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NiFe2O4 thin films prepared by solution deposition technique
Ceramics International 44 (2018) 15965–15971
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Magnetic properties of multilayer BaTiO3/NiFe2O4 thin films prepared by solution deposition technique
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Branimir Bajaca, , Marija Milanovica, Zeljka Cvejicb, Adelina Ianculescuc, Petronel Postolached, Liliana Mitoseriud, Vladimir V. Srdica a
Department of Materials Engineering, Faculty of Technology Novi Sad, University of Novi Sad, Bul. Cara Lazara 1, 21000 Novi Sad, Serbia Department of Physics, Faculty of Sciences, University of Novi Sad, Trg D. Obradovića 4, 21000 Novi Sad, Serbia c Department of Oxide Materials Science & Engineering, Politehnica University of Bucharest, 1-7 Gh. Polizu, 011061 Bucharest, Romania d Faculty of Physics, Al. I. Cuza University, Bv. Carol I no. 11, Iasi 700506, Romania b
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Keywords: BaTiO3/NiFe2O4 Multilayer thin films Solution deposition technique Magnetic properties
In this work multilayered structures, composed of perovskite BaTiO3 and spinel NiFe2O4 layers, were obtained by solution deposition technique. The multiferroic films with different geometries, i.e. different numbers and thicknesses of titanate and ferrite layers, were prepared. Structural characterization of the sintered thin films confirmed the well-defined layered structure with overall thickness around 500 nm, crystalline nature of perovskite BaTiO3 and spinel NiFe2O4 phases without secondary phases (after sintering at 750 °C) and grains on nanometer scale. It was also shown that there are no significant changes of the film microstructure with variation of the layer geometry. The influence of the film geometries on magnetic properties of the multiferroic films sintered at 750 and 900 °C were also investigated. The well saturated hysteresis loops with low coercitivity, typical for nickel-ferrite based materials, were recorded. It was shown that the saturation magnetization decreased with decrease of the titanate and ferrite layer thicknesses (i.e. increased amount of their contact surfaces). It is believed that the observed magnetic behaviors are mainly influenced by the interlayer straining, whereas contribution of the microstructural differences is not so pronounced.
1. Introduction Few decades ago, new discoveries in field of electronic materials [1–11] and components have revolutionized the concept of computers and other electronic device, in terms of size and general performance quality. Development of ceramic thin film technologies, down to submicrometer thickness, is crucial for fabrication of actuators, computer memory, transistors, capacitors, integrated circuits etc. [1,3,5]. Multiferroics are an attractive group of materials with potential application in microelectronic industry, due to their unique properties to exhibit multiple ferroic orders simultaneously [3–5,7–9]. It is not just important for multiferroic material to possess multiple ferroic abilities, but it is crucial to achieve significant coupling interaction between them, defined as magnetoelectric effect [2–4,7–9]. Until now, several review articles [8–11] have shown that single phase multiferroics cannot match the functionality of composite multiferroics, where thin film structures are a favorable choice against the bulk composites. In addition, composite horizontal multilayer thin film structures have attracted special interest because of better
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Corresponding author. E-mail address: [email protected] (B. Bajac).
https://doi.org/10.1016/j.ceramint.2018.06.023 Received 27 April 2018; Received in revised form 1 June 2018; Accepted 4 June 2018
Available online 05 June 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
magnetoelectric coupling than particulate thin films. Moreover, such kind of structures might solve the current leakage problems due to the perfect insulation of low resistivity magnetic phase with highly resistive ferroelectric layers. Thus, this can be a good perspective for further development, especially if relatively easy and cheap solution deposition technique could be used for preparation of these multilayer thin films [7–11]. Some recent articles have addressed the functional properties and layer interactions of layered multiferroic films trough different approaches [12–16]. Among available literature, Dai et al. [14] explored the layer thickness effect on ferroelectric and ferromagnetic properties of BaTiO3/CoFe2O4 multilayers obtained by sol-gel process, Verma et al. [13] addressed the origin of enhanced magnetoelectric coupling of BaTiO3/NiFe2O4 films obtained by pulsed laser deposition, while Patil et al. [17] investigated the longitudinal magnetoelectric coupling in BaTiO3/NiFe2O4 thick film systems. None of the thin films systems obtained by the sol-gel technique explore 12 layer systems thick below 1 µm, thus we think that some valuable data for further research in this field may be obtained by modification of layer geometry, and trough
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investigation of magnetic properties interlayer interactions may be recognized. In this work the multiferroic films with different geometries, i.e. different numbers and thicknesses of BaTiO3 and NiFe2O4 layers, were prepared. Structural characterization was performed to provide base for understanding the functional properties. Magnetic hysteresis loops of the multilayer films with specific geometries were measured to identify extent of interface effects and investigate the influence of layer number and thickness on the magnetic behavior.
2. Experimental The multilayer BaTiO3/NiFe2O4 (B/N) thin films were prepared by chemical solution deposition technique. The precursor solution for the deposition of BaTiO3 phase was prepared by mixing of BaCO3 (Merck, Germany) dissolved in glacial acetic acid with tetrabutyl-orthotitanate (Ti(OC4H9)4, Fluka, Switzerland) at room temperature and pH = 1. After 20 min of stirring at room temperature a clear, colorless precursor sol with concentration of 0.25 M was obtained. Ferrite phase was deposited from a clear, brownish precursor solution, prepared by dissolving stoichiometric amounts of Fe(NO)3 × 9H2O and Ni(NO)3 × 6H2O (Fluka, Switzerland) in 2-methoxyethanol at room temperature and pH = 1.5. Up to twelve depositions were processed by spin coating (using speed of 3000 rpm) on ultrasonically cleaned commercially available Pt/TiO2/SiO2/Si substrate with BaTiO3 as a bottom layer. Every single deposit was calcined at 500 °C for 5 min immediately after deposition in order to completely evaporate traces of residual solvents, because reaction between them leads to surface defects in the prepared films [18]. The multilayer films with different geometries, i.e. different numbers and thicknesses of titanate and ferrite layers, were prepared where one single layer composed of one (Fig. 1a), two (Fig. 1b) or three deposits (Fig. 1c). The prepared multilayer thin films are denoted as BxNy-Z, where x is number of depositions used to form one single BaTiO3 layer, y is number of depositions used to form one single NiFe2O4 layer and Z total number of depositions. The deposited multilayer B/N films were sintered in tube furnace at different temperatures (up to 900 °C) in air atmosphere. The pure NiFe2O4 thin film with six spin coated deposits (the sample denoted N-6) was also prepared as a reference. X-ray diffraction analyses of the investigated samples were performed on a Rigaku, MiniFlex 600 instrument, using Ni-filtered CuKα radiation. Raman spectra were obtained by using Thermo Scientific DXR Raman Microscope, equipped with green laser (wavelength 532 nm). Morphology and layer structure of the films were investigated with TEM (Tecnai TM G2 F30 S-TWIN transmission electron microscope FEI, the Netherlands) operating at 300 kV. Room temperature ferromagnetic loops were measured up to a magnetic field of 10 kOe using
an Alternating Gradient Magnetometer, MicroMag™ VSM/AGM model 3900, Princeton Measurements Co.
3. Results and discussion 3.1. Structure of BaTiO3/NiFe2O4 films The aim of structural characterization of the prepared multilayer BaTiO3/NiFe2O4 films with complex geometry (i.e. different layer numbers and thicknesses) was to confirm uniformity and homogeneity of the obtained layers as well as to investigate the influence of their geometry on microstructure and phase composition. TEM images of the multilayer B1N1-10 sample structure, with 10 deposits, thermally treated at 500 °C for 5 min are presented in Fig. 2. It can be observed that the film possesses well defined layered structure, with thickness of one single titanate layer around 60 nm, one deposited ferrite layer about 40 nm, and overall thickness around 500 nm (Fig. 2a and c). The formation of nanosized crystalline grains of NiFe2O4 phase is clearly observed even after thermal treatment at 500 °C (Fig. 2b), which is characteristic for the formation of spinel structure in general [19,20]. The ferrite layer has a polycrystalline nature, with well-defined grains of sizes in the range of ~5 nm, while the BaTiO3 layer has a finer grainy aspect (Fig. 2d), with only some small single crystalline regions (average diameters below 10 nm) embedded into an amorphous matrix (red circles in HRTEM image, Fig. 2d), because the formation of BaTiO3 perovskite phase requires higher sintering temperatures [21]. The interfaces between the spinel and perovskite layers are smooth and clean, demonstrating a high structural compatibility between the ferrite and ferroelectric layers. STEM and mapping through the cross section of the multilayer B1N1-10 film (Fig. 3) confirmed the layered structure and even distribution of elements over the cross section. In line profile, from zero point starting with the silicon substrate, 10 layers in alternating order can be observed, with overlapping of Ba and Ti lines. Phase composition of the multilayer thin film and possible chemical interaction between BaTiO3 and NiFe2O4 layers were analyzed by XRD and only a few important results are presented here. Typical XRD patterns of the multilayer BaTiO3/NiFe2O4 thin film sintered at 750 and 900 °C for 30 min are given in Fig. 4. It can be concluded that the diffractograms of the multilayer films sintered at 750 °C consist of the characteristic XRD peaks of BaTiO3, NiFe2O4 and Pt only, without any additional undesirable phase. Further increase of the sintering temperature to 900 °C causes appearance of very small amount of some other phases. Thus, XRD peak at ~ 27.4° belongs to TiO2 (rutile) most probably from the substrate, and a very weak peak at 28.6° might belong to some secondary phase formed in the reaction between the ferrite and titanate layers. The influence of layer thickness and geometry on structure of the
Fig. 1. Multilayer film structures with different BaTiO3 and NiFe2O4 layer thickness: a) B1N1-12, b) B2N2-12 and c) B3N3-12. 15966
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Fig. 2. TEM images of B1N1-10 thin film thermal treated at 500 °C: a) TEM of film cross section (low magnification), b) HRTEM of ferrite layer, c) TEM of film cross section (high magnification) and d) HRTEM of titanate layer.
multilayer thin films was also evaluated by XRD (Fig. 5) using three different films sintered at 750 °C with the same phase ratio and similar thickness (around 500 nm), but with different numbers of shared B/N interfaces and layer thickness. A gradual increase of the layer thickness does not change the phase composition, but causes some small modification of XRD peaks which become more intense and narrow (due to a small increase of grain size). Thus, it can be concluded that change in layer thicknesses causes only small differences in the film microstructure. In regard to that, only SEM surface image of sample B3N3-12 had been added as inset in Fig. 5, confirming dense, crack free structure, and grains of NiFe2O4 around 20–25 nm. In all presented XRD patterns (Figs. 4 and 5), the tetragonal phase of BaTiO3 could not be clearly identified, since there was no obvious peak
split characteristic for tetragonal phase at around 2θ~45°. A gradual decrease of tetragonal distortion c/a and corresponding spontaneous polarization with decreasing the size towards a pseudocubic state was commonly reported for nanostructured BaTiO3 instead of the tetragonal one specific to bulk material [22,23]. At a critical size, tetragonality vanishes and ferroelectricity is not anymore sustained, which was interpreted as a size-driven ferroelectric-to-paraelectric transition. Due to the resolution limit of XRD experiment, the presence of tetragonal phase in principle could be detected by Raman spectroscopy, since vibrational spectroscopy is more sensitive to detect modifications of local crystalline symmetry with respect to the average one detected by XRD. Raman spectra of all the multilayer samples are very similar. Hence, only the Raman spectrum of the sample B2N2-12 is shown in Fig. 6,
Fig. 3. STEM and mapping of the multilayer B1N1-10 film structure thermally treated at 500 °C. 15967
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Fig. 4. XRD patterns of B1N1-12 thin films on Pt/TiO2/SiO2/Si substrate sintered at 750 and 900 °C.
Raman active modes [24], since F1u mode is completely optically silent and F2u is only infrared active. On the other hand, ferroelectric tetragonal phase has a number of optical modes 3(A1 + E) + E + B1, which all are Raman active [25]. As observed by XRD analysis, pseudocubic state or low tetragonal distortion may be expected in titanate phase, which explains a weak Raman response of barium titanate component (Fig. 6). However, inverse spinel structure of nickel ferrite is suitable for Raman analysis, due to a number of strong Raman active modes [26]. Thus, under broad 702 cm−1 peak with the well-defined shoulder two peaks can be observed at 661 cm−1 and 702 cm−1 due to contribution of Eg and A1g modes of NiFe2O4, respectively. Broad 576 cm−1 peak corresponds to A1g vibration mode of NiFe2O4. Two weak characteristic tetragonal Raman modes of BaTiO3 may be observed at 520 cm−1 and 305 cm−1, representing E(TO) and E modes, respectively [27]. In addition, Eg and T2g modes of NiFe2O4 are also noticeable at 486 cm−1 with 451 cm−1 shoulder and 333 cm−1 with very weak 308 cm−1 shoulder, respectively. Raman spectroscopy complemented X-ray diffraction and confirmed the presence of inverse spinel NiFe2O4 phase, and a weak response from BaTiO3 phase. Since the Raman modes are so weak, it is difficult to assign the tetragonal phase to the BaTiO3 layer. In any case, the observed Raman active modes indicate that a local noncentrosymmetric structure is typical at room temperature to the nanostructured BaTiO3 layers, even it appears as globally pseudo-cubic within the resolution limit of XRD analysis 3.2. Magnetic properties of BaTiO3/NiFe2O4 films
Fig. 5. XRD patterns of multilayer thin films with different geometry sintered at 750 °C (inset: SEM image of top NiFe2O4 surface layer).
Fig. 6. Room temperature Raman spectrum of B2N2-12 thin film sintered at 750 °C.
where a strong response of the ferrite phase is noticed, overlapped onto weak response of titanate phase, with slightly stronger noise at lower values of wavenumber. Cubic BaTiO3 crystal phase theoretically has no
3.2.1. NiFe2O4 films Room temperature magnetic loops for the pure ferrite films (N-6), sintered at 750 and 900 °C, were analyzed by AGM device. The well saturated hysteresis loops were recorded in range from 10 to − 10 kOe (Fig. 7) and their shapes resemble ferrimagnetic behavior typical for nickel-ferrite based materials. Saturation magnetization (Ms) of the pure NiFe2O4 thin films sintered at 750 and 900 °C are 60 and 77 emu/ cm3, respectively. Higher Ms value of the ferrite film sintered at 900 °C is due to its higher crystallinity and larger grains. These values are lower than the saturation magnetization of the bulk NiFe2O4 ceramics (~ 270 emu/cm3) [28], but also smaller than reported in other NiFe2O4 films, as for example Ms = 200 emu/cm3 in the pure NFO films produced by polymer assisted deposition and calcined at higher temperature of 950 °C/1 h [29]. As explained by Tong et al. [30], the grain morphology combined with density and stress tuning can strongly modulate the magnetic, but also the optical and surface characteristics in such NFO films. Therefore, the differences found in the magnetization values with respect to the reported ones are related to microstructural characteristics as grain size, sample density, texturing and to strain-stress interface mediated properties generated by various synthesis methods, calcination temperatures and substrate type. The larger grain size leads to better crystallinity, which induces higher magnetocrystalline anisotropy and finally leads the high saturation magnetization and coercivity, similar as found in many iron oxide-based materials [31]. The increase in saturation magnetization with increased grain size in such ferrites may be also caused by changes in the cation distribution (i.e. cation disorder), which usually are affected by the synthesis conditions and/or the strain the film [32,33]. The increase of sintering temperature from 750 to 900°C reduces the coercive field (Hc) from around 200–120 Oe and remanent magnetization (Mr), from 15 to 8 emu/cm3 (Fig. 7). Reduced intergranular straining and better crystallinity of the pure ferrite N-6 film sintered at higher temperature (i.e. 900 °C) insures easier domain orientation and a lower Hc. 3.2.2. BaTiO3/NiFe2O4 films The hysteresis loops of the multilayer films sintered at 750 and 900 °C are presented in Figs. 8 and 9, respectively. Magnetization values were calculated in accordance to the amount of magnetic (NiFe2O4)
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Fig. 7. Magnetic M(H) loop of NiFe2O4 (N-6) films sintered at 750 and 900 °C (a) and detail showing the low field region (b).
phase (whereas insets show hysteresis loops when the presence of both phases were considered). The hysteresis loops of the multilayer films sintered at 750 °C, presented in the inset of Fig. 8, clearly indicate considerably lower magnetization of the multilayer films than the pure NiFe2O4 films, due to high amount of nonmagnetic BaTiO3 phase. However, if only the amount of ferrite phase is considered (Fig. 8), it can be seen that the multilayer B3N3-12 thin film has almost the same Ms as the pure ferrite film (N-6). In addition, there is clear decrease of saturation magnetization with increase of the contact B/N surface number, from B3N3-12 to B1N1-12. Two main effects might explain such a result: i) thicker ferrite layers promote the formation of somewhat larger grains and ii) more contact B/N surfaces and thinner layers produce stronger straining effect over the surface (desirable effect in multiferroic composite materials) and increase the amount of defectrich disordered or amorphous interfaces. Since there is no considerable change in crystallinity and the grain size between the multilayer B1N112, B2N2-12 and B3N3-12 films (Fig. 5) it seems that the effect of interlayer straining is a dominant one. The strong effect of straining in layered nanostructures, gradually increasing with reduction of layer thickness, has been reported in similar systems [14,17]. Similar behavior was observed for the multilayer films sintered at 900 °C (Fig. 9), only the saturation magnetization of the pure ferrite film (N-6) is higher that Ms of the multilayer B3N3-12 thin film. It might
be due to somewhat faster growth of ferrite grains in the pure N-6 film at 900 °C which is not constrained by the titanate layers. Another characteristic of all multilayer films sintered at 750 and 900 °C is decrease of magnetization at higher magnetic fields (Figs. 8 and 9). It is most probably due to the contribution from diamagnetic substrate, but it could be also explained by with interlayer straining (as it seems that this behavior is more pronounced for the multilayer films with more contact B/N surfaces, i.e. sample B1N1-12). The remanent magnetization (Mr) of the multulayer films is in a direct proportion with Ms. Compared to the pure ferrite, somewhat lower Hc of the multilayer films has been recorded for the samples sintered at 750 and 900 °C (Figs. 8 and 9). This may be ascribed to strain relaxation induced by reaching Ms. The values of Mr and Hc point out strain meditated magnetization of multilayer films, as a dominant effect over microstructure coarsening with sintering temperature.
4. Conclusions Multilayer thin film structures composed of BaTiO3 and NiFe2O4 layers deposited in alternating order, with different layer thickness, were prepared by solution deposition technique on platinum coated silicon substrates. Structural characterization of the sintered samples confirmed the well-defined layered structure with overall thickness
Fig. 8. Magnetic properties of thin films sintered at 750 °C: a) magnetic hysteresis loops determined in relation to ferrite phase alone (inset - hysteresis loops determined in relation to complete film thickness) and b) enlarged part of magnetic hysteresis loops in the low field region. 15969
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Fig. 9. Magnetic properties of thin films sintered at 900 °C: a) magnetic hysteresis loops determined in relation to ferrite phase alone (inset - hysteresis loops determined in relation to complete film thickness) and b) enlarged part of magnetic hysteresis loops in the low field region.
around 500 nm, crystalline nature (with perovskite BaTiO3 and spinel NiFe2O4 phases) without secondary phases (after sintering below 900 °C) and grains on nanometer scale. The influence of the film geometries on magnetic properties of the multiferroic films sintering temperature at 750 and 900 °C were also investigated. The well saturated hysteresis loops with low coercitivity, typical for nickel-ferrite based materials, were recorded. Magnetic saturation of the pure ferrite films increased with higher sintering temperature, from 750° to 900°C, due to promoted grain growth, making them more susceptible to magnetization. It was also shown that the saturation magnetization decreased with decrease of the titanate and ferrite layer thicknesses (i.e. increased amount of their contact surfaces). The reason could be the interlayer strain relaxation which is more pronounced when there are more sharing ferrite/titanate interfaces and thinner ferrite layers. Low values of Hc support the strain mediated magnetization process. Acknowledgements The authors gratefully acknowledge the financial support provided by the Ministry of Science of the Republic of Serbia, project III45021 and COST Project IC1208. The authors thank Bogdan Vasile, from Dept. Oxide Materials Science & Engineering, “Politehnica” University of Bucharest, Romania, for performed TEM/STEM analysis. References [1] L.W. Martin, Y.H. Chu, R. Ramesh, Advances in the growth and characterization of magnetic, ferroelectric, and multiferroic oxide thin films, Mater. Sci. Eng. R68 (2010) 89–133. [2] J. Ma, J. Hu, Z. Li, C.-W. Nan, Recent progress in multiferroic magnetoelectric composites: from bulk to thin films, Adv. Mater. 23 (2011) 1062–1087. [3] S. Roy, S.B. Majumder, Recent advances in multiferroic thin films and composites, J. Alloy. Compd. 538 (2012) 153–159. [4] L.W. Martin, D.G. Schlom, Advanced synthesis techniques and routes to new singlephase multiferroics, Curr. Opin. Solid State Mater. Sci. 16 (2012) 199–215. [5] M.I. Bichurinn, V.M. Petrov, R.V. Petrov, Direct and inverse magnetoelectric effect in layered composites in electromechanical resonance range: a review, J. Magn. Magn. Mater. 324 (2012) 3548–3550. [6] G. Schileo, Recent developments in ceramic multiferroic composites based oncore/ shell and other heterostructures obtained by sol-gel routes, Prog. Solid State Chem. 41 (2013) 87–98. [7] N. Ortega, A. Kumar, J.F. Scott, R.S. Katiyar, Multifunctional magnetoelectric materials for device applications, J. Appl. Phys. 113 (2013) 074103. [8] J.F. Scott, Room-temperature multiferroic magnetoelectrics, NPG Asia Mater. 5 (2013) e72. [9] N.A. Spaldin, S.W. Cheong, R. Ramesh, Multiferroics: past, present, and future, Phys. Today 63 (10) (2010) 38–43. [10] C.A.F. Vaz, J. Hoffman, C.H. Ahn, R. Ramesh, Magnetoelectric coupling effects in multiferroic complex oxide composite structures, Adv. Mater. 22 (2010)
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Magnetic properties of multilayer BaTiO3/NiFe2O4 thin films prepared by solution deposition technique
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Branimir Bajaca, , Marija Milanovica, Zeljka Cvejicb, Adelina Ianculescuc, Petronel Postolached, Liliana Mitoseriud, Vladimir V. Srdica a
Department of Materials Engineering, Faculty of Technology Novi Sad, University of Novi Sad, Bul. Cara Lazara 1, 21000 Novi Sad, Serbia Department of Physics, Faculty of Sciences, University of Novi Sad, Trg D. Obradovića 4, 21000 Novi Sad, Serbia c Department of Oxide Materials Science & Engineering, Politehnica University of Bucharest, 1-7 Gh. Polizu, 011061 Bucharest, Romania d Faculty of Physics, Al. I. Cuza University, Bv. Carol I no. 11, Iasi 700506, Romania b
A R T I C LE I N FO
A B S T R A C T
Keywords: BaTiO3/NiFe2O4 Multilayer thin films Solution deposition technique Magnetic properties
In this work multilayered structures, composed of perovskite BaTiO3 and spinel NiFe2O4 layers, were obtained by solution deposition technique. The multiferroic films with different geometries, i.e. different numbers and thicknesses of titanate and ferrite layers, were prepared. Structural characterization of the sintered thin films confirmed the well-defined layered structure with overall thickness around 500 nm, crystalline nature of perovskite BaTiO3 and spinel NiFe2O4 phases without secondary phases (after sintering at 750 °C) and grains on nanometer scale. It was also shown that there are no significant changes of the film microstructure with variation of the layer geometry. The influence of the film geometries on magnetic properties of the multiferroic films sintered at 750 and 900 °C were also investigated. The well saturated hysteresis loops with low coercitivity, typical for nickel-ferrite based materials, were recorded. It was shown that the saturation magnetization decreased with decrease of the titanate and ferrite layer thicknesses (i.e. increased amount of their contact surfaces). It is believed that the observed magnetic behaviors are mainly influenced by the interlayer straining, whereas contribution of the microstructural differences is not so pronounced.
1. Introduction Few decades ago, new discoveries in field of electronic materials [1–11] and components have revolutionized the concept of computers and other electronic device, in terms of size and general performance quality. Development of ceramic thin film technologies, down to submicrometer thickness, is crucial for fabrication of actuators, computer memory, transistors, capacitors, integrated circuits etc. [1,3,5]. Multiferroics are an attractive group of materials with potential application in microelectronic industry, due to their unique properties to exhibit multiple ferroic orders simultaneously [3–5,7–9]. It is not just important for multiferroic material to possess multiple ferroic abilities, but it is crucial to achieve significant coupling interaction between them, defined as magnetoelectric effect [2–4,7–9]. Until now, several review articles [8–11] have shown that single phase multiferroics cannot match the functionality of composite multiferroics, where thin film structures are a favorable choice against the bulk composites. In addition, composite horizontal multilayer thin film structures have attracted special interest because of better
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Corresponding author. E-mail address: [email protected] (B. Bajac).
https://doi.org/10.1016/j.ceramint.2018.06.023 Received 27 April 2018; Received in revised form 1 June 2018; Accepted 4 June 2018
Available online 05 June 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
magnetoelectric coupling than particulate thin films. Moreover, such kind of structures might solve the current leakage problems due to the perfect insulation of low resistivity magnetic phase with highly resistive ferroelectric layers. Thus, this can be a good perspective for further development, especially if relatively easy and cheap solution deposition technique could be used for preparation of these multilayer thin films [7–11]. Some recent articles have addressed the functional properties and layer interactions of layered multiferroic films trough different approaches [12–16]. Among available literature, Dai et al. [14] explored the layer thickness effect on ferroelectric and ferromagnetic properties of BaTiO3/CoFe2O4 multilayers obtained by sol-gel process, Verma et al. [13] addressed the origin of enhanced magnetoelectric coupling of BaTiO3/NiFe2O4 films obtained by pulsed laser deposition, while Patil et al. [17] investigated the longitudinal magnetoelectric coupling in BaTiO3/NiFe2O4 thick film systems. None of the thin films systems obtained by the sol-gel technique explore 12 layer systems thick below 1 µm, thus we think that some valuable data for further research in this field may be obtained by modification of layer geometry, and trough
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investigation of magnetic properties interlayer interactions may be recognized. In this work the multiferroic films with different geometries, i.e. different numbers and thicknesses of BaTiO3 and NiFe2O4 layers, were prepared. Structural characterization was performed to provide base for understanding the functional properties. Magnetic hysteresis loops of the multilayer films with specific geometries were measured to identify extent of interface effects and investigate the influence of layer number and thickness on the magnetic behavior.
2. Experimental The multilayer BaTiO3/NiFe2O4 (B/N) thin films were prepared by chemical solution deposition technique. The precursor solution for the deposition of BaTiO3 phase was prepared by mixing of BaCO3 (Merck, Germany) dissolved in glacial acetic acid with tetrabutyl-orthotitanate (Ti(OC4H9)4, Fluka, Switzerland) at room temperature and pH = 1. After 20 min of stirring at room temperature a clear, colorless precursor sol with concentration of 0.25 M was obtained. Ferrite phase was deposited from a clear, brownish precursor solution, prepared by dissolving stoichiometric amounts of Fe(NO)3 × 9H2O and Ni(NO)3 × 6H2O (Fluka, Switzerland) in 2-methoxyethanol at room temperature and pH = 1.5. Up to twelve depositions were processed by spin coating (using speed of 3000 rpm) on ultrasonically cleaned commercially available Pt/TiO2/SiO2/Si substrate with BaTiO3 as a bottom layer. Every single deposit was calcined at 500 °C for 5 min immediately after deposition in order to completely evaporate traces of residual solvents, because reaction between them leads to surface defects in the prepared films [18]. The multilayer films with different geometries, i.e. different numbers and thicknesses of titanate and ferrite layers, were prepared where one single layer composed of one (Fig. 1a), two (Fig. 1b) or three deposits (Fig. 1c). The prepared multilayer thin films are denoted as BxNy-Z, where x is number of depositions used to form one single BaTiO3 layer, y is number of depositions used to form one single NiFe2O4 layer and Z total number of depositions. The deposited multilayer B/N films were sintered in tube furnace at different temperatures (up to 900 °C) in air atmosphere. The pure NiFe2O4 thin film with six spin coated deposits (the sample denoted N-6) was also prepared as a reference. X-ray diffraction analyses of the investigated samples were performed on a Rigaku, MiniFlex 600 instrument, using Ni-filtered CuKα radiation. Raman spectra were obtained by using Thermo Scientific DXR Raman Microscope, equipped with green laser (wavelength 532 nm). Morphology and layer structure of the films were investigated with TEM (Tecnai TM G2 F30 S-TWIN transmission electron microscope FEI, the Netherlands) operating at 300 kV. Room temperature ferromagnetic loops were measured up to a magnetic field of 10 kOe using
an Alternating Gradient Magnetometer, MicroMag™ VSM/AGM model 3900, Princeton Measurements Co.
3. Results and discussion 3.1. Structure of BaTiO3/NiFe2O4 films The aim of structural characterization of the prepared multilayer BaTiO3/NiFe2O4 films with complex geometry (i.e. different layer numbers and thicknesses) was to confirm uniformity and homogeneity of the obtained layers as well as to investigate the influence of their geometry on microstructure and phase composition. TEM images of the multilayer B1N1-10 sample structure, with 10 deposits, thermally treated at 500 °C for 5 min are presented in Fig. 2. It can be observed that the film possesses well defined layered structure, with thickness of one single titanate layer around 60 nm, one deposited ferrite layer about 40 nm, and overall thickness around 500 nm (Fig. 2a and c). The formation of nanosized crystalline grains of NiFe2O4 phase is clearly observed even after thermal treatment at 500 °C (Fig. 2b), which is characteristic for the formation of spinel structure in general [19,20]. The ferrite layer has a polycrystalline nature, with well-defined grains of sizes in the range of ~5 nm, while the BaTiO3 layer has a finer grainy aspect (Fig. 2d), with only some small single crystalline regions (average diameters below 10 nm) embedded into an amorphous matrix (red circles in HRTEM image, Fig. 2d), because the formation of BaTiO3 perovskite phase requires higher sintering temperatures [21]. The interfaces between the spinel and perovskite layers are smooth and clean, demonstrating a high structural compatibility between the ferrite and ferroelectric layers. STEM and mapping through the cross section of the multilayer B1N1-10 film (Fig. 3) confirmed the layered structure and even distribution of elements over the cross section. In line profile, from zero point starting with the silicon substrate, 10 layers in alternating order can be observed, with overlapping of Ba and Ti lines. Phase composition of the multilayer thin film and possible chemical interaction between BaTiO3 and NiFe2O4 layers were analyzed by XRD and only a few important results are presented here. Typical XRD patterns of the multilayer BaTiO3/NiFe2O4 thin film sintered at 750 and 900 °C for 30 min are given in Fig. 4. It can be concluded that the diffractograms of the multilayer films sintered at 750 °C consist of the characteristic XRD peaks of BaTiO3, NiFe2O4 and Pt only, without any additional undesirable phase. Further increase of the sintering temperature to 900 °C causes appearance of very small amount of some other phases. Thus, XRD peak at ~ 27.4° belongs to TiO2 (rutile) most probably from the substrate, and a very weak peak at 28.6° might belong to some secondary phase formed in the reaction between the ferrite and titanate layers. The influence of layer thickness and geometry on structure of the
Fig. 1. Multilayer film structures with different BaTiO3 and NiFe2O4 layer thickness: a) B1N1-12, b) B2N2-12 and c) B3N3-12. 15966
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Fig. 2. TEM images of B1N1-10 thin film thermal treated at 500 °C: a) TEM of film cross section (low magnification), b) HRTEM of ferrite layer, c) TEM of film cross section (high magnification) and d) HRTEM of titanate layer.
multilayer thin films was also evaluated by XRD (Fig. 5) using three different films sintered at 750 °C with the same phase ratio and similar thickness (around 500 nm), but with different numbers of shared B/N interfaces and layer thickness. A gradual increase of the layer thickness does not change the phase composition, but causes some small modification of XRD peaks which become more intense and narrow (due to a small increase of grain size). Thus, it can be concluded that change in layer thicknesses causes only small differences in the film microstructure. In regard to that, only SEM surface image of sample B3N3-12 had been added as inset in Fig. 5, confirming dense, crack free structure, and grains of NiFe2O4 around 20–25 nm. In all presented XRD patterns (Figs. 4 and 5), the tetragonal phase of BaTiO3 could not be clearly identified, since there was no obvious peak
split characteristic for tetragonal phase at around 2θ~45°. A gradual decrease of tetragonal distortion c/a and corresponding spontaneous polarization with decreasing the size towards a pseudocubic state was commonly reported for nanostructured BaTiO3 instead of the tetragonal one specific to bulk material [22,23]. At a critical size, tetragonality vanishes and ferroelectricity is not anymore sustained, which was interpreted as a size-driven ferroelectric-to-paraelectric transition. Due to the resolution limit of XRD experiment, the presence of tetragonal phase in principle could be detected by Raman spectroscopy, since vibrational spectroscopy is more sensitive to detect modifications of local crystalline symmetry with respect to the average one detected by XRD. Raman spectra of all the multilayer samples are very similar. Hence, only the Raman spectrum of the sample B2N2-12 is shown in Fig. 6,
Fig. 3. STEM and mapping of the multilayer B1N1-10 film structure thermally treated at 500 °C. 15967
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Fig. 4. XRD patterns of B1N1-12 thin films on Pt/TiO2/SiO2/Si substrate sintered at 750 and 900 °C.
Raman active modes [24], since F1u mode is completely optically silent and F2u is only infrared active. On the other hand, ferroelectric tetragonal phase has a number of optical modes 3(A1 + E) + E + B1, which all are Raman active [25]. As observed by XRD analysis, pseudocubic state or low tetragonal distortion may be expected in titanate phase, which explains a weak Raman response of barium titanate component (Fig. 6). However, inverse spinel structure of nickel ferrite is suitable for Raman analysis, due to a number of strong Raman active modes [26]. Thus, under broad 702 cm−1 peak with the well-defined shoulder two peaks can be observed at 661 cm−1 and 702 cm−1 due to contribution of Eg and A1g modes of NiFe2O4, respectively. Broad 576 cm−1 peak corresponds to A1g vibration mode of NiFe2O4. Two weak characteristic tetragonal Raman modes of BaTiO3 may be observed at 520 cm−1 and 305 cm−1, representing E(TO) and E modes, respectively [27]. In addition, Eg and T2g modes of NiFe2O4 are also noticeable at 486 cm−1 with 451 cm−1 shoulder and 333 cm−1 with very weak 308 cm−1 shoulder, respectively. Raman spectroscopy complemented X-ray diffraction and confirmed the presence of inverse spinel NiFe2O4 phase, and a weak response from BaTiO3 phase. Since the Raman modes are so weak, it is difficult to assign the tetragonal phase to the BaTiO3 layer. In any case, the observed Raman active modes indicate that a local noncentrosymmetric structure is typical at room temperature to the nanostructured BaTiO3 layers, even it appears as globally pseudo-cubic within the resolution limit of XRD analysis 3.2. Magnetic properties of BaTiO3/NiFe2O4 films
Fig. 5. XRD patterns of multilayer thin films with different geometry sintered at 750 °C (inset: SEM image of top NiFe2O4 surface layer).
Fig. 6. Room temperature Raman spectrum of B2N2-12 thin film sintered at 750 °C.
where a strong response of the ferrite phase is noticed, overlapped onto weak response of titanate phase, with slightly stronger noise at lower values of wavenumber. Cubic BaTiO3 crystal phase theoretically has no
3.2.1. NiFe2O4 films Room temperature magnetic loops for the pure ferrite films (N-6), sintered at 750 and 900 °C, were analyzed by AGM device. The well saturated hysteresis loops were recorded in range from 10 to − 10 kOe (Fig. 7) and their shapes resemble ferrimagnetic behavior typical for nickel-ferrite based materials. Saturation magnetization (Ms) of the pure NiFe2O4 thin films sintered at 750 and 900 °C are 60 and 77 emu/ cm3, respectively. Higher Ms value of the ferrite film sintered at 900 °C is due to its higher crystallinity and larger grains. These values are lower than the saturation magnetization of the bulk NiFe2O4 ceramics (~ 270 emu/cm3) [28], but also smaller than reported in other NiFe2O4 films, as for example Ms = 200 emu/cm3 in the pure NFO films produced by polymer assisted deposition and calcined at higher temperature of 950 °C/1 h [29]. As explained by Tong et al. [30], the grain morphology combined with density and stress tuning can strongly modulate the magnetic, but also the optical and surface characteristics in such NFO films. Therefore, the differences found in the magnetization values with respect to the reported ones are related to microstructural characteristics as grain size, sample density, texturing and to strain-stress interface mediated properties generated by various synthesis methods, calcination temperatures and substrate type. The larger grain size leads to better crystallinity, which induces higher magnetocrystalline anisotropy and finally leads the high saturation magnetization and coercivity, similar as found in many iron oxide-based materials [31]. The increase in saturation magnetization with increased grain size in such ferrites may be also caused by changes in the cation distribution (i.e. cation disorder), which usually are affected by the synthesis conditions and/or the strain the film [32,33]. The increase of sintering temperature from 750 to 900°C reduces the coercive field (Hc) from around 200–120 Oe and remanent magnetization (Mr), from 15 to 8 emu/cm3 (Fig. 7). Reduced intergranular straining and better crystallinity of the pure ferrite N-6 film sintered at higher temperature (i.e. 900 °C) insures easier domain orientation and a lower Hc. 3.2.2. BaTiO3/NiFe2O4 films The hysteresis loops of the multilayer films sintered at 750 and 900 °C are presented in Figs. 8 and 9, respectively. Magnetization values were calculated in accordance to the amount of magnetic (NiFe2O4)
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Fig. 7. Magnetic M(H) loop of NiFe2O4 (N-6) films sintered at 750 and 900 °C (a) and detail showing the low field region (b).
phase (whereas insets show hysteresis loops when the presence of both phases were considered). The hysteresis loops of the multilayer films sintered at 750 °C, presented in the inset of Fig. 8, clearly indicate considerably lower magnetization of the multilayer films than the pure NiFe2O4 films, due to high amount of nonmagnetic BaTiO3 phase. However, if only the amount of ferrite phase is considered (Fig. 8), it can be seen that the multilayer B3N3-12 thin film has almost the same Ms as the pure ferrite film (N-6). In addition, there is clear decrease of saturation magnetization with increase of the contact B/N surface number, from B3N3-12 to B1N1-12. Two main effects might explain such a result: i) thicker ferrite layers promote the formation of somewhat larger grains and ii) more contact B/N surfaces and thinner layers produce stronger straining effect over the surface (desirable effect in multiferroic composite materials) and increase the amount of defectrich disordered or amorphous interfaces. Since there is no considerable change in crystallinity and the grain size between the multilayer B1N112, B2N2-12 and B3N3-12 films (Fig. 5) it seems that the effect of interlayer straining is a dominant one. The strong effect of straining in layered nanostructures, gradually increasing with reduction of layer thickness, has been reported in similar systems [14,17]. Similar behavior was observed for the multilayer films sintered at 900 °C (Fig. 9), only the saturation magnetization of the pure ferrite film (N-6) is higher that Ms of the multilayer B3N3-12 thin film. It might
be due to somewhat faster growth of ferrite grains in the pure N-6 film at 900 °C which is not constrained by the titanate layers. Another characteristic of all multilayer films sintered at 750 and 900 °C is decrease of magnetization at higher magnetic fields (Figs. 8 and 9). It is most probably due to the contribution from diamagnetic substrate, but it could be also explained by with interlayer straining (as it seems that this behavior is more pronounced for the multilayer films with more contact B/N surfaces, i.e. sample B1N1-12). The remanent magnetization (Mr) of the multulayer films is in a direct proportion with Ms. Compared to the pure ferrite, somewhat lower Hc of the multilayer films has been recorded for the samples sintered at 750 and 900 °C (Figs. 8 and 9). This may be ascribed to strain relaxation induced by reaching Ms. The values of Mr and Hc point out strain meditated magnetization of multilayer films, as a dominant effect over microstructure coarsening with sintering temperature.
4. Conclusions Multilayer thin film structures composed of BaTiO3 and NiFe2O4 layers deposited in alternating order, with different layer thickness, were prepared by solution deposition technique on platinum coated silicon substrates. Structural characterization of the sintered samples confirmed the well-defined layered structure with overall thickness
Fig. 8. Magnetic properties of thin films sintered at 750 °C: a) magnetic hysteresis loops determined in relation to ferrite phase alone (inset - hysteresis loops determined in relation to complete film thickness) and b) enlarged part of magnetic hysteresis loops in the low field region. 15969
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Fig. 9. Magnetic properties of thin films sintered at 900 °C: a) magnetic hysteresis loops determined in relation to ferrite phase alone (inset - hysteresis loops determined in relation to complete film thickness) and b) enlarged part of magnetic hysteresis loops in the low field region.
around 500 nm, crystalline nature (with perovskite BaTiO3 and spinel NiFe2O4 phases) without secondary phases (after sintering below 900 °C) and grains on nanometer scale. The influence of the film geometries on magnetic properties of the multiferroic films sintering temperature at 750 and 900 °C were also investigated. The well saturated hysteresis loops with low coercitivity, typical for nickel-ferrite based materials, were recorded. Magnetic saturation of the pure ferrite films increased with higher sintering temperature, from 750° to 900°C, due to promoted grain growth, making them more susceptible to magnetization. It was also shown that the saturation magnetization decreased with decrease of the titanate and ferrite layer thicknesses (i.e. increased amount of their contact surfaces). The reason could be the interlayer strain relaxation which is more pronounced when there are more sharing ferrite/titanate interfaces and thinner ferrite layers. Low values of Hc support the strain mediated magnetization process. Acknowledgements The authors gratefully acknowledge the financial support provided by the Ministry of Science of the Republic of Serbia, project III45021 and COST Project IC1208. The authors thank Bogdan Vasile, from Dept. Oxide Materials Science & Engineering, “Politehnica” University of Bucharest, Romania, for performed TEM/STEM analysis. References [1] L.W. Martin, Y.H. Chu, R. Ramesh, Advances in the growth and characterization of magnetic, ferroelectric, and multiferroic oxide thin films, Mater. Sci. Eng. R68 (2010) 89–133. [2] J. Ma, J. Hu, Z. Li, C.-W. Nan, Recent progress in multiferroic magnetoelectric composites: from bulk to thin films, Adv. Mater. 23 (2011) 1062–1087. [3] S. Roy, S.B. Majumder, Recent advances in multiferroic thin films and composites, J. Alloy. Compd. 538 (2012) 153–159. [4] L.W. Martin, D.G. Schlom, Advanced synthesis techniques and routes to new singlephase multiferroics, Curr. Opin. Solid State Mater. Sci. 16 (2012) 199–215. [5] M.I. Bichurinn, V.M. Petrov, R.V. Petrov, Direct and inverse magnetoelectric effect in layered composites in electromechanical resonance range: a review, J. Magn. Magn. Mater. 324 (2012) 3548–3550. [6] G. Schileo, Recent developments in ceramic multiferroic composites based oncore/ shell and other heterostructures obtained by sol-gel routes, Prog. Solid State Chem. 41 (2013) 87–98. [7] N. Ortega, A. Kumar, J.F. Scott, R.S. Katiyar, Multifunctional magnetoelectric materials for device applications, J. Appl. Phys. 113 (2013) 074103. [8] J.F. Scott, Room-temperature multiferroic magnetoelectrics, NPG Asia Mater. 5 (2013) e72. [9] N.A. Spaldin, S.W. Cheong, R. Ramesh, Multiferroics: past, present, and future, Phys. Today 63 (10) (2010) 38–43. [10] C.A.F. Vaz, J. Hoffman, C.H. Ahn, R. Ramesh, Magnetoelectric coupling effects in multiferroic complex oxide composite structures, Adv. Mater. 22 (2010)
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