Pulsed laser deposition of rocksalt magnetic binary oxides
Alireza Kashir1*, Hyeon-Woo Jeong1, Gil-ho Lee1, Pavlo Mikheenko2, and Yoon Hee Jeong1*
1Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea
2 Department of Physics, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
Corresponding Authors:
Abstract
Magnetic binary oxides with the rocksalt structure constitute an important class of materials for potential applications as electronic or electrochemical devices. Moreover, they often become a theoretical playground, due to the simple electronic and crystal structures, in the quest for novel phenomena. For these possibilities to be realized, a necessary prerequisite would be to grow atomically ordered and controllably-strained binary oxides on proper substrates. Pulsed laser deposition (PLD) is one of the simplest deposition techniques for this purpose; however, there has been no comprehensive study of the deposition of binary oxides by this technique. Here we explore systematically the growth of three basic oxides with rocksalt structure having different chemical stability in the ambient atmosphere: NiO (stable), MnO (metastable) and EuO (unstable). It is shown that by tuning laser fluence, an epitaxial single-phase nickel oxide thin-film growth can be achieved in a wide range of temperatures from 10 to 750 °C. At the lowest growth temperature, the out-of-plane strain raises to 1.5%, which is five times bigger than that for a thick NiO film grown at 750 °C. The growth of metastable MnO phase presents a bigger challenge and is strongly influenced by the laser fluence as well as the temperature of substrates. In particular, by properly tuning the deposition parameters, long-range ordered MnO thin films are successfully deposited on the MgO substrates. The unstable EuO phase is found to be strongly sensitive to oxygen contents during the growth.
Controlling this parameter, EuO films with satisfactory quality are also deposited by PLD.
Overall, it is proven that PLD is a quick and reliable method to grow binary oxides with rocksalt structure in high quality that would satisfy requirements for applications as well as needs for basic research. Specific details of the growth are highlighted.
Keywords:Pulsed laser deposition; Strain engineering; Long-range ordered thin films; NiO; MnO;
EuO;
1. Introduction
Magnetic binary oxides with cubic rocksalt structure and the molecular formula MO, where M is a magnetic metal, like MnO, NiO, CoO, FeO, EuO, etc. form a group of insulating materials with antiferromagnetic alignment of atomic magnetic moments achieved through the superexchange interaction [1–3], except EuO, that is a ferromagnet [4]. These oxides are the best candidates for studying electronic behavior and exploring the new phenomena emerged in the strongly correlated electronic systems. For decades, they attracted a huge attention in solid states electronics. The simplicity of their crystal, atomic and electronic structure enable theorists to mathematically calculate and analyze their electronic behavior under diverse conditions. Moreover, they are the best materials to experimentally investigate the accuracy of the new mechanisms proposed for the electronic behavior, which emerge in the solid state physics, e.g. spin-phonon coupling [5–8], magnetic ordering [9] and insulator-metal transition [10,11]. The role of this family of oxides in the development of 20th-century physics is such remarkable that it is not an overstatement to claim that the physics owe them greatly for its dazzling progress.
Among the vast different approaches to induce new properties in materials, application of strain is a technique allowing to create a range of systems with a continuous change of physical properties [12]. Inducing insulator-metal transition, transformation of non- ferroelectric materials into ferroelectrics and creation of multiferroics by enhancing spin- phonon coupling are all possible by applying an appropriate strain [13–19]. This role of stress is such remarkable, that the field of strain engineering appeared in the beginning of the 21st century and speedily becomes one of the hottest topics in solid-state electronics. With applying strain and also inducing point defects, it is shown theoretically and experimentally that some interesting features can be induced in magnetic binary oxides with rocksalt structure. Feng and Harrison by using density functional theory (DFT) calculations [20] and Gavriliuk, et al. [10] experimentally, showed that a highly compressed NiO single crystal passes through an insulator-metal Mott transition expressed in a strong nonlinear decrease in resistance by about three orders of magnitude. Using DFT calculations, Bousquet, et al. [14] and Bog G. Kim [17]
independently proved that by applying a proper strain, the ferroelectricity can be induced in a binary compound, specifically in EuO. This was recently confirmed by Djermouni et al. [21]. Xiangang Wan and his co-workers demonstrated that there might exist a magnetically-induced electric polarization in a strained magnetic binary oxide with a superexchange chain of atoms (e. g. MnO) [19].
Thin-film deposition is a technique allowing to control strain [22]. It was shown that the Curie temperature (TCurie) of EuO thin film is strongly dependent on the strain and also film thickness [23]. Sugiyama and his coworkers discovered that dislocations in NiO thin films locally change the magnetic ordering from an antiferromagnetic to the ferromagnetic one [24]. Such ‘ferromagnetic’ dislocations originate from a local non-stoichiometry at the
dislocation cores with the Ni deficiency. A slight deviation from the stoichiometry also changes the conduction behavior in the nickel oxide thin films [25]. The oxygen-deficient EuO shows a metallic condiction with a substantial increase of the TCurie up to 120 K [26].
In a recent work published by Fukumura’s group, a superconducting transition was detected in the rocksalt LaO epitaxial films [27]. Consequently, the deposition of high- quality magnetic binary oxides, and also the ability to control the level of strain and density of defects in their structure, became one of the most exciting, albeit challenging, tasks in materials science and solid state physics.
Among the various thin-film techniques, pulsed laser deposition technique has many advantages for growing high-quality oxides [28–33]. Stoichiometric preservation of the target materials in the film composition, growing the complicated oxides and deposition of ceramic thin films at relatively low temperatures, provide diverse opportunities for fabrication of many different systems and design of unique electronic properties in the materials. On the other hand, applying high-energy pulses in this method causes appearance of undesirable defects in the structure, which changes electronic behavior of the films. Many reports on the deposition of various binary compounds via PLD have been published [34–43]. However, the lack of detailed and comprehensive study, the demand for obtaining higher-quality thin films and the requirement to control the level of strain in order to check recent theories, was the motivation behind this study. The focus of it is the growth behavior of three basic binary oxides with different chemical stabilities in the atmosphere, i. e. NiO, MnO, and EuO which are stable, metastable and unstable, respectively.
In this work, by optimizing the growth conditions, the highest-quality binary oxide thin films were produced and the details of the growth behavior are discussed. A valuable information about how to induce strain and achieve long-range atomic order in the films, will be revealed.
2. Experiments
2.1. Substrate preparation
As-received substrates, SrTiO3 (001) (STO), MgO (001) (MgO), LaAlO3 (001) (LAO) and YAlO3 (110) (YAO) (all from Crystech) were cleaned ultrasonically in acetone and methanol for 10 minutes in each to remove all contaminations from the top surface. For STO and LAO substrates, a 5-minute ultrasonic soaking in DI-water followed by a 30- second etching process in buffered hydrogen fluoride (NH4F: HF=7:1, pH~ 4.5) and buffered hydrogen chloride (pH~ 4.5), respectively, were performed. After drying all four substrates under a high purity nitrogen gas flow, STO, LAO and YAO substrates passed through an annealing process at 1000 °C for 2.5 hours in the air. A 3-hour heat treatment at 1150°C was carried out on MgO substrates.
2.2. Film Deposition
To have a comprehensive study on the pulsed laser deposition of magnetic binary oxides, the growth of the three oxides different in their chemical stability in the atmosphere was investigated in detail. These are: NiO, which is a stable oxide in Ni-O system [44] at ambient conditions; MnO, which represents a metastable phase in Mn-O system [45], and, finally, EuO, which is an unstable phase in the Eu-O phase diagram that tends to receive more oxygen and transform itself to Eu2O3 [46]. Each of these oxides could be a representative material for a group of oxides with the same stability. To grow the epitaxial films, we used PLD, as the capability of this technique to fabricate the high-quality oxide thin films was already established.
2.2.1. Nickel Monoxide
A NiO polycrystalline ceramic pellet (99.99% purity) was used as a target for the growth of NiO film. An Nd-YAG laser (266 nm) with the energy fluence of 2 J/cm2 was used for films deposition. Films were grown on TiO2–terminated SrTiO3 (001) substrates at temperatures ranging from 10 to 750 oC. To compensate for the expected oxygen deficiency in the deposited materials, the chamber was filled with high-purity (99.999%) oxygen gas. The pressure of the gas was 15 mTorr during the deposition process. After the deposition, the films were cooled down to the room temperature with the rate of 12.5
oC /min.
2.2.2. Manganese Monoxide
To deposit MnO film, the polycrystalline MnO ceramic pellet of the purity 99.99% was used as a target. A KrF laser with 248 nm wavelength was used for the film growth. All samples were grown in vacuum of 10-6 Torr. We used different substrate temperatures and laser fluences to achieve the best conditions for growing MnO film without inclusions of other phases. Since there are four different types of oxides in the Mn-O system: MnO, Mn3O4, Mn2O3 and MnO2, tuning the deposition parameters to grow pure rocksalt binary oxide MnO proved to be a challenging task.
2.2.3. Europium Monoxide
Growing EuO with PLD system is a more complicated process in comparison with the growth of NiO and MnO, because both Eu and EuO are extremely sensitive to the oxygen.
Because of this, the chosen deposition target was with reduced oxygen content: Eu2O3
polycrystalline ceramic target of the purity of 99.99%. Later it was replaced with pure Eu target of 99.9% purity. The deposition temperature, substrate material and oxygen pressure were the most important parameters at focus for the deposition of EuO.
Normally, the growth of EuO requires ultra-high vacuum. In our case, the chamber was pumped to the base pressure of 5×10-8 Torr. The KrF excimer laser with the wavelength of 248 nm was also used for this deposition.
In all depositions, the substrates were placed at the distance of 50 mm, right above the targets, and the repetition rate of the laser pulses was set to 10 Hz.
2.3.
Characterization
2.3.1. Surface morphology
The surface morphology of the substrates and the deposited samples was investigated using Atomic Force Microscopy (AFM) operating in a non-contact mode.
2.3.2. Phase analysis and crystal structure
The crystal structure of the films was characterized by Ɵ-2Ɵ X-ray diffraction (XRD) using CuKα radiation from a source operating at 40 kV with current of 200 mA. This study was followed by a Ɵ rocking around the detected Bragg peaks to investigate the configuration of the crystalline lattice in more detail. An X-ray reflectometer was used to measure the film thickness and the quality of the surface and the film/substrate interface.
3. Results and Discussion
3.1. Substrates
Atomic force microscopy revealed atom-scale high steps and flat terrace structure [see fig.
1] on the surface of all treated substrates. Such flattness was also achieved by other research groups [47–51]. YAO shows an atomically smooth surface which is proven by a line scan through its surface. In the case of MgO substrate, Bonholzer, et al. [51] showed that a step-terrace surface is obtainable after a vacuum annealing at 950 °C. This process, however, may cause the formation of point defects on the substrate surface, which subsequently can change its electronic structure. In our treatment, the annealing environment was changed to the air, and to avoid the hindering effect of atmospheric pressure on the surface diffusion, the annealing temperature was increased above1000
°C. After several experiments, it was found that an optimal temperature for surface treatment of MgO (001) is 1150 °C.
3.2. Nickel monoxide
X-ray diffraction study on NiO films grown at 750 °C under 15 mTorr oxygen pressure shows a single peak of NiO (002) [see fig. 2]. In this scan, no additional peaks were observed indicating the absence of any other phases, like Ni and Ni2O3.
As the film thickness decreases, the NiO (002) peak shifts to the left side, which is a result of the out-of-plane elongation of lattice planes. It shows that the deposited films are possibly under in-plane compressive strain. Repulsion of the similar charges induced by point defects could also be a reason for initial lattice expansion in the PLD grown films [52]. Figure 2.b shows that the highest achieved out-of-plane strain is 0.5% for a 5-nm film and that nearly total relaxation happens for 200 nm film. The relaxation processes start in the early stages of growth, so that the films cannot reach theoretically-predicted
strain of 6.5% for this type of substrate. Indeed, there is a so-called fast relaxation of strain for NiO thin films grown on the SrTiO3 substrates at the mentioned temperature.
Such relaxation was already described by Chern and Cheng [53]. As they suggested, the stronger adsorbate-adsorbate interaction compared to the adsorbate-substrate interaction, could be a reason for the fast relaxation. The film thickness itself seems to be not a crucial parameter for the relaxation process in the NiO films grown at 750 oC. The growth temperature, however, is likely to play important role in the fast relaxation process.
Heated substrate provides thermal energy for the coming ablated particles, sufficient to overcome the diffusion barriers at the surface, accelerates the surface diffusion and finally releases the mismatch strain between the NiO film and the SrTiO3 substrate.
The Ɵ rocking around NiO (002) peak yields more information on the structural features of the grown films. Fig.2c shows that the general shape of curves changes as a function of film thickness. The films thicker than 40 nm show a simple broad peak and that with a thickness of 2.5 nm reveals a single sharp peak. A mixture of these two peaks forms the rocking curves of other grown films. These two components imply two different structural correlation lengths in the films [54–59]. The narrow peak, width of which is limited by instrumental resolution, reveals a long range-ordered phase. On the other hand, the emergence of the broad component shows the formation of sub-grains (mosaic structure).
As the film thickness decreases, the shape of the rocking curve undergoes two remarkable changes. The width of broad peak increases, and the sharp peak appears and intensifies. The ratio of the broad to sharp components’ intensities becomes negligible for the films thinner than 5 nm. These features indicate the spread of the long- range ordered phase and reduction in mosaicity. The introduction of the threading dislocations as films become thicker, is likely to be a reason for the appearance of the broad peak mosaics, because the dislocations can break the long-range ordered phase by forming subgrain boundaries [57]. The strain relaxation of thicker films might originate from the emergence of dislocations from the interface between the film and substrate.
Figure 3 shows the evolution of the surface morphology as the thickness of the film increases from 15 to 200 nm. At the increase, surface structure changes from a perfect steps-and-terraces structure to an atomically smooth surface without clear steps and to an island-like structure for relatively thick films. It seems that at the initial stages of growth, the deposited materials follow the atomic structure of the substrate, and gradually the clarity of the steps present there fades out .The gradual change of the surface morphology during the growth from a perfect steps-and-terraces structure of STO to a defective NiO crystal might be the reason behind different growth behaviors of the films.
Also, the defects, influincing this behaviour, provide the preferred places for the initial growth of the films.
In the further investigation, NiO thin films were deposited at a wide range of substrate temperature, while the other parameters remained the same as for the films described above.
XRD study on a room temperature-grown NiO film shows a single NiO (002) peak (see Fig.4). This feature helps to study the strain relaxation behavior as a function of the growth temperature. By the lattice constant calculation using the position of the Bragg peak, it was found that for a 15-nm NiO film grown at 10 °C, the out-of-plane strain is 5 times higher than in the film grown at 750 °C with the same thickness. It supports the idea that the fast strain relaxation is due to the thermal energy, which is provided by the substrate during the growth. Therefore, to obtain a strained NiO thin film, the substrate temperature should be kept low.
The Ɵ-rocking curve around the NiO (002) diffraction peak shows that general shape of the peak is strongly altered by the reduction of the growth temperature (Fig. 4c). The sharp peak, which is clear for the film grown at 750 °C, is totally suppressed at the lower deposition temperature of 600 °C. Additionally, the width of the broad peak increases as the growth temperature decreases, which is an indication of a decrease of the crystallite sizes in the mosaic structure, and, therefore, degradation of the film quality. Reducing the surface diffusion for the particles supplied to the substrate due to the lower thermal energy, might be responsible for the resulting smaller crystallite sizes in the films grown at low temperatures. Surprisingly, the AFM images show an atomically flat steps-and- terraces surface structure for an NiO thin film grown at room temperature (see Fig. 5). X- ray reflectivity spectra from the15-nm films grown at different temperature confirm that all films have smooth surfaces and interfaces with substrates (see fig. 5c).
3.3. Manganese Monoxide
According to the Mn-O phase diagram [45], at ambient conditions, MnO2 (highly oxidized manganese) is the stable phase, and to form MnO, the high temperature and low oxygen pressure are required. Normally this condition is not achievable in the PLD chamber, but by taking advantage of the non-equilibrium processes, at certain conditions it can be done.
In the deposition of MnO, we started with the setting the substrate temperature at 750 °C and placing the substrate at the distance of 50 mm from the target and then tried to achieve deposition of MnO single phase at different oxygen pressures using different energies of laser pulses. Both parameters at the given deposition conditions strongly influence the quality and composition of the films.
According to the XRD data in figure 6, to grow a pure MnO phase in vacuum of ~ 10-6 Torr, the laser fluence should be kept lower than 1.5 J/cm2. When the fluence in laser pulses increases to 3 J/cm2, the higher oxidized phases emerge in the films. The reason
might be in the presence of higher thermally activated fragments and subsequently their higher reactivity with the oxygen. As a result, the Mn3+ valance sites appear in the film.
For the deposition under the energy fluence of 4 J/cm2 or higher, there is no film peak in the XRD spectrum indicating an amorphous film growth or the absence of any deposited material. Re-sputtering of the ablated species due to their high kinetic energy can be one of the reasons of this, as there is no background gas to reduce the kinetic energy of particles when they are landing on the substrate surface [60].
Substrate temperature plays two important roles in growing the high-quality and high- purity MnO thin films. It speeds up the surface diffusion allowing to form a well-crystallized film, and the second effect is based on the specifics of the phase diagram discussed previously. Figure 7 shows the XRD results for the films grown at four different temperatures of substrate, i. e. 750, 650, 600 and 550 °C. First, one can notice that lowering the growth temperature to 550 °C is accompanied by the emergence of Mn3O4
phase, which is predictable from the Mn-O phase diagram [45]. At the same time, the position of MnO (002) peak shifts to the left side, which is due to an out-of-plane lattice expansion. The lattice mismatch of the film and substrate and also the point charges created by the defects, might be responsible for this expansion.
X-Ray reflectivity and Ɵ-rocking measurements reveal that the films grown at 650 °C have better quality compared to the ones grown at 750 °C (see Fig. 8). The Ɵ-rocking around the (002) Bragg peak for MnO film grown at 750 °C shows a single broad peak, which is a signature of mosaic structure formed in it. This is a typical feature of the films deposited in a vacuum, because in this condition, there is no decelerating force able to reduce the kinetic energy of the ablated species. Subsequently, the highly energetic particles create defects in the grown films and break their atomic order. Reduction of temperature to 650
°C is accompanied by the emergence of a specular peak, which indicates the formation of long-range ordered phase in the film. This phenomenon is completely against the expectations, because usually, thermal energy drives the system to higher atomic order.
Substrates at higher temperature might make the deposited material more sensitive to the incidence of subsequently arriving particles during the growth. It yields a more defective structure, which is clearly displayed in AFM images. The surface structure of the film grown at 750 ˚C is granular, whereas a smooth surface appears for the film deposited at 650 ˚C. Therefore, considering influence of both parameters: crystal quality and phase purity of MnO, one can conclude that there is an optimum temperature for the deposition.
3.4. Europium Monoxide
Elemental europium is so sensitive to the oxygen, that even in a fraction of a minute, the polished surface gets oxidized in an ambient atmosphere, and the Eu2O3 is formed.
Growing EuO, which is not stable at typical PLD conditions, is a very challenging task. To
establish a condition for the deposition of a pure single-phase EuO, the focus in this study was on most effective parameters, especially the amount of oxygen available in the PLD chamber and the substrate temperature. Usually, there are four sources of oxygen in the PLD process: oxygen in the target material, background gas, residual oxygen in a vacuum chamber and as a component of substrate itself. In the deposition, the growth temperature was set to 350 oC. At this temperature, EuO is stable in a very narrow region of oxygen pressure [46], which is difficult to achieve in PLD chamber. However, the non-equilibrium processes in the PLD can help in growing EuO even at relatively high pressure of oxygen.
To take advantage of it, the laser fluence of 2.5 J/cm2 was used to deposit the samples.
To prevent the atmospheric degradation of the grown films during the measurements, they were capped with a ~ 2-nm MgO layer right after the deposition, at the same conditions of growth.
Firstly, we have tried to grow at different conditions EuO thin film using Eu2O3 ceramic target, but due to the highly oxidized target, it was not possible to achieve EuO composition even in high vacuum, with the residual pressure on the order of ~10-8 Torr.
Eu3O4 and Eu2O3 phases were the only phases, which were found in XRD scans.
To remove the major source of oxygen, we replaced the target with a high-purity Eu metal disk and grew samples at different conditions on three different substrates: LAO (001), YAO (110) and MgO (001). For the films grown in the vacuum of 5.0 10-8 Torr at 350 °C, the XRD spectra show a single phase of EuO grown on LAO and YAO (see Fig. 9). On the MgO substrate, the EuO (002) peak is not strong in comparison with that on films deposited to LAO and YAO. It supports the idea that Eu-ablated particles use oxygen from the substrate to form EuO [61]. The surface structure of the substrate (lattice spacing and atomic arrangement) is also important for growing high-quality thin films. The dominant atomic spacing on the surface of MgO (001) is 0.4212 nm, therefore in both x and y directions, the lattice mismatch is about -18%, but in the case of YAO (110) and LAO (001), the lattice mismatch is less than 4% which is much smaller than that for MgO substrate.
Lowering the growth temperature to 300 °C is accompanied by the emergence of the secondary phaseEu2O3(see fig. 10).
One of the most important and usually missing parts in analysis for growing EuO is the composition of the surface of target. In this work, it was realized that the cleaning time of the target surface by laser pulses before the growth strongly affects the composition of the subsequently deposited films. Figure 11 reveals that the film, which was produced after cleaning target for a long time, shows higher purity compared to the one produced after a short cleaning time. The oxidation of the Eu surface inside the chamber before the deposition, may result in the appearance of Eu2O3 phase in the deposited film. To grow a pure EuO phase, one should first remove oxide layer from the target.
XRR scan for a 20-nm film grown at 350 °C on YAO (110) substrate shows a good surface and interface quality and the Ɵ-rocking around the (002) diffraction peak gives a single sharp peak indicating a full long-range order in the film without considerable mosaic blocks (see Fig. 12). The atomic force microscopy also shows a homogeneous and smooth surface of the film grown on LAO substrate at this temperature.
4. Conclusions
The purity, crystal structure and surface morphology of the pulsed laser deposited magnetic binary oxides: NiO, MnO, and EuO have been investigated using X-ray diffractometry and atomic force microscopy. It was shown that unlike common expectations, by setting the growth parameters properly, PLD can deposit high-quality binary oxides even in high vacuum. Growing epitaxial NiO thin films in a large range of temperatures enabled us to study the evolution of crystal structure and the strain relaxation as a function of film thickness and substrate temperature. It was clarified that the growth temperature can greatly affect the strain relaxation in the NiO thin films. To obtain a pure MnO film, the laser fluence and substrate temperature appeared to be the crucial parameters. By tuning these parameters, an atomically long-range ordered film was deposited. The unstable EuO, which is very sensitive to oxygen, was grown in a typical PLD chamber condition by choosing appropriate target and substrate materials.
Despite many instrumental limitations, a perfectly-crystallized epitaxial EuO film was deposited by PLD.
Acknowledgement
This work was partially supported by National Research Foundation (NRF) of Korea (2015R1D1A1A02062239 and 2016R1A5A1008184) funded by the Korean
Government.
References
[1] C.N.R. Rao, Transition-metal oxides, in: Solid State Chem., 1974.
[2] T. Archer, C.D. Pemmaraju, S. Sanvito, C. Franchini, J. He, A. Filippetti, P.
Delugas, D. Puggioni, V. Fiorentini, R. Tiwari, P. Majumdar, Exchange interactions and magnetic phases of transition metal oxides: Benchmarking advanced ab initio methods, Phys. Rev. B - Condens. Matter Mater. Phys. 84 (2011) 1–14. doi:10.1103/PhysRevB.84.115114.
[3] P. Gopal, R. De Gennaro, M.S.D.S. Gusmao, R. Al Rahal Al Orabi, H. Wang, S.
Curtarolo, M. Fornari, M. Buongiorno Nardelli, Improved electronic structure and magnetic exchange interactions in transition metal oxides, J. Phys. Condens.
Matter. 29 (2017). doi:10.1088/1361-648X/aa8643.
[4] B.T. Matthias, R.M. Bozorth, J.H. Van Vleck, Ferromagnetic Interaction in EuO, Phys. Rev. Lett. 7 (1961) 160–161. doi:10.1103/PhysRevLett.7.160.
[5] C. Kant, F. Mayr, T. Rudolf, M. Schmidt, F. Schrettle, J. Deisenhofer, A. Loidl, Spin-phonon coupling in highly correlated transition-metal monoxides, Eur. Phys.
J. Spec. Top. 180 (2009) 43–59. doi:10.1140/epjst/e2010-01211-6.
[6] C. Kant, M. Schmidt, Z. Wang, F. Mayr, V. Tsurkan, J. Deisenhofer, A. Loidl, Universal exchange-driven phonon splitting in antiferromagnets, Phys. Rev. Lett.
108 (2012) 1–5. doi:10.1103/PhysRevLett.108.177203.
[7] S. Massidda, M. Posternak, A. Baldereschi, R. Resta, Noncubic behavior of antiferromagnetic transition-metal monoxides with the rocksalt structure, Phys.
Rev. Lett. 82 (1999) 430.
[8] R. Pradip, P. Piekarz, A. Bosak, D.G. Merkel, O. Waller, A. Seiler, A.I. Chumakov, R. Rüffer, A.M. Oleś, K. Parlinski, Lattice dynamics of EuO: evidence for giant spin-phonon coupling, Phys. Rev. Lett. 116 (2016) 185501.
[9] P.W. Anderson, Antiferromagnetism. Theory of Suyerexchange, Phys. Rev. 79 (1950). doi:10.1103/PhysRev.79.350.
[10] A.G. Gavriliuk, I.A. Trojan, V. V. Struzhkin, Insulator-metal transition in highly compressed NiO, Phys. Rev. Lett. 109 (2012) 1–5.
doi:10.1103/PhysRevLett.109.086402.
[11] J. Kuně, A. V. Lukoyanov, V.I. Anisimov, R.T. Scalettar, W.E. Pickett, Collapse of magnetic moment drives the Mott transition in MnO, Nat. Mater. 7 (2008) 198–
202. doi:10.1038/nmat2115.
[12] J. Cao, J. Wu, Strain effects in low-dimensional transition metal oxides, Mater.
Sci. Eng. R Reports. 71 (2011) 35–52. doi:10.1016/j.mser.2010.08.001.
[13] D.G. Schlom, L.-Q. Chen, C.-B. Eom, K.M. Rabe, S.K. Streiffer, J.-M. Triscone, Strain Tuning of Ferroelectric Thin Films, Annu. Rev. Mater. Res. 37 (2007) 589–
626. doi:10.1146/annurev.matsci.37.061206.113016.
[14] E. Bousquet, N.A. Spaldin, P. Ghosez, Strain-induced ferroelectricity in simple rocksalt binary oxides, Phys. Rev. Lett. 104 (2010) 1–4.
doi:10.1103/PhysRevLett.104.037601.
[15] D.G. Schlom, L.Q. Chen, C.J. Fennie, V. Gopalan, D.A. Muller, X. Pan, R.
Ramesh, R. Uecker, Elastic strain engineering of ferroic oxides, MRS Bull. 39 (2014) 118–130. doi:10.1557/mrs.2014.1.
[16] K.J. Choi, Enhancement of Ferroelectricity in Strained BaTiO3 Thin Films.pdf, 1005 (2010) 1005–1010. doi:10.1126/science.1103218.
[17] B.G. Kim, Epitaxial strain induced ferroelectricity in rocksalt binary compound:
Hybrid functional Ab initio calculation and soft mode group theory analysis, Solid State Commun. 151 (2011) 674–677. doi:10.1016/j.ssc.2011.02.023.
[18] J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury, W.
Tian, M.E. Hawley, B. Craigo, Room-temperature ferroelectricity in strained SrTiO 3, Nature. 430 (2004) 758.
[19] X. Wan, H.C. Ding, S.Y. Savrasov, C.G. Duan, Short range magnetic exchange interaction favors ferroelectricity, Sci. Rep. 6 (2016) 1–7. doi:10.1038/srep22743.
[20] X.B. Feng, N.M. Harrison, Metal-insulator and magnetic transition of NiO at high pressures, Phys. Rev. B - Condens. Matter Mater. Phys. 69 (2004) 1–5.
doi:10.1103/PhysRevB.69.035114.
[21] M. Djermouni, A. Zaoui, S. Kacimi, N. Benayad, A. Boukortt, Ferromagnetism and ferroelectricity in EuX (X= O, S): pressure effects, Eur. Phys. J. B. 91 (2018) 28.
[22] A. Biswas, M. Talha, A. Kashir, Y.H. Jeong, A thin film perspective on quantum functional oxides, Curr. Appl. Phys. (2018) 0–1. doi:10.1016/j.cap.2018.07.013.
[23] A. Melville, T. Mairoser, A. Schmehl, T. Birol, T. Heeg, B. Holländer, J. Schubert, C.J. Fennie, D.G. Schlom, Effect of film thickness and biaxial strain on the curie temperature of EuO, Appl. Phys. Lett. 102 (2013) 1–6. doi:10.1063/1.4789972.
[24] I. Sugiyama, N. Shibata, Z. Wang, S. Kobayashi, T. Yamamoto, Y. Ikuhara, Ferromagnetic dislocations in antiferromagnetic NiO, Nat. Nanotechnol. 8 (2013) 266–270. doi:10.1038/nnano.2013.45.
[25] P. Gupta, T. Dutta, S. Mal, J. Narayan, Controlled p-type to n-type conductivity transformation in NiO thin films by ultraviolet-laser irradiation, J. Appl. Phys. 111 (2012) 13706.
[26] T. Yamasaki, K. Ueno, A. Tsukazaki, T. Fukumura, M. Kawasaki, Observation of anomalous Hall effect in EuO epitaxial thin films grown by a pulse laser
deposition, Appl. Phys. Lett. 98 (2011) 96–99. doi:10.1063/1.3557050.
[27] K. Kaminaga, D. Oka, T. Hasegawa, T. Fukumura, Superconductivity of Rock-Salt Structure LaO Epitaxial Thin Film, J. Am. Chem. Soc. 140 (2018) 6754–6757.
doi:10.1021/jacs.8b03009.
[28] D.H.A. Blank, M. Dekkers, G. Rijnders, Pulsed laser deposition in Twente: From research tool towards industrial deposition, J. Phys. D. Appl. Phys. 47 (2014).
doi:10.1088/0022-3727/47/3/034006.
[29] P.R. Willmott, J.R. Huber, Pulsed laser vaporization and deposition, Rev. Mod.
Phys. 72 (2000) 315–328. doi:10.1103/RevModPhys.72.315.
[30] R. Groenen, J. Smit, K. Orsel, A. Vailionis, B. Bastiaens, M. Huijben, K. Boller, G.
Rijnders, G. Koster, Research Update: Stoichiometry controlled oxide thin film growth by pulsed laser deposition, APL Mater. 3 (2015). doi:10.1063/1.4926933.
[31] J. Cheung, J. Horwitz, Pulsed Laser Deposition History and Laser-Target Interactions, MRS Bull. 17 (1992) 30–36. doi:10.1557/S0883769400040598.
[32] D.H. Lowndes, D.B. Geohegan, A.A. Puretzky, D.P. Norton, C.M. Rouleau, Synthesis of novel thin-film materials by pulsed laser deposition, Science (80-. ).
273 (1996) 898–903.
[33] Ian W. Boyd, Thin Film Growth by Pulsed Laser Deposition, Ceram. Int. 47 (2010) 790–805. doi:10.1177/0300985810372508.
[34] I. Fasaki, A. Koutoulaki, M. Kompitsas, C. Charitidis, Structural, electrical and mechanical properties of NiO thin films grown by pulsed laser deposition, Appl.
Surf. Sci. 257 (2010) 429–433. doi:10.1016/j.apsusc.2010.07.006.
[35] H. Search, C. Journals, A. Contact, M. Iopscience, I.P. Address, Room- Temperature Heteroepitaxial Growth of NiO Thin Films using Pulsed Laser Deposition Room-Temperature Heteroepitaxial Growth of NiO Thin Films using Pulsed Laser Deposition, 1817 (1817).
[36] Y. Kakehi, S. Nakao, K. Satoh, T. Kusaka, Room-temperature epitaxial growth of NiO(1 1 1) thin films by pulsed laser deposition, J. Cryst. Growth. 237–239 (2002) 591–595. doi:10.1016/S0022-0248(01)01964-9.
[37] R. Yamauchi, Y. Hamasaki, T. Shibuya, A. Saito, N. Tsuchimine, K. Koyama, A.
Matsuda, M. Yoshimoto, Layer matching epitaxy of NiO thin films on atomically stepped sapphire (0001) substrates, Sci. Rep. 5 (2015) 1–9.
doi:10.1038/srep14385.
[38] V. Verma, M. Katiyar, Effect of the deposition parameters on the structural and magnetic properties of pulsed laser ablated NiO thin films, Thin Solid Films. 527 (2013) 369–376. doi:10.1016/j.tsf.2012.12.020.
[39] K. Oka, T. Yanagida, K. Nagashima, H. Tanaka, T. Kawai, Growth atmosphere dependence of transport properties of NiO epitaxial thin films, J. Appl. Phys. 104 (2008). doi:10.1063/1.2952012.
[40] S.R. Lee, K. Char, D.C. Kim, R. Jung, S. Seo, X.S. Li, G.S. Park, I.K. Yoo,
Resistive memory switching in epitaxially grown NiO, Appl. Phys. Lett. 91 (2007).
doi:10.1063/1.2815658.
[41] W. Neubeck, L.L. Neel, Epitaxial MnO thin films grown by pulsed laser deposition,
(1999) 195–198. doi:10.1016/S0169-4332(98)00421-8.
[42] S. Isber, E. Majdalani, M. Tabbal, T. Christidis, K. Zahraman, B. Nsouli, Study of manganese oxide thin films grown by pulsed laser deposition, Thin Solid Films.
517 (2009) 1592–1595. doi:10.1016/j.tsf.2008.09.097.
[43] F. Liu, T. Makino, T. Yamasaki, K. Ueno, A. Tsukazaki, T. Fukumura, Y. Kong, M.
Kawasaki, Ultrafast time-resolved faraday rotation in EuO thin films, Phys. Rev.
Lett. 108 (2012) 1–5. doi:10.1103/PhysRevLett.108.257401.
[44] J.P. Neumann, T. Zhong, Y.A. Chang, The Ni-O (Nickel-Oxygen) system, Bull.
Alloy Phase Diagrams. 5 (1984) 141–144. doi:10.1007/BF02868949.
[45] S. Fritsch, A. Navrotsky, Thermodynamic properties of manganese oxides, J. Am.
Ceram. Soc. 79 (1996) 1761–1768.
[46] G.J. McCarthy, W.B. White, On the stabilities of the lower oxides of the rare earths, J. Less Common Met. 22 (1970) 409–417.
[47] A. Biswas, P.B. Rossen, C.H. Yang, W. Siemons, M.H. Jung, I.K. Yang, R.
Ramesh, Y.H. Jeong, Universal Ti-rich termination of atomically flat SrTiO3(001), (110), and (111) surfaces, Appl. Phys. Lett. 98 (2011) 2009–2012.
doi:10.1063/1.3549860.
[48] R. Gunnarsson, A.S. Kalabukhov, D. Winkler, Evaluation of recipes for obtaining single terminated perovskite oxide substrates, Surf. Sci. 603 (2009) 151–157.
[49] A. Biswas, C.H. Yang, R. Ramesh, Y.H. Jeong, Atomically flat single terminated oxide substrate surfaces, Prog. Surf. Sci. 92 (2017) 117–141.
doi:10.1016/j.progsurf.2017.05.001.
[50] M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuchiya, M. Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto, H. Koinuma, Atomic control of the SrTiO3 crystal surface, Science (80-. ). 266 (1994) 1540–1542.
[51] M. Bonholzer, M. Lorenz, M. Grundmann, Layer-by-layer growth of TiN by pulsed laser deposition on in-situ annealed (100) MgO substrates, Phys. Status Solidi Appl. Mater. Sci. 211 (2014) 2621–2624. doi:10.1002/pssa.201431458.
[52] T. Ohnishi, M. Lippmaa, T. Yamamoto, S. Meguro, H. Koinuma, Improved stoichiometry and misfit control in perovskite thin film formation at a critical fluence by pulsed laser deposition, Appl. Phys. Lett. 87 (2005) 1–3.
doi:10.1063/1.2146069.
[53] G. Chern, C. Cheng, Interface matching in oxides of rocksalt/rocksalt(001) and rocksalt/perovskite(001), J. Vac. Sci. Technol. A. 17 (1999) 1097–1102.
doi:10.1116/1.581780.
[54] B. Wölfing, K. Theis-Bröhl, C. Sutter, H. Zabel, AFM and x-ray studies on the growth and quality of Nb (110) on, J. Phys. Condens. Matter. 11 (1999) 2669.
[55] M. Becht, F. Wang, J.G. Wen, T. Morishita, Evolution of the microstructure of
oxide thin films, J. Cryst. Growth. 170 (1997) 799–802. doi:10.1016/S0022- 0248(96)00563-5.
[56] O. Durand, A. Letoublon, D.J. Rogers, F. Hosseini Teherani, Interpretation of the two-components observed in high resolution X-ray diffraction ω scan peaks for mosaic ZnO thin films grown on c-sapphire substrates using pulsed laser deposition, Thin Solid Films. 519 (2011) 6369–6373.
doi:10.1016/j.tsf.2011.04.036.
[57] P.F. Miceli, J. Weatherwax, T. Krentsel, C.J. Palmstrøm, Specular and diffuse reflectivity from thin films containing misfit dislocations, Phys. B Condens. Matter.
221 (1996) 230–234. doi:10.1016/0921-4526(95)00930-2.
[58] A.R. Wildes, J. Mayer, K. Theis-Bröhl, The growth and structure of epitaxial niobium on sapphire, Thin Solid Films. 401 (2001) 7–34. doi:10.1016/S0040- 6090(01)01631-5.
[59] P. Review, X-ray scattering, 51 (1995) 5506–5509.
[60] S.K. Hau, K.H. Wong, P.W. Chan, C.L. Choy, Intrinsic resputtering in pulsed‐laser deposition of lead‐zirconate‐titanate thin films, Appl. Phys. Lett. 66 (1995) 245–
247. doi:10.1063/1.113560.
[61] J.N. Beukers, J.E. Kleibeuker, G. Koster, D.H.A. Blank, G. Rijnders, H.
Hilgenkamp, A. Brinkman, Epitaxial EuO thin films by pulsed laser deposition monitored by in situ x-ray photoelectron spectroscopy, Thin Solid Films. 518 (2010) 5173–5176. doi:10.1016/j.tsf.2010.04.071.
Figure. 1.
Fig.1. AFM topographic 5ˣ5 μm2 images of the treated substrates, (a) SrTiO3 (001), (b) MgO (001), (c) LaAlO3 (001) and (d) YAlO3 (110).
Figure. 2.
Fig. 2. (a) XRD spectra of NiO films with different thickness from 15 to 200 nm grown at 750 °C on STO (001) substrate. (b) The out-of-plane strain derived from the position of NiO (002) peak.
(c) Ɵ rocking curves around the NiO (002) diffraction peak.
Figure. 3.
Fig. 3. AFM images (5ˣ5 μm2) of the NiO thin films with different thickness of (a) 15 nm, (b) 20 nm, (c) 30 nm, (d) 100 nm and (e) 200 nm grown on STO (001) substrate at 750 °C.
Figure. 4.
Fig. 4. (a) XRD spectra of the 15-nm NiO films grown at different temperatures on the STO (001) substrate. (b) The out of plane strain derived from the NiO (002) peak position. (c) Ɵ-rocking curve around the NiO (002) diffraction peak.
Figure. 5.
Fig. 5. AFM images of the 15-nm NiO thin films grown on STO (001) substrates at different temperatures: (a) 400 °C and (b) 10 °C; (c) X-ray reflectivity of films grown at different temperatures.
Figure. 6.
Fig. 6. XRD spectra of the MnO films grown at 750 °C with different laser energy fluences.
Figure. 7.
Fig. 7. XRD spectra of the MnO films grown at different substrate temperatures from 550 to 750° shown in the legend.
Figure. 8.
Fig. 8. (a) Ɵ-rocking scans around the MnO (002) peak for the films grown at two different temperatures on the MgO (001) substrates.(b) X-ray reflectivity spectra. (c, d) AFM topographic images for the films grown at 750 and 650 °C, respectively.
Figure. 9.
Fig. 9. XRD scans for EuO thin films grown on different substrates at 350 °C.
Figure. 10.
Fig.10. The effect of growth temperature on the composition of the deposited film on YAO (110) substrate
Figure. 11.
Fig.11. The effect of target cleaning time on the XRD spectrum of the EuO films.
Figure. 12.
Fig. 12. (a) X-ray reflectivity spectrum for a 20-nm EuO film deposited on YAO (110) at 350 °C (b) Ɵ-rocking curve around EuO (002) peak and (c) a 5ˣ5 μm2 AFM image of the film.