Journal of the Korean Physical Society, Vol. 65, No. 3, August 2014, pp. 342∼345
Effect of Annealing Temperature on the Magnetoelectric Properties of CoFe2 O4 /Pt/Pb(Zr0.52 Ti0.48 )O3 Multilayer Films You Jeong Eum, Sung-Ok Hwang, Chang Young Koo, Jai-Yeoul Lee and Hee Young Lee∗ School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Korea
Jungho Ryu Functional Ceramics Group, Korea Institute of Materials Science (KIMS), Changwon 641-831, Korea
Jung Min Park Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan (Received 14 January 2014, in final form 25 February 2014) CoFe2 O4 (CoFO)/Pt/Pb(Zr0.52 Ti0.48 )O3 (PZT) multilayer films were grown on Pt/Ti/SiO2 /Si substrates. A thin Pt layer was inserted between the ferrimagnetic and the ferroelectric layers in order to suppress diffusion at high temperatures and thereby to prevent possible interfacial reactions. The effect of annealing on the film’s microstructure and multiferroic properties was then investigated using thin film stacks heat-treated at temperatures ranging from 550 to 650 ◦ C. The magnetoelectric coefficients were calculated from the magnetoelectric voltages measured using a magnetoelectric measurement system. The effect of annealing temperature on the magnetoelectric coupling in the CoFO/Pt/PZT multilayer thin film is discussed in detail. PACS numbers: 85.80.Jm, 77.65.-j, 85.70.Ec Keywords: Magnetoelectric, Ferroelectric, Ferromagnetic, CoFe2 O4 , PZT DOI: 10.3938/jkps.65.342
I. INTRODUCTION
unique advantages such as controllability for nanoscale processing and easy integration into micro electro mechanical systems (MEMS) devices [12, 13]. Among various ME composite thin films, CoFe2 O4 -Pb(Zr,Ti)O3 (CoFO-PZT) layered composite thin film could be attractive and were chosen for this work due primarily to its superior magnetostriction coefficient and piezoelectric coefficient among oxide materials. In addition, the lattice mismatch between cubic spinel CoFO (a = 8.39) and perovskite PZT (a = b = 4.03 ˚ A and c = 4.14 ˚ A) was reported to be less than 4% [14]. Also, the magnetic properties of CoFO were reported to vary with the annealing temperature. For example, the magnetization value increased with increasing annealing temperature from 550 ◦ C to 650 ◦ C [15]. However, there have been no reports on the ME effect of CoFO-PZT multilayer films, which led us to investigate this interesting thin film composite in more detail. In this paper, the change in the ME coefficient in CoFO/Pt/PZT multilayer films is discussed as a function of the annealing temperature.
Multiferroic materials simultaneously exhibit two or more of the so-called “ferroic” phenomena such as ferroelectricity, ferromagnetism, and ferroelasticity [1–3]. Such materials have a wide range of potential applications, e.g., sensors, data storage devices, energy harvesters and multifunctional devices [4, 5]. The magnetoelectric (ME) coupling effect is defined as the magnetization’s response to an applied electric field or reversely the polarization’s response to an applied external magnetic field [6–8]. The ME materials are usually classified into single phase or composite groups. Except for bismuth ferrite (BiFeO3 ), almost no single-phase multiferroic materials show ME coupling at room temperature because they have very low transition temperatures. Therefore, bulk composite materials consisting of magnetic and ferroelectric phases such as PZT/TerfenolD and Pb(Zr0.52 Ti0.48 )O3 /CoFe2 O4 (PZT/CoFO) have been reported by several researchers [9–11]. Recently, research on bulk composites has widely been explored due to their ME effects being large compared to those of thin films. Nonetheless, ME composite thin films show many ∗ E-mail:
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Effect of Annealing Temperature on the Magnetoelectric Properties· · · – You Jeong Eum et al.
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Fig. 1. (Color online) Schematic of the CoFO/Pt/PZT thin film.
II. EXPERIMENTS AND DISCUSSION In the fabrication of CoFO/Pt/PZT multilayer films, each layer was deposited using different methods. First, the PZT layer was deposited using a simple solution process and spin-coating technique of a PZT sol solution (Zr: Ti = 52:48) with a concentration of 0.4 M . Stock solution was spin-coated onto a Pt/Ti/SiO2 /Si substrate at a rate of 2000 rpm for 25 sec; then, the coated film was dried at 450 ◦ C for 10 min and pyrolized at 650 ◦ C for 2 min. The process was repeated eleven times to obtain a film with a thickness of 1 μm; then, the film was given a final annealing at 650 ◦ C for 30 min in air. Pt dots, 80-nm thick and 500-μm-diameter, were deposited at room temperature onto the resulting PZT layer by using ion-beam sputtering. The Pt dots play two roles: the top electrode of the PZT thin film and a diffusion barrier between the CoFO and the PZT thin films. Prior to the deposition of the Pt layer, the base pressure and the working pressure were fixed at 5.0 × 10−6 Torr and 4.2 × 10−4 Torr at room temperature under an argon atmosphere of 4 sccm. The CoFO dots were then deposited on top of the Pt dots through a 200-μm-diameter circular shadow mask by using a pulsed-laser deposition (PLD) technique. The PLD process was performed by using a Nd:YAG excimer laser of 355 nm in wavelength and 10 Hz in repetition frequency with a laser of ∼1.0 J/cm2 . The CoFO layer deposition was carried out under an oxygen atmosphere of 10 sccm for 3 hours while deposition temperature was maintained at 500 ◦ C. Figure 1 shows the configuration of the final CoFO/Pt/PZT multilayer film’s structure. After deposition, the final CoFO/Pt/PZT multilayer films were annealed under an oxygen atmosphere for 30 min at different temperature from 550 to 650 ◦ C in 50 ◦ C interval. The crystallinity and the phase structure of the CoFO/Pt/PZT multilayer films were observed by using X-ray diffraction (XRD, PANalytical MPD) with Cu Kα radiation. The surface morphology and the film thickness of the CoFO/Pt/PZT multilayer films were examined by using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800). The magnetic properties of the films were identified using a vibrating sample magnetometer (VSM, Lake Shore Model 7404). The electrical and the ferroelectric hysteresis loops were mea-
Fig. 2. (Color online) X-ray diffraction patterns of the CoFO/Pt/PZT thin films.
Fig. 3. SEM micrographs of the CoFO/Pt/PZT thin film: (a)-(c) surface images of CoFO and (d) cross-sectional image of CoFO/Pt/PZT thin films.
sured at various electric fields by using a ferroelectric test system (Radiant, Precision-Pro). In order to analyze the ME coupling of the multilayer thin films, we applied an electromagnet was field to a DC magnetic field, and the sample was placed at the center of a Helmholtz coil under an AC magnetic field of 1 Oe. The ME voltage was generated by becoming an AC magnetic field (1 kHz), and the measurement was attained using a lock-in amplifier (Stanford Research Systems R850) [10]. The ME coefficient was calculated using the measured PZT film’s thickness and were converted to units of mV/cmOe. Figure 2 present the XRD patterns of the CoFO/Pt/PZT multilayer films. From the patterns, the PZT peaks are clearly identified. All diffraction peaks were indexed and matched with the perovskite PZT and the cubic spinel CoFO phases except for the one corresponding to the Pt/Ti/SiO2 /Si substrate. Thus, PZT and CoFO films had been successfully formed into a multilayer thin film without any noticeable
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Journal of the Korean Physical Society, Vol. 65, No. 3, August 2014
Fig. 4. (Color online) Magnetization hysteresis loops of the CoFO/Pt/PZT films for different annealing temperatures.
interfacial reaction at various annealing temperatures. The intensities of the CoFO (111) peaks from the XRD patterns increased slightly with increasing annealing temperature. However, the peaks from the CoFO phase had relatively weak intensities in comparison with those from PZT. This is because the total area of the CoFO layer was much smaller than that of PZT layer. In the XRD patterns, PZT has a (111) preferential orientation while CoFO does not show such a behavior. The surface morphology and the cross-sectional images of the CoFO/Pt/PZT multilayer films are shown in Fig. 3. Figures 3 (a) − (c) show surface images of CoFO films after annealing treatment at temperature from 550 to 650 ◦ C in intervals of 50 ◦ C. At lower annealing temperature such as 550 ◦ C shown in Fig. 3(a), grains and grain boundaries are not clearly visible. However, the grain size of the CoFO films increased with increasing annealing temperature from 550 to 650 ◦ C, which indicated that the annealing temperature range in this work was high enough for grain growth in the CoFO film [15]. This is reasonable from an energy viewpoint because nucleation occurring inside the grains can be completed by means of rearrangement and short-distance diffusion [15, 16]. A cross-sectional image (Fig. 3(d)) was used to examine the CoFO/Pt/PZT multilayer film’s structure with a clean interface at 650 ◦ C. The thickness of the layers were approximately 670-nm for the CoFO layer on the top, 80 nm for the Pt layer, and 1 μm for the PZT layer at the bottom. Figure 4 shows the in-plane magnetization behavior of the CoFO/Pt/PZT composite films given different annealing treatments upon the application of an external magnetic field. The magnetization was measured at room temperature while applying a magnetic field of up to ±10 kOe. Saturated magnetization values of 134, 156 and 209 emu/cm3 were measured at annealing temperatures of 550, 600, 650 ◦ C respectively. The magnetization of the CoFO/Pt/PZT multilayer film at 550 ◦ C
Fig. 5. (Color online) P − E hysteresis loops: (a) PZT(52/48) and (b)-(d) CoFO/Pt/PZT at for annealing temperature from 550 to 650 ◦ C. (e) 2Pr versus electric field for each film.
was relatively lower than that those of the films annealed at higher temperature, which could be explained by the relatively-poor crystallinity shown in the SEM images (Fig. 2). Therefore, clearly the magnetic behavior of the film was related to the grain size and was, thus, affected by the annealing treatment. Increasing the annealing temperature resulted in an increased grain size, which was confirmed by the enhanced crystallinity; thus, the magnetization values increased. This result was in a good agreement with other studies in which the magnetization of ferrite increased with increasing annealing temperature [16,17]. Figures 5(a) − (d) show the ferroelectric behavior (polarization versus electric field; P-E) while Fig. 5(e) shows the dependence on the annealing temperature of CoFO for the curve of 2Pr (remnant polarization) versus the electric field. The P − E hysteresis loops were measured at 1 kHz and room temperature. The shapes of the PE loops from both single layer PZT and multilayer ME films showed a typical symmetric and saturated ferroelec-
Effect of Annealing Temperature on the Magnetoelectric Properties· · · – You Jeong Eum et al.
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ondary phases, indicating that there were negligible interfacial reactions and that a clean interface was realized between each layer. The magnetization values were influenced by the grain size, which was associated with the annealing temperature. Compared to the single layer PZT film, the 2Pr values in the multilayer films were decreased, which was attributed to the interfacial constraint effect. In this study, the highest ME coefficient, with the value of 60 mV/cmOe was obtained at 650 ◦ C.
ACKNOWLEDGMENTS
Fig. 6. (Color online) Magnetoelectric coefficients of CoFO/Pt/PZT thin films for various annealing temperatures.
tric nature. The P − E hysteresis loops from each film, however, showed slight differences in the Pr /Ps and the Ec values. Figure 5 shows the change in the 2Pr values in each film. From the 2Pr versus electric field plot, the 2Pr values in the multilayer films were smaller than that in the single-layer PZT film. The decreasing Pr value was attributed to the clamping effect between the substrate and the CoFO top layer, which occurred during the switching of the ferroelectric domains [18]. Figure 6 shows the ME coefficient of the CoFO/Pt/PZT multilayer films measured by applying a 1 kHz magnetic field at room temperature. Prior to the measurement, a corona poling treatment was performed using a 6.5 kV dc power supply for 30 min at 100 ◦ C. The CoFO film exhibited a magnetic field-dependent magnetostriction caused by the mechanical stress in the underlying PZT film; thus, a ME voltage developed across the film due to the piezoelectric properties of the PZT layer. The increasing ME coefficient values of the multilayer films were found to be due to an increase in the magnetic field and in the non-linear magnetization behavior of the CoFO layer with increasing annealing temperature. We note that both the polarization and the magnetization values increased upon increasing annealing treatment. The maximum saturation value of 60 mV/cmOe was reached at an applied magnetic field of about 0.65 to 0.7 T in the case of CoFO/Pt/PZT multilayer thin films annealing at 650 ◦ C.
III. CONCLUSION CoFO/Pt/PZT multilayer films were successfully fabricated on Pt/Ti/SiO2 /Si substrates by using pulsed laser deposition, ion-beam sputtering and sol-gel spincoating methods, respectively. XRD and SEM analyses showed that both the ferromagnetic CoFO and the ferroelectric PZT layers were crystallized without any sec-
The work at Korea Institute of Materials Science (KIMS) was supported by the Global Frontier R&D Program (2013-073298) at the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.
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