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Strong sub-resonance magnetoelectric coupling in PZT-NiFe2 O4 -PZT thin film composite
Nano-Structures & Nano-Objects 18 (2019) 100272
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Strong sub-resonance magnetoelectric coupling in PZT-NiFe2 O4 -PZT thin film composite Li Jian a , Ajith S. Kumar b , C.S. Chitra Lekha b , S. Vivek b , Isabel Salvado c , Andrei ∗ L. Kholkin c,d , Swapna S. Nair a,b,c , a
Departamento de Engenharia Cerâmica e do Vidro & CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal Department of Physics, Central University of Kerala, Kasaragod 671316, India c Departamento de Física & I3N, Universidade de Aveiro, 3810-193 Aveiro, Portugal d ITMO University, Saint-Petersburg, 197101, Russia b
graphical
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abstract
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Article history: Received 12 October 2018 Received in revised form 12 February 2019 Accepted 22 February 2019 Keywords: Magnetoelectric Piezoelectric Sol–gel Thin film PZT Nickel ferrite
a b s t r a c t Magnetoelectric composite thin films of 2–2 type attract significant attention due to potentially high magnetoelectric (ME) coupling and ease of miniaturization. PbZr0.52 Ti0.48 O3 (PZT) and NiFe2 O4 (NFO) have been among the materials of interest for the development of such composites, however, the obtained values of ME coefficients were sufficiently low so far. Also, these works report ME coupling in micrometer thick composite films. Here, we report strong magnetoelectric coupling (maximum longitudinal ME coefficient ≈ 1.2 V/cm·Oe at 100 kHz) obtained in nanograined PZT-NFO-PZT thin films of nanometer thickness fabricated on Si/SiO2 /Ti/Pt substrates by sol–gel spin coating technique. This is a strong magnetoelectric coupling recorded in thin film composites involving ferrites at subresonant conditions and can be used in miniature transducers and sensors and for data processing devices. © 2019 Published by Elsevier B.V.
1. Introduction
∗
Corresponding author at: Department of Physics, Central University of Kerala, Kasaragod 671316, India. E-mail address: [email protected] (S.S. Nair). https://doi.org/10.1016/j.nanoso.2019.100272 2352-507X/© 2019 Published by Elsevier B.V.
Multiferroic materials which display both ferroelectric and ferro/ferrimagnetic characteristics are currently hot topic of research owing to their tremendous application potential in novel
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functional devices [1–3]. The most interesting aspect of multiferroic materials is the cross coupling between the ferroic order parameters called magnetoelectric (ME) coupling. However, single-phase compounds that exhibit ferroelectric as well as magnetic properties are very rare, and their magnetoelectric response is typically very weak at room temperature, which makes them practically useless from the device point of view [4]. An alternative method of simultaneously obtaining ferroelectric and magnetic properties as well as a higher degree of magnetoelectric coupling is the synthesis of multiferroic composite materials [5]. In this case, the ME coupling is mediated via strain. The magnetostrictive layer is deformed under an applied magnetic field and this strain/stress is transferred onto the piezoelectric material that generates voltage due to direct piezoelectric effect. Hence, a composite made of a magnetostrictive and a piezoelectric layer can yield an elevated coupling as the constituent phases are individually optimized for room temperature. The ME composites can theoretically be 0–0, 0–3, 2–2, or 1– 1 connectivity type [6–9]. The 2–2 type layered structures are one of the preferred structures as they are easier to fabricate and miniaturize. Also, 2–2 structures help to avoid the leakage problems, which is a major problem causing decrease in ME coupling. Leakage current is a major problem in the case of composite structures, which arises due the higher conductivity of the magnetic component when compared to the ferroelectric part. It is also observed that this type of 2–2 structures are easier to polarize to increase the piezoelectric property of the ferroelectric component [10]. In addition, ferroelectric layers connected in series lead to enhanced voltage output in an analogy with multilayer actuators [11]. Bulk composites [9], composite films [12] and multilayer [7] comprising ferroelectric and ferro/ferrimagnetic phases are thus a viable design for the fabrication of highly sensitive magnetoelectric sensors and transducers. It is necessary to mention that 2–2 magnetoelectric composites based on thin films deposited on rigid substrates suffer from intrinsic disadvantage — clamping effect that limits the deformation of the films in lateral dimensions and thus magnetoelectric coupling. Layered structuring can reduce this adverse effect. This work is therefore focused on increasing ME coupling coefficients in thin film 2–2 multilayer design using a modified sol–gel approach. Lead zirconate titanate (PZT) is chosen as the piezoelectric phase and Nickel ferrite (NFO) as the magnetostrictive phase. PZT is a highly employed piezoelectric material for applications in transducers, capacitors, sensors and actuators, as well as in non-volatile random-access memories, owing to its outstanding piezoelectric and ferroelectric properties and chemical stability [13–17]. The particular composition of PZT (Zr/Ti = 52/48) employed here, is characterized by a morphotropic phase boundary (MPB). The MPB consists of different possible domain states of both tetragonal (6) and rhombohedral (8) phases. These phases are energetically equally favorable and their presence causes instability in polarization states [18]. Due to this instability, the direction of polarization can be easily rotated by external triggers like applied stress or electric field, resulting in very high piezoelectric properties [19]. Nickel ferrite and nickel-doped ferrites possess excellent potential for ME applications as they exhibit high initial permeability, high magnetostriction, low hysteresis losses and nearly ideal interface coupling with PZT [11]. Also, small saturation field and high magnetostriction results in very high piezomagnetic coefficient for NFO, which is the key factor in determining the strength of the ME coupling. In ferrites, domains are spontaneously deformed in the magnetization direction and the Joule magnetostriction is caused by domain wall motion and domain rotation in the presence of an external magnetic field. Since the ME effects involve dynamic magnetoelastic coupling, unimpeded domain motion is required in order to get a large
ME coupling. Hence, in the present work, we present a magnetoelectric thin film composite of 2–2 multilayer type (PZT-NFO-PZT sandwich), employing PZT as the piezoelectric and NFO as the magnetostrictive components. 2. Experimental techniques The trilayered thin films with the structure PZT-NFO-PZT were fabricated by sol–gel spin coating technique. The sol for the synthesis of NFO was prepared using Ni(NO3 )2 ·5H2 O and Fe(NO3 )3 ·9H2 O dissolved in a mixture of ethylene glycol-ethanol (9:1). For PZT preparation, stoichiometric amounts of lead acetate (Pb(O2 C2 H3 )2 ·3H2 O), titanium (IV) propoxide (Ti(OC3 H7 )4 ) and zirconium (IV) propoxide Zr(OC3 H7 )4 precursors were dissolved in 2-methoxypropanol. The Zr/Ti ratio was maintained as 52/48 and was aged for 48 h at 35 ◦ C. The details of the film preparation are reported elsewhere [20]. Thin films were prepared by spin coating the corresponding sol (3 layers PZT, 3 layers NFO and 3 layers PZT) at a speed of 6000 rpm during 30 s on chemically and ultrasonically cleaned Si/SiO2 /Ti/Pt substrates using a programmable spin coater. The films were sequentially pyrolyzed in air at 250 ◦ C for 30 min after the deposition of each individual layer. Finally, the ferroelectric–ferrimagnetic multilayer was annealed in air at 750 ◦ C for 30 min. The sample was prepoled at room temperature by the pulses of variable height before the ferroelectric characterization. A continuous DC high voltage supply was used for the poling. The pulsed DC was obtained by mechanically tapping the voltage source on to the surface of the films with the help of a stepper motor. The height of the pulses was varied from 10 kV/cm to 35 kV/cm. The duty cycle of the pulses was maintained as 25% with a period of oscillation of 40 s. The sample was maintained under the poling conditions for 12 h. The crystal structure and the morphology of the films were investigated by an X-ray Diffractometer (Rigaku D-Max, Japan) and Scanning Electron Microscope (HR-SEM-SE/EDS: SEM marca Hitachi, modelo SU-70e). Ferroelectric properties of the PZT films were probed locally with the help of Piezoresponse Force Microscopy (PFM, Bruker Nanoscope) capable to provide qualitative and quantitative information on the value of piezoelectric coefficients and polarization switchability. The magnetic characterization of the samples was done using a SQUID magnetometer (Quantum Design SQUID/MPMS). ME coupling has been measured in a home-made apparatus based on a Bruker EPR spectrometer extended with an external lock-in amplifier controlled by a LabView program. 3. Results and discussion 3.1. Structural and morphological characterization The structural properties of PZT-NFO-PZT sandwich layers were investigated by XRD and SEM. The XRD (Fig. 1) displayed only the peaks of PZT and NFO phases with well-formed perovskite and spinel structure. The broad peaks are indicative of the formation of small crystallites in the film [21]. The SEM analysis of the multilayers are presented in Fig. 2. Cross-sectional SEM revealed that the individual layers are well adhered to each other showing dense nanograin structure with thicknesses of 74, 89 and 60 nm respectively for PZT, NFO and PZT (top) layers. The surface morphology is clear from Fig. 2(b). It confirms the formations of nanograins in the top PZT layer.
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Fig. 1. The X-ray diffractogram of the PZT-NFO-PZT multilayers.
3.2. Piezoelectric characterization Ferroelectric properties of the PZT films were probed locally with the help of Piezoresponse Force Microscopy (PFM) capable of providing qualitative and quantitative information on the value of piezoelectric coefficients and polarization switchability [22]. Fig. 3 presents representative hysteresis loop taken on the bare surface of the top PZT layer when the film was biased between the top surface and the Pt bottom electrode. First, the sample was pre-poled by pulses of variable height and then the piezoelectric response was measured at the same location in the absence of electric field. The results unambiguously confirm the existence of piezo/ferroelectricity in PZT with the value of the saturation effective d33 coefficients comparable to those in single layer films [22]. The multilayer films were capped with top metallic electrodes and piezoelectric coefficients were also measured by the photonic sensor coupled with the lock-in amplifier [23] and results were similar of those typically observed in sol–gel PZT thin films of comparable thickness [24]. 3.3. Magnetic characterization Magnetic characterization of the composite samples was done using a SQUID at 300 and 100 K and is shown in Fig. 4. Here, in order to calculate the magnetization in emu/cc, the volume of NFO alone is taken into account. The magnetization curves present ferromagnetic ordering in NFO layers with a reasonably high saturation magnetization of
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Fig. 3. Piezoresponse hysteresis loop traced in the PZT-NFO-PZT heterostructure.
≈58 emu/cm3 at room temperature. However, the observed value is less than the reported value of bulk NFO (270 emu/cm3 ) [25]. The reduction in magnetization as compared to bulk NFO can be due to the small grain size of the films. The thermal energy in the samples has a significant effect on the magnetization. As the grain size decreases, thermal fluctuations increases, resulting in the reduction in magnetization. However, a higher magnetization (78 emu/ cm3 ) is recorded at 100 K. At low temperatures, the thermal energy is small so that the domains can easily be oriented along the applied field. Therefore the increase in magnetization at low temperature can be attributed to the reorientation of the magnetic domains. The obtained saturation magnetization is comparable to the values previously reported in NFO-PZT heterostructures [26]. The coercivity of the sample is also found to increase when the temperature is decreased (from 130 Oe to 450 Oe). This too, can be attributed to the reduction in thermal fluctuations. 3.4. Magnetoelectric coupling studies The ME coupling has been measured in a home-made apparatus based on a Bruker EPR spectrometer extended with an external lock-in amplifier controlled by a LabView program. The transverse (E-field perpendicular to H-field) ME coupling coefficient was determined at room temperature as a function of the bias dc magnetic field Hdc swept in between 0 and 10 kOe (Fig. 5(a)). While measuring the ME voltage output, a high frequency magnetic field (100 kHz) with amplitude of 10 Oe was
Fig. 2. (a) Cross-sectional SEM and (b) surface SEM of a PZT-NFO-PZT heterostructure.
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Fig. 4. (a) Magnetic hysteresis of a PZT-NFO-PZT multilayer measured in a SQUID at 100 K and 300 K. (b) The enlarged version of the M-H curve.
applied to the structure. Then the frequency dependence of the ME coupling coefficient was studied at fixed frequencies available in EPR spectrometer (Fig. 5(b)). The amplitude of the dc bias field is kept at 50 Oe during the frequency measurements. The measured coupling coefficient was very high, attaining 1225 mV/ (cm Oe) at low dc bias field, notably decreasing with increasing bias. This is typical behavior reflecting saturation in the magnetization and magnetostriction response at high bias field [21,27]. The magnetostriction reaches a maximum value, corresponds to the magnetic saturation, at a particular bias field beyond which the magnetostriction produces a nearly constant change in polarization. As a result, the coupling coefficient decreases with increasing magnetic field. Here, the maximum ME coupling coefficient occurs at very low magnetic biasing field, which corresponds to the low saturation magnetic field of the NFO thinfilms at room temperature. However, the ME coefficient did not vanish at saturation signifying that it is not determined solely by the magnetostriction mechanism [28]. In order to shed light onto the mechanism of such a high ME coefficient, we measured a frequency dependence of the ME coefficient (at low bias field of 50 Oe). ME coupling was found to notably increase from 890 to 1225 mV/ (cm Oe) with increasing frequency from 1.56 to 100 kHz (Fig. 5(b)). This behavior is typical for ME response approaching to resonance [29]. Thus some resonant enhancement due to the transverse electromechanical resonance of the composite (expected at the frequency of 100–200 kHz) cannot be ruled out. The values of the obtained ME coefficients should be compared with those reported in the literature for 2–2 configuration of both materials. Zhai et al. [28] have measured ME coupling coefficients in laminated PZT-NFO-PZT ceramics, in the same 2–2 configuration as used in this work but for millimeter thicknesses. The obtained maximum value of ME coefficient was 160 mV/cm Oe for low frequency (1 kHz) for the transverse configuration of E and H-fields. It was explicitly shown that the ME coupling is due to magnetostriction and direct piezoelectric effect in NFO and PZT layers, respectively. Even stronger effects have been reported by Srinivasan et al. [11] in NFO-PZT bilayers and multilayers prepared by doctor blade method with a thickness of individual layers of 10–200 µm. The transverse coefficient varied from 460 mV/ (cm Oe) in bilayers to 1400 mV/ (cm Oe) in multilayers. Again, coupling via magnetostriction was confirmed by the direct measurements of magnetostriction coefficients and by rigorous calculations. Much higher values of the ME coefficient (23 V/cm Oe) have been achieved in the same system by using resonance enhancement, especially in the bending mode [26,30]. This is much higher than in 3–1 composites [31] and particulate thin film composites [32] achieved for the same or similar systems.
One of the reasons of the deterioration of the ME coupling in thin films is the clamping effect of the substrate that prevents a transfer of the mechanical deformation from the magnetic to piezoelectric component of the composite [33]. In this work, we obtained sufficiently high ME coefficients of the order of 1000 mV/(cm Oe) at sub-resonance frequencies (i.e., below the peak corresponding to transverse electromechanical resonance) and these values are very close to those in the multilayer NFO-PZT system [11]. This is considerably higher than the value of 180 mV/(cm Oe) predicted in [11] for the same volume fraction of the magnetostrictive component (NFO) at low frequency. Such a high value cannot be explained by the resonance enhancement as the resonance frequency is not reached yet. The presence of the shoulder on the frequency curve (Fig. 4(b)) indicates a possibility of another mechanism such as bending resonance [30] in the system and thus the enhancement of the ME response may come from the coupled resonance behavior due to the interaction of closely positioned fundamental modes and their harmonics [34]. Currently, no provision can be made regarding the frequency dependence of the observed effect as the ME measurements could be done only at a limited number of fixed frequencies provided by the EPR spectrometer and the maximum frequency is limited by the used lock-in amplifier. The increased ME coupling at sub-resonance frequencies can be due to the fact that the thin film consists of nanosized nickel ferrite particles. It has already been reported that the magnetostriction is almost four times larger in nanosized nickel ferrite as compared to its bulk counterpart [28]. Thus, owing to the large magnetostriction and large surface area, the ME coupling in the present system could be much higher than in the micron-sized counterparts even in the presence of the rigid substrate (clamping effect). The middle magnetostrictive layer can be deformed locally by the application of magnetic field, and the deformation can be transferred locally to the neighboring PZT nanograins and induce large polarization even in the presence of the substrate. First PZT layer (next to substrate) may play a role of the buffer thus weakening the apparent clamping which is reduced with increasing film thickness [35]. Hence, the strain/stress transfer can be enhanced at the top NFO-PZT interface with concurrent increase of the ME coefficients. Due to the small thickness of the top PZT layer, the deformation produced in the magnetic nanoparticles is sufficiently high to create large electric fields in a PZT layer. Further measurements in a wider frequency range and as a function of the film thickness (or volume ratio of ferromagnetic and ferroelectric components) are needed to fully explain the observed coupling effect. It should be noted that observation of the large ME effects in simple thin-film structures opens up an avenue for the miniaturization of the devices and thus prepared magnetoelectric transducers and sensors can be a part of
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Fig. 5. (a) Magnetoelectric coupling coefficient vs. dc bias magnetic field in a PZT-NFO-PZT multilayer measured at 100 kHz. (b) MECC at a biasing magnetic field of 50 Oe for different frequencies.
multifunctional fully integrated devices. The main feature of the observed effect (i.e. non-resonance character) is advantageous for many applications such as magnetoelectric energy harvesting or broad-band magnetic sensors [36]. 4. Conclusion We observed a strong ME coupling effect in PZT-NFO-PZT trilayers prepared by sol–gel method. Thin film heterostructures demonstrated a very high ME transverse coefficient of 1225 mV/ (cm Oe) at 100 kHz and a dc biasing magnetic field of 50 Oe. The high coupling is retained (91% of the maximum value) even at applied dc magnetic field of 10 kOe. This is quite important from the application point of view, especially for the transducers working under high magnetic fields. The origin of the strong coupling is tentatively attributed to the increased magnetostriction in the magnetic nanograins. However further measurements are needed to confirm this assumption. Acknowledgments SSN acknowledges the funding provided by the Fundação para a Ciência e a Tecnologia of Portugal (grant SFRH/BPD/42136/2007) and UGC of India (F.No. 20-26/2013). ASK acknowledges UGC, India for the financial support in the form of JRF and SRF (F.17131/2012 (SA-1)) and CSC acknowledges DST, INDIA for the financial support through WOS-A project (SR/WOS-A/PS-14/2014). ASK, CSC, VS and SSN acknowledges Central University of Kerala, India for the financial support. ALK wish to acknowledge Russian Science Foundation for the grant (No: 18-19-00512). The authors are thankful to Drs. Igor Bdikin and Vladimir Shvartsman for the help with piezoelectric and magnetic measurements, respectively. Conflict of interest The authors declare that there is no conflict of interest in this paper. References [1] X.-Z. Chen, M. Hoop, F. Mushtaq, E. Siringil, C. Hu, B.J. Nelson, S. Pané, Recent developments in magnetically driven micro- and nanorobots, Appl. Mater. Today. 9 (2017) 37–48, http://dx.doi.org/10.1016/J.APMT.2017.04. 006. [2] C.M. Leung, J. Li, D. Viehland, X. Zhuang, A review on applications of magnetoelectric composites: from heterostructural uncooled magnetic sensors, energy harvesters to highly efficient power converters, J. Phys. D. Appl. Phys. 51 (2018) 263002, http://dx.doi.org/10.1088/1361-6463/aac60b.
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Strong sub-resonance magnetoelectric coupling in PZT-NiFe2 O4 -PZT thin film composite Li Jian a , Ajith S. Kumar b , C.S. Chitra Lekha b , S. Vivek b , Isabel Salvado c , Andrei ∗ L. Kholkin c,d , Swapna S. Nair a,b,c , a
Departamento de Engenharia Cerâmica e do Vidro & CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal Department of Physics, Central University of Kerala, Kasaragod 671316, India c Departamento de Física & I3N, Universidade de Aveiro, 3810-193 Aveiro, Portugal d ITMO University, Saint-Petersburg, 197101, Russia b
graphical
article
abstract
info
Article history: Received 12 October 2018 Received in revised form 12 February 2019 Accepted 22 February 2019 Keywords: Magnetoelectric Piezoelectric Sol–gel Thin film PZT Nickel ferrite
a b s t r a c t Magnetoelectric composite thin films of 2–2 type attract significant attention due to potentially high magnetoelectric (ME) coupling and ease of miniaturization. PbZr0.52 Ti0.48 O3 (PZT) and NiFe2 O4 (NFO) have been among the materials of interest for the development of such composites, however, the obtained values of ME coefficients were sufficiently low so far. Also, these works report ME coupling in micrometer thick composite films. Here, we report strong magnetoelectric coupling (maximum longitudinal ME coefficient ≈ 1.2 V/cm·Oe at 100 kHz) obtained in nanograined PZT-NFO-PZT thin films of nanometer thickness fabricated on Si/SiO2 /Ti/Pt substrates by sol–gel spin coating technique. This is a strong magnetoelectric coupling recorded in thin film composites involving ferrites at subresonant conditions and can be used in miniature transducers and sensors and for data processing devices. © 2019 Published by Elsevier B.V.
1. Introduction
∗
Corresponding author at: Department of Physics, Central University of Kerala, Kasaragod 671316, India. E-mail address: [email protected] (S.S. Nair). https://doi.org/10.1016/j.nanoso.2019.100272 2352-507X/© 2019 Published by Elsevier B.V.
Multiferroic materials which display both ferroelectric and ferro/ferrimagnetic characteristics are currently hot topic of research owing to their tremendous application potential in novel
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functional devices [1–3]. The most interesting aspect of multiferroic materials is the cross coupling between the ferroic order parameters called magnetoelectric (ME) coupling. However, single-phase compounds that exhibit ferroelectric as well as magnetic properties are very rare, and their magnetoelectric response is typically very weak at room temperature, which makes them practically useless from the device point of view [4]. An alternative method of simultaneously obtaining ferroelectric and magnetic properties as well as a higher degree of magnetoelectric coupling is the synthesis of multiferroic composite materials [5]. In this case, the ME coupling is mediated via strain. The magnetostrictive layer is deformed under an applied magnetic field and this strain/stress is transferred onto the piezoelectric material that generates voltage due to direct piezoelectric effect. Hence, a composite made of a magnetostrictive and a piezoelectric layer can yield an elevated coupling as the constituent phases are individually optimized for room temperature. The ME composites can theoretically be 0–0, 0–3, 2–2, or 1– 1 connectivity type [6–9]. The 2–2 type layered structures are one of the preferred structures as they are easier to fabricate and miniaturize. Also, 2–2 structures help to avoid the leakage problems, which is a major problem causing decrease in ME coupling. Leakage current is a major problem in the case of composite structures, which arises due the higher conductivity of the magnetic component when compared to the ferroelectric part. It is also observed that this type of 2–2 structures are easier to polarize to increase the piezoelectric property of the ferroelectric component [10]. In addition, ferroelectric layers connected in series lead to enhanced voltage output in an analogy with multilayer actuators [11]. Bulk composites [9], composite films [12] and multilayer [7] comprising ferroelectric and ferro/ferrimagnetic phases are thus a viable design for the fabrication of highly sensitive magnetoelectric sensors and transducers. It is necessary to mention that 2–2 magnetoelectric composites based on thin films deposited on rigid substrates suffer from intrinsic disadvantage — clamping effect that limits the deformation of the films in lateral dimensions and thus magnetoelectric coupling. Layered structuring can reduce this adverse effect. This work is therefore focused on increasing ME coupling coefficients in thin film 2–2 multilayer design using a modified sol–gel approach. Lead zirconate titanate (PZT) is chosen as the piezoelectric phase and Nickel ferrite (NFO) as the magnetostrictive phase. PZT is a highly employed piezoelectric material for applications in transducers, capacitors, sensors and actuators, as well as in non-volatile random-access memories, owing to its outstanding piezoelectric and ferroelectric properties and chemical stability [13–17]. The particular composition of PZT (Zr/Ti = 52/48) employed here, is characterized by a morphotropic phase boundary (MPB). The MPB consists of different possible domain states of both tetragonal (6) and rhombohedral (8) phases. These phases are energetically equally favorable and their presence causes instability in polarization states [18]. Due to this instability, the direction of polarization can be easily rotated by external triggers like applied stress or electric field, resulting in very high piezoelectric properties [19]. Nickel ferrite and nickel-doped ferrites possess excellent potential for ME applications as they exhibit high initial permeability, high magnetostriction, low hysteresis losses and nearly ideal interface coupling with PZT [11]. Also, small saturation field and high magnetostriction results in very high piezomagnetic coefficient for NFO, which is the key factor in determining the strength of the ME coupling. In ferrites, domains are spontaneously deformed in the magnetization direction and the Joule magnetostriction is caused by domain wall motion and domain rotation in the presence of an external magnetic field. Since the ME effects involve dynamic magnetoelastic coupling, unimpeded domain motion is required in order to get a large
ME coupling. Hence, in the present work, we present a magnetoelectric thin film composite of 2–2 multilayer type (PZT-NFO-PZT sandwich), employing PZT as the piezoelectric and NFO as the magnetostrictive components. 2. Experimental techniques The trilayered thin films with the structure PZT-NFO-PZT were fabricated by sol–gel spin coating technique. The sol for the synthesis of NFO was prepared using Ni(NO3 )2 ·5H2 O and Fe(NO3 )3 ·9H2 O dissolved in a mixture of ethylene glycol-ethanol (9:1). For PZT preparation, stoichiometric amounts of lead acetate (Pb(O2 C2 H3 )2 ·3H2 O), titanium (IV) propoxide (Ti(OC3 H7 )4 ) and zirconium (IV) propoxide Zr(OC3 H7 )4 precursors were dissolved in 2-methoxypropanol. The Zr/Ti ratio was maintained as 52/48 and was aged for 48 h at 35 ◦ C. The details of the film preparation are reported elsewhere [20]. Thin films were prepared by spin coating the corresponding sol (3 layers PZT, 3 layers NFO and 3 layers PZT) at a speed of 6000 rpm during 30 s on chemically and ultrasonically cleaned Si/SiO2 /Ti/Pt substrates using a programmable spin coater. The films were sequentially pyrolyzed in air at 250 ◦ C for 30 min after the deposition of each individual layer. Finally, the ferroelectric–ferrimagnetic multilayer was annealed in air at 750 ◦ C for 30 min. The sample was prepoled at room temperature by the pulses of variable height before the ferroelectric characterization. A continuous DC high voltage supply was used for the poling. The pulsed DC was obtained by mechanically tapping the voltage source on to the surface of the films with the help of a stepper motor. The height of the pulses was varied from 10 kV/cm to 35 kV/cm. The duty cycle of the pulses was maintained as 25% with a period of oscillation of 40 s. The sample was maintained under the poling conditions for 12 h. The crystal structure and the morphology of the films were investigated by an X-ray Diffractometer (Rigaku D-Max, Japan) and Scanning Electron Microscope (HR-SEM-SE/EDS: SEM marca Hitachi, modelo SU-70e). Ferroelectric properties of the PZT films were probed locally with the help of Piezoresponse Force Microscopy (PFM, Bruker Nanoscope) capable to provide qualitative and quantitative information on the value of piezoelectric coefficients and polarization switchability. The magnetic characterization of the samples was done using a SQUID magnetometer (Quantum Design SQUID/MPMS). ME coupling has been measured in a home-made apparatus based on a Bruker EPR spectrometer extended with an external lock-in amplifier controlled by a LabView program. 3. Results and discussion 3.1. Structural and morphological characterization The structural properties of PZT-NFO-PZT sandwich layers were investigated by XRD and SEM. The XRD (Fig. 1) displayed only the peaks of PZT and NFO phases with well-formed perovskite and spinel structure. The broad peaks are indicative of the formation of small crystallites in the film [21]. The SEM analysis of the multilayers are presented in Fig. 2. Cross-sectional SEM revealed that the individual layers are well adhered to each other showing dense nanograin structure with thicknesses of 74, 89 and 60 nm respectively for PZT, NFO and PZT (top) layers. The surface morphology is clear from Fig. 2(b). It confirms the formations of nanograins in the top PZT layer.
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Fig. 1. The X-ray diffractogram of the PZT-NFO-PZT multilayers.
3.2. Piezoelectric characterization Ferroelectric properties of the PZT films were probed locally with the help of Piezoresponse Force Microscopy (PFM) capable of providing qualitative and quantitative information on the value of piezoelectric coefficients and polarization switchability [22]. Fig. 3 presents representative hysteresis loop taken on the bare surface of the top PZT layer when the film was biased between the top surface and the Pt bottom electrode. First, the sample was pre-poled by pulses of variable height and then the piezoelectric response was measured at the same location in the absence of electric field. The results unambiguously confirm the existence of piezo/ferroelectricity in PZT with the value of the saturation effective d33 coefficients comparable to those in single layer films [22]. The multilayer films were capped with top metallic electrodes and piezoelectric coefficients were also measured by the photonic sensor coupled with the lock-in amplifier [23] and results were similar of those typically observed in sol–gel PZT thin films of comparable thickness [24]. 3.3. Magnetic characterization Magnetic characterization of the composite samples was done using a SQUID at 300 and 100 K and is shown in Fig. 4. Here, in order to calculate the magnetization in emu/cc, the volume of NFO alone is taken into account. The magnetization curves present ferromagnetic ordering in NFO layers with a reasonably high saturation magnetization of
3
Fig. 3. Piezoresponse hysteresis loop traced in the PZT-NFO-PZT heterostructure.
≈58 emu/cm3 at room temperature. However, the observed value is less than the reported value of bulk NFO (270 emu/cm3 ) [25]. The reduction in magnetization as compared to bulk NFO can be due to the small grain size of the films. The thermal energy in the samples has a significant effect on the magnetization. As the grain size decreases, thermal fluctuations increases, resulting in the reduction in magnetization. However, a higher magnetization (78 emu/ cm3 ) is recorded at 100 K. At low temperatures, the thermal energy is small so that the domains can easily be oriented along the applied field. Therefore the increase in magnetization at low temperature can be attributed to the reorientation of the magnetic domains. The obtained saturation magnetization is comparable to the values previously reported in NFO-PZT heterostructures [26]. The coercivity of the sample is also found to increase when the temperature is decreased (from 130 Oe to 450 Oe). This too, can be attributed to the reduction in thermal fluctuations. 3.4. Magnetoelectric coupling studies The ME coupling has been measured in a home-made apparatus based on a Bruker EPR spectrometer extended with an external lock-in amplifier controlled by a LabView program. The transverse (E-field perpendicular to H-field) ME coupling coefficient was determined at room temperature as a function of the bias dc magnetic field Hdc swept in between 0 and 10 kOe (Fig. 5(a)). While measuring the ME voltage output, a high frequency magnetic field (100 kHz) with amplitude of 10 Oe was
Fig. 2. (a) Cross-sectional SEM and (b) surface SEM of a PZT-NFO-PZT heterostructure.
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Fig. 4. (a) Magnetic hysteresis of a PZT-NFO-PZT multilayer measured in a SQUID at 100 K and 300 K. (b) The enlarged version of the M-H curve.
applied to the structure. Then the frequency dependence of the ME coupling coefficient was studied at fixed frequencies available in EPR spectrometer (Fig. 5(b)). The amplitude of the dc bias field is kept at 50 Oe during the frequency measurements. The measured coupling coefficient was very high, attaining 1225 mV/ (cm Oe) at low dc bias field, notably decreasing with increasing bias. This is typical behavior reflecting saturation in the magnetization and magnetostriction response at high bias field [21,27]. The magnetostriction reaches a maximum value, corresponds to the magnetic saturation, at a particular bias field beyond which the magnetostriction produces a nearly constant change in polarization. As a result, the coupling coefficient decreases with increasing magnetic field. Here, the maximum ME coupling coefficient occurs at very low magnetic biasing field, which corresponds to the low saturation magnetic field of the NFO thinfilms at room temperature. However, the ME coefficient did not vanish at saturation signifying that it is not determined solely by the magnetostriction mechanism [28]. In order to shed light onto the mechanism of such a high ME coefficient, we measured a frequency dependence of the ME coefficient (at low bias field of 50 Oe). ME coupling was found to notably increase from 890 to 1225 mV/ (cm Oe) with increasing frequency from 1.56 to 100 kHz (Fig. 5(b)). This behavior is typical for ME response approaching to resonance [29]. Thus some resonant enhancement due to the transverse electromechanical resonance of the composite (expected at the frequency of 100–200 kHz) cannot be ruled out. The values of the obtained ME coefficients should be compared with those reported in the literature for 2–2 configuration of both materials. Zhai et al. [28] have measured ME coupling coefficients in laminated PZT-NFO-PZT ceramics, in the same 2–2 configuration as used in this work but for millimeter thicknesses. The obtained maximum value of ME coefficient was 160 mV/cm Oe for low frequency (1 kHz) for the transverse configuration of E and H-fields. It was explicitly shown that the ME coupling is due to magnetostriction and direct piezoelectric effect in NFO and PZT layers, respectively. Even stronger effects have been reported by Srinivasan et al. [11] in NFO-PZT bilayers and multilayers prepared by doctor blade method with a thickness of individual layers of 10–200 µm. The transverse coefficient varied from 460 mV/ (cm Oe) in bilayers to 1400 mV/ (cm Oe) in multilayers. Again, coupling via magnetostriction was confirmed by the direct measurements of magnetostriction coefficients and by rigorous calculations. Much higher values of the ME coefficient (23 V/cm Oe) have been achieved in the same system by using resonance enhancement, especially in the bending mode [26,30]. This is much higher than in 3–1 composites [31] and particulate thin film composites [32] achieved for the same or similar systems.
One of the reasons of the deterioration of the ME coupling in thin films is the clamping effect of the substrate that prevents a transfer of the mechanical deformation from the magnetic to piezoelectric component of the composite [33]. In this work, we obtained sufficiently high ME coefficients of the order of 1000 mV/(cm Oe) at sub-resonance frequencies (i.e., below the peak corresponding to transverse electromechanical resonance) and these values are very close to those in the multilayer NFO-PZT system [11]. This is considerably higher than the value of 180 mV/(cm Oe) predicted in [11] for the same volume fraction of the magnetostrictive component (NFO) at low frequency. Such a high value cannot be explained by the resonance enhancement as the resonance frequency is not reached yet. The presence of the shoulder on the frequency curve (Fig. 4(b)) indicates a possibility of another mechanism such as bending resonance [30] in the system and thus the enhancement of the ME response may come from the coupled resonance behavior due to the interaction of closely positioned fundamental modes and their harmonics [34]. Currently, no provision can be made regarding the frequency dependence of the observed effect as the ME measurements could be done only at a limited number of fixed frequencies provided by the EPR spectrometer and the maximum frequency is limited by the used lock-in amplifier. The increased ME coupling at sub-resonance frequencies can be due to the fact that the thin film consists of nanosized nickel ferrite particles. It has already been reported that the magnetostriction is almost four times larger in nanosized nickel ferrite as compared to its bulk counterpart [28]. Thus, owing to the large magnetostriction and large surface area, the ME coupling in the present system could be much higher than in the micron-sized counterparts even in the presence of the rigid substrate (clamping effect). The middle magnetostrictive layer can be deformed locally by the application of magnetic field, and the deformation can be transferred locally to the neighboring PZT nanograins and induce large polarization even in the presence of the substrate. First PZT layer (next to substrate) may play a role of the buffer thus weakening the apparent clamping which is reduced with increasing film thickness [35]. Hence, the strain/stress transfer can be enhanced at the top NFO-PZT interface with concurrent increase of the ME coefficients. Due to the small thickness of the top PZT layer, the deformation produced in the magnetic nanoparticles is sufficiently high to create large electric fields in a PZT layer. Further measurements in a wider frequency range and as a function of the film thickness (or volume ratio of ferromagnetic and ferroelectric components) are needed to fully explain the observed coupling effect. It should be noted that observation of the large ME effects in simple thin-film structures opens up an avenue for the miniaturization of the devices and thus prepared magnetoelectric transducers and sensors can be a part of
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Fig. 5. (a) Magnetoelectric coupling coefficient vs. dc bias magnetic field in a PZT-NFO-PZT multilayer measured at 100 kHz. (b) MECC at a biasing magnetic field of 50 Oe for different frequencies.
multifunctional fully integrated devices. The main feature of the observed effect (i.e. non-resonance character) is advantageous for many applications such as magnetoelectric energy harvesting or broad-band magnetic sensors [36]. 4. Conclusion We observed a strong ME coupling effect in PZT-NFO-PZT trilayers prepared by sol–gel method. Thin film heterostructures demonstrated a very high ME transverse coefficient of 1225 mV/ (cm Oe) at 100 kHz and a dc biasing magnetic field of 50 Oe. The high coupling is retained (91% of the maximum value) even at applied dc magnetic field of 10 kOe. This is quite important from the application point of view, especially for the transducers working under high magnetic fields. The origin of the strong coupling is tentatively attributed to the increased magnetostriction in the magnetic nanograins. However further measurements are needed to confirm this assumption. Acknowledgments SSN acknowledges the funding provided by the Fundação para a Ciência e a Tecnologia of Portugal (grant SFRH/BPD/42136/2007) and UGC of India (F.No. 20-26/2013). ASK acknowledges UGC, India for the financial support in the form of JRF and SRF (F.17131/2012 (SA-1)) and CSC acknowledges DST, INDIA for the financial support through WOS-A project (SR/WOS-A/PS-14/2014). ASK, CSC, VS and SSN acknowledges Central University of Kerala, India for the financial support. ALK wish to acknowledge Russian Science Foundation for the grant (No: 18-19-00512). The authors are thankful to Drs. Igor Bdikin and Vladimir Shvartsman for the help with piezoelectric and magnetic measurements, respectively. Conflict of interest The authors declare that there is no conflict of interest in this paper. References [1] X.-Z. Chen, M. Hoop, F. Mushtaq, E. Siringil, C. Hu, B.J. Nelson, S. Pané, Recent developments in magnetically driven micro- and nanorobots, Appl. Mater. Today. 9 (2017) 37–48, http://dx.doi.org/10.1016/J.APMT.2017.04. 006. [2] C.M. Leung, J. Li, D. Viehland, X. Zhuang, A review on applications of magnetoelectric composites: from heterostructural uncooled magnetic sensors, energy harvesters to highly efficient power converters, J. Phys. D. Appl. Phys. 51 (2018) 263002, http://dx.doi.org/10.1088/1361-6463/aac60b.
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