluminescence spectra of pristine and SHI irradiated Zn0.9Mg0.1O thin films. The spectra of all the samples consist of sharp near band edge (NBE) in the UV region at ~ 388 nm and broad deep level emission (DLE) band in the green region at ~ 527 nm. It has been reported
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that the NBE (UV emission) band arises due to the most preferred orientation (002), which is the characteristic of wurtzite structure of ZnO [19]. The origin of broad DLE band (Green luminescence) [29] is controversial and found to be originate from several kinds of defects and vacancies present in the system. Most commonly observed defects in ZnO that contribute
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in the origin of magnetization are Zn interstitials (Zni), Zn vacancies (VZn) and O vacancies (VO). It has been reported that the green emission is attributed to Zn vacancy (VZn: 490 nm),
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O vacancy (VO: 527 nm) and O interstitial (Oi: 580 nm) [30]. There are many studies that support oxygen related bands are supposed to be the origin of magnetism [25, 31-32]. On the other hand, it has been reported that Zn related defects (VZn) are mainly responsible for magnetism rather than VO [13-14, 33]. Detailed analysis using XRD, PL and XAS were carried out to understand the exact origin for the magnetic moment. The presence of tensile stress from XRD pattern is an indicative of the VO present in the system [33]. Additionally, the presence of C and D peaks in O K-edge and no variation in Zn L-edge of XAS spectra confirms the presence of VO defects. Further, the defects related emision as observed through PL suggest the presence of high concentration of oxygen related defects. Based on above measurements, one can conclude that VO formed during growth are the key factor, playing
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ACCEPTED MANUSCRIPT crucial role for the modification of magnetic properties. It is reported that thermal annealing (heating/sintering) at different temperature could drastically influence the magnetic properties of ZnO. It is well known that thermal annealing and ion irradiation processes affect the strain in the system. However, both these processes induce the modifications depending upon the difference in energy supplied. The heat treatment provides sufficient thermal energy to the
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atoms so that lattice reconstruction can occur. In thermal annealing process, the temperature of the films as a whole increases while SHI irradiation increases the local temperature of the films under the effect of transit heat which dissipates into a single track via electron–phonon coupling [22, 34]. Therefore, the thermal energy provided to the system via SHI irradiation
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causes lattice deformation. This would results in the decrease in lattice defects and hence explains the reason behind the reduction in saturation magnetization with the increase in temperature [35]. SHI irradiation also provides the thermal energy to the atoms, due to which
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lattice defect reduces, as confirmed using PL spectra. The DLE band peak intensity for 5×1012 ions/cm2 sample is lower than the pristine Zn0.9Mg0.1O thin films, implying the reduction in defect states and hence the magnetization. Interestingly, the further increase in ion fluence (for 1×1013 ions/cm2) results in the enhancement of magnetization corresponding to the increase in defect concentration. The increase in defect concentration for the highest
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value is confirmed from PL measurements that show the maximum value. The variation in peak intensities (indicating the variation in defects concentration) is in well correlation with the XAS results.
4. Conclusion
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Defect induced ferromagnetism has been investigated on RF sputtered Zn0.9Mg0.1O thin films. The defects in the deposited films were induced by 200 MeV Ag15+ ion beam
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irradiation. XRD study confirms the formation of highly c-axis oriented Zn0.9Mg0.1O thin films. Initially, crystallite size decreases with the ion fluence of irradiation whereas it increases for the highest fluence. Similar type of variation in grain size has been observed from AFM measurements. It has been found that pristine Zn0.9Mg0.1O thin film exhibits ferromagnetic order, which decreases initially and then increases for the highest fluence. Therefore, depending upon the fluence variation, one could tune the defects concentrations in the host lattice. Apparantly, this study reveals that defects play the significant role in modifying the room temperature ferromagnetism. These results may open a new approach for oxides semiconductors just by controlling oxygen vacancies to fabricate potential candidates for future spintronic devices.
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Acknowledgement: One of the authors (PK) is grateful to the Department of Science and Technology for providing the fnancial support under DST Inspire Faculty Scheme [No. DST/INSPIRE/04/2015/003149] and SUM, University of Leipzig, Germany for their help in measurements.
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RF sputtered Zn0.9Mg0.1O thin films Investigation on magnetic properties of Zn0.9Mg0.1O films caused by SHI irradiation Presence of defects were confirmed through PL and XAS.
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• • •
S0925-8388(17)31106-4
DOI:
10.1016/j.jallcom.2017.03.304
Reference:
JALCOM 41342
To appear in:
Journal of Alloys and Compounds
Received Date: 14 February 2017 Revised Date:
23 March 2017
Accepted Date: 26 March 2017
Please cite this article as: P. Kumar, H.K. Malik, S. Gautam, K.H. Chae, K. Asokan, D. Kanjilal, Modifications in room temperature ferromagnetism by dense electronic excitations in Zn0.9Mg0.1O thin films, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.304. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Modifications in Room Temperature Ferromagnetism by Dense Electronic Excitations in Zn0.9Mg0.1O Thin Films Parmod Kumar1,2*, Hitendra K. Malik1, S. Gautam3$, K. H. Chae3 and K. Asokan2, D. Kanjilal2 1
Department of Physics, Indian Institute of Technology Delhi, New Delhi - 110016, India Materials Science Division, Inter University Accelerator Centre, New Delhi - 110067, India 3 Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136791, South Korea
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Abstract
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Present study shows that dense electronic excitations caused by ion beam interaction in Zn0.9Mg0.1O films lead to significant changes in room temperature ferromagnetism. It is found that defects are playing dominant role in the modification of various physical properties including ferromagnetic behaviour. Presence of defects were investigated through photoluminescence spectroscopy along with local electronic structure study using X-ray absorption spectroscopy. This novel observation is relevant to get further insight into ferromagnetic origin and maybe applied to tune magnetic properties for spintronic applications. A strong correlation exists between the ferromagnetic properties and defects produced during high energy irradiation.
Keywords: Swift heavy ion irradiation, defects induced magnetism, photoluminescence spectroscopy and local electronic structure
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Corresponding author: Parmod Kumar *Email:[email protected] $ Presently at: Dr. SSB UICET, Panjab University, Chandigarh−160014, India.
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1. Introduction Defect induced ferromagntism has been a topic of current interest in the scientific community due to lack of a well established theory. These reports created great excitement and threw a wider debate on the origin and understanding of magnetism in the oxide semiconductors, as it is believed that partially filled 3d and 4f-shell electrons are mainly
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responsible for the ferromagnetism [1]. This unconventional ferromagnetism have been found in various undoped and doped oxides systems, such as ZnO, HfO2, TiO2, SnO2 and In2O3 [2, 3] and origin of magnetism is still not well understood. Among various oxides, ZnO has been choosen as it is extensively preferred for optoelectronic device applications due to its wide
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bandgap (3.37 eV) and large exciton binding energy (60 meV) at room temperature [4-5]. The observation of room temperature ferromagnetism adds another dimension towards the multifunctionality of this material.
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Preliminary studies on transition metal (TM) / rare earth (RE) ions doped ZnO depicts that it is not clear whether ferromagnetism is originating from the uniformly distributed TM / RE ions in the host matrix or it is due to the extrinsic magnetization arising from precipitation of secondary phases such as magnetic cluster formation [6-9]. After these complications, researchers put efforts to understand the origin of magnetization in pure and
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non-magntic ions doped ZnO. Mg proves to be an ideal dopant in ZnO host matrix since it enhances optical bandgap and also exhibits unusual ferromagnetic behaviour. In addition, similar ionic radii of the Mg2+ (0.57 Å) and Zn2+ (0.60 Å) ions makes the incorporation of Mg2+ ions into ZnO lattice quite feasible. The origin of magnetism in pure and non-magnetic
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dopants is supposed to be strongly dependent on defects such as O vacancies (VO), Zn interstitials (Zni) and other surface effects [10-11]. A report by Hong et al. [12] showed that
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defects could tune ferromagnetic (FM) behaviour and a perfect crystallinity might destroy the FM, or presence of more oxygen could degrade the magnetic ordering. On the other hand, Lorite et al. [13] found that Zn related defects are mainly responsible for the magnetic ordering in ZnO. Additionally, the induced strain by doping and other factors also play dominant role. Few reports also depicts that the presence of threshold value of defects and the excitation of spin state to higher state in effects of thermal energy is required for the inducing magnetism [14]. Therefore, control and engineering of defects become a very interesting and challenging issue. It would be very useful if we are able to tune the defects as per the requirement for the enhancement of magnetic ordering in these materials [15]. It has been realized from recent observations that ion beam irradiation is very effective approach for
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ACCEPTED MANUSCRIPT creating and engineering the defects in controlled fashion. Swift heavy ions (SHI) irradation induces point/cluster/columnar defects and structural disorder depending upon the extent of electronic energy loss mechanism in the system [16]. There are a limited number of reports on magnetism observed in ZnO by Mg doping and scarcely any reports available on SHI introduced defects in ZnMgO. In our earlier study, we focused on magnetic behaviour
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introduced by varying Mg content in host ZnO lattice for bulk samples. The present investigation aims to understand the role of defects on magnetic properties of Mg doped ZnO thin films. In this study, SHI (200 MeV, Ag beam) has been used as a tool for the creation of controlled defects in Zn0.9Mg0.1O thin films. X-ray absorption sectroscopy (XAS) in
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combination with photoluminescence (PL) have been used to understand the underlying mechanism and corroborate it with defects and hence magnetism in these samples.
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2. Experimental
Zn0.9Mg0.1O thin films were deposited on a silicon (Si) substrate using RF sputtering technique. Sputtering was done in Ar gas environment at a substrate temperature of 500 °C and 150 W RF power for 30 min. The grown films (having thickness ~ 150 nm) were irradiated with 200 MeV Ag15+ ions using the 15UD Pelletron Accelerator at IUAC, New Delhi at increased fluences varying from 1×1012 ions/cm2 to 1×1013 ions/cm2. The pristine
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and irradiated films were then studied for their structural, magnetic, optical and electronic properties. The crystal structure and topography were examined by X-ray diffraction (Bruker D8 X-ray diffractometer) and atomic force microscope (Nanoscope-IIIa). The room
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temperature ferromagnetic behaviour of the films was studied using SQUID magnetometer. Further, XAS measurements were performed at PAL 10D XAS-KIST beam line.
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3. Results and Discussion 3.1 X-ray Diffraction
XRD pattern show the growth of highly c-axis (002) oriented Zn0.9Mg0.1O thin films
(Figure 1). A visual inspection indicates that crystalline phase of the films is maintained even at the highest fluence. It can be seen from the figure that the (002) peak shifts systematically from 34.15° for pristine sample to the higher angle side 34.36° for the fluence of 1×1013 ions/cm2. This continuous shift towards higher angle is related with the reduction in c-axis lattice constant, which is calculated using the relation c=λ/sin θ [5] and found to vary from 5.245 Å to 5.213 Å with the increase in the ion fluence. The passage of high energy beam through the material creates defects due to large electronic energy transfer [17]. This creates a 3
ACCEPTED MANUSCRIPT high density of defects which induces the stress in the films and responsible for the systematic shift in the peak or the decrease in lattice constant. The induced stress in the films is estimated using the relation [5]. σ = − 453.6
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The irraditation induced stress is found to be tensile in nature and increases
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monotonically with the increase in ion fluences (Table 1). Vijayalakshmi et al. [18] reported that the compressive stress may be attributed to the presence of Zni, and the tensile stress is likely to be associated with the presence of VO. Further investigations on the crystal structure shows that the variation in broadening of (002) peak results in the change in crystallite size,
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which is estimated using Scherrer’s formula D = 0.9λ/β cosθ [19]. Here D is the crystallite size, λ is the wavelength and β is the FWHM. Initially, there is a reduction in the crystallite
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size up to the fluence of 5×1012 ions/cm2. However, further increase in fluence results in small increase of crystallite size. The origin of such kind of variation in crystallite size is
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discussed in the next section.
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Figure 1 : XRD pattern of SHI irradiated Zn0.9Mg0.1O thin films. Inset shows the enlarged view of (002) peak.
Table 1:Various parameters observed from XRD pattern of SHI irradiated Zn0.9Mg0.1O thin films.
Ion-fluence (ions/cm2)
Crystallite Size (nm)
c-axis lattice parameter (Å)
Strain
Stress (GPa)
Pristine
12.9
5.245
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1×1012
10.9
5.243
-0.343E-3
0.16
5×1012
10.6
5.229
-3.050E-3
1.38
1×1013
13.2
5.213
-6.100E-3
2.78
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3.2 Atomic Force Microscopy The detailed investigation on modifications in the topological nature of Zn0.9Mg0.1O thin films after ion beam irradiation has been carried out by atomic force microscopy. Figure 2 shows the AFM images of pristine and SHI irradited Zn0.9Mg0.1O thin films. It is observed that pristine film exhibits larger grain size and there is a reduction in the grain size with an
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increase in ion fluence. However, for highest fluence (for 1×1013 ions/cm2), the grain size again increases. Singh et al. [20] also observed similar kind of behaviour of grain size in MgO thin films and explained on the basis of total energy deposited by electronic excitations/ionizations (i.e. Se×Φ) in the films by energetic ions based on thermal spike and
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Coulomb’s explosion models. The incoming SHI imparts sufficient energy to the surface of the grains resulting in the fragmentation and leads to the reduction in grain size of Zn0.9Mg0.1O thin films. However, grain growth occurs (increase in grain size) after a
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particular ion fluence. Kaoumi et al. [21] have proposed that grain growth occurs under the effect of ion irradiation after a threshold value due to the increase in local temperature of the grains. It is worth mentioning here that SHI irradiation induced ordering and disordering has already been reported in ZnO thin films [22]. The estimated grain size from AFM micrographs is found to be larger as compared to the one calculated using XRD pattern.
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Usually the XRD peaks are broadened due to the presence of internal stress and defects that cause the reduction in size. It is also known that AFM measures the particles that constitute of several nanograins. Hence, the mean grain size estimated from Scherrer’s formula is
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normally smaller than that measured from AFM micrographs [23].
Figure 2: AFM micrographs of Pristine, 5×1012 and 1×1013 ions/cm2 irradited Zn0.9Mg0.1O thin films.
3.3 Magnetic Measurement 5
ACCEPTED MANUSCRIPT Figure 3 shows the M-H curves of SHI irradited Zn0.9Mg0.1O thin films. The diamagnetic signal from the Si substrate has been substracted carefully. It is observed that pristine sample (Zn0.9Mg0.1O thin film) exhibits ferromagnetic behaviour. For pristine thin film, the room temperature saturation magnetization (Ms) value is found to be ~ 7.88 emu/cm3, which is consistent with the previous reports on oxide semiconductors [24]. A
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visual inspection of the magnetic data shows that the saturation magnetization value decreases initally with the increase in ion fluence (5×1012 ions/cm2) and then again increases
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for the maximum ion fluence (1×1013 ions/cm2).
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Figure 3: Magnetization curves of Pristine, 5×1012 and 1×1013 ions/cm2 irradiated Zn0.9Mg0.1O thin films. Inset shows the magnified view of hysteresis curve of sample irradiated with ion fluence of 5×1012 ions/cm2 .
There are reports mentioning that the origin of ferromagnetism can either be extrinsic or intrinsic in nature. Therefore, one must be very careful while assigning the origin of ferromagnetism in these systems. Since there is no reason to attribute the introduction of
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ferromagnetism to dopant (Mg) as it is non-magnetic in nature. ZnO doesnot contain any vacant 3d electrons and hence no interaction originates due to 3d orbitals. Moreover, the
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absence of any impurity phase from the XRD pattern rules out the existence of ferromangetism due to any extrinsic origin. [25]. Thus, one must reconsider the possibility that was previously assumed for undoped ferromagnetic oxides. Many authors proposed that uncontrolled formation of lattice defects can generate carriers that can mediate ferromagnetic ordering. There are mainly two kinds of lattice defects present in the system i.e. Zn interstitials and O vacancies, which contribute towards the magnetization. Krishnamurti et al. [26] pointed out the origin of magnetism on the basis of oxygen vacancies probed by XAS. To understand the local electronic structure and find out the type of defects that are contributing towards magnetization, XAS and PL measurements were carried out.
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3.4 X-ray Absorption & Photoluminescence Spectroscopy XAS is well known technique for gathering information related to the local electronic structure and the defects present in the system [27]. Figure 4 (a) represents the normalized room temperature O K-edge spectra of the pristine and irradiated Zn0.9Mg0.1O thin films. The O K-edge spectra consist of spectral features at ~ 530, 535, 538, 541 and 552eV. On the basis
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of band structure calculations and existing reports on ZnO, the peaks originating in the energy range 530−536 eV (peaks A and B) arise due to the hybridization of O 2p electrons with highly dispersive Zn 4s states [26, 27]. The spectral features in the energy range 537−548eV (peaks C and D) are due to the hybridization between the O 2p states with the Zn
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4p states and above 548 eV, the features arises due to O 2p states that extend to Zn higher orbitals. The observed O K-edge spectra of Zn0.9Mg0.1O thin films are similar to that of the
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earlier reports [28] however, there is change in the peak intensities with varying fluences. A visual inspection over the XAS spectra shows that there is an increase in the peak intensity with the increase in fluence value up to 5×1012 ions/cm2 irradiated sample; however, there is a decrease in the peak intensity for the highest fluence (1×1013 ions/cm2). As evident, the variation in peak intensities of O K-edge XAS spectra is associated with the change in number of unoccupied O 2p-derived states and oxygen content. Therefore, the decrease in
and vice-versa.
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overall peak intensities infers to the reduction in number of unoccupied O 2p-derived states
In order to get better insight of the defects present in the system contributing towards magnetism, the O K-edge (Figure 4 (a)) and Zn L-edge XAS spectra (Inset of Figure 4 (a))
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were analysed in detail. Zn L-edge spectra depict that the spectra of all the samples are similar. This indicates that SHI irradiation does not affect the electronic states of Zn, and the
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presence of Zn interstitials is neglected [29]. Further, it has been reported that peaks at 538− 541eV (peak C and D) in O K-edge spectra corresponds to the defects (O vacancies). Presence of these peaks in all the samples confirms that VO are the prominent defects, which results the change in magnetization with SHI irradiation. The variation in peak intensities is also consistent with the XRD and AFM results.
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Figure 4:(a). O K-edge XAS spectra of Pristine, 5×1012 and 1×1013 ions/cm2 irradiated Zn0.9Mg0.1O thin films and the inset shows the Zn L-edge for pristine and irradiated (1×1013 ions/cm2), (b) room temperature photoluminescence spectra of Pristine, 5×1012 and 1×1013 ions/cm2 irradiated Zn0.9Mg0.1O films.
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Photoluminescence spectroscopy has been employed to elucidate the origin of room temperature ferromagnetism in ZnO based systems. Figure 4(b) shows the room temperature photoluminescence spectra of pristine and SHI irradiated Zn0.9Mg0.1O thin films. The spectra of all the samples consist of sharp near band edge (NBE) in the UV region at ~ 388 nm and broad deep level emission (DLE) band in the green region at ~ 527 nm. It has been reported
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that the NBE (UV emission) band arises due to the most preferred orientation (002), which is the characteristic of wurtzite structure of ZnO [19]. The origin of broad DLE band (Green luminescence) [29] is controversial and found to be originate from several kinds of defects and vacancies present in the system. Most commonly observed defects in ZnO that contribute
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in the origin of magnetization are Zn interstitials (Zni), Zn vacancies (VZn) and O vacancies (VO). It has been reported that the green emission is attributed to Zn vacancy (VZn: 490 nm),
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O vacancy (VO: 527 nm) and O interstitial (Oi: 580 nm) [30]. There are many studies that support oxygen related bands are supposed to be the origin of magnetism [25, 31-32]. On the other hand, it has been reported that Zn related defects (VZn) are mainly responsible for magnetism rather than VO [13-14, 33]. Detailed analysis using XRD, PL and XAS were carried out to understand the exact origin for the magnetic moment. The presence of tensile stress from XRD pattern is an indicative of the VO present in the system [33]. Additionally, the presence of C and D peaks in O K-edge and no variation in Zn L-edge of XAS spectra confirms the presence of VO defects. Further, the defects related emision as observed through PL suggest the presence of high concentration of oxygen related defects. Based on above measurements, one can conclude that VO formed during growth are the key factor, playing
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ACCEPTED MANUSCRIPT crucial role for the modification of magnetic properties. It is reported that thermal annealing (heating/sintering) at different temperature could drastically influence the magnetic properties of ZnO. It is well known that thermal annealing and ion irradiation processes affect the strain in the system. However, both these processes induce the modifications depending upon the difference in energy supplied. The heat treatment provides sufficient thermal energy to the
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atoms so that lattice reconstruction can occur. In thermal annealing process, the temperature of the films as a whole increases while SHI irradiation increases the local temperature of the films under the effect of transit heat which dissipates into a single track via electron–phonon coupling [22, 34]. Therefore, the thermal energy provided to the system via SHI irradiation
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causes lattice deformation. This would results in the decrease in lattice defects and hence explains the reason behind the reduction in saturation magnetization with the increase in temperature [35]. SHI irradiation also provides the thermal energy to the atoms, due to which
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lattice defect reduces, as confirmed using PL spectra. The DLE band peak intensity for 5×1012 ions/cm2 sample is lower than the pristine Zn0.9Mg0.1O thin films, implying the reduction in defect states and hence the magnetization. Interestingly, the further increase in ion fluence (for 1×1013 ions/cm2) results in the enhancement of magnetization corresponding to the increase in defect concentration. The increase in defect concentration for the highest
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value is confirmed from PL measurements that show the maximum value. The variation in peak intensities (indicating the variation in defects concentration) is in well correlation with the XAS results.
4. Conclusion
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Defect induced ferromagnetism has been investigated on RF sputtered Zn0.9Mg0.1O thin films. The defects in the deposited films were induced by 200 MeV Ag15+ ion beam
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irradiation. XRD study confirms the formation of highly c-axis oriented Zn0.9Mg0.1O thin films. Initially, crystallite size decreases with the ion fluence of irradiation whereas it increases for the highest fluence. Similar type of variation in grain size has been observed from AFM measurements. It has been found that pristine Zn0.9Mg0.1O thin film exhibits ferromagnetic order, which decreases initially and then increases for the highest fluence. Therefore, depending upon the fluence variation, one could tune the defects concentrations in the host lattice. Apparantly, this study reveals that defects play the significant role in modifying the room temperature ferromagnetism. These results may open a new approach for oxides semiconductors just by controlling oxygen vacancies to fabricate potential candidates for future spintronic devices.
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Acknowledgement: One of the authors (PK) is grateful to the Department of Science and Technology for providing the fnancial support under DST Inspire Faculty Scheme [No. DST/INSPIRE/04/2015/003149] and SUM, University of Leipzig, Germany for their help in measurements.
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RF sputtered Zn0.9Mg0.1O thin films Investigation on magnetic properties of Zn0.9Mg0.1O films caused by SHI irradiation Presence of defects were confirmed through PL and XAS.
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