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Magnetic properties of nanocrystalline nickel incorporated CuO thin films
Accepted Manuscript Magnetic properties of nanocrystalline nickel incorporated CuO thin films S. Dolai, S.N. Sarangi, S. Hussain, R. Bhar, A.K. Pal PII: DOI: Reference:
S0304-8853(18)33015-4 https://doi.org/10.1016/j.jmmm.2019.02.005 MAGMA 64924
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
18 September 2018 4 January 2019 1 February 2019
Please cite this article as: S. Dolai, S.N. Sarangi, S. Hussain, R. Bhar, A.K. Pal, Magnetic properties of nanocrystalline nickel incorporated CuO thin films, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.02.005
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Magnetic properties of nanocrystalline nickel incorporated CuO thin films S. Dolai, S. N. Sarangi1, S. Hussain2, R. Bhar and A. K. Pal* Department of Instrumentation Science, Jadavpur University, Kolkata-700032, India 1 2
Institute of Physics, P.O. Sainik School, Sachivalaya Marg, Bhubaneswar-751005, India UGC-DAE CSR, Kalpakkam Node, Kokilamedu-603104, India
Abstract CuO films were deposited on fused silica substrates by using sol-gel method. Nanocrystalline nickel was incorporated in CuO in varying concentrations to form Ni:CuO films. Field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD), and Raman spectroscopy measurements were carried out to characterize the films. X-ray photoelectron spectroscopy (XPS) studies indicated that Ni ions successfully substituted Cu in the CuO lattice. Superconducting quantum interface device (SQUID) measurements were carried out to measure the magnetization of the films which decreased with increase in temperature. Coercive field and residual magnetization in the Ni:CuO films were measured as a function of nickel content in the CuO films.
Keywords: CuO; DMS; Raman spectroscopy; SQUID
1
1. Introduction Studies of dilute magnetic semiconductors (DMSs) are of current scientific interest for spintronic [1] application. The main criteria for such material is are that they would exhibit ferromagnetism at room temperature for effective device applications. In the past few years, research has been carried out on observation of room temperature ferromagnetism (RTFM) in metal oxide semiconductor (MOS) materials. They used either pure semiconductor or doped ones with small amount of transition metal [TM] in the MOS lattice. Lots of work has been carried out for studying DMS properties with magnetic impurity in ZnO [2-12], TiO2 [13, 14], GaN [15] SnO2 [16], AlN [17]. However, the origin of ferromagnetism in DMS based materials is still highly debatable. Cupric oxide (CuO) is one of the most important antiferromagnetic oxide semiconductors that display p-type conductivity with narrow band gap (Eg~1.2eV). This material was widely used in the fields of solar cells [18, 19], supercapacitors [20, 21], DMS [22], photocatalytic hydrogen evolution [23], gas sensors [24], high temperature superconductivity [25], etc. Currently, CuO is being considered as an alternative host matrix [26-29] to ZnO for DMS applications. It is known that Cu can have three oxidation states: Cu+, Cu2+, and Cu3+ [14]. Additionally, low production cost, non-toxic nature and abundance of the materials constituting the compound makes this material technologically important [4]. Doping in CuO, thus modulating the magnetic properties of CuO nanoparticles was reported by Wesselinowa et al. [26]. Exchange bias effect and its dependence on nanoparticle size in Fe-doped CuO nanoparticles samples were reported by Yin et al. [27]. Basith et al. [28] used rapid microwave-assisted combustion technique to prepare CuO and nickel-doped CuO nanomaterials. These materials showed ferromagnetic properties at room temperature. The co-precipitation technique was adopted by Meneses et al. [29] to deposit Cu1-xTMxO samples using Ni and Fe as transition materials (TM). Mariammal et al. [30] synthesized both undoped and Mn doped (5% and 10%) CuO nanoflakes using wet chemical methods. -----------------------------------------------------------------------------------------------------------*Corresponding author: email: [email protected]
2
But room temperature ferromagnetism in Mn-CuO was not observed by them. This was attributed to the formation of clusters on the surface of the film. Yang et al. [31] prepared Mn doped CuO by co-precipitation method. They observed ferromagnetic properties with an increased doping concentration at low temperature (T~5K). A transition temperature (TC) from ferromagnetic to paramagnetic phase at ~ 80K was reported by them. One dimensional CuO and Fe doped CuO nanorods were prepared via template free solution phase hydrothermal method by Manna and Dey [32]. Ferromagnetism was observed even at room temperature which was related to either excess level of oxygen or oxygen vacancy levels in the samples. Shi et al. [33] also reported the magnetic measurement studies on Cu2O/Cu composites which showed clear ferromagnetism. Presence of Cu vacancies was ascribed to the observed ferromagnetism. Cu vacancies were found to modulate the interface quality of Cu2O/Cu composites.
Recently, synthesis of 2D
transition metal dichalcogenides (TMDs) based on ferromagnetic nanosheets was reported by Xia et al. [34]. Cu-doped MoS2 nanosheets were prepared by them using a hydrothermal
method.
Magnetic
measurement
studies
resulted
in
inducing
ferromagnetism in MoS2 nanosheets by doping of Cu ions. Saturation magnetization was seen to increase with increased in Cu concentration. The above study opened a new path to induce spintronic properties by non-magnetic atom doping in pristine 2D nanostructures. From the above reports on CuO-based DMS materials, it is apparent that need still exists for depositing transition metal doped CuO material of high quality showing room temperature ferromagnetism. A reproducible, scalable and easy technique for the deposition of Ni-doped CuO films for DMS application is required even today to critically study them for understanding the origin of room temperature ferromagnetism in these systems. We present a report on the synthesis of high quality CuO and Ni-doped CuO films (Cu1-xNixO) films with increasing Ni concentration (x= 0.00, 0.03M, 0.05M, 0.08M). Films were deposited on fused silica substrate by simple sol-gel technique. The films are critically characterized by measuring their microstructural, compositional and Raman properties. Magnetic behaviour of Ni:CuO films were recorded by using superconducting quantum interference device (SQUID) magnetometer. 3
2. Experimental 2.1. Synthesis process: Materials: Nickel acetate tetra hydrate (Ni (CH3COO)2. 4H2O) (Merck Specialities Private Limited, 98%), N-N Dimethyl formamide (Merck Specialities Private Limited, 99.9%), Diethanolamine (Merck Specialities Private Limited, 98%) and Copper acetate monohydrate (Cu (CH3COO)2. H2O) (Alfa aesar). Fused silica substrates were cleaned properly. 0.8 gm copper acetate monohydrate (Cu (CH3COO)2. H2O) was taken in a conical flask and was dissolved by stirring in 80 ml DMF at room temperature. This solution (0.2M) was kept in four different conical flasks containing 20 ml solution each. Different amounts of nickel acetate tetra hydrate (Ni(CH3COO)2.4H2O) were added to three conical flasks containing 0.2 M
copper
acetate monohydrate solution. Nickel acetate tetra hydrate was dissolved in the above three solutions by stirring at room temperature to obtain 0.03M, 0.05M, 0.08M solutions of nickel acetate tetra hydrate in copper acetate monohydrate solution. The fourth solution was kept undoped for depositing pristine CuO film. All the above solutions were then heated to temperature ~120o C for 1 hr with continuous stirring. After that aqueous NH3 solution was then added drop by drop to the above solution to maintain a pH value ~11.0. As a result, the color of the solutions turned black. Finally, films were casted using a spin coater. Before casting the films, the fused silica substrates were dried at 700C. To obtain the desired layer thickness, the films were coated for several times. These films were subjected to rapid thermal annealing (RTA) at 5000C for 5 minutes with maintaining partial pressure of Ar (~10-2 Torr) inside the RTA chamber to eliminate the volatile solvents from the films. 2.2. Characterization of the films Surface morphology of the films was recorded by using a Carl Zeiss AURIGA Field emission scanning electron microscopey (FESEM). This FESEM was operated at a voltage of 5kV in secondary emission mode. Nominal composition was determined by using energy dispersive X-ray (EDAX) studies. Micro-structural information was obtained by using a Rigaku MiniFlex X-ray diffractometer (0.154 nm Cu Kα line). 4
Charged states of the elemental constituents were obtained by using X-ray Photoelectron Spectroscopy (XPS) measurements. We have used Al Kα X-ray source at 1486.74 eV. Approximate rate of sputtering in the system was ~10 angstroms per minute. A PHOIBOS 150 HSA3500 analyzer with a delay-line detector was used to collect the spectra. The resolution and pass energy were 0.6 eV for 656 kcps and 12 eV, respectively. A Renishaw inVia micro-Raman spectrometer was used to record the Raman spectra. Wavelength of the argon laser was 514 nm. Variation of magnetization with temperature (M-T curve) and magnetic hysteresis (M-H) curves were recorded by using a superconducting quantum interference device (SQUID) magnetometer (SQUID– VSM based MPMS System, Quantum Design-make). Magnetization was measured at different temperatures in the zero-field-cooled (ZFC) and field cooled (FC) mode at the applied field of 200 Oe. Magnetization was also recorded as a function of field in the range of -10KOe H 10KOe at 10 K and 300 K.
3. Results and discussion 3.1. Microstructural studies Scotch tape adhesion tests were performed on the CuO and Cu1-xNixO (Ni~0.1, ~1.9 and ~4.7 at %) films deposited here. The films adhered strongly to the fused silica substrates. Figs. 1(a-d) show the FESEM micrographs of the above films. It may be observed that the texture of the films changed (Fig.1b-d) significantly with the incorporation of nickel nano-crystallites in CuO (Fig.1a).
Insets of Fig.1 show the
corresponding EDAX spectra recorded in line scan mode. The presence of the peaks related to Cu, Ni and O dominated the spectra. Amount of Ni (in at %) present in the Ni:CuO films were evaluated from the EDAX studies and are shown in Table-I. The structural properties of the CuO and Ni doped CuO films were investigated by X-ray diffraction studies. XRD traces recorded for the representative CuO and Cu1xNixO
(Ni~0.1, ~1.9 and ~4.7 at %) films are shown in Fig.2. The films were found to be
polycrystalline in nature. Figure.2 (curve-a) indicates the presence of the characteristic peaks located at 2θ~32.6o, ~35.8o, ~38.9o, ~58.3° and ~72.4° originating from the reflections from (110), (-111/002), (111), (202) and (311) planes of monoclinic phase of 5
CuO, respectively. Addition of nickel with varying concentration in the CuO modulated the intensities of the peaks. The intensity of peak for CuO started decreasing (Fig.2: curve-b-d) with the addition of nickel. The intensities of the peaks located at 2θ ~58.2° and ~72.4° decreasing significantly (Fig.2: curve-d). Additional peaks related to elemental Ni and NiO related phases were not observed and the concentration of Ni is very low compared to CuO. This would indicate that Ni2+ ions have effectively substituted in Cu2+ sites without changing the monoclinic structure of CuO lattice. This may be due to the fact that Cu2+ and Ni2+ have comparable ionic radii (Cu2+ = 0.73Å and Ni2+ = 0.70Å) facilitating the above substitution of Cu2+ by Ni2+ [35]. It may be observed that the peak intensity related to (002)/ (-111) and (111) planes are much stronger than that of other peak (Fig.2) which affirms the preferential orientation of planes in CuO lattice [36]. The average crystal sizes for both CuO and Ni-doped CuO were calculated by Scherrer relation [37]: D = (Kλ)/ (β cosθ)
(1)
where K is a constant with a value of K is ~0.92. β is the full width at half maximum of the peaks related to the [h k l] reflection plane and
is the Bragg’s angle. By considering
the reflection planes (002)/ (-111) and (111), we have calculated the average crystallize size. The value of average crystallite size varied between 9 nm-16 nm (Table-1). Crystallite size decreased with the inclusion of nickel in CuO.
3.2. XPS studies It may be worthwhile now to report the results on XPS studies performed on the CuO and Ni-doped CuO films. This study will add information on the formation of CuO thin films and the effect of Ni incorporation on the charge states of the elements constituting the films. Fig.3a and Fig.3b show the general survey spectra of the above films. C1s peak appeared at ~ 284.5 eV and it was used a reference to calibrate the XPS spectra. Both the general survey spectra for CuO (Fig.3a) and Cu1-xNixO (Fig.3b) are dominated by peaks arising from the binding energies of Cu2p, O1s and C1s. Additional peak for nickel could be observed for Ni:CuO film (Fig.3b). All the core level spectra of elements comprising the compounds were recorded separately in high resolution mode. 6
If we examine the core level spectra critically, they would reveal important information on the charge states of those elements. The core level Cu2p spectra for pristine CuO film (Fig.3c) indicated a doublet. The peaks located at ~933.15 eV (Cu2p3/2) and ~953.5 eV (Cu2p1/2), respectively arose due to L-S coupling of Cu2p state. The binding energy difference between these two peaks is ~20.35 eV. It is known that the Cu2p3/2 and Cu2p1/2 spectra are indicative of the Cu2+ oxidation state [38] in CuO lattice. The above doublets are associated with shake-up satellite peaks located at higher energies (~942.0 eV and ~961.5 eV) than that of parent peak [39]. The separation between the satellite peaks is ~19.5 eV. Now, upon examining the core level Cu2p spectra for Ni doped (4.72 at%) CuO films, one can see a shift in peak positions for both the peaks for Cu2p3/2 (located at ~933.15 eV) and Cu2p1/2 (located at ~953.5 eV) towards higher energy (Fig.3d). The corresponding satellites also showed a shift to higher energy (~941.8 eV and ~961.5 eV) (Fig.3d). The separations between both the parent and satellite peaks are~20.35 eV and ~19.5 eV, respectively. This observation will also indicate Cu2+ oxidation state of copper in Ni doped CuO samples. A doublet structure for Ni2p core level spectra is also visible (Fig.3e). The doublets are located at ~855.6 eV and ~873.4 eV for Ni2p3/2 and Ni2p1/2, respectively. These doublets also have satellites located at ~ 860.4 eV and ~ 879 eV. The energy shift between the two parent peaks is ~ 17.8 eV while that between the satellites is ~18.6 eV. The observed differences in energies between the peaks for Ni2p3/2 and Ni2p1/2 is lower than that reported (19.7 eV) by Liu et al. [40]. The presence of the satellite peak at ~860 eV affirms the inclusion of Ni as oxide of Ni. The peak position of Ni2p3/2 peak at ~855.6 eV is higher than the binding energy value of 852.7 eV for metallic Ni. This value matches more or less well with that for Ni2+ in NiO (~853.5 eV). The other peak for Ni2p1/2 is located at ~873.1 eV. The binding energies of Ni in metallic form and in NiO are different. The spinorbit splitting energy difference is also different in the above cases. The observed binding-energy difference in this study is ~17-20 eV. The binding-energy separation is 17.4 eV for metallic nickel and 18.4 eV Ni in NiO [41]. The experimental results obtained in this study indicated the separation between the parent peaks is ~17.5 eV and that for the satellites is ~18 eV. The observed binding-energy difference confirms that Ni 7
present in this CuO matrix would resemble that in NiO. From the above studies, it may be stated that Ni ions in these Ni:CuO films have successfully substituted Cu in the CuO lattice [42]. 3.3. Raman spectra Raman spectra of all the films were recorded at room temperature. Three Raman active optical phonons have generally been identified in the literature [43]. The Raman spectra for a representative pristine CuO film are shown in Fig.4 (curve-a). This spectra confirmed the existence of the above three known bands at ~292 cm-1, ~346 cm-1 and ~631 cm-1. The origin of the peaks located at ~292 cm-1 could easily be associated with Ag mode. The peaks located at ~346 cm-1 and ~631 cm-1 could be associated with Bg modes [44]. The broad peaks centered ~1357 cm-1 and a high energy cutoff ~1560 cm-1 may have the origin from one-magnon density of states. This observation is in conformity with that recorded from neutron scattering experiment by Ain et al. [45]. The values for Raman modes observed in our films agreed quite well with the previously reported data [43-45]. With nickel inclusion in CuO matrix, the Raman spectra changed drastically. The intensities of the peaks located at ~1357 cm-1 and ~1560 cm-1 decreased significantly (Fig.4: curves b-d). Additional peaks started appearing. Films with highest Ni content (4.7 at%: curve-d) showed peaks centered ~809-900 cm-1, ~1135 cm-1 and ~1440 cm−1. These peaks may arise due to two phonon scattering which would confirm the presence of a single phase CuO for all the films deposited here. With the inclusion of nickel in the CuO matrix, the Raman spectra got modulated significantly (Fig.4: curve b-d). With increasing nickel content in CuO, the peaks at ~943 cm−1 and ~1430 cm−1 became stronger and distinct (Fig.4d: curve-d). Raman peaks showed a slight shift towards lower wavenumber with increased nickel content in CuO lattice. This shift could be attributed to the reduction in nickel particle size. This shift could be associated with Heisenberg uncertainty principle suggesting a relationship between crystallite size and phonon [46, 47]. 3.4. Magnetization Studies The magnetic properties of CuO and Ni-doped CuO (Ni at % ~0.1, 1.9 and 4.7) have been performed by SQUID measurements. Variation of magnetization with applied 8
magnetic field (M-H) was recorded at 10 K and 300 K for the undoped CuO and Ni:CuO films. M-H curves for both T~10K and 300K showed distinct hysteresis in Fig. 5a and Fig.5b, respectively. The ferromagnetic nature of the Ni:CuO films were distinctly visible. The magnetic field was applied parallel to the substrate surface of Ni:CuO films. The shape and width of hysteresis loop changed with amount of nickel incorporation in CuO films. The soft magnetic nature of the films was indicated from the hysteresis loop. The strongest ferromagnetic moments observed in the nickel doped samples might be due to the higher magnetic moments of Ni2+ ions substituting Cu2+ ions with lower magnetic moment. It is known that the magnetic moments of Cu2+ and Ni2+are 1.73 and 2.83 µB, respectively, in the high spin state [28]. The magnetic moments of Ni2+ ions are quite larger than that of the Cu2+ ions. This would mean that increased substitution of Ni2+ ions would induce stronger ferromagnetic behavior in Ni:CuO films. Meneses et al. [29] and Arbuzova et al. [48] reported similar observations. Ni:CuO films indicated higher remanence (MR) for films containing higher amount of nickel in the Ni:CuO films (Fig.5d). It may be noted here that the MS was seen to increase monotonously with the amount of nickel incorporated (Fig.5c). This increase was found to be steeper when measured at 10 K than 300K. It may also be observed that magnetization in the samples did not show saturation in the range of magnetic field under this study. Then the necessary question would arise if there is an upper limit of incorporation of nickel for MS in the sample showing saturation in this observed magnetic field. As this study was focused on the observation of the onset of ferromagnetism in Ni-doped CuO films, we refrained ourselves from adding larger amounts of nickel for observing the upper limit of incorporation nickel that might exist for MS in the sample. Contrary to the above, the coercive force decreased for films containing increased amounts of nickel (Fig.5e). This observation would not support the possible formation of NiO at the surface of the Ni:CuO composite films. It may be pointed here that the contribution from the paramagnetic nature of the substrate (fused silica here) cannot be ruled out. Background effect arising due to the substrate was taken care of during measurement. But one must remember that the mass of the substrate materials is quite larger than that of the film material under study. Thus the observed continuous increase in high field magnetization may have the origin of the above paramagnetic contributor present in the sample. Core-shell 9
morphology would be a more probable surface structure in films having surface oxide layer. The above structure would mean that ferromagnetic metallic nanoparticle (Ni) would be surrounded by an anti-ferromagnetic oxide layer (CuO) around it. In such a case, the magnetic properties would be greatly influenced due to the exchange interaction between the ferromagnetic core material and antiferromagnetic shell material [29, 47]. Formation of oxygen vacancies might also be another cause of enhanced ferromagnetism in Ni-doped CuO samples. Thus, Ni ion in CuO lattice might ‘‘trap’’ the free electrons originated from the oxygen vacancies. This might result in the enhancement of the ferromagnetic spin–spin interaction between nickel and copper atoms. This would be resulting in an enhancement of ferromagnetism in this system. The M(T) curves related to zero-field cooling (ZFC) and field-cooling (FC) of representative undoped CuO and Ni:CuO samples containing three different Ni concentrations (0.1 at %, 1.9 at% and 4.7 at%) are shown in Figs.6(a-d). ZFC and FC measurements were carried out from 20 K to 330 K at a fixed applied field ~200 Oe. The magnetic field was applied parallel to the substrate surface of Ni:CuO films and the contribution from the substrate (fused silica) was subtracted from the raw data for all the films. The samples show ferromagnetic behavior persisting up to 330 K and Currie temperature (TC) is thus estimated to be higher than 330 K. Magnetization measured was found to be essentially temperature independent between 20 K and 330 K. A model combining both diamagnetic and ferromagnetic components may account for the initial decrease of magnetization from 20 K to 200 K and remaining non-zero up to 330 K [49]. It is known that CuO may be either paramagnetic or anti-ferromagnetic in nature [50, 51]. As we increased the nickel content in CuO, there would be increased probability of an increase in NiO to Ni ratio due to increased surface oxidation. NiO is also well known anti-ferromagnets with the Neel temperature of 523 K. It may be stressed here that the amount of anti-ferreomagnetic NiO component would be insignificantly small compared to anti-ferreomagnetic CuO component. Thus, the observed sharp decrease in magnetization for Ni:CuO film deposited with higher nickel content (Fig. 6) would mainly be related to the anti-ferromagentic CuO component only (Fig.6 c, d). The ferromagnetic behavior in the films would thus mainly arise from the inclusion of nanocrystalline Ni in the CuO lattice. 10
Ferromagnetic properties of Ni:CuO is further demonstrated by the separation between the field cooling (FC) and zero field cooling (ZFC) curves. The FC curve was measured after recording the ZFC curve. The magnetization was measured with field cooled from 330 K to 20 K under 200 Oe. The ZFC-FC studies are carried out for studying static magnetization. Generally, an open loop is indicative of ferromagnetic behavior. A closed loop is observed in superparamagnetic materials. Paramagnetic behavior is indicated by a linear M-H curve. ZFC measurements are carried out by cooling the sample without application of any magnetic field. The data are collected upon heating and in presence of some magnetic filed. For FC curves, Sample are cooled with an applied magnetic field for recording FC behavior. The data can be collected in two ways: may be during cooling or heating [52,53] . At room temperature, the spins would be oriented by the magnetic field. When cooled in presence of an applied, field, the spins would have less chance to disorient. In presence of strong anisotropy, it would increase at low temperatures. The observed open loop between ZFC and FC disappeared at ~330 K. This would support the observed ferromagnetism in these Ni:CuO samples under this study At this point, the particles are free to align with the field during the measurement time. The sample showed ferromagnetic behavior which persisted above 300 K and TC is thus estimated to be higher than 330 K. Magnetization was found to be essentially temperature dependent between 20 and 330 K. 4. Conclusions
Ni:CuO composite films were deposited by sol-gel technique. The films contained 0.1 at %, 1.9 at% and 4.7 at% of nickel. The size of the Ni nanocrystallites varied between 9.0 nm and 16 nm. The Ni 2p core level XPS spectra contained two peaks located at ~852.7 and ~870.1 eV for Ni 2p3/2 and Ni 2p1/2, respectively. This implied that Ni mainly existed in the Ni2+ states in the Ni:CuO films. Ferromagnetic behavior persisted above 300 K in all the Ni:CuO films studied here. TC estimated was higher than 300 K. Magnetization decreased with increasing temperature. Thus, it may be concluded that observed ferromagnetism in Ni-doped CuO samples can be due to: (i) substitution of Cu2+ ions having a lower magnetic moment by Ni2+ ions having larger magnetic moments and/or ii) the formation of oxygen vacancies. Ni ion in CuO matrix may ‘‘trap’’ the free 11
electrons emitted by the oxygen vacancies. This may result in the enhancement of the ferromagnetic spin–spin interaction between nickel and copper atoms. Magnetization measured was found to be essentially temperature independent between 20 K and 330 K. The observed sharp decrease in the magnetization for Ni:CuO film deposited with higher nickel content would mainly be related to the anti-ferromagentic CuO component only. Acknowledgement The authors wish to thank the Department of Science and Technology, Government of India, Government of India for supporting the fellowship. The authors thank, Dr. D. Samal, Institute of Physics, Bhubaneswar for providing SQUID facility.
12
Caption of Figures Fig. 1 FESEM pictures of representative (a) CuO and three of Ni:CuO films deposited with different concentration of nickel in CuO: (b) 0.1 at %, (c) 1.9 at%, (d) 4.7 at %. Insets show corresponding EDAX spectra of the samples. Fig. 2 XRD traces of four representative Ni:CuO films deposited with different amount of nickel in CuO: (a) Pure CuO, (b) Ni~0.1 at %, (c) Ni~1.9 at%, (d) Ni~4.7 at %. Fig. 3 General survey XPS spectra for a representative: (a) undoped CuO film and (b) Ni:CuO film (Ni~4.7 at%); Core level spectra for: (c) Cu2p of undoped CuO and (d) Cu2p of Ni:CuO film (Ni~4.7 at%); (e) Ni 2p core level spectra of Ni:CuO film (Ni~4.7 at%). Fig.4 (a) Raman spectra for four representative Ni:CuO films: (a) Pure CuO, (b) Ni~0.1 at %, (c) Ni~1.9 at%, (d) Ni~4.7 at %. Fig. 5 M-H curves for Ni:CuO films (a) at 10 K and (b) at 300 K. The magnetization was measured in parallel configurations to the substrate of Ni:CuO films. (c) Variation of saturation magnetic moment as a function nickel concentration, (d) Variation of reminiscent magnetization as a function nickel concentration and (e) Variation of coercive force with the Ni content (At %) in the films Fig.6. FC and ZFC behavior of: (a) Pure CuO, (b) Ni~0.1 at %, (c) Ni~1.9 at%, (d) Ni~4.7 at %.
13
Table-1: Different properties of the Ni:CuO films deposited here
Sample
Ni
Crystal
(EDAX)
planes
(At %)
Crystal Average size
crystals
(XRD)
sizes
(nm)
(XRD)
Remanence
Coercivity
Saturation
(MR)
( HC)
magnetization
(KOe)
(MS)
2
(A.m /kg)
(A.m2/kg)
(nm)
Cu1.00Ni0.0O
Cu0.97Ni0.03O
Cu0.95Ni0.05O
Cu0.92Ni0.08O
0
0.1
1.9
4.7
[-111]
16.14
[111]
12.23
[-111]
12.28
[111]
12.39
[-111]
9.8
[111]
12.22
[-111]
9.69
[111]
9.26
10 K
300 K
( x10-4)
(x10-4)
14.18
1.43
0.0413
0.228
12.33
2.35
1.641
11.01
3.99
9.48
11.8
14
10 K
300 K
10 K
300 K
(x10-3)
(x10-3)
0.156
0.873
0.194
0.196
0.142
5.76
0.565
2.548
0.149
0.105
7.22
1.35
5.459
0.103
0.077
11.2
3.45
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Highlights:
High quality Ni-doped CuO films is synthesized by sol-gel technique Ni2+ atomic concentration was found to dominate the magnetic property
The magnetization increases with Ni inclusion in CuO lattice.
27
S0304-8853(18)33015-4 https://doi.org/10.1016/j.jmmm.2019.02.005 MAGMA 64924
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
18 September 2018 4 January 2019 1 February 2019
Please cite this article as: S. Dolai, S.N. Sarangi, S. Hussain, R. Bhar, A.K. Pal, Magnetic properties of nanocrystalline nickel incorporated CuO thin films, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.02.005
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Magnetic properties of nanocrystalline nickel incorporated CuO thin films S. Dolai, S. N. Sarangi1, S. Hussain2, R. Bhar and A. K. Pal* Department of Instrumentation Science, Jadavpur University, Kolkata-700032, India 1 2
Institute of Physics, P.O. Sainik School, Sachivalaya Marg, Bhubaneswar-751005, India UGC-DAE CSR, Kalpakkam Node, Kokilamedu-603104, India
Abstract CuO films were deposited on fused silica substrates by using sol-gel method. Nanocrystalline nickel was incorporated in CuO in varying concentrations to form Ni:CuO films. Field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD), and Raman spectroscopy measurements were carried out to characterize the films. X-ray photoelectron spectroscopy (XPS) studies indicated that Ni ions successfully substituted Cu in the CuO lattice. Superconducting quantum interface device (SQUID) measurements were carried out to measure the magnetization of the films which decreased with increase in temperature. Coercive field and residual magnetization in the Ni:CuO films were measured as a function of nickel content in the CuO films.
Keywords: CuO; DMS; Raman spectroscopy; SQUID
1
1. Introduction Studies of dilute magnetic semiconductors (DMSs) are of current scientific interest for spintronic [1] application. The main criteria for such material is are that they would exhibit ferromagnetism at room temperature for effective device applications. In the past few years, research has been carried out on observation of room temperature ferromagnetism (RTFM) in metal oxide semiconductor (MOS) materials. They used either pure semiconductor or doped ones with small amount of transition metal [TM] in the MOS lattice. Lots of work has been carried out for studying DMS properties with magnetic impurity in ZnO [2-12], TiO2 [13, 14], GaN [15] SnO2 [16], AlN [17]. However, the origin of ferromagnetism in DMS based materials is still highly debatable. Cupric oxide (CuO) is one of the most important antiferromagnetic oxide semiconductors that display p-type conductivity with narrow band gap (Eg~1.2eV). This material was widely used in the fields of solar cells [18, 19], supercapacitors [20, 21], DMS [22], photocatalytic hydrogen evolution [23], gas sensors [24], high temperature superconductivity [25], etc. Currently, CuO is being considered as an alternative host matrix [26-29] to ZnO for DMS applications. It is known that Cu can have three oxidation states: Cu+, Cu2+, and Cu3+ [14]. Additionally, low production cost, non-toxic nature and abundance of the materials constituting the compound makes this material technologically important [4]. Doping in CuO, thus modulating the magnetic properties of CuO nanoparticles was reported by Wesselinowa et al. [26]. Exchange bias effect and its dependence on nanoparticle size in Fe-doped CuO nanoparticles samples were reported by Yin et al. [27]. Basith et al. [28] used rapid microwave-assisted combustion technique to prepare CuO and nickel-doped CuO nanomaterials. These materials showed ferromagnetic properties at room temperature. The co-precipitation technique was adopted by Meneses et al. [29] to deposit Cu1-xTMxO samples using Ni and Fe as transition materials (TM). Mariammal et al. [30] synthesized both undoped and Mn doped (5% and 10%) CuO nanoflakes using wet chemical methods. -----------------------------------------------------------------------------------------------------------*Corresponding author: email: [email protected]
2
But room temperature ferromagnetism in Mn-CuO was not observed by them. This was attributed to the formation of clusters on the surface of the film. Yang et al. [31] prepared Mn doped CuO by co-precipitation method. They observed ferromagnetic properties with an increased doping concentration at low temperature (T~5K). A transition temperature (TC) from ferromagnetic to paramagnetic phase at ~ 80K was reported by them. One dimensional CuO and Fe doped CuO nanorods were prepared via template free solution phase hydrothermal method by Manna and Dey [32]. Ferromagnetism was observed even at room temperature which was related to either excess level of oxygen or oxygen vacancy levels in the samples. Shi et al. [33] also reported the magnetic measurement studies on Cu2O/Cu composites which showed clear ferromagnetism. Presence of Cu vacancies was ascribed to the observed ferromagnetism. Cu vacancies were found to modulate the interface quality of Cu2O/Cu composites.
Recently, synthesis of 2D
transition metal dichalcogenides (TMDs) based on ferromagnetic nanosheets was reported by Xia et al. [34]. Cu-doped MoS2 nanosheets were prepared by them using a hydrothermal
method.
Magnetic
measurement
studies
resulted
in
inducing
ferromagnetism in MoS2 nanosheets by doping of Cu ions. Saturation magnetization was seen to increase with increased in Cu concentration. The above study opened a new path to induce spintronic properties by non-magnetic atom doping in pristine 2D nanostructures. From the above reports on CuO-based DMS materials, it is apparent that need still exists for depositing transition metal doped CuO material of high quality showing room temperature ferromagnetism. A reproducible, scalable and easy technique for the deposition of Ni-doped CuO films for DMS application is required even today to critically study them for understanding the origin of room temperature ferromagnetism in these systems. We present a report on the synthesis of high quality CuO and Ni-doped CuO films (Cu1-xNixO) films with increasing Ni concentration (x= 0.00, 0.03M, 0.05M, 0.08M). Films were deposited on fused silica substrate by simple sol-gel technique. The films are critically characterized by measuring their microstructural, compositional and Raman properties. Magnetic behaviour of Ni:CuO films were recorded by using superconducting quantum interference device (SQUID) magnetometer. 3
2. Experimental 2.1. Synthesis process: Materials: Nickel acetate tetra hydrate (Ni (CH3COO)2. 4H2O) (Merck Specialities Private Limited, 98%), N-N Dimethyl formamide (Merck Specialities Private Limited, 99.9%), Diethanolamine (Merck Specialities Private Limited, 98%) and Copper acetate monohydrate (Cu (CH3COO)2. H2O) (Alfa aesar). Fused silica substrates were cleaned properly. 0.8 gm copper acetate monohydrate (Cu (CH3COO)2. H2O) was taken in a conical flask and was dissolved by stirring in 80 ml DMF at room temperature. This solution (0.2M) was kept in four different conical flasks containing 20 ml solution each. Different amounts of nickel acetate tetra hydrate (Ni(CH3COO)2.4H2O) were added to three conical flasks containing 0.2 M
copper
acetate monohydrate solution. Nickel acetate tetra hydrate was dissolved in the above three solutions by stirring at room temperature to obtain 0.03M, 0.05M, 0.08M solutions of nickel acetate tetra hydrate in copper acetate monohydrate solution. The fourth solution was kept undoped for depositing pristine CuO film. All the above solutions were then heated to temperature ~120o C for 1 hr with continuous stirring. After that aqueous NH3 solution was then added drop by drop to the above solution to maintain a pH value ~11.0. As a result, the color of the solutions turned black. Finally, films were casted using a spin coater. Before casting the films, the fused silica substrates were dried at 700C. To obtain the desired layer thickness, the films were coated for several times. These films were subjected to rapid thermal annealing (RTA) at 5000C for 5 minutes with maintaining partial pressure of Ar (~10-2 Torr) inside the RTA chamber to eliminate the volatile solvents from the films. 2.2. Characterization of the films Surface morphology of the films was recorded by using a Carl Zeiss AURIGA Field emission scanning electron microscopey (FESEM). This FESEM was operated at a voltage of 5kV in secondary emission mode. Nominal composition was determined by using energy dispersive X-ray (EDAX) studies. Micro-structural information was obtained by using a Rigaku MiniFlex X-ray diffractometer (0.154 nm Cu Kα line). 4
Charged states of the elemental constituents were obtained by using X-ray Photoelectron Spectroscopy (XPS) measurements. We have used Al Kα X-ray source at 1486.74 eV. Approximate rate of sputtering in the system was ~10 angstroms per minute. A PHOIBOS 150 HSA3500 analyzer with a delay-line detector was used to collect the spectra. The resolution and pass energy were 0.6 eV for 656 kcps and 12 eV, respectively. A Renishaw inVia micro-Raman spectrometer was used to record the Raman spectra. Wavelength of the argon laser was 514 nm. Variation of magnetization with temperature (M-T curve) and magnetic hysteresis (M-H) curves were recorded by using a superconducting quantum interference device (SQUID) magnetometer (SQUID– VSM based MPMS System, Quantum Design-make). Magnetization was measured at different temperatures in the zero-field-cooled (ZFC) and field cooled (FC) mode at the applied field of 200 Oe. Magnetization was also recorded as a function of field in the range of -10KOe H 10KOe at 10 K and 300 K.
3. Results and discussion 3.1. Microstructural studies Scotch tape adhesion tests were performed on the CuO and Cu1-xNixO (Ni~0.1, ~1.9 and ~4.7 at %) films deposited here. The films adhered strongly to the fused silica substrates. Figs. 1(a-d) show the FESEM micrographs of the above films. It may be observed that the texture of the films changed (Fig.1b-d) significantly with the incorporation of nickel nano-crystallites in CuO (Fig.1a).
Insets of Fig.1 show the
corresponding EDAX spectra recorded in line scan mode. The presence of the peaks related to Cu, Ni and O dominated the spectra. Amount of Ni (in at %) present in the Ni:CuO films were evaluated from the EDAX studies and are shown in Table-I. The structural properties of the CuO and Ni doped CuO films were investigated by X-ray diffraction studies. XRD traces recorded for the representative CuO and Cu1xNixO
(Ni~0.1, ~1.9 and ~4.7 at %) films are shown in Fig.2. The films were found to be
polycrystalline in nature. Figure.2 (curve-a) indicates the presence of the characteristic peaks located at 2θ~32.6o, ~35.8o, ~38.9o, ~58.3° and ~72.4° originating from the reflections from (110), (-111/002), (111), (202) and (311) planes of monoclinic phase of 5
CuO, respectively. Addition of nickel with varying concentration in the CuO modulated the intensities of the peaks. The intensity of peak for CuO started decreasing (Fig.2: curve-b-d) with the addition of nickel. The intensities of the peaks located at 2θ ~58.2° and ~72.4° decreasing significantly (Fig.2: curve-d). Additional peaks related to elemental Ni and NiO related phases were not observed and the concentration of Ni is very low compared to CuO. This would indicate that Ni2+ ions have effectively substituted in Cu2+ sites without changing the monoclinic structure of CuO lattice. This may be due to the fact that Cu2+ and Ni2+ have comparable ionic radii (Cu2+ = 0.73Å and Ni2+ = 0.70Å) facilitating the above substitution of Cu2+ by Ni2+ [35]. It may be observed that the peak intensity related to (002)/ (-111) and (111) planes are much stronger than that of other peak (Fig.2) which affirms the preferential orientation of planes in CuO lattice [36]. The average crystal sizes for both CuO and Ni-doped CuO were calculated by Scherrer relation [37]: D = (Kλ)/ (β cosθ)
(1)
where K is a constant with a value of K is ~0.92. β is the full width at half maximum of the peaks related to the [h k l] reflection plane and
is the Bragg’s angle. By considering
the reflection planes (002)/ (-111) and (111), we have calculated the average crystallize size. The value of average crystallite size varied between 9 nm-16 nm (Table-1). Crystallite size decreased with the inclusion of nickel in CuO.
3.2. XPS studies It may be worthwhile now to report the results on XPS studies performed on the CuO and Ni-doped CuO films. This study will add information on the formation of CuO thin films and the effect of Ni incorporation on the charge states of the elements constituting the films. Fig.3a and Fig.3b show the general survey spectra of the above films. C1s peak appeared at ~ 284.5 eV and it was used a reference to calibrate the XPS spectra. Both the general survey spectra for CuO (Fig.3a) and Cu1-xNixO (Fig.3b) are dominated by peaks arising from the binding energies of Cu2p, O1s and C1s. Additional peak for nickel could be observed for Ni:CuO film (Fig.3b). All the core level spectra of elements comprising the compounds were recorded separately in high resolution mode. 6
If we examine the core level spectra critically, they would reveal important information on the charge states of those elements. The core level Cu2p spectra for pristine CuO film (Fig.3c) indicated a doublet. The peaks located at ~933.15 eV (Cu2p3/2) and ~953.5 eV (Cu2p1/2), respectively arose due to L-S coupling of Cu2p state. The binding energy difference between these two peaks is ~20.35 eV. It is known that the Cu2p3/2 and Cu2p1/2 spectra are indicative of the Cu2+ oxidation state [38] in CuO lattice. The above doublets are associated with shake-up satellite peaks located at higher energies (~942.0 eV and ~961.5 eV) than that of parent peak [39]. The separation between the satellite peaks is ~19.5 eV. Now, upon examining the core level Cu2p spectra for Ni doped (4.72 at%) CuO films, one can see a shift in peak positions for both the peaks for Cu2p3/2 (located at ~933.15 eV) and Cu2p1/2 (located at ~953.5 eV) towards higher energy (Fig.3d). The corresponding satellites also showed a shift to higher energy (~941.8 eV and ~961.5 eV) (Fig.3d). The separations between both the parent and satellite peaks are~20.35 eV and ~19.5 eV, respectively. This observation will also indicate Cu2+ oxidation state of copper in Ni doped CuO samples. A doublet structure for Ni2p core level spectra is also visible (Fig.3e). The doublets are located at ~855.6 eV and ~873.4 eV for Ni2p3/2 and Ni2p1/2, respectively. These doublets also have satellites located at ~ 860.4 eV and ~ 879 eV. The energy shift between the two parent peaks is ~ 17.8 eV while that between the satellites is ~18.6 eV. The observed differences in energies between the peaks for Ni2p3/2 and Ni2p1/2 is lower than that reported (19.7 eV) by Liu et al. [40]. The presence of the satellite peak at ~860 eV affirms the inclusion of Ni as oxide of Ni. The peak position of Ni2p3/2 peak at ~855.6 eV is higher than the binding energy value of 852.7 eV for metallic Ni. This value matches more or less well with that for Ni2+ in NiO (~853.5 eV). The other peak for Ni2p1/2 is located at ~873.1 eV. The binding energies of Ni in metallic form and in NiO are different. The spinorbit splitting energy difference is also different in the above cases. The observed binding-energy difference in this study is ~17-20 eV. The binding-energy separation is 17.4 eV for metallic nickel and 18.4 eV Ni in NiO [41]. The experimental results obtained in this study indicated the separation between the parent peaks is ~17.5 eV and that for the satellites is ~18 eV. The observed binding-energy difference confirms that Ni 7
present in this CuO matrix would resemble that in NiO. From the above studies, it may be stated that Ni ions in these Ni:CuO films have successfully substituted Cu in the CuO lattice [42]. 3.3. Raman spectra Raman spectra of all the films were recorded at room temperature. Three Raman active optical phonons have generally been identified in the literature [43]. The Raman spectra for a representative pristine CuO film are shown in Fig.4 (curve-a). This spectra confirmed the existence of the above three known bands at ~292 cm-1, ~346 cm-1 and ~631 cm-1. The origin of the peaks located at ~292 cm-1 could easily be associated with Ag mode. The peaks located at ~346 cm-1 and ~631 cm-1 could be associated with Bg modes [44]. The broad peaks centered ~1357 cm-1 and a high energy cutoff ~1560 cm-1 may have the origin from one-magnon density of states. This observation is in conformity with that recorded from neutron scattering experiment by Ain et al. [45]. The values for Raman modes observed in our films agreed quite well with the previously reported data [43-45]. With nickel inclusion in CuO matrix, the Raman spectra changed drastically. The intensities of the peaks located at ~1357 cm-1 and ~1560 cm-1 decreased significantly (Fig.4: curves b-d). Additional peaks started appearing. Films with highest Ni content (4.7 at%: curve-d) showed peaks centered ~809-900 cm-1, ~1135 cm-1 and ~1440 cm−1. These peaks may arise due to two phonon scattering which would confirm the presence of a single phase CuO for all the films deposited here. With the inclusion of nickel in the CuO matrix, the Raman spectra got modulated significantly (Fig.4: curve b-d). With increasing nickel content in CuO, the peaks at ~943 cm−1 and ~1430 cm−1 became stronger and distinct (Fig.4d: curve-d). Raman peaks showed a slight shift towards lower wavenumber with increased nickel content in CuO lattice. This shift could be attributed to the reduction in nickel particle size. This shift could be associated with Heisenberg uncertainty principle suggesting a relationship between crystallite size and phonon [46, 47]. 3.4. Magnetization Studies The magnetic properties of CuO and Ni-doped CuO (Ni at % ~0.1, 1.9 and 4.7) have been performed by SQUID measurements. Variation of magnetization with applied 8
magnetic field (M-H) was recorded at 10 K and 300 K for the undoped CuO and Ni:CuO films. M-H curves for both T~10K and 300K showed distinct hysteresis in Fig. 5a and Fig.5b, respectively. The ferromagnetic nature of the Ni:CuO films were distinctly visible. The magnetic field was applied parallel to the substrate surface of Ni:CuO films. The shape and width of hysteresis loop changed with amount of nickel incorporation in CuO films. The soft magnetic nature of the films was indicated from the hysteresis loop. The strongest ferromagnetic moments observed in the nickel doped samples might be due to the higher magnetic moments of Ni2+ ions substituting Cu2+ ions with lower magnetic moment. It is known that the magnetic moments of Cu2+ and Ni2+are 1.73 and 2.83 µB, respectively, in the high spin state [28]. The magnetic moments of Ni2+ ions are quite larger than that of the Cu2+ ions. This would mean that increased substitution of Ni2+ ions would induce stronger ferromagnetic behavior in Ni:CuO films. Meneses et al. [29] and Arbuzova et al. [48] reported similar observations. Ni:CuO films indicated higher remanence (MR) for films containing higher amount of nickel in the Ni:CuO films (Fig.5d). It may be noted here that the MS was seen to increase monotonously with the amount of nickel incorporated (Fig.5c). This increase was found to be steeper when measured at 10 K than 300K. It may also be observed that magnetization in the samples did not show saturation in the range of magnetic field under this study. Then the necessary question would arise if there is an upper limit of incorporation of nickel for MS in the sample showing saturation in this observed magnetic field. As this study was focused on the observation of the onset of ferromagnetism in Ni-doped CuO films, we refrained ourselves from adding larger amounts of nickel for observing the upper limit of incorporation nickel that might exist for MS in the sample. Contrary to the above, the coercive force decreased for films containing increased amounts of nickel (Fig.5e). This observation would not support the possible formation of NiO at the surface of the Ni:CuO composite films. It may be pointed here that the contribution from the paramagnetic nature of the substrate (fused silica here) cannot be ruled out. Background effect arising due to the substrate was taken care of during measurement. But one must remember that the mass of the substrate materials is quite larger than that of the film material under study. Thus the observed continuous increase in high field magnetization may have the origin of the above paramagnetic contributor present in the sample. Core-shell 9
morphology would be a more probable surface structure in films having surface oxide layer. The above structure would mean that ferromagnetic metallic nanoparticle (Ni) would be surrounded by an anti-ferromagnetic oxide layer (CuO) around it. In such a case, the magnetic properties would be greatly influenced due to the exchange interaction between the ferromagnetic core material and antiferromagnetic shell material [29, 47]. Formation of oxygen vacancies might also be another cause of enhanced ferromagnetism in Ni-doped CuO samples. Thus, Ni ion in CuO lattice might ‘‘trap’’ the free electrons originated from the oxygen vacancies. This might result in the enhancement of the ferromagnetic spin–spin interaction between nickel and copper atoms. This would be resulting in an enhancement of ferromagnetism in this system. The M(T) curves related to zero-field cooling (ZFC) and field-cooling (FC) of representative undoped CuO and Ni:CuO samples containing three different Ni concentrations (0.1 at %, 1.9 at% and 4.7 at%) are shown in Figs.6(a-d). ZFC and FC measurements were carried out from 20 K to 330 K at a fixed applied field ~200 Oe. The magnetic field was applied parallel to the substrate surface of Ni:CuO films and the contribution from the substrate (fused silica) was subtracted from the raw data for all the films. The samples show ferromagnetic behavior persisting up to 330 K and Currie temperature (TC) is thus estimated to be higher than 330 K. Magnetization measured was found to be essentially temperature independent between 20 K and 330 K. A model combining both diamagnetic and ferromagnetic components may account for the initial decrease of magnetization from 20 K to 200 K and remaining non-zero up to 330 K [49]. It is known that CuO may be either paramagnetic or anti-ferromagnetic in nature [50, 51]. As we increased the nickel content in CuO, there would be increased probability of an increase in NiO to Ni ratio due to increased surface oxidation. NiO is also well known anti-ferromagnets with the Neel temperature of 523 K. It may be stressed here that the amount of anti-ferreomagnetic NiO component would be insignificantly small compared to anti-ferreomagnetic CuO component. Thus, the observed sharp decrease in magnetization for Ni:CuO film deposited with higher nickel content (Fig. 6) would mainly be related to the anti-ferromagentic CuO component only (Fig.6 c, d). The ferromagnetic behavior in the films would thus mainly arise from the inclusion of nanocrystalline Ni in the CuO lattice. 10
Ferromagnetic properties of Ni:CuO is further demonstrated by the separation between the field cooling (FC) and zero field cooling (ZFC) curves. The FC curve was measured after recording the ZFC curve. The magnetization was measured with field cooled from 330 K to 20 K under 200 Oe. The ZFC-FC studies are carried out for studying static magnetization. Generally, an open loop is indicative of ferromagnetic behavior. A closed loop is observed in superparamagnetic materials. Paramagnetic behavior is indicated by a linear M-H curve. ZFC measurements are carried out by cooling the sample without application of any magnetic field. The data are collected upon heating and in presence of some magnetic filed. For FC curves, Sample are cooled with an applied magnetic field for recording FC behavior. The data can be collected in two ways: may be during cooling or heating [52,53] . At room temperature, the spins would be oriented by the magnetic field. When cooled in presence of an applied, field, the spins would have less chance to disorient. In presence of strong anisotropy, it would increase at low temperatures. The observed open loop between ZFC and FC disappeared at ~330 K. This would support the observed ferromagnetism in these Ni:CuO samples under this study At this point, the particles are free to align with the field during the measurement time. The sample showed ferromagnetic behavior which persisted above 300 K and TC is thus estimated to be higher than 330 K. Magnetization was found to be essentially temperature dependent between 20 and 330 K. 4. Conclusions
Ni:CuO composite films were deposited by sol-gel technique. The films contained 0.1 at %, 1.9 at% and 4.7 at% of nickel. The size of the Ni nanocrystallites varied between 9.0 nm and 16 nm. The Ni 2p core level XPS spectra contained two peaks located at ~852.7 and ~870.1 eV for Ni 2p3/2 and Ni 2p1/2, respectively. This implied that Ni mainly existed in the Ni2+ states in the Ni:CuO films. Ferromagnetic behavior persisted above 300 K in all the Ni:CuO films studied here. TC estimated was higher than 300 K. Magnetization decreased with increasing temperature. Thus, it may be concluded that observed ferromagnetism in Ni-doped CuO samples can be due to: (i) substitution of Cu2+ ions having a lower magnetic moment by Ni2+ ions having larger magnetic moments and/or ii) the formation of oxygen vacancies. Ni ion in CuO matrix may ‘‘trap’’ the free 11
electrons emitted by the oxygen vacancies. This may result in the enhancement of the ferromagnetic spin–spin interaction between nickel and copper atoms. Magnetization measured was found to be essentially temperature independent between 20 K and 330 K. The observed sharp decrease in the magnetization for Ni:CuO film deposited with higher nickel content would mainly be related to the anti-ferromagentic CuO component only. Acknowledgement The authors wish to thank the Department of Science and Technology, Government of India, Government of India for supporting the fellowship. The authors thank, Dr. D. Samal, Institute of Physics, Bhubaneswar for providing SQUID facility.
12
Caption of Figures Fig. 1 FESEM pictures of representative (a) CuO and three of Ni:CuO films deposited with different concentration of nickel in CuO: (b) 0.1 at %, (c) 1.9 at%, (d) 4.7 at %. Insets show corresponding EDAX spectra of the samples. Fig. 2 XRD traces of four representative Ni:CuO films deposited with different amount of nickel in CuO: (a) Pure CuO, (b) Ni~0.1 at %, (c) Ni~1.9 at%, (d) Ni~4.7 at %. Fig. 3 General survey XPS spectra for a representative: (a) undoped CuO film and (b) Ni:CuO film (Ni~4.7 at%); Core level spectra for: (c) Cu2p of undoped CuO and (d) Cu2p of Ni:CuO film (Ni~4.7 at%); (e) Ni 2p core level spectra of Ni:CuO film (Ni~4.7 at%). Fig.4 (a) Raman spectra for four representative Ni:CuO films: (a) Pure CuO, (b) Ni~0.1 at %, (c) Ni~1.9 at%, (d) Ni~4.7 at %. Fig. 5 M-H curves for Ni:CuO films (a) at 10 K and (b) at 300 K. The magnetization was measured in parallel configurations to the substrate of Ni:CuO films. (c) Variation of saturation magnetic moment as a function nickel concentration, (d) Variation of reminiscent magnetization as a function nickel concentration and (e) Variation of coercive force with the Ni content (At %) in the films Fig.6. FC and ZFC behavior of: (a) Pure CuO, (b) Ni~0.1 at %, (c) Ni~1.9 at%, (d) Ni~4.7 at %.
13
Table-1: Different properties of the Ni:CuO films deposited here
Sample
Ni
Crystal
(EDAX)
planes
(At %)
Crystal Average size
crystals
(XRD)
sizes
(nm)
(XRD)
Remanence
Coercivity
Saturation
(MR)
( HC)
magnetization
(KOe)
(MS)
2
(A.m /kg)
(A.m2/kg)
(nm)
Cu1.00Ni0.0O
Cu0.97Ni0.03O
Cu0.95Ni0.05O
Cu0.92Ni0.08O
0
0.1
1.9
4.7
[-111]
16.14
[111]
12.23
[-111]
12.28
[111]
12.39
[-111]
9.8
[111]
12.22
[-111]
9.69
[111]
9.26
10 K
300 K
( x10-4)
(x10-4)
14.18
1.43
0.0413
0.228
12.33
2.35
1.641
11.01
3.99
9.48
11.8
14
10 K
300 K
10 K
300 K
(x10-3)
(x10-3)
0.156
0.873
0.194
0.196
0.142
5.76
0.565
2.548
0.149
0.105
7.22
1.35
5.459
0.103
0.077
11.2
3.45
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Highlights:
High quality Ni-doped CuO films is synthesized by sol-gel technique Ni2+ atomic concentration was found to dominate the magnetic property
The magnetization increases with Ni inclusion in CuO lattice.
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