PLEASE SCROLL DOWN FOR ARTICLE. Full terms and conditions of use:

Transcription

1 This article was downloaded by:[neicon Consortium] [NEICON Consortium] On: 13 April 2007 Access Details: [subscription number ] Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Ferroelectrics Publication details, including instructions for authors and subscription information: AC Switching of Relaxor PLZT Ceramics V. Ya. Shur a ; E. L. Rumyantsev a ; G. G. Lomakin a ; O. V. Yakutova a ; D. V. Pelegov a ; A. Sternberg b ; M. Kosec c a Ferroelectric Laboratory, Ural State University , Ekaterinburg. Russia b Institute of Solid State Physics, University of Latvia. LV-1063, Riga. Latvia c Jozef Stefan Institute. 1000, Ljubljana. Slovenia To cite this Article: V. Ya. Shur, E. L. Rumyantsev, G. G. Lomakin, O. V. Yakutova, D. V. Pelegov, A. Sternberg and M. Kosec, 'AC Switching of Relaxor PLZT Ceramics', Ferroelectrics, 314:1, To link to this article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Taylor and Francis 2007

2 Ferroelectrics, 314: , 2005 Copyright Taylor & Francis Inc. ISSN: print / online DOI: / AC Switching of Relaxor PLZT Ceramics V. YA. SHUR, 1 E. L. RUMYANTSEV, 1 G. G. LOMAKIN, 1 O. V. YAKUTOVA, 1 D. V. PELEGOV, 1 A. STERNBERG, 2 AND M. KOSEC 3 1 Ferroelectric Laboratory, Ural State University, Ekaterinburg, Russia 2 Institute of Solid State Physics, University of Latvia, LV-1063 Riga, Latvia 3 Jozef Stefan Institute, 1000 Ljubljana, Slovenia The switching under application of bipolar ac field is studied by recording of the hysteresis loops in wide temperature and field range in PLZT x/65/35 ceramics. The qualitative difference of the shape of hysteresis loops above and below the freezing temperature is attributed to backswitching in relaxor phase due to the depolarization field produced by the bound charges located at the interphase boundaries. The application of the quasistatic approach to analysis of the experimental data allows us to obtain parameters of the distribution function of local coercive fields from the field dependence of the switching charge derivative on field. Keywords Nanodomains; polarization reversal; heterophase structure; hysteresis loop; charged domain walls Introduction The experimental and theoretical study of the relaxor ferroelectrics is due to their practical importance resulting from prominent electromechanical properties [1]. The nature of the diffuse phase transition in relaxors is still a subject of controversy [1 3], since similar features have been observed both in ordinary ferroelectrics and dipole glasses [4]. Two models for explanation of unusual properties of relaxors are discussed. The relaxor phase is considered: (1) as a dipole-glass state, or (2) as a nano-scale mixture of polar regions, broken up into nano-domains, and nonpolar ones [5]. It was shown recently by high-resolution electron microscopy that the heterophase structure of the relaxor state consists of polar nano-regions with diameters down to 10 nm embedded in a nonpolar matrix [6]. The formation of polar phase with complicated structure of nanoscale domains was observed after cooling through the Curie point in strontium barium niobate single crystals (Sr,Ba)Nb 2 O 6 as pure so doped by Ce, using the piezoelectric force microscopy PFM [7, 8]. The finger-print sub-micro-scale domain patterns have been observed using PFM in single-crystalline PMN:PT and PZN:PT [9 11]. The similar domain patterns have been revealed recently in PLZT ceramics by scanning force microscopy in acoustic mode and PFM [12]. The absence of micro-scale polarization is due to random orientation of spontaneous polarization P S of individual polar nano-regions. The alignment of polar nano-regions under application of the strong electric field leads to appearance of averaged P S.Itwasshown that the field induced state in relaxors is instable in contrast with the normal ferroelectrics 245

3 246 V. Ya. Shur et al. [13, 14]. Thus even restoration of the initial state (complete spontaneous backswitching ) can be observed after external field switch off. Experimental study of the switching behavior in relaxor (Sr,Ba)Nb 2 O 6 single crystals reveals the absence of the definite coercive field attributed to essential inhomogeneity of the crystal structure [15, 16]. In PLZT of various compositions as conventional, so double shapes of hysteresis loops have been observed in wide temperature range as below so above freezing temperature [17]. The doubling of loops observed in PLZT 8/65/35 was attributed to aging effect [18]. In this paper we report results of investigation of the switching behavior under application of bipolar ac field in relaxor PLZT x/65/35 ceramics with La concentration from 5 to 8 atomic percents. The proposed approach to analysis of the field dependence of the switching charge derivative on field dq(e)/de extracted from the recorded hysteresis loops allows us to obtain the parameters of the distribution function of local coercive fields. The backswitching process in relaxor phase was attributed to existence of the inhomogeneous depolarization field produced by the bound charges located at the boundaries of nonpolar inclusions. Experiment We have studied the hot-pressed high-density transparent lanthanum-doped lead zirconatetitanate ceramics (Pb 1 x La x ) (Zr 0.65 Ti 0.35 )O 3 PLZT (5-8)/65/35, which exhibits a classic relaxor behavior within a broad temperature range. The sample thickness varied from 90 µm to 300 µm. Two types of continuous electrodes were used for field application: (1) transparent ones based on indium and tin oxides deposited by magnetron sputtering, and (2) gold ones deposited by thermal evaporation. The hysteresis loops were measured in temperature range from 20 Cto200 C under the action of sine ac voltage in frequency range from 0.02 Hz to 0.2 Hz. The field amplitude reached 8 kv/cm. The switching current data were recorded by measuring the field drop on the series resistor 1 M. The switching charge was obtained by digital integration of the current data. The low frequency range was chosen to insure the quasi-static switching regime. The precision of the temperature measurement was about 0.2 C. The heating/cooling rate in the vicinity of a freezing point did not exceed 0.4 C/min. Each cycle of measurements represents recording of the hysteresis loops during heating and subsequent cooling. Before measurements the samples have been treated by zero field heating and subsequent zero field cooling. The dielectric measurements were carried out in the temperature range from 25 Cto 200 Catfrequency 1 khz. Each cycle of measurements consists of zero field heating and zero field cooling stages with heating/cooling rate about 1 C/min. The samples were poled before each cycle by dc electric field (E = 10 kv/cm) applied during about one second at the temperature below the freezing point for the given composition. Hysteresis Loops In all studied samples the shape of the hysteresis loops qualitatively differs for measurements above and below the freezing point. The typical loops measured at different temperatures for PLZT 8/65/35 and 6.5/65/35 are shown on Fig. 1. It is seen that the classical ferroelectric loop observed at low temperatures changes to the double loop at the high temperatures. Such a special shape of hysteresis loop is usual for antiferroelectrics or ferroelectrics with inhomogeneous sign of the internal bias field [19].

4 AC Switching of Relaxor PLZT Ceramics 247 Figure 1. Hysteresis loops for different temperatures recorded during heating: (a) PLZT 6.5/65/35, and (b) PLZT 8/65/35. Frequency 0.04 Hz. For detail study of the evolution of the switching process during heating/cooling we analyzed a derivative of the switched charge on field dq/de versus applied field E at various temperatures (Fig. 2). It must be pointed out that dq(e)/de coincides with the switching current data for switching in ac linear increasing field (under the action of ac triangular pulses). Figure 2. Typical (a), (c) hysteresis loops Q(E), and (b), (d) field dependence of dq/de at temperatures (a), (b) below freezing temperature T fs (T = 50 C), and (c), (d) above T fs (T= 120 C). The arrows show direction of the field scan. PLZT 6.5/65/35. T fs = 106 C. Frequency 0.04 Hz.

5 248 V. Ya. Shur et al. Figure 3. Field dependences of dq/de at different temperatures for low branch of hysteresis loop during cooling. PLZT 6.5/65/35. Frequency 0.04 Hz. It is seen that dq(e)/de qualitatively changes during heating (Fig. 2). At the temperatures essentially below freezing point T f the dq(e)/de dependences have two distinct peaks typical for switching in normal ferroelectrics [Fig. 2(b)]. The value of T f is defined from the temperature dependence of the dielectric permittivity measured in the same sample polarized at low temperatures [20, 21]. The fields E m, corresponding to two peak maximums, are close to the values of coercive fields E c determined by conventional methods. The splitting of both peaks is observed for temperatures above T f [Fig. 2(d)]. The shapes of dq(e)/de for saturated hysteresis loops were usually similar on both branches for all investigated compositions. The evolution of dq(e)/de data during heating for the low branch of the hysteresis loop (the field increasing from the negative to positive values) for PLZT 6.5/65/35 is presented on Fig. 3. The position of maximum of the first peak is strongly temperature dependent while the second one demonstrates weak temperature dependence. Analysis of the Experimental Data For analysis of the experimental data during slow switching we have used quasi-static approach. This approach has been originally proposed by Preisach for explanation of hysteresis loop shape in ferromagnets [22] and applied for ferroelectrics by Turik [23]. Recently applications of Preisach approach in ferroelectric thin films were made [24]. In classical Preisach model observed hysteresis loop is described as a sum of responses from a collection of a large number of elementary bistable units (small regions of the inhomogeneous ferroelectric), each characterized by the local values of coercive fields for switching in opposite directions, which switches independently when applied field reaches

6 AC Switching of Relaxor PLZT Ceramics 249 these values. The field dependence of dq/de is very informative for the description of the switching process in inhomogeneous media because according to this model dq(e)/de corresponds to the distribution function of the local coercive fields. In this case the observed splitting of dq(e)/de data is the evidence of coexistence of two switching processes happened in distinct regions. Switching processes in opposite directions can be characterized by their own distribution function with positions of the maxima representing the value of the local coercive fields averaged over the corresponding region. The integral of the given distribution function is proportional to the relative charge switched in the given region. It must be stressed that similar splitting of dq(e)/de can be obtained from the hysteresis loop data recorded while ac switching after aging or imprint in ferroelectrics kept in multidomain static state [19, 25]. It is common to attribute such behavior to the existence of the regions with opposite sign of internal bias field [25, 26]. In this case the first peak obtained at each branch of the hysteresis loop corresponds to the switching in the regions with E b facilitating the switching process. The second peak corresponds to the regions with opposite direction of E b, which hampers the switching. It is important to stress that in this case the order of the switched regions reverses for different branches of the loop [Fig. 4(a)]. The change of the peak order can be clearly observed only for considerably different volumes of the regions with the opposite sign of E b. Figure 4. Schematic double loops and corresponding dq(e)/de for: (a) ferroelectric with inhomogeneous sign of the internal bias field E b, and (b) relaxor with uncompensated depolarization field E dep. (a) Grey and white areas differ by the sign of E b. (b) Grey areas regions with E dep produced by bound charges located at the boundaries of nonpolar inclusions (grey ellipses). Black arrows directions of P s. White arrows direction of E dep.wide black lines domain walls.

7 250 V. Ya. Shur et al. Figure 5. Scheme showing the depolarization field E dep produced by bound charges located at the boundaries of nonpolar inclusions (grey ellipses) for two single domain states with opposite directions of P s shown by black arrows. White arrows show directions of E dep. In our experiments the shapes of the loops and dq(e)/de are qualitatively different [Fig. 4(b)]. It is seen that the order of the peaks does not change and the ratio of the first peak integral to the second one is equal for both branches of hysteresis loop. The obtained behavior demonstrates that in contrast to situation in normal ferroelectrics with internal bias the order of the switched regions remains the same for different branches of the loop in relaxors. The difference between relaxors and normal ferroelectrics can be attributed to the existence of the depolarization field produced by bound charges situated in the bulk at the boundaries of nonpolar regions existing in relaxor phase. In normal ferroelectric the bound charges are situated mostly at the polar surfaces of the finite sample and can be substantially screened by fast external screening processes [3, 27]. In contrast this mechanism is ineffective for screening of the depolarization fields produced by bound charges existing in the bulk. Thus the depolarization field can be compensated by slow bulk screening only. In our experimental situation, when the period of applied field is essentially shorter than the bulk screening time constant, the first peak is caused by acceleration of the switching by uncompensated depolarization field. It must be understood that the sign of depolarization field reverses with change of the direction of spontaneous polarization [Fig. 5(b)]. That is why the regions situated in the vicinity of the nonpolar inclusions are switched first of all at both branches, while starting from any single-domain state. The first switching process represents backswitching from the field-induced singledomain state to multi-domain one with averaged value of local coercive field E mbs. The second process corresponds to switching in the regions where the influence of the depolarization fields produced by bound charges situated at the boundaries of nonpolar inclusions is negligible. It is characterized by its own averaged value of local coercive field E ms. The pronounced temperature dependence of E mbs is due to increase of the relative volume of nonpolar inclusions during heating [Fig. 6(a)]. The averaged value of the effective backswitching depolarization field E dep, defined by the difference between the averaged local coercive fields E dep = E ms E mbs, increases during heating [Fig. 6(b)]. The temperature dependence of E dep was fitted by the following formula E dep (T) = A(T T fs ) α (1) where A and T fs are parameters, and α is a critical index. The critical temperature dependence of E dep defines the temperature point T fs (a freezing temperature extracted by ac switching) characterizing transition between stable and unstable field-induced domain states due to the influence of the nonpolar inclusions. The characteristic temperatures extracted by dielectric and switching methods for different PLZT

8 AC Switching of Relaxor PLZT Ceramics 251 Figure 6. Temperature dependences of: (a) averaged values of the local coercive fields for switching E ms and backswitching E mbs, and (b) depolarization field E dep. Experimental points for low branch of hysteresis loop during heating were fitted by Eq. (1). PLZT 7.5/65/35. compositions are presented in Table 1. T m is the temperature corresponding to wide dielectric maximum, while T fd is the freezing temperature extracted from position of the narrow dielectric maximum measured during heating after poling. It must be pointed out that T fs values are essentially lower than T fd [20, 21]. The value of critical index α is about 0.5 for all studied compositions. It was shown earlier that the Gaussian can be chosen as a distribution function of the local coercive field for fitting of dq(e)/de data in inhomogeneous ferroelectrics [25, 26]. Our attempt to fit dq(e)/de data for the temperatures below T f by Gaussian reveals the additional wide input which can be attributed to nonlinear dielectric input caused by existence of the charged domain walls [28, 29]. High concentration of the charged domain walls in the ferroelectric state after zero field cooling have been shown in PLZT experimentally [12]. The attempt to fit the data recorded above T f by sum of two Gaussians corresponding to backswitching and switching processes also reveal the wide dielectric response. The obtained increase of that dielectric response during cooling to T f, which leads to increasing of the relative volume of the polar phase, confirms our supposition that this response is Table 1 Characteristic Temperatures for PLZT with Different La Concentration %La T m, C T fd, C T fs, C

9 252 V. Ya. Shur et al. due to charged domain walls [28, 29]. The details of the analysis of the role of the charged domain walls will be published by us elsewhere. Conclusion We have proposed the interpretation of the switching behavior under application of bipolar ac field in relaxor PLZT ceramics as polarization reversal in inhomogeneous ferroelectric with nonpolar inclusions. The qualitative difference of the shape of hysteresis loops above and below the freezing temperature has been attributed to backswitching in relaxor phase under the action of the depolarization field produced by the bound charges existing at the boundaries of nonpolar inclusions. The qualitative difference between double loops caused by depolarization fields and internal bias fields is demonstrated. It was shown that the application of the quasistatic approach can be used for obtaining of the distribution functions of local coercive fields from the field dependence of the switching charge derivative on field. The research was made possible in part by Grants p2004ural and of RFBR, by Grant UR of Program Basic Research in Russian Universities of Federal Agency of Education RF and by Award No.EK-005-X1 of the U.S. CRDF and Federal Agency of Education RF. References 1. L. E. Cross, Ferroelectrics 151, 305 (1994). 2. G. A. Smolenskii and A. I. Agranovskaya, Sov. Phys. Solid State 1, 1429 (1960). 3. V. Ya. Shur, Phase Transitions 65, 49(1998). 4. Z. Kutnjak, R. Pirc, and R. Blinc, Appl. Phys. Letters 80(17), 3162 (2002). 5. R. Blinc, R. Pirc, B. Zalar, A. Gregorovic, and V. Bobnar, Ferrolectrics 299, 1(2004). 6. X. Dai, Z. Xu, and D. Viehland, Phil. Mag. B70, 33(1994). 7. P. Lehnen, W. Kleemann, Th. Woeike, and R. Pankrath, Physical Review B64, (2001). 8. K. Terabe, S. Takekawa, M. Nakamura, K. Kitamura, S. Higuchi, Y. Gotoh, and A. Gruverman, Appl. Phys. Letters 81(11), 2044 (2002). 9. M. Abplanalp, D. Baroshava, P. Bridenbaugh, J. Erhart, J. Fousek, P. Guenter, J. Nosek, and M. Shulc, J. Appl. Phys. 91(6), 3797 (2002). 10. I. K. Bdikin, V. V. Shvartsman, and A. L. Kholkin, J. Appl. Phys. 83(20), 4232 (2003). 11. F. Bai, J. F. Li, and D. Viehland, J. Appl. Phys. 85(12), 2313 (2004). 12. Q. R. Yin, G. R. Li, H. R. Zeng, X. X. Liu, R. Heiderhoff, and L. J. Balk, Appl. Phys. A 78, 699 (2004). 13. V. Ya. Shur, G. G. Lomakin, V. P. Kuminov, D. V. Pelegov, S. S. Beloglazov, S. V. Slovikovski, and I. L. Sorkin, Phys. Solid State 41, 453 (1999). 14. V. Ya. Shur, G. G. Lomakin, E. L. Rumyantsev, S. S. Beloglazov, D. V. Pelegov, A. Sternberg, and A. Krumins, Ferroelectrics 299, 75(2004). 15. V. V. Gladkii, V. A. Kirikov, S. V. Nekhlyudov, T. R. Volk, and L. I. Ivleva, Phys. Solid State 42(7), 1334 (2000). 16. V. V. Gladkii, V. A. Kirikov, and T. R. Volk, Phys. Solid State 44(2), 365 (2002). 17. Z. Xu, X. Dai, and D. Veihland, Appl. Phys. Lett. 65(25), 3287 (1994). 18. G. Schmidt, H. Arndt, G. Borchhardt, J. von Cieminski, T. Petzsche, K. Borman, A. Sternberg, A. Zirnite, and V. A. Isupov, Phys Stat. Sol. 63, 501 (1981). 19. P. V. Lambeck and G. H. Jonker, Ferroelectrics 22, 729 (1978); J. Phys. Chem. Solids 47(5), 453 (1986). 20. J. L. Dellis, M. El. Marssi, P. Tilloloy, R. Farhi, and D. Vieland, Ferroelectrics 201, 167 (1997). 21. V. Ya. Shur, G. G. Lomakin, E. L. Rumyantsev, O. V. Yakutova, D. V. Pelegov, A. Sternberg, and M. Kosec, Phys. Solid State 47(7) (2005).

10 AC Switching of Relaxor PLZT Ceramics F. Preisach, Z. Phys. 94, 277 (1935). 23. A. V. Turik, Sov. Phys. Solid State 5, 885 (1963). 24. G. Robert, D. Damjanovic, and N. Setter, Appl. Phys. Lett. 77, 4413 (2000); A. Bartic, D. Wouters, H. Maes, J. Rickes, and R. Waser, J. Appl. Phys. 89, 3420 (2001). 25. V. Ya. Shur, I. S. Baturin, E. I. Shishkin, and M. V. Belousova, Ferroelectrics 291, 27(2003). 26. V. Ya. Shur, I. S. Baturin, E. I. Shishkin, and M. V. Belousova, Integrated Ferroelectrics 53, 379 (2003). 27. V.Ya. Shur, Phase Transitions 65, 49(1998). 28. L. E. Cross and T. W. Cline, Ferroelectrics 11, 333 (1976). 29. V. Ya. Shur, E. L. Rumyantsev, E. V. Nikolaeva, and E. I. Shishkin, Appl. Phys. Letters 77(22), 3636 (2000).