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Thickness dependent ferromagnetism in thermally decomposed NiO thin films
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Thickness dependent ferromagnetism in thermally decomposed NiO thin films Patta Ravikumar, Bhagaban Kisan, Alagarsamy Perumal n Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 26 November 2015 Received in revised form 10 February 2016 Accepted 27 February 2016
We report the effects of film thickness, annealing temperature and annealing environments on thermal decomposition behavior and resulting magnetic properties of NiO (t ¼50–300 nm) thin films. All the NiO films were prepared directly on thermally oxidized Si at ambient temperature using magnetron sputtering technique and post annealed at different temperatures (TA) under vacuum and oxygen atmospheres. As-deposited films exhibit face centered cubic structure with large lattice constant due to strain induced during sputtering process. With increasing TA, the lattice constant decreases due to the release of strain and thickness dependent thermal decomposition reaction of NiO into Ni has been observed for the NiO films annealed at 500 °C under vacuum condition. As a result, the antiferromagnetic nature of the asdeposited NiO films transforms into ferromagnetic one with dominant thickness dependent ferromagnetic behavior at room temperature. In addition, the existence of both Ni and NiO phases in the annealed NiO films shows noticeable exchange bias under field cooling condition. The behavior of thermal decomposition was not observed for the NiO films annealed under oxygen condition which results in no detectable change in the magnetic properties. The observed results are discussed on the basis of thickness dependent thermal decomposition in NiO films with increasing TA and changing annealing conditions. & 2016 Elsevier B.V. All rights reserved.
Keywords: NiO films Annealing Microstructure Thermal decomposition Antiferromagnetism Ferromagnetism
1. Introduction Recently nanostructured nickel oxide (NiO) materials have received enormous interest due to their novel properties, which are considerably different from those observed in bulk material, and their potential applications in a wide variety of technological areas such as spin valves, magnetic recording, resistive random access memory, electrochromic, supercapacitors, alkaline batteries, etc [1–6]. The proper control of size, shape, morphology and surface of nanostructures remains a challenge as the properties are strongly depending on the structures at the nanoscale. In particular, the development of magnetism in NiO based thin films was focused to obtain ferromagnetism above room temperature such that these oxides with cubic structure could easily facilitate integration of spintronic devices. As a result, different fabrication methods such as sol–gel [7], vacuum evaporation [8], pulsed laser deposition [9], magnetron sputtering [10–13] and reactive ion beam sputtering [14] were employed for the preparation of NiO films. It is well known that the bulk NiO is an antiferromagnetic (AFM) material with a Néel temperature (TN) of 523 K and n
Corresponding author. E-mail address: [email protected] (A. Perumal).
transforms from cubic to rhombohedral structure below TN [15,16]. However, NiO thin films interestingly show unusual properties due to the occurrence of structural disorder, vacancies of nickel and/or oxygen, aligned dislocations into NiO crystal, finite size effect, etc. For instance, Sugiyama et al. [9] demonstrated that dislocations in NiO crystals show unique magnetic properties with high coercivity due to the strong interaction between the ferromagnetic dislocations and surrounding AFM bulk phase. While Jang et al. [17] reported that the enhanced electrical properties of sputtered NiO thin films are mainly due to the nickel vacancy based defects, Yang et al. [18] shown that the non-stoichiometric NiO thin films should contain excess oxygens at interstitial positions, which diffuse out during the annealing process and hence tune the electrical, chemical and optical properties. Bruckner et al. [19] reported that the sputtered NiO films having non-stoichiometric NiO decompose during heating at high temperatures (TA) into thermally stable oxygen poorer NiO and/or metallic Ni. Similarly, Kawai et al. [20] reported that annealing of NiO films above 400 °C under high vacuum environment produces conducting filaments in single crystalline NiO, which results in memory cell. Recently, Jang et al. [21] reported that the decomposition reaction of NiO films strongly depends on TA and thickness of the films (t). These results reveal that the thermal decomposition of sputtered NiO is very much important for selected practical applications such as
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resistive random access memory and spintronic devices. However, the related mechanism of thermal decomposition as a function of thickness and annealing temperature, and the resulting magnetic properties are still unclear. Therefore, in this study, we report the thickness dependent thermal decomposition behavior and the resulting magnetic properties of NiO thin films deposited using magnetron sputtering technique at ambient temperature directly on thermally oxidized Si substrate and post annealed at different TA under high vacuum environment. The observed results are also compared with the results obtained by annealing the as-deposited NiO films under oxygen condition.
2. Experimental details NiO (t nm) single layer thin films with various thicknesses (t ¼50–300 nm) were deposited directly on thermally oxidized Si substrate by sputtering NiO target (50.8 mm diameter and 3 mm thickness) using magnetron sputtering technique with various process parameters. NiO sputtering target was prepared by ball milling the high purity NiO (99.9% Sigma Aldrich, USA) powders using high energy planetary ball mill followed by sintering process. Before starting the deposition, the base pressure of the chamber was evacuated below 5 10 7 Torr. Sputtering was carried out at ambient temperature in an argon atmosphere of 21 mTorr. The optimization of argon gas pressure was done mainly by analyzing the formation of NiO thin films with AFM nature. The deposition rate of NiO films was pre-calibrated using ex-situ surface profilometer (Vecco, Dektak 150 model). Accordingly, the deposition rate for NiO thin film under the above sputtering conditions was optimized to be 1.67 nm/min. All the as-deposited films were post annealed ex-situ in a separate high vacuum (o10 6 Torr) annealing setup at different TA: 300 °C, 400 °C and 500 °C for 70 min duration. In order to compare the resulting properties, the NiO films were also annealed at 500 °C under oxygen atmosphere. The annealing temperature and annealing time were optimized based on the thermal decomposition behavior of NiO thin films. Crystal structure and microstructure properties of the as-deposited and annealed films were analyzed through X-ray diffraction (XRD) obtained using high-power X-ray diffractometer (Rigaku TTRAX III, 18 kW) with Cu-Kα radiation (λ ¼1.54056 Å) and transmission electron microscopy (TEM, JEOL 2100) techniques, respectively. XRD data were collected at a slow scan rate of 0.005°/ s for analyzing the structural parameters. Magnetic properties of the films were analyzed by using vibrating sample magnetometer (VSM, Lake Shore Model 7410) by performing magnetic hysteresis loops (M H) along the film plane at different constant temperatures in the temperature range of 30–300 K.
diffractometer confirms the formation of high purity NiO thin films. (ii) However, a careful comparison between the as-deposited films and bulk NiO powder confirms that the peaks are substantially broadened and the peak positions are shifted noticeably to lower angles. This suggests that the as-deposited films have fine crystallites and large lattice constant as compared to the bulk NiO. (iii) With increasing TA up to 400 °C, the peak broadening decreases gradually along with a considerable shift in the peak position to higher angles as shown in Fig. 1b and c. This confirms that the average crystallite size increases and lattice constant decreases with increasing TA. (iv) On further increasing TA to 500 °C, the NiO films with tr100 nm exhibit no detectable peaks related to fcc phase of NiO, but display only the peak corresponding to Ni phase. This reveals that the NiO films decompose into Ni. With increasing t4100 nm, we observed the existence of two peaks corresponding to (200) peak of NiO and (111) peak of Ni. Interestingly, the relatively intensity of the NiO(200) peak increases with increasing NiO film thickness from 200 nm to 300 nm. This suggests that the amount of decomposition reaction from NiO into Ni decreases with increasing NiO film thickness [21]. These results clearly suggest that the thermal decomposition reaction of NiO into Ni
3. Results and discussion 3.1. Structural properties of NiO thin film Fig. 1 depicts the room temperature XRD patterns of the asdeposited and annealed NiO (t nm) thin films at different temperatures under vacuum and oxygen annealing conditions. In order to compare the XRD patterns of the thin films with the bulk counterpart, XRD pattern of the bulk NiO powder with rescaled intensity is also included in Fig. 1a. The XRD patterns show the following features: (i) All the NiO films grown with different thicknesses exhibit face centered cubic (fcc) structure and highly oriented along (200) plane. The intensity of the (200) peak increases with increasing NiO film thickness. The absence of any other impurity peaks within the detection limit of X-ray
Fig. 1. Room temperature XRD patterns of as-deposited (a) and annealed NiO thin films with different thicknesses at different temperatures under vacuum (b,c,d) and oxygen (e) conditions.
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strongly depends on the NiO film thickness. To understand the decomposition reaction under different environments, we have carried out annealing at 500 °C under oxygen atmosphere. Fig. 1e displays the XRD patterns of the annealed NiO films at 500 °C under oxygen atmosphere. It is clearly evident that the annealing under oxygen atmosphere has not promoted any decomposition behavior and exhibited highly oriented NiO(200) plane. In addition, a considerable reduction in peak broadening is observed and the peak position of NiO(200) almost matches with the peak position of the bulk NiO powder. This indicates that the average crystallite size increases and the lattice constant approaches to that of bulk NiO. In order to quantify the changes in the structural parameters such as lattice constant (a), crystallite size (D) and strain (η) in the as-deposited and annealed films, the XRD data were analyzed using the following formulae:
⎛ ⎞ λ h2 + k 2 + l2 ⎟ a = ⎜⎜ ⎟ 2 sin θ ⎝ ⎠
(1)
⎛ 0.94λ ⎞ D=⎜ ⎟ ⎝ β cos θ ⎠
(2)
⎛ afilm − abulk ⎞ η(%) = ⎜ ⎟ × 100 abulk ⎝ ⎠
(3)
where, λ is the wavelength of the X-ray (λ ¼1.54056 Å), h,k,l are the miller indices, D is the crystallite size calculated using the full width at half maximum (β) of (200) peak for NiO and (111) peak for Ni from the Scherrer's formula, abulk is unstrained lattice constant taken from the bulk NiO and its value is 4.1687 Å and afilm is the lattice constant of the NiO thin film. The strain in NiO thin films is mainly caused by the combined effect of thickness and dynamics of the deposition. Fig. 2 depicts the variations of a and η with increasing TA for NiO thin films with different thicknesses. As-deposited films exhibit significantly a large lattice constant in the range between 4.239 Å and 4.251 Å, but decreases slightly with increasing film thickness. On other hand, the lattice constant decreases with increasing TA and approaches to bulk value for the films annealed at 500 °C. Kuo et al. [22] reported that (i) the intensity of NiO(111) and NiO(220) peaks decreases, (ii) the intensity of NiO(200) peak intensity increases, and (iii) a considerable shift in the peak position of all the peaks to lower angle was observed with increasing argon gas pressure. These behaviors were correlated to the increase of argon content in sputtered NiO film deposited at higher argon pressure, which expands the lattice parameter when argon atoms occupy the interstitial positions. On the other hand, Bruckner et al. [19] and Kim et al. [23] confirmed that the sputtered NiO films have non-stoichiometric and poor crystallinity due to excess oxygen state, which makes them thermally unstable. Youssef et al. [10] reported that the NiO microstructure can be modified by changing the stoichiometry through the control of argon gas pressure during the sputtering growth. These results in correlation with the presently observed results indicate that the observation of large lattice constant in the as-deposited NiO thin films could be attributed to the strain induced in the film possibly due to the existence of interstitial argon atoms in the NiO lattice, which expands the lattice constant when argon pressure is high and/or increase of nickel vacancies created from the nonstoichiometry. With increasing TA up to 400 °C, the lattice constant decreases gradually and approaches to bulk NiO value for the NiO films
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Fig. 2. The variations of lattice constant, a and strain, η as a function of annealing temperature, TA for the NiO thin films with different thicknesses.
annealed at TA ¼500 °C. This could be related to the release of interstitial argon atoms from NiO during the post annealing process. In addition, NiO films annealed at 500 °C exhibit thermal decomposition reaction, which is strongly depending on the film thickness. The decomposition reaction could be attributed to the existence of considerable nickel vacancies. According to the ideal model, the vacancy of each Ni in NiO lattice results in six oxygen neighbors to become incompletely bonded, which might induce decomposition under less stringent conditions [17,18]. Furthermore, this decomposition behavior typically occurs from the upper surface of the films, which is evident from the thickness dependent decomposition process [21]. Das et al. [24] have also demonstrated that the creation of oxygen vacancies at the freshly deposited NiO film surface by ultrahigh vacuum annealing is more effective in case of thinner NiO films. These results further confirm that there is a critical thickness, which is currently 100 nm, for the decomposition reaction under vacuum annealing conditions. On the other hand, the strain, calculated using Eq. (3) for NiO thin films annealed at different TA, displays the variations almost similar to those observed for lattice constant with increasing TA. This could be correlated to the release of interstitial argon atoms with increasing TA. The average crystallite size of the NiO phase calculated from the (200) peak exhibits weak dependence on the film thickness, but increases from about 5 nm to 10 nm with increasing TA. Similarly, for the NiO films annealed at 500 °C, the average crystallite size of the Ni, calculated from (111) peak, increases from about 10 nm for the 50 nm thick film to 18 nm for the 200 nm thick film and then decreases slightly to 15 nm for the 300 nm thick decomposed film. This could be attributed to the thickness dependent decomposition behavior of NiO into Ni with increasing film thickness. In order to understand the changes in the microstructure of NiO films with annealing, we have analyzed the microstructure of the as-deposited and annealed films using TEM. Fig. 3 displays typical TEM micrographs and selected area electron diffraction (SAED) patterns of the as-deposited and annealed NiO (t ¼200 nm) thin films at 500 °C. The bright-field TEM micrograph and the SAED pattern of as-deposited film displays very fine crystals and the diffraction rings corresponding to fcc structure, respectively. In order to understand the microstructure more in detail, dark-field TEM micrograph corresponding to NiO(200) plane and high-resolution TEM micrograph are shown in Fig. 3b and c, respectively. While the dark-field TEM micrograph clearly reveals the existence of fine nanocrystals with the size of around 5 nm, the high-resolution TEM micrograph confirms the NiO phase with enhanced lattice constant. On the other hand, the annealed film exhibits well-defined polycrystalline nature mainly due to the formation of Ni nanocrystals out of NiO matrix with the size ranging between 12 and 20 nm. The presence of both the NiO and Ni phases is also supported by the SAED pattern. The lattice constant calculated using SAED pattern are found to be 4.167 Å and 3.517 Å for NiO and
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Fig. 3. Bright-field TEM micrograph and SAED pattern (a), dark-field TEM micrograph (b) and high-resolution TEM micrograph (c) for as-deposited NiO (200 nm) film, and bright-field TEM and SAED pattern for annealed NiO (200 nm) film at 500 °C under vacuum condition (d).
Ni, respectively. These results are in good agreement with the results obtained from XRD analysis. These structural changes with annealing temperature in NiO films with different thicknesses are also expected to play a significant role on the magnetic properties. 3.2. Magnetic properties of NiO thin film To study the changes in the magnetic properties, we have measured room temperature and temperature dependent in-plane M–H loops in the temperature range of 30–300 K for the as-deposited and annealed films. Fig. 4 displays the typical M–H loops of as-deposited and annealed films at different TA, temperature dependent M–H loops for as-deposited NiO (t ¼300 nm) films, and the extracted values of magnetization at 2.5 kOe (MS) and coercivity (HC) as a function of film thickness for the annealed films at TA ¼500 °C under vacuum condition. All the as-deposited films (Fig. 4a) and the films annealed up to 400 °C (see inset (i) of Fig. 4c) exhibit weak magnetic signals without any clear trace of hysteresis loop. In addition, the noise in the substrate corrected data is mainly due to the small moment from the antiferromagnetic nature of the as-deposited films [12] to be detected
by the instrument in the presence of large diamagnetic contribution. It has been reported that the nanoscale NiO powders show induced ferromagnetic behavior due to uncompensated surface spins and the average magnetization increases with decreasing the average crystal size [25–27]. In order to understand the role of presence of fine NiO crystals (see Fig. 3) on the magnetic properties of the as-deposited films, M–H loops were obtained at low temperatures down to 30 K and shown in Fig. 4b. It is observed that the magnetization increases slightly with decreasing temperature, but the nature of M–H loops does not change substantially down to 30 K. This could be explained as follows: Although the size reduction in NiO is expected to produce induced ferromagnetic properties due to uncompensated spins, the observation of pronounced lattice expansion (see Fig. 2) in the presence of many broken bonds and defects is most likely to give an increased bond length of Ni–O and thereby significantly weakening the strength of the super-exchange interactions of magnetic ions [28]. As a result, no induced ferromagnetic properties within the detection limit of the instrument could clearly be seen in the presently investigated samples down to 30 K. On the other hand, as depicted in Fig. 4c, the NiO thin films annealed at 500 °C show
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ferromagnetic nature with clear hysteresis loops, which saturates at moderate applied magnetic fields. However, with increasing t, the shape of the M–H loop changes and the field required for saturating the films' magnetization increases with increasing film thickness. Interestingly, the NiO thin films annealed at 500 °C under oxygen atmosphere (see inset (ii) of Fig. 4c) also exhibit only a weak magnetic signal without any hysteresis behavior. This could be correlated to the non-observation of decomposition reaction in NiO films under oxygen conditions. The values of MS for the NiO films annealed at 500 °C under vacuum condition increase from 95 emu/cc to 130 emu/cc with increasing t from 50 to 100 nm and then decrease to 80 emu/cc and 71 emu/cc with the further increase in film thickness between 200 and 300 nm, respectively. This reveals that MS is strongly depending on the film thickness. While the increase in MS can be explained to the complete decomposition of NiO into Ni in lower thickness films up to 100 nm, the decrease in MS for higher thickness NiO films is due to
Fig. 4. Room temperature (a) in-plane M–H loops of as-deposited NiO films with different thicknesses, (b) in-plane M–H loops of as-deposited NiO (300 nm) films measured at different temperatures, (c) Room temperature in-plane M–H loops of annealed NiO thin films at 500 °C, and (d) the variations of MS and HC with NiO film thickness annealed at 500 °C. Inset of (c): Room temperature M–H loops of NiO films with different thicknesses annealed at (i) 400 °C under vacuum condition and (ii) 500 °C under oxygen condition.
Fig. 5. M–H loops measured at different temperatures for NiO films with different thicknesses annealed at 500 °C under vacuum condition.
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reduction in the decomposition reaction with increasing NiO film thickness above 100 nm resulting in a reduced amount of Ni in the NiO matrix, as demonstrated in the structural properties using XRD and TEM. Zhang et al. [29] reported that MS values in Fe doped NiO thin films increases initially with increasing film thickness up to 250 nm and then decreases due to the enhanced stress in the film which induces the antiferromagnetic exchange interaction and resulting in a weaker magnetization. On the other hand, HC increases from 43 Oe to 100 Oe with increasing t, but the amount of increase in HC decreases with increasing t above 100 nm. While the low value of HC in smaller thickness is related to soft magnetic nature of Ni, the increase in HC may be attributed to the strong interaction between Ni and NiO phases resulting in a high anisotropy and hence enhances the value of HC. Miller et al. [30] reported that HC in pure Ni film increases initially with increasing Ni thickness nearly up to 80 nm, which is in good agreement with the presently observed results. Yi et al. [31] reported that high values of HC in NiO films prepared under different O2 partial pressures are mainly due to the interface structure of Ni/ NiO during the formation of NiO under magnetic field annealing. To study the magnetic properties of the NiO films annealed at 500 °C under vacuum annealing in more detail, we have carried out M–H loops at different temperatures under zero-field cooled and field-cooled conditions. Fig. 5 shows the typical M–H loops measured under zero-field cooled conditions for NiO thin films annealed at 500 °C with different thicknesses. It is clear from the figure that the shape of the M–H loops does not change with temperature for each film thickness. However, the area under the loop increases gradually with decreasing temperature as expected for a typical ferromagnetic material and resulting in an increase in HC. The extracted values of HC from the M–H loops at different temperatures for different film thicknesses are summarized in Fig. 6. HC increases almost linearly in logarithmic scale for all the films with decreasing temperature and the rate of increase in HC considerably depends on the film thickness. To study the exchange bias properties in these NiO and Ni systems, we have also carried out M-H loop measurements at different temperatures from 30 to 300 K under the field of 2 kOe. Fig. 6b shows the determined values of exchange bias, HE (¼(|HC |-|HC þ |)/2) under zero-field cooled and field-cooled conditions as a function of temperature for 300 nm thick NiO film annealed at 500 °C. It is clearly evident from the figure that HE in zero-field cooled films is nearly zero down to 150 K and then increases weakly at low temperatures. On the other hand, the field cooled films exhibit significantly large HE at low temperature. Rinaldi-Montes et al. [27] reported that the size effects in NiO induce surface spin frustration which competes with the expected antiferromagnetic order and giving rise to a threshold size for the antiferromagnetic phase to nucleate. While the larger NiO
Fig. 6. The variations of (a) HC with temperature and (b) HE with temperature under zero-field-cooled (ZFC) and field-cooled (FC) conditions for the NiO (300 nm) thin films annealed at 500 °C under vacuum condition.
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particles form uncompensated antiferromagnetic core with a net magnetic moment surrounded by a spin-glass shell resulting in exchange bias effect, the NiO nanoparticle of size less than 3 nm seems to be in a spin-glass state. Kisan et al. [32] reported that the NiO nanoparticles with different particle sizes prepared by sol-gel process showed large variation in exchange bias under field-cooled condition due to the existence of spin-glass phase, which induce additional anisotropy between surface spins and compensated core spins. On the other hand, Valladares et al. [11] reported that the magnitude of exchange bias field in annealed NiO thin films strongly depends on the relative fraction of Ni and NiO phases providing more prominent interfaces between Ni and NiO crystals. Therefore, the exchange bias in the presently investigated system is mainly due to the enhanced anisotropy caused by the improved interaction between Ni and NiO crystallites having sharp and small mixed interface areas [11,31]. The disappearance of HE above 200 K reflects the existence of thermal dependence of anisotropy in these films. The observed results reveal that the NiO films with cubic structure are potential candidates for resistive random access memory and spintronic applications.
4. Conclusion We have studied the effects of NiO film thickness and annealing temperature under different environments on the thermal decomposition reaction of NiO into Ni and the resulting magnetic properties of the NiO films deposited directly on thermally oxidized Si substrate using magnetron sputtering technique at ambient temperature. All the as-deposited films exhibited fcc structure and highly oriented along (200) plane. The lattice constant of the as-deposited films was observed to be large as compared to bulk NiO due to induced strain by the presence of interstitial argon atoms in the sputtered NiO films and possible nickel vacancy. With increasing annealing temperature, the lattice constant decreases due to the release of strain and approaches to the bulk NiO value. Furthermore, the thickness dependent decomposition reaction of NiO into Ni occurs typically from the upper surface of the NiO thin films annealed at 500 °C under vacuum annealing. However, the decomposition reaction was completely suppressed for the NiO films annealed under oxygen condition. All the as-deposited films and the films annealed up to 400 °C under vacuum condition and 500 °C under oxygen conditions show only antiferromagnetic nature, but the NiO films annealed at 500 °C under vacuum condition exhibit room temperature ferromagnetism due to the decomposition process. A close correlation between the structural and magnetic properties was observed for all the NiO films.
Acknowledgments The authors acknowledge the financial support from Council of Scientific and Industrial Research vide project No. 03(1337)/15/
EMR-II, New Delhi. Infrastructural facilities provided by the Department of Science and Technology, India vide project No: [SR/S2/ CMP-19/2006, SR/FST/PII-020/2009] and Central Instruments Facility (CIF), IIT, Guwahati are gratefully acknowledged.
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Thickness dependent ferromagnetism in thermally decomposed NiO thin films Patta Ravikumar, Bhagaban Kisan, Alagarsamy Perumal n Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India
art ic l e i nf o
a b s t r a c t
Article history: Received 26 November 2015 Received in revised form 10 February 2016 Accepted 27 February 2016
We report the effects of film thickness, annealing temperature and annealing environments on thermal decomposition behavior and resulting magnetic properties of NiO (t ¼50–300 nm) thin films. All the NiO films were prepared directly on thermally oxidized Si at ambient temperature using magnetron sputtering technique and post annealed at different temperatures (TA) under vacuum and oxygen atmospheres. As-deposited films exhibit face centered cubic structure with large lattice constant due to strain induced during sputtering process. With increasing TA, the lattice constant decreases due to the release of strain and thickness dependent thermal decomposition reaction of NiO into Ni has been observed for the NiO films annealed at 500 °C under vacuum condition. As a result, the antiferromagnetic nature of the asdeposited NiO films transforms into ferromagnetic one with dominant thickness dependent ferromagnetic behavior at room temperature. In addition, the existence of both Ni and NiO phases in the annealed NiO films shows noticeable exchange bias under field cooling condition. The behavior of thermal decomposition was not observed for the NiO films annealed under oxygen condition which results in no detectable change in the magnetic properties. The observed results are discussed on the basis of thickness dependent thermal decomposition in NiO films with increasing TA and changing annealing conditions. & 2016 Elsevier B.V. All rights reserved.
Keywords: NiO films Annealing Microstructure Thermal decomposition Antiferromagnetism Ferromagnetism
1. Introduction Recently nanostructured nickel oxide (NiO) materials have received enormous interest due to their novel properties, which are considerably different from those observed in bulk material, and their potential applications in a wide variety of technological areas such as spin valves, magnetic recording, resistive random access memory, electrochromic, supercapacitors, alkaline batteries, etc [1–6]. The proper control of size, shape, morphology and surface of nanostructures remains a challenge as the properties are strongly depending on the structures at the nanoscale. In particular, the development of magnetism in NiO based thin films was focused to obtain ferromagnetism above room temperature such that these oxides with cubic structure could easily facilitate integration of spintronic devices. As a result, different fabrication methods such as sol–gel [7], vacuum evaporation [8], pulsed laser deposition [9], magnetron sputtering [10–13] and reactive ion beam sputtering [14] were employed for the preparation of NiO films. It is well known that the bulk NiO is an antiferromagnetic (AFM) material with a Néel temperature (TN) of 523 K and n
Corresponding author. E-mail address: [email protected] (A. Perumal).
transforms from cubic to rhombohedral structure below TN [15,16]. However, NiO thin films interestingly show unusual properties due to the occurrence of structural disorder, vacancies of nickel and/or oxygen, aligned dislocations into NiO crystal, finite size effect, etc. For instance, Sugiyama et al. [9] demonstrated that dislocations in NiO crystals show unique magnetic properties with high coercivity due to the strong interaction between the ferromagnetic dislocations and surrounding AFM bulk phase. While Jang et al. [17] reported that the enhanced electrical properties of sputtered NiO thin films are mainly due to the nickel vacancy based defects, Yang et al. [18] shown that the non-stoichiometric NiO thin films should contain excess oxygens at interstitial positions, which diffuse out during the annealing process and hence tune the electrical, chemical and optical properties. Bruckner et al. [19] reported that the sputtered NiO films having non-stoichiometric NiO decompose during heating at high temperatures (TA) into thermally stable oxygen poorer NiO and/or metallic Ni. Similarly, Kawai et al. [20] reported that annealing of NiO films above 400 °C under high vacuum environment produces conducting filaments in single crystalline NiO, which results in memory cell. Recently, Jang et al. [21] reported that the decomposition reaction of NiO films strongly depends on TA and thickness of the films (t). These results reveal that the thermal decomposition of sputtered NiO is very much important for selected practical applications such as
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resistive random access memory and spintronic devices. However, the related mechanism of thermal decomposition as a function of thickness and annealing temperature, and the resulting magnetic properties are still unclear. Therefore, in this study, we report the thickness dependent thermal decomposition behavior and the resulting magnetic properties of NiO thin films deposited using magnetron sputtering technique at ambient temperature directly on thermally oxidized Si substrate and post annealed at different TA under high vacuum environment. The observed results are also compared with the results obtained by annealing the as-deposited NiO films under oxygen condition.
2. Experimental details NiO (t nm) single layer thin films with various thicknesses (t ¼50–300 nm) were deposited directly on thermally oxidized Si substrate by sputtering NiO target (50.8 mm diameter and 3 mm thickness) using magnetron sputtering technique with various process parameters. NiO sputtering target was prepared by ball milling the high purity NiO (99.9% Sigma Aldrich, USA) powders using high energy planetary ball mill followed by sintering process. Before starting the deposition, the base pressure of the chamber was evacuated below 5 10 7 Torr. Sputtering was carried out at ambient temperature in an argon atmosphere of 21 mTorr. The optimization of argon gas pressure was done mainly by analyzing the formation of NiO thin films with AFM nature. The deposition rate of NiO films was pre-calibrated using ex-situ surface profilometer (Vecco, Dektak 150 model). Accordingly, the deposition rate for NiO thin film under the above sputtering conditions was optimized to be 1.67 nm/min. All the as-deposited films were post annealed ex-situ in a separate high vacuum (o10 6 Torr) annealing setup at different TA: 300 °C, 400 °C and 500 °C for 70 min duration. In order to compare the resulting properties, the NiO films were also annealed at 500 °C under oxygen atmosphere. The annealing temperature and annealing time were optimized based on the thermal decomposition behavior of NiO thin films. Crystal structure and microstructure properties of the as-deposited and annealed films were analyzed through X-ray diffraction (XRD) obtained using high-power X-ray diffractometer (Rigaku TTRAX III, 18 kW) with Cu-Kα radiation (λ ¼1.54056 Å) and transmission electron microscopy (TEM, JEOL 2100) techniques, respectively. XRD data were collected at a slow scan rate of 0.005°/ s for analyzing the structural parameters. Magnetic properties of the films were analyzed by using vibrating sample magnetometer (VSM, Lake Shore Model 7410) by performing magnetic hysteresis loops (M H) along the film plane at different constant temperatures in the temperature range of 30–300 K.
diffractometer confirms the formation of high purity NiO thin films. (ii) However, a careful comparison between the as-deposited films and bulk NiO powder confirms that the peaks are substantially broadened and the peak positions are shifted noticeably to lower angles. This suggests that the as-deposited films have fine crystallites and large lattice constant as compared to the bulk NiO. (iii) With increasing TA up to 400 °C, the peak broadening decreases gradually along with a considerable shift in the peak position to higher angles as shown in Fig. 1b and c. This confirms that the average crystallite size increases and lattice constant decreases with increasing TA. (iv) On further increasing TA to 500 °C, the NiO films with tr100 nm exhibit no detectable peaks related to fcc phase of NiO, but display only the peak corresponding to Ni phase. This reveals that the NiO films decompose into Ni. With increasing t4100 nm, we observed the existence of two peaks corresponding to (200) peak of NiO and (111) peak of Ni. Interestingly, the relatively intensity of the NiO(200) peak increases with increasing NiO film thickness from 200 nm to 300 nm. This suggests that the amount of decomposition reaction from NiO into Ni decreases with increasing NiO film thickness [21]. These results clearly suggest that the thermal decomposition reaction of NiO into Ni
3. Results and discussion 3.1. Structural properties of NiO thin film Fig. 1 depicts the room temperature XRD patterns of the asdeposited and annealed NiO (t nm) thin films at different temperatures under vacuum and oxygen annealing conditions. In order to compare the XRD patterns of the thin films with the bulk counterpart, XRD pattern of the bulk NiO powder with rescaled intensity is also included in Fig. 1a. The XRD patterns show the following features: (i) All the NiO films grown with different thicknesses exhibit face centered cubic (fcc) structure and highly oriented along (200) plane. The intensity of the (200) peak increases with increasing NiO film thickness. The absence of any other impurity peaks within the detection limit of X-ray
Fig. 1. Room temperature XRD patterns of as-deposited (a) and annealed NiO thin films with different thicknesses at different temperatures under vacuum (b,c,d) and oxygen (e) conditions.
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strongly depends on the NiO film thickness. To understand the decomposition reaction under different environments, we have carried out annealing at 500 °C under oxygen atmosphere. Fig. 1e displays the XRD patterns of the annealed NiO films at 500 °C under oxygen atmosphere. It is clearly evident that the annealing under oxygen atmosphere has not promoted any decomposition behavior and exhibited highly oriented NiO(200) plane. In addition, a considerable reduction in peak broadening is observed and the peak position of NiO(200) almost matches with the peak position of the bulk NiO powder. This indicates that the average crystallite size increases and the lattice constant approaches to that of bulk NiO. In order to quantify the changes in the structural parameters such as lattice constant (a), crystallite size (D) and strain (η) in the as-deposited and annealed films, the XRD data were analyzed using the following formulae:
⎛ ⎞ λ h2 + k 2 + l2 ⎟ a = ⎜⎜ ⎟ 2 sin θ ⎝ ⎠
(1)
⎛ 0.94λ ⎞ D=⎜ ⎟ ⎝ β cos θ ⎠
(2)
⎛ afilm − abulk ⎞ η(%) = ⎜ ⎟ × 100 abulk ⎝ ⎠
(3)
where, λ is the wavelength of the X-ray (λ ¼1.54056 Å), h,k,l are the miller indices, D is the crystallite size calculated using the full width at half maximum (β) of (200) peak for NiO and (111) peak for Ni from the Scherrer's formula, abulk is unstrained lattice constant taken from the bulk NiO and its value is 4.1687 Å and afilm is the lattice constant of the NiO thin film. The strain in NiO thin films is mainly caused by the combined effect of thickness and dynamics of the deposition. Fig. 2 depicts the variations of a and η with increasing TA for NiO thin films with different thicknesses. As-deposited films exhibit significantly a large lattice constant in the range between 4.239 Å and 4.251 Å, but decreases slightly with increasing film thickness. On other hand, the lattice constant decreases with increasing TA and approaches to bulk value for the films annealed at 500 °C. Kuo et al. [22] reported that (i) the intensity of NiO(111) and NiO(220) peaks decreases, (ii) the intensity of NiO(200) peak intensity increases, and (iii) a considerable shift in the peak position of all the peaks to lower angle was observed with increasing argon gas pressure. These behaviors were correlated to the increase of argon content in sputtered NiO film deposited at higher argon pressure, which expands the lattice parameter when argon atoms occupy the interstitial positions. On the other hand, Bruckner et al. [19] and Kim et al. [23] confirmed that the sputtered NiO films have non-stoichiometric and poor crystallinity due to excess oxygen state, which makes them thermally unstable. Youssef et al. [10] reported that the NiO microstructure can be modified by changing the stoichiometry through the control of argon gas pressure during the sputtering growth. These results in correlation with the presently observed results indicate that the observation of large lattice constant in the as-deposited NiO thin films could be attributed to the strain induced in the film possibly due to the existence of interstitial argon atoms in the NiO lattice, which expands the lattice constant when argon pressure is high and/or increase of nickel vacancies created from the nonstoichiometry. With increasing TA up to 400 °C, the lattice constant decreases gradually and approaches to bulk NiO value for the NiO films
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Fig. 2. The variations of lattice constant, a and strain, η as a function of annealing temperature, TA for the NiO thin films with different thicknesses.
annealed at TA ¼500 °C. This could be related to the release of interstitial argon atoms from NiO during the post annealing process. In addition, NiO films annealed at 500 °C exhibit thermal decomposition reaction, which is strongly depending on the film thickness. The decomposition reaction could be attributed to the existence of considerable nickel vacancies. According to the ideal model, the vacancy of each Ni in NiO lattice results in six oxygen neighbors to become incompletely bonded, which might induce decomposition under less stringent conditions [17,18]. Furthermore, this decomposition behavior typically occurs from the upper surface of the films, which is evident from the thickness dependent decomposition process [21]. Das et al. [24] have also demonstrated that the creation of oxygen vacancies at the freshly deposited NiO film surface by ultrahigh vacuum annealing is more effective in case of thinner NiO films. These results further confirm that there is a critical thickness, which is currently 100 nm, for the decomposition reaction under vacuum annealing conditions. On the other hand, the strain, calculated using Eq. (3) for NiO thin films annealed at different TA, displays the variations almost similar to those observed for lattice constant with increasing TA. This could be correlated to the release of interstitial argon atoms with increasing TA. The average crystallite size of the NiO phase calculated from the (200) peak exhibits weak dependence on the film thickness, but increases from about 5 nm to 10 nm with increasing TA. Similarly, for the NiO films annealed at 500 °C, the average crystallite size of the Ni, calculated from (111) peak, increases from about 10 nm for the 50 nm thick film to 18 nm for the 200 nm thick film and then decreases slightly to 15 nm for the 300 nm thick decomposed film. This could be attributed to the thickness dependent decomposition behavior of NiO into Ni with increasing film thickness. In order to understand the changes in the microstructure of NiO films with annealing, we have analyzed the microstructure of the as-deposited and annealed films using TEM. Fig. 3 displays typical TEM micrographs and selected area electron diffraction (SAED) patterns of the as-deposited and annealed NiO (t ¼200 nm) thin films at 500 °C. The bright-field TEM micrograph and the SAED pattern of as-deposited film displays very fine crystals and the diffraction rings corresponding to fcc structure, respectively. In order to understand the microstructure more in detail, dark-field TEM micrograph corresponding to NiO(200) plane and high-resolution TEM micrograph are shown in Fig. 3b and c, respectively. While the dark-field TEM micrograph clearly reveals the existence of fine nanocrystals with the size of around 5 nm, the high-resolution TEM micrograph confirms the NiO phase with enhanced lattice constant. On the other hand, the annealed film exhibits well-defined polycrystalline nature mainly due to the formation of Ni nanocrystals out of NiO matrix with the size ranging between 12 and 20 nm. The presence of both the NiO and Ni phases is also supported by the SAED pattern. The lattice constant calculated using SAED pattern are found to be 4.167 Å and 3.517 Å for NiO and
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Fig. 3. Bright-field TEM micrograph and SAED pattern (a), dark-field TEM micrograph (b) and high-resolution TEM micrograph (c) for as-deposited NiO (200 nm) film, and bright-field TEM and SAED pattern for annealed NiO (200 nm) film at 500 °C under vacuum condition (d).
Ni, respectively. These results are in good agreement with the results obtained from XRD analysis. These structural changes with annealing temperature in NiO films with different thicknesses are also expected to play a significant role on the magnetic properties. 3.2. Magnetic properties of NiO thin film To study the changes in the magnetic properties, we have measured room temperature and temperature dependent in-plane M–H loops in the temperature range of 30–300 K for the as-deposited and annealed films. Fig. 4 displays the typical M–H loops of as-deposited and annealed films at different TA, temperature dependent M–H loops for as-deposited NiO (t ¼300 nm) films, and the extracted values of magnetization at 2.5 kOe (MS) and coercivity (HC) as a function of film thickness for the annealed films at TA ¼500 °C under vacuum condition. All the as-deposited films (Fig. 4a) and the films annealed up to 400 °C (see inset (i) of Fig. 4c) exhibit weak magnetic signals without any clear trace of hysteresis loop. In addition, the noise in the substrate corrected data is mainly due to the small moment from the antiferromagnetic nature of the as-deposited films [12] to be detected
by the instrument in the presence of large diamagnetic contribution. It has been reported that the nanoscale NiO powders show induced ferromagnetic behavior due to uncompensated surface spins and the average magnetization increases with decreasing the average crystal size [25–27]. In order to understand the role of presence of fine NiO crystals (see Fig. 3) on the magnetic properties of the as-deposited films, M–H loops were obtained at low temperatures down to 30 K and shown in Fig. 4b. It is observed that the magnetization increases slightly with decreasing temperature, but the nature of M–H loops does not change substantially down to 30 K. This could be explained as follows: Although the size reduction in NiO is expected to produce induced ferromagnetic properties due to uncompensated spins, the observation of pronounced lattice expansion (see Fig. 2) in the presence of many broken bonds and defects is most likely to give an increased bond length of Ni–O and thereby significantly weakening the strength of the super-exchange interactions of magnetic ions [28]. As a result, no induced ferromagnetic properties within the detection limit of the instrument could clearly be seen in the presently investigated samples down to 30 K. On the other hand, as depicted in Fig. 4c, the NiO thin films annealed at 500 °C show
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ferromagnetic nature with clear hysteresis loops, which saturates at moderate applied magnetic fields. However, with increasing t, the shape of the M–H loop changes and the field required for saturating the films' magnetization increases with increasing film thickness. Interestingly, the NiO thin films annealed at 500 °C under oxygen atmosphere (see inset (ii) of Fig. 4c) also exhibit only a weak magnetic signal without any hysteresis behavior. This could be correlated to the non-observation of decomposition reaction in NiO films under oxygen conditions. The values of MS for the NiO films annealed at 500 °C under vacuum condition increase from 95 emu/cc to 130 emu/cc with increasing t from 50 to 100 nm and then decrease to 80 emu/cc and 71 emu/cc with the further increase in film thickness between 200 and 300 nm, respectively. This reveals that MS is strongly depending on the film thickness. While the increase in MS can be explained to the complete decomposition of NiO into Ni in lower thickness films up to 100 nm, the decrease in MS for higher thickness NiO films is due to
Fig. 4. Room temperature (a) in-plane M–H loops of as-deposited NiO films with different thicknesses, (b) in-plane M–H loops of as-deposited NiO (300 nm) films measured at different temperatures, (c) Room temperature in-plane M–H loops of annealed NiO thin films at 500 °C, and (d) the variations of MS and HC with NiO film thickness annealed at 500 °C. Inset of (c): Room temperature M–H loops of NiO films with different thicknesses annealed at (i) 400 °C under vacuum condition and (ii) 500 °C under oxygen condition.
Fig. 5. M–H loops measured at different temperatures for NiO films with different thicknesses annealed at 500 °C under vacuum condition.
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reduction in the decomposition reaction with increasing NiO film thickness above 100 nm resulting in a reduced amount of Ni in the NiO matrix, as demonstrated in the structural properties using XRD and TEM. Zhang et al. [29] reported that MS values in Fe doped NiO thin films increases initially with increasing film thickness up to 250 nm and then decreases due to the enhanced stress in the film which induces the antiferromagnetic exchange interaction and resulting in a weaker magnetization. On the other hand, HC increases from 43 Oe to 100 Oe with increasing t, but the amount of increase in HC decreases with increasing t above 100 nm. While the low value of HC in smaller thickness is related to soft magnetic nature of Ni, the increase in HC may be attributed to the strong interaction between Ni and NiO phases resulting in a high anisotropy and hence enhances the value of HC. Miller et al. [30] reported that HC in pure Ni film increases initially with increasing Ni thickness nearly up to 80 nm, which is in good agreement with the presently observed results. Yi et al. [31] reported that high values of HC in NiO films prepared under different O2 partial pressures are mainly due to the interface structure of Ni/ NiO during the formation of NiO under magnetic field annealing. To study the magnetic properties of the NiO films annealed at 500 °C under vacuum annealing in more detail, we have carried out M–H loops at different temperatures under zero-field cooled and field-cooled conditions. Fig. 5 shows the typical M–H loops measured under zero-field cooled conditions for NiO thin films annealed at 500 °C with different thicknesses. It is clear from the figure that the shape of the M–H loops does not change with temperature for each film thickness. However, the area under the loop increases gradually with decreasing temperature as expected for a typical ferromagnetic material and resulting in an increase in HC. The extracted values of HC from the M–H loops at different temperatures for different film thicknesses are summarized in Fig. 6. HC increases almost linearly in logarithmic scale for all the films with decreasing temperature and the rate of increase in HC considerably depends on the film thickness. To study the exchange bias properties in these NiO and Ni systems, we have also carried out M-H loop measurements at different temperatures from 30 to 300 K under the field of 2 kOe. Fig. 6b shows the determined values of exchange bias, HE (¼(|HC |-|HC þ |)/2) under zero-field cooled and field-cooled conditions as a function of temperature for 300 nm thick NiO film annealed at 500 °C. It is clearly evident from the figure that HE in zero-field cooled films is nearly zero down to 150 K and then increases weakly at low temperatures. On the other hand, the field cooled films exhibit significantly large HE at low temperature. Rinaldi-Montes et al. [27] reported that the size effects in NiO induce surface spin frustration which competes with the expected antiferromagnetic order and giving rise to a threshold size for the antiferromagnetic phase to nucleate. While the larger NiO
Fig. 6. The variations of (a) HC with temperature and (b) HE with temperature under zero-field-cooled (ZFC) and field-cooled (FC) conditions for the NiO (300 nm) thin films annealed at 500 °C under vacuum condition.
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particles form uncompensated antiferromagnetic core with a net magnetic moment surrounded by a spin-glass shell resulting in exchange bias effect, the NiO nanoparticle of size less than 3 nm seems to be in a spin-glass state. Kisan et al. [32] reported that the NiO nanoparticles with different particle sizes prepared by sol-gel process showed large variation in exchange bias under field-cooled condition due to the existence of spin-glass phase, which induce additional anisotropy between surface spins and compensated core spins. On the other hand, Valladares et al. [11] reported that the magnitude of exchange bias field in annealed NiO thin films strongly depends on the relative fraction of Ni and NiO phases providing more prominent interfaces between Ni and NiO crystals. Therefore, the exchange bias in the presently investigated system is mainly due to the enhanced anisotropy caused by the improved interaction between Ni and NiO crystallites having sharp and small mixed interface areas [11,31]. The disappearance of HE above 200 K reflects the existence of thermal dependence of anisotropy in these films. The observed results reveal that the NiO films with cubic structure are potential candidates for resistive random access memory and spintronic applications.
4. Conclusion We have studied the effects of NiO film thickness and annealing temperature under different environments on the thermal decomposition reaction of NiO into Ni and the resulting magnetic properties of the NiO films deposited directly on thermally oxidized Si substrate using magnetron sputtering technique at ambient temperature. All the as-deposited films exhibited fcc structure and highly oriented along (200) plane. The lattice constant of the as-deposited films was observed to be large as compared to bulk NiO due to induced strain by the presence of interstitial argon atoms in the sputtered NiO films and possible nickel vacancy. With increasing annealing temperature, the lattice constant decreases due to the release of strain and approaches to the bulk NiO value. Furthermore, the thickness dependent decomposition reaction of NiO into Ni occurs typically from the upper surface of the NiO thin films annealed at 500 °C under vacuum annealing. However, the decomposition reaction was completely suppressed for the NiO films annealed under oxygen condition. All the as-deposited films and the films annealed up to 400 °C under vacuum condition and 500 °C under oxygen conditions show only antiferromagnetic nature, but the NiO films annealed at 500 °C under vacuum condition exhibit room temperature ferromagnetism due to the decomposition process. A close correlation between the structural and magnetic properties was observed for all the NiO films.
Acknowledgments The authors acknowledge the financial support from Council of Scientific and Industrial Research vide project No. 03(1337)/15/
EMR-II, New Delhi. Infrastructural facilities provided by the Department of Science and Technology, India vide project No: [SR/S2/ CMP-19/2006, SR/FST/PII-020/2009] and Central Instruments Facility (CIF), IIT, Guwahati are gratefully acknowledged.
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Please cite this article as: P. Ravikumar, et al., Journal of Magnetism and Magnetic Materials (2016), http://dx.doi.org/10.1016/j. jmmm.2016.02.091i