ISSN 1063-7850, Technical Physics Letters, 2009, Vol. 35, No. 1, pp. 43–46. © Pleiades Publishing, Ltd., 2009. Original Russian Text © Yu.E. Kalinin, L.N. Korotkov, A.V. Sitnikov, D.P. Tarasov, 2009, published in Pis’ma v Zhurnal Tekhnicheskoœ Fiziki, 2009, Vol. 35, No. 1, pp. 90–97.
Anomalies in the Elastic and Inelastic Properties of Ferromagnet–Ferroelectric Composites Yu. E. Kalinin*, L. N. Korotkov, A. V. Sitnikov, and D. P. Tarasov Voronezh State Technical University, Voronezh, 394026 Russia *e-mail:
[email protected] Received June 26, 2008
Abstract—We have studied the elastic and inelastic properties of composite materials of the Cox(PZT)1 – x system (x = 0.23–0.79) possessing a nonequilibrium nanogranular structure with an average grain size of ~3 nm. The results of mechanical tests performed in a temperature range of 300–900 K revealed a significant increase in the level of mechanical losses (Q–1) above 750 K, which is caused by the thermoactivated migration of point defects. An additional heat treatment leads to grain coarsening and the appearance of a ferroelectric state in the dielectric matrix. The temperature dependences of Q–1 in annealed samples exhibit two maxima, one of which is due to the interaction of domain boundaries with lattice defects and the other is related to the motion of interfaces in the region of the ferroelectric phase transition. PACS numbers: 62.40.+i, 81.40.Jj DOI: 10.1134/S1063785009010131
varying from 23 to 72 at % in one technological cycle, depending on the mutual arrangement of the target and substrate.
Granular composites comprising ferromagnetic nanoparticles dispersed in a dielectric matrix possess a number of physical properties different from those of the usual materials. Such composites exhibit giant magnetoresistance [1], improved magnetic characteristics in the high-frequency and microwave range [2], and the possibility of controlled variation of their resistivity in a broad range [3]. The use of a ferroelectric material as the dielectric component in a composite allows us to expect the appearance of new properties, in particular, the magnetoelectric effect. This effect can take place due to a striction coupling mediated by the elastic interaction between the ferromagnet particles and the ferroelectric matrix.
The structure of deposited films was studied by scanning electron microscopy (SEM) on a JSM-6380 instrument (JEOL, Japan). Figure 1 presents the typical SEM micrograph, which reveals a nanogranular structure with an average grain size of ~3 nm. The film composition was determined using electron-probe microanalysis. The samples for the investigation of internal friction and elastic properties had the form of rectangular 5 × 18 × 0.4 mm Sitall substrates with deposited nanocomposite films. The temperature dependences of the elastic modulus (G) and internal friction (Q–1) were measured using the bending oscillation decay technique [5]. These measurements were performed in a broad range of frequency of ~20 Hz at a temperature varied from 300 to 900 K at a heating rate of 3 K/min. The internal friction of the composite was determined as a difference between the Q–1 values measured for the film/substrate system and the pure substrate. The errors of determining Q–1 and G did not exceed 3 and 1%, respectively.
In this context, the present study was aimed at the synthesis and evaluation of the elastic and inelastic properties of granular composite materials of the Cox(PZT)1 – x system. The metal component in this composite (Co) exhibits the transition to a ferromagnetic state below 1394 K. The dielectric matrix represents lead zirconate titanate PbZrTiO3 (PZT), the wellknown ferroelectric compound [4] exhibiting the transition to a polar phase at ~573 K. The samples of Cox(PZT)1 – x composites with a nanogranular structure were prepared using ion-beam sputtering of a composite target onto Sitall (glass-ceramic composite) substrates. The composite target comprised a 280 × 80 × 10 mm cobalt plate with 80 × 10 × 2 mm pieces of PZT ceramics fastened to the plate surface. Using this target, it was possible to obtain samples of 3-µm-thick composite films with a metal phase content
The temperature of a ferroelectric phase transition (TC) in composites with a cobalt content x ≤ 0.5 was determined by measuring the temperature dependence of the capacitance (C) and the dielectric loss tangent ( tan δ ). In these experiments, a sample was placed between 5 × 7 mm electrodes pressed from the film and 43
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KALININ et al. Q–1 × 104
G, a.u. 1.00
100
1'
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ln(Q–1)
80
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0.5 µm
(a)
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60
3.9 3.6
0.88
40 0.00116 0.00120 1/T, K–1
1 20 0.84 550
650
700 750 T, K
800
850
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Fig. 2. Temperature dependences of (1) internal friction Q–1 and (1') elastic modulus G of an as-deposited Co0.77PZT0.23 film sample. The inset shows a plot of lnQ–1 versus 1/T.
0.5 µm
(b)
600
Fig. 1. SEM micrographs of the surface of a Co0.54PZT0.46 film: (a) as-deposited; (b) upon annealing at 923 K.
substrate sides. The electrical measurements were performed in the course of sample heating at a frequency of 1 kHz. The temperature dependences of the capacitance measured for as-deposited samples revealed no anomalies (characteristic of a ferroelectric phase transition) in the dielectric response in the entire range of temperatures studied. The temperature dependences of the elastic modulus and internal friction showed a monotonic decrease in G, which was accompanied by an increase in mechanical losses with increasing temperature (Fig. 2). At temperatures above ~750 K, the variation of Q–1(T) obeyed the following law [6]: Q
–1
–1
E –1 = Q 0 exp ⎛ – ------⎞ , ⎝ kT ⎠
(1)
where Q 0 is a constant factor, E is the activation energy of the high-temperature background, k is the Boltzmann constant. This approximation is confirmed by a linear plot of lnQ–1(1/T) presented in the inset in Fig. 2. It can be suggested that the exponential growth of Q–1 is related to the thermoactivated migration of point defects [7]. It was found that the Q–1 background
activation energy slightly increases (from 0.8 to 1.1 eV) with growing metal fraction content. Thermal annealing of the composite samples in air at 923 K for 2 h led to a pronounced grain coarsening, the average grain size reaching ~100 nm (Fig. 1b). The structural transformation leads to the appearance of a ferroelectric state in the dielectric matrix of the composite, which is manifested by a clearly pronounced peak in the temperature dependences of the capacitance (see the inset in Fig. 3), which corresponds to the point of a transition from the ferroelectric to paraelectric state. It should be noted that the ferroelectric phase transition in the composite material studied takes pace at the same temperature as that in the pure PZT ceramics [4]. The temperature dependences of the elastic modulus and internal friction measured for the heat-treated samples showed anomalies in the vicinity of TC. The plots of G(T) also exhibit a characteristic minimum in the region of the ferroelectric phase transition, which corresponds to the maximum in Q–1 (Fig. 3). These anomalies are especially clearly pronounced in the samples with high PZT concentrations. On the contrary, as the metal fraction content is increased, the magnitude of these anomalies decreases. It can be suggested that the nature of the observed peak in Q–1 is the same as that in the bulk samples of TECHNICAL PHYSICS LETTERS
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PZT ceramics. Indeed, a peak in the internal friction observed during the first-order ferroelectric phase transition [8] is related to the motion of polar phase nuclei in a nonpolar matrix. Within the framework of the lowfrequency fluctuation mechanism of internal friction, which takes into account this motion, the peak in the internal friction is described by the following expression [8]:
G, a.u.
2πGβx s dT /dt 1 - -------, = ---------------------------------kT m ω ∆T
(2)
30
9
1.00
8
2'
540
570
600 T, K
535 K
27
1'
–1
Substituting the experimental values of ∆T, G, and Tm into formula (2) and assuming that the critical nucleus volume is β ≈ 10–25 m3, the jump in the spontaneous deformation can be evaluated as xs ≈ 1.7 × 10–2. This value is close in the order of magnitude to the jump in the spontaneous deformation observed in bulk PZT samples [9]. At temperatures below TC (in the region of 530 K), the temperature dependence of the internal friction exhibits another maximum (Fig. 3b), which was also not observed for the freshly prepared (as-deposited) samples. As analogous maximum of Q–1 at ~515 K was previously observed for some PZT ceramics [10]. It was demonstrated [11, 12] that this maximum is related to the interaction of domain boundaries with lead vacancies. This circumstance indicates that a domain structure appears at temperatures below TC in the composites under consideration with small concentrations of the metal phase. In conclusion, the results of our investigation showed that the composite materials under consideration in the initial (as-deposited) state represent nanogranular systems with an average grain size of ~3 nm. Subsequent thermal annealing leads to the formation of a polycrystalline matrix structure with an average grain size of about 100 nm. In Cox(PZT)1 – x compositions with x ≤ 0.5, a ferroelectric state is realized at temperatures below the Curie point. The temperature of the ferroelectric phase transition approximately coincides with the Curie temperature of PZT ceramics with the same composition. These results allow us to suggest that Co atoms in the samples upon annealing segregate outside the growing grains rather than form a solid solution with the matrix material. The existence of an additional maximum of internal friction below TC, –1 Qm ,
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570 K
where 4 Q m is the peak height, ∆T is the peak full width at half height (FWHM), Tm is the temperature of the peak, β is the critical nucleus volume, xs is the jump of spontaneous deformation at the point of the phase transition, ω = 2πf, f is the frequency of mechanical oscillations of the sample at Tm; and dT/dt is the temperature variation rate. According to expression (2), the peak height must linearly increase with the sample heating rate dT/dt, in agreement with the observed behavior.
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–1 Qm
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24 0.99 1 21 0.98 2 520
540
560
580
T, K Fig. 3. Temperature dependences of the (1, 2) internal friction Q–1 and (1', 2') elastic modulus G of annealed Cox(PZT)1 – x composite films with various concentrations of the metallic phase (x, at %): (1, 1') 0.24; (2, 2') 0.58. The inset shows the temperature dependences of the capacitance of a Co0.24PZT0.76 film sample.
which is analogous to the peak observed in some PZT ceramics and attributed to the interaction of domain boundaries with lattice defects, suggests that a domain structure is also formed upon annealing in the composite materials under consideration. Acknowledgments. This study was supported in part by the Russian Foundation for Basic Research, project no. 08-02-01089. REFERENCES 1. O. V. Stognei, Yu. E. Kalinin, I. V. Zolotukhin, A. V. Sitnikov, V. Wagner and F. J. Ahlers, J. Phys. Condens. Matter 15, 4267 (2003). 2. Yu. E. Kalinin, L. N. Kotov, S. N. Petrunev, and A. V. Sitnikov, Izv. Ross. Akad. Nauk, Ser. Phys. 69, 1195 (2005). 3. I. V. Zolotukhin, Yu. E. Kalinin, P. V. Neretin, A. V. Sitnikov, and O. V. Stognei, Al’tern. Énerg. Ékol., No. 2, 7 (2002). 4. G. A. Smolenskii, V. A. Bokov, V. A. Isupov, et al., Ferroelectrics and Antiferroelectrics (Nauka, Leningrad, 1971) [in Russian].
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5. Yu. E. Kalinin, A. V. Sitnikov, and D. P. Tarasov, Pis’ma Zh. Tekh. Fiz. 34 (11), 12 (2008) [Tech. Phys. Lett. 34, 459 (2008)]. 6. V. S. Postnikov, Internal Friction in Metals (Metallurgiya, Moscow, 1974) [in Russian]. 7. I. V. Zolotukhin and Yu. E. Kalinin, Fiz. Tverd. Tela (St. Petersburg) 37, 536 (1995) [Phys. Solid State 37, 290 (1995)]. 8. S. A. Gridnev, Ferroelectrics 112, 107 (1990) 9. S. A. Gridnev, Mechanism of Internal Friction in Ferroelectrics and Ferroelastics, Doctoral Dissertation in Mathematical Physics (Voronezh, 1983).
10. V. S. Pavlov, S. A. Turkov, and É. N. Bessonova, Influence of Point Defect Concentration on the Internal Friction in Polycrystalline Lead Zirconate Titanate (Nauka, Moscow, 1972), pp. 151–156 [in Russian]. 11. V. S. Postnikov, S. A. Gridnev, et al., Fiz. Tverd. Tela (Laningrad) 10, 1599 (1968) [Sov. Phys. Solid State 10, (1968)]. 12. V. S. Postnikov, V. S. Pavlov, et al., Izv. Akad. Nauk SSSR, Ser. Phys. 11, 1845 (1967).
Translated by P. Pozdeev
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