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Correlation between interfacial defects and ferromagnetism of BaTiO3 nanocrystals studied by positron annihilation
Applied Surface Science 258 (2011) 19–23
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Correlation between interfacial defects and ferromagnetism of BaTiO3 nanocrystals studied by positron annihilation Zhi-Yuan Chen a,b , Z.Q. Chen a,∗ , D.D. Wang a , S.J. Wang a,∗ a b
Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan 430072, PR China School of Nuclear Engineering and Technology, Xianning University, Xianning 437100, PR China
a r t i c l e
i n f o
Article history: Received 30 January 2011 Received in revised form 26 June 2011 Accepted 29 July 2011 Available online 4 August 2011 Keywords: BaTiO3 nanocrystal Annealing Interfacial defect Ferromagnetism
a b s t r a c t High purity BaTiO3 nanopowders were pressed into pellets and annealed between 100 and 1200 ◦ C. The crystal quality and grain size of the BaTiO3 nanocrystals were characterized by X-ray diffraction measurements. Annealing induces an increase in the grain size from 44 to 82 nm with temperature increasing up to 1200 ◦ C. XRD and Raman spectroscopy studies confirm that all the samples were single phase with a tetragonal structure after annealing at different temperatures. Positron annihilation measurements reveal large number of vacancy defects in the grain boundary region. These interfacial defects remain stable after annealing at temperatures below 400 ◦ C and begin to disappear rapidly above 700 ◦ C. After annealing at 1200 ◦ C, most of the interfacial defects have been removed. Hysteresis loops are observed for the 100 ◦ C annealed samples, which indicate ferromagnetism in BaTiO3 nanocrystals. The ferromagnetism becomes a little weaker after annealing at 700 ◦ C, and it disappears after 1200 ◦ C annealing. This change of ferromagnetism coincides with the defect recovery process after annealing, suggesting that ferromagnetism might originate from the interfacial defects. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Barium titanate (BaTiO3 , abbreviated as BTO) is an important ferroelectric material. It has a cubic perovskite structure and is paraelectric when it is above the Curie temperature of 120 ◦ C. When it is below the Curie point, the crystal transforms to a tetragonal phase which is ferroelectric at ambient condition. It is well known that BaTiO3 has interesting properties, which have a variety of electronic applications. Due to the positive temperature coefficient of resistivity, BaTiO3 is used as a basic material for thermistors, protective sensors for motors, self-controlled heaters and degausses [1,2]. The high dielectric constant of BaTiO3 allows its application in ceramic capacitors [1,2]. These properties might strongly depend on defects that exist in large concentrations [3]. Various defect types may exist, among them vacancies are expected to be the dominant intrinsic defects because BaTiO3 is an ionic compound making the formation of other defects such as antisites or interstitials less probable [4]. The intrinsic vacancies types are acceptor-type Ba vacancies (VBa ) and Ti vacancies (VTi ), and donortype oxygen vacancies (VO ) [5,6]. In addition, the grain boundaries presented in the polycrystalline BaTiO3 might contain open volume
∗ Corresponding authors. E-mail addresses: [email protected] (Z.Q. Chen), [email protected] (S.J. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.132
defects and can have a strong influence on the materials properties [7]. The structure and electrical properties of BaTiO3 is known to be also crucially determined by the grain size, especially when it decreases down to nanometer scale [8–11]. For BaTiO3 nanoparticles, it is well known that ferroelectricity disappears below a critical size (40 nm) [12]. The reason is that ferroelectricity is suppressed at the nanoscale due to depolarization fields arising from the bound charges at the nanoparticle surface [13,14]. Recently, both experiments and first-principles calculations have revealed that the classic ferroelectric material BaTiO3 even shows ferromagnetism at room temperature when it is made at the nanoscale [15,16]. Systematic work has also shown that ferromagnetism appears in nanoparticles of the otherwise non-magnetic oxides such as CeO2 , Al2 O3 , MgO, ZnO, In2 O3 and SnO2 , etc,[17] but decreases with increasing particle size. In fact, ferromagnetism has been envisaged to be a universal feature of nanoparticles of inorganic materials [18]. Conversely, bulk samples obtained by sintering the nanoparticles at high temperatures became diamagnetic. The magnetism in these nanoparticles has been suggested to be intrinsic and probably originates from cation or anion vacancies at the surfaces of nanoparticles. For nanomaterials, surface phenomena dominate over their respective bulk features due to high surface to volume ratio. Generally, there is a high fraction of grain boundary region, which separates the crystallites. It was reported that the structure of grain boundaries in nanocrystalline materials has neither
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long-range nor short-range order [19]. Therefore, the interfaces of nanomaterials are rich of defects. These defects may induce ferromagnetism of materials. A detailed characterization of these interfacial defects is thus necessary to find its correlation with the magnetic behavior. Positron annihilation spectroscopy (PAS) has been proved to be a powerful tool to investigate vacancy defects in solids [20,21]. Due to the Coulomb repulsion from the positive ion cores, positrons are trapped preferentially by vacancy defects where the atom is missing. Annihilation characteristics of positrons are different in the perfect bulk state and vacancy trapped state, which makes the identification of vacancies very straightforward. A longer positron lifetime or a narrower Doppler broadening spectrum compared to the value in the perfect bulk thus gives evidence of defects in materials. This method could provide information on the nature and abundance of defects. By using coincidence Doppler broadening (CDB) technique, the chemical environment around the defects can be also detected [22]. Positron annihilation spectroscopy is particularly useful in characterizing the interfacial defects in nanomaterials. This is because that the grain size is usually smaller than the positron diffusion length (generally on the order of 100 nm). The interfacial defects are deep positron trapping centers, and almost all the positrons will diffuse to the grain boundary region and get trapped by vacancy defects. Thus the effective grain boundary region seen by positrons is magnified, and the sensitivity of positrons to the interfacial defects is greatly enhanced. Schaefer et al. [23] first reported positron annihilation study of the defects in Fe nanocrystals. Later several works were published about PAS studies in various nanomaterials [24–27]. In this paper, we studied the effect of thermal treatment on the interfacial defects in BTO nanocrystals. Positron lifetime, CDB, X-ray diffraction (XRD) and Raman spectroscopy were performed to get a comprehensive understanding of the defect characteristics. Correlation between the interfacial defects and the ferromagnetism in these BTO nanocrystals was observed, and the origin of magnetization will be discussed.
2. Experiment Samples for this study were prepared from commercially available high-purity BTO (grain size ∼50 nm, purity ∼99.9%) nanopowders. The powders were hand milled in agate mortar with pestle for 2 h. After hand milling, the powders were pressed into pellets under a static pressure of about 6 MPa for 1 min at room temperature. The pellet samples were in disk shape having a diameter of 15 mm and a thickness of 1–2 mm. They were subsequently annealed in open air at 12 different temperatures ranging from 100 to 1200 ◦ C for 2 h in an electric furnace. The 22 Na positron source was prepared by depositing 22 NaCl onto a Ni thin film with a thickness of about 3 m, and was sandwiched between two identical plane-faced pellets. Positron lifetime measurements were performed using a conventional fast-fast coincidence system with a time resolution of about 280 ps in full width at half maximum (FWHM). The source activity was ∼5 Ci, and the count rate was about 40 cps. More than 106 total counts were accumulated in each measurement. The lifetime spectra were analyzed by the computer program PATFIT [28]. The contribution of positrons annihilating within the source and the foil was determined to be 530 ps with an intensity of 2% using a defect free silicon single crystal. Coincidence Doppler broadening spectra were measured using two high purity (HP)-Ge detectors with energy resolutions of about 1.76 and 1.64 keV (FWHM) at 1.33 MeV, respectively. At least 4 × 106 total counts were collected in each spectrum. The spectrum was characterized by the S and W parameter which is defined as the ratio of low momentum (|PL | < 0.68 keV) and high
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50
60
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Fig. 1. XRD patterns for the 100, 400, 700, 1000 and 1200 ◦ C annealed BTO nanocrystals.
momentum (2.86 keV < | PL | < 5.73 keV) region to the total region of the spectrum, respectively. The positron lifetime and CDB spectra were measured simultaneously at room temperature. X-ray diffraction measurements were performed using Cu K˛ radiation (Bruker D8 Advance) with a Ni filter. The scanning rate was 1◦ /min with a step of 0.02◦ . Raman spectra excited with the 515 nm line of an Ar+ laser were recorded in the backscattering geometry with a power level of about 4.8 mW arriving at the sample surface using a Renishaw confocal micro-Raman spectroscopy RM-1000. The wavenumber of Raman shift ranged from 100 to 800 cm−1 . The magnetization behaviors of the BTO nanocrystals annealed at different temperatures were investigated by a physical properties measurement system (quantum design). All the above measurements were carried out at room temperature. 3. Results and Discussion 3.1. XRD characterization To study the grain size and the crystalline phase of the nanopowders, five samples were subjected to X-ray diffraction measurements. The samples selected were 100, 400, 700,1000 and 1200 ◦ C annealed BTO. The XRD patterns are shown in Fig. 1. For each sample, all the observed peaks can be indexed with the tetragonal phase of BaTiO3 (JCPDS cards No. 05-0626). The BTO samples exhibit good crystallinity with no obvious indication for the existence of amorphous domains. In the annealing temperature interval between 100 and 1200 ◦ C, no phase transition was observed. XRD patterns confirmed that all the samples were single phase with a tetragonal structure. In addition, the intensity of the peaks increases and the FWHM decreases with the increasing annealing temperature, which indicates a possible change in the grain size. The average grain size of the annealed BTO samples is calculated by Scherrer’s formula:[29] Dh k l =
K , ˇ cos
(1)
Z.-Y. Chen et al. / Applied Surface Science 258 (2011) 19–23
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Fig. 4. Average positron lifetime as a function of annealing temperature for BTO nanocrystals. Fig. 2. Variation in the average grain size with annealing temperature for BTO nanocrystals.
where Dh k l is the average grain size perpendicular to the (h k l) plane, K is the shape factor (usually taken as 0.89), is taken as the X-ray wavelength of Cu K˛1 radiation ( = 0.15418 nm), ˇ is the FWHM of the XRD peak (every observed peak in the spectra was fitted with a Gaussian function), and is the Bragg angle. Standard method to deduct the contribution of instrumental broadening in FWHM has been taken into account [29]. The calculated BTO grain size as a function of annealing temperature is shown in Fig. 2. It can be seen that obvious grain growth occurs only above 600 ◦ C and the fast growth starts above 900 ◦ C. The average grain size of the sample annealed below 600 ◦ C was estimated to be around 44 nm. With increasing annealing temperature, it increases continuously and reaches up to about 82 nm at 1200 ◦ C. 3.2. Raman scattering measurement Raman spectroscopy is one of the useful methods for investigating phase transformation behavior. There are Raman active lattice vibration modes in the tetragonal BaTiO3 (P 4mm), while there is no Raman active mode in the cubic BaTiO3 (Pm 3m) [30]. The Raman peaks in tetragonal BaTiO3 powder are at 720 cm−1 [E(4LO) + A1 (3LO)], 515 cm−1 [E(4TO) + A1 (3TO)], 305 cm−1 [E(3TO) + E(2LO) + B1 ], 260 cm−1 [A1 (2TO)], and 185 cm−1 [E(2TO) + E(1LO) +A1 (1TO) + A1 (1LO)],[31] where the lattice vibration modes are shown in parentheses. Using the present Raman spectroscopy apparatus, however, the peak at 185 cm−1 was not observed, because the Raman scattering light in the range from 0 to 250 cm−1 is attenuated owing to a notch filter which cuts off the Rayleigh scattering light.
Fig. 3. Evolution of Raman spectra as a function of annealing temperature for BTO nanocrystals.
Fig. 3 shows the evolution of the Raman spectra measured for BTO samples annealed at various temperatures. In the samples heated at 100, 400, 700, 1000 and 1200 ◦ C, the tetragonal peaks of 720, 515, 305 and 260 cm−1 were confirmed. Hayashi et al. [32] reported that a peak at around 310 cm−1 appeared below the Curie point and vanished above the Curie point in BaTiO3 ceramics, suggesting that the peak at 305 cm−1 is an intrinsic peak for tetragonal BaTiO3 . Our BTO samples annealed at various temperatures show a slight peak at 305 cm−1 , indicating that all the samples are single phase with a tetragonal structure. 3.3. Positron annihilation measurements A detailed study of the microstructural defects in the BTO nanocrystals was performed by positron lifetime measurements. Every lifetime spectrum was evaluated to two lifetime components. In the low temperature annealed sample, the short lifetime component 1 is around 182 ps, while the second lifetime 2 has a value of about 300 ps. Theoretical calculation predicted a bulk positron lifetime of 152 ps for BaTiO3 , 293 ps for VBa , 204 ps for VTi , and 162 ps for VO , respectively [33]. Oxygen vacancies are believed to be effective positron traps in other perovskites [34–36]. Therefore, 1 might be an average value of free positron lifetime and the lifetime in smaller cation vacancies like VTi or VO . It might be also due to the saturated trapping of positrons in some vacancies. The second lifetime component 2 is probably attributed to VBa –VO divacancies or larger vacancy clusters. All the defects revealed by 1 and 2 are most probably located in the grain boundary region, which are called interfacial defects in this study. Due to the complexity of the defects in our BTO nanocrystals, decomposition result of the positron lifetime spectra might be not so reliable. Therefore we need to rely on the average positron lifetime. The variation of average positron lifetime av as a function of the annealing temperature for BTO nanocrystals is presented in Fig. 4. It is seen that with the increase of annealing temperature, the average lifetime av keeps constant at around 245 ps up to annealing temperature of 400 ◦ C, then it begins to decrease slowly with temperature increasing up to 700 ◦ C, subsequently it decreases rapidly above 700 ◦ C. Thus it can be inferred that the interfacial defects in the grain boundary region begin to recover above 400 ◦ C, and they disappear rapidly above 700 ◦ C, and most of them may have been removed at 1200 ◦ C. However, even after annealing at 1200 ◦ C, the average positron lifetime is still higher than the bulk lifetime. This suggests that some interfacial defects still remain after high temperature annealing. In general, due to the tendency to form agglomerates, nanosize particles can lead to microstructures with defects that cannot be eliminated during sintering [37].
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Fig. 5. The Doppler broadening S-parameter as a function of annealing temperature for BTO nanocrystals.
The variation of S-parameter with the annealing temperature, shown in Fig. 5, further supports above arguments. It is clear from Fig. 5 that the S-parameter decreases slowly from the annealing temperature 400–700 ◦ C, and after that it decreases rapidly. The variation of S-parameter with annealing temperature gives a preliminary idea of the changing defective state of the sample. In general, the decrease of S-parameter is a signature of lower annihilation probability of positrons with the lower momentum valence electrons. This occurs when the relative concentrations of defects decrease in the material. Therefore the same conclusion about the defect recovery process can be drawn from the Doppler broadening measurements, i.e, the defects have a rapid recovery after annealing at above 700 ◦ C. To identify the different positron trapping sites, the correlation between S- and W-parameters for the sample during annealing process is also studied. This plot can provide complementary information on the defects, and is usually used to verify the change of defect species or the chemical surrounding of the defects. The Sparameter versus W-parameter graph is drawn in Fig. 6. All the data points were concentrated on a straight line, suggesting that the defect species do not change during annealing. There is only a variation of their respective concentrations. 3.4. Ferromagnetic behavior Fig. 7 illustrates the magnetization versus applied magnetic field M–H curves of the BTO nanocrystals annealed at 100, 700 and 1200 ◦ C. Hysteresis loops can be observed clearly in the M–H curves of the BTO nanocrystals annealed at 100 and 700 ◦ C. Since there are no magnetic elements involved in the preparation of BTO nanoparticles and there was no contamination of magnetic materials during characterization, the ferromagnetism is intrinsic to the nanoparticles. Considering that the bulk BTO is diamag-
Fig. 6. The variation of S-parameter versus W-parameter for the nanocrystalline BTO after annealing at different temperatures.
Fig. 7. M–H curves for the BTO nanocrystals annealed at different temperatures. The diamagnetic contribution was subtracted for the samples annealed at 100 and 700 ◦ C.
netic, the room-temperature ferromagnetism is likely associated with the surface state of the BTO nanocrystal which has high surface to volume ratio. After deducting the diamagnetic signal of the bulk region of BTO samples, the saturation magnetization of the annealed samples changes slightly with the annealing temperature in the interval between 100 and 700 ◦ C. However, for the BTO nanocrystals annealed at 1200 ◦ C, no apparent ferromagnetism can be observed. The variation in ferromagnetism in annealed BTO nanocrystals is consistent with the recovery process of defects located at the grain boundary region indicated by the above positron lifetime results. After lower temperature annealing (<700 ◦ C), the interfacial defects remain little change, while the magnetic moment also shows only slight change. With increasing annealing temperature, gradual disappearance of the surface defects takes place. Simultaneously, a decrease of the magnetization is observed. After annealing at 1200 ◦ C, most surface defects are annealed out. At the same time, hysteresis loop cannot be observed in the magnetization versus field data within the resolution of the measurements. Therefore, the change in the interfacial defects is consistent with the variation in the magnetic behavior. In other words, the experimental results support the interfacial defects playing a role in the ferromagnetism. From our results it can be also inferred that a critical concentration of defects is necessary to induce the ferromagnetism in BTO sample. Though the annealing induces an increasing of grain size from 44 to 82 nm, which causes a decrease in the ratio of surface to bulk region, this is not the sole reason for the reduction in magnetization. Even after annealing at 1200 ◦ C, the grain size increases to only 82 nm, and the volume fraction of the surface and interface region is still about 8%. On the other hand, the grain size of 82 nm is still smaller than the positron diffusion length (generally on the order of 100 nm), Thus, all positrons probably diffuse towards the grain boundaries. In other words, the decrease in positron lifetime
Z.-Y. Chen et al. / Applied Surface Science 258 (2011) 19–23
or Doppler broadening S-parameter after annealing is primarily due to the recovery of vacancy defects in the interface region. Mangalam et al. [15] still observed ferromagnetism in BaTiO3 nanoparticles with a size of 40–300 nm. This may suggest that the room temperature ferromagnetism of the BaTiO3 nanocrystals originates mainly from the surface and interfacial defects, rather than the surface state. Vacancy-induced magnetism in nonmagnetic ferroelectric BaTiO3 has been studied by first principle calculation [38,39]. According to their calculation, the magnetism is induced by the partial spin-polarized O 2p states around Ti vacancies and the partially filled d-states Ti around the oxygen vacancies. 4. Conclusion In conclusion, we have studied the effect of annealing on the interfacial defects in high purity BTO nanocrystals. The grain size shows rapid increase after annealing at above 900 ◦ C and reaches around 82 nm at 1200 ◦ C. In the annealing temperature interval between 100 and 1200 ◦ C, no phase transition was observed. XRD patterns confirm that all the samples are single phase with a tetragonal structure. Raman scattering measurements confirm the tetragonal peaks at 720, 515, 305 and 260 cm−1 . The BTO nanocrystals annealed at various temperatures show a slight peak at 305 cm−1 in Raman spectra, indicating that all the samples were tetragonal structure. The interfacial defects evidenced by positrons in the grain boundary region do not change after annealing at 100–400 ◦ C, while recovery of the vacancies takes place above 400 ◦ C and begins to disappear rapidly above 700 ◦ C. The magnetic measurements indicate that the 100 and 700 ◦ C annealed BTO samples clearly show ferromagnetism at room temperature whereas the 1200 ◦ C annealed BTO shows diamagnetic. The disappearance of ferromagnetism in BTO nanocrystals coincides with the disappearance of interfacial defects, suggesting that the surface defects in BTO nanoparticles are the most probable origin of the ferromagnetism. Our results clearly show a correlation between positron response and magnetic behavior, but perhaps not directly. The mechanism through which defects may induce ferromagnetism still requires further study. Acknowledgments This work was supported by the Program for New Century Excellent Talents in University, the National Natural Science Foundation of China under grant nos. 10875088, 11075120, and the Natural Science Foundation of Xianning University under grant no. ZX1025.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Correlation between interfacial defects and ferromagnetism of BaTiO3 nanocrystals studied by positron annihilation Zhi-Yuan Chen a,b , Z.Q. Chen a,∗ , D.D. Wang a , S.J. Wang a,∗ a b
Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan 430072, PR China School of Nuclear Engineering and Technology, Xianning University, Xianning 437100, PR China
a r t i c l e
i n f o
Article history: Received 30 January 2011 Received in revised form 26 June 2011 Accepted 29 July 2011 Available online 4 August 2011 Keywords: BaTiO3 nanocrystal Annealing Interfacial defect Ferromagnetism
a b s t r a c t High purity BaTiO3 nanopowders were pressed into pellets and annealed between 100 and 1200 ◦ C. The crystal quality and grain size of the BaTiO3 nanocrystals were characterized by X-ray diffraction measurements. Annealing induces an increase in the grain size from 44 to 82 nm with temperature increasing up to 1200 ◦ C. XRD and Raman spectroscopy studies confirm that all the samples were single phase with a tetragonal structure after annealing at different temperatures. Positron annihilation measurements reveal large number of vacancy defects in the grain boundary region. These interfacial defects remain stable after annealing at temperatures below 400 ◦ C and begin to disappear rapidly above 700 ◦ C. After annealing at 1200 ◦ C, most of the interfacial defects have been removed. Hysteresis loops are observed for the 100 ◦ C annealed samples, which indicate ferromagnetism in BaTiO3 nanocrystals. The ferromagnetism becomes a little weaker after annealing at 700 ◦ C, and it disappears after 1200 ◦ C annealing. This change of ferromagnetism coincides with the defect recovery process after annealing, suggesting that ferromagnetism might originate from the interfacial defects. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Barium titanate (BaTiO3 , abbreviated as BTO) is an important ferroelectric material. It has a cubic perovskite structure and is paraelectric when it is above the Curie temperature of 120 ◦ C. When it is below the Curie point, the crystal transforms to a tetragonal phase which is ferroelectric at ambient condition. It is well known that BaTiO3 has interesting properties, which have a variety of electronic applications. Due to the positive temperature coefficient of resistivity, BaTiO3 is used as a basic material for thermistors, protective sensors for motors, self-controlled heaters and degausses [1,2]. The high dielectric constant of BaTiO3 allows its application in ceramic capacitors [1,2]. These properties might strongly depend on defects that exist in large concentrations [3]. Various defect types may exist, among them vacancies are expected to be the dominant intrinsic defects because BaTiO3 is an ionic compound making the formation of other defects such as antisites or interstitials less probable [4]. The intrinsic vacancies types are acceptor-type Ba vacancies (VBa ) and Ti vacancies (VTi ), and donortype oxygen vacancies (VO ) [5,6]. In addition, the grain boundaries presented in the polycrystalline BaTiO3 might contain open volume
∗ Corresponding authors. E-mail addresses: [email protected] (Z.Q. Chen), [email protected] (S.J. Wang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.132
defects and can have a strong influence on the materials properties [7]. The structure and electrical properties of BaTiO3 is known to be also crucially determined by the grain size, especially when it decreases down to nanometer scale [8–11]. For BaTiO3 nanoparticles, it is well known that ferroelectricity disappears below a critical size (40 nm) [12]. The reason is that ferroelectricity is suppressed at the nanoscale due to depolarization fields arising from the bound charges at the nanoparticle surface [13,14]. Recently, both experiments and first-principles calculations have revealed that the classic ferroelectric material BaTiO3 even shows ferromagnetism at room temperature when it is made at the nanoscale [15,16]. Systematic work has also shown that ferromagnetism appears in nanoparticles of the otherwise non-magnetic oxides such as CeO2 , Al2 O3 , MgO, ZnO, In2 O3 and SnO2 , etc,[17] but decreases with increasing particle size. In fact, ferromagnetism has been envisaged to be a universal feature of nanoparticles of inorganic materials [18]. Conversely, bulk samples obtained by sintering the nanoparticles at high temperatures became diamagnetic. The magnetism in these nanoparticles has been suggested to be intrinsic and probably originates from cation or anion vacancies at the surfaces of nanoparticles. For nanomaterials, surface phenomena dominate over their respective bulk features due to high surface to volume ratio. Generally, there is a high fraction of grain boundary region, which separates the crystallites. It was reported that the structure of grain boundaries in nanocrystalline materials has neither
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long-range nor short-range order [19]. Therefore, the interfaces of nanomaterials are rich of defects. These defects may induce ferromagnetism of materials. A detailed characterization of these interfacial defects is thus necessary to find its correlation with the magnetic behavior. Positron annihilation spectroscopy (PAS) has been proved to be a powerful tool to investigate vacancy defects in solids [20,21]. Due to the Coulomb repulsion from the positive ion cores, positrons are trapped preferentially by vacancy defects where the atom is missing. Annihilation characteristics of positrons are different in the perfect bulk state and vacancy trapped state, which makes the identification of vacancies very straightforward. A longer positron lifetime or a narrower Doppler broadening spectrum compared to the value in the perfect bulk thus gives evidence of defects in materials. This method could provide information on the nature and abundance of defects. By using coincidence Doppler broadening (CDB) technique, the chemical environment around the defects can be also detected [22]. Positron annihilation spectroscopy is particularly useful in characterizing the interfacial defects in nanomaterials. This is because that the grain size is usually smaller than the positron diffusion length (generally on the order of 100 nm). The interfacial defects are deep positron trapping centers, and almost all the positrons will diffuse to the grain boundary region and get trapped by vacancy defects. Thus the effective grain boundary region seen by positrons is magnified, and the sensitivity of positrons to the interfacial defects is greatly enhanced. Schaefer et al. [23] first reported positron annihilation study of the defects in Fe nanocrystals. Later several works were published about PAS studies in various nanomaterials [24–27]. In this paper, we studied the effect of thermal treatment on the interfacial defects in BTO nanocrystals. Positron lifetime, CDB, X-ray diffraction (XRD) and Raman spectroscopy were performed to get a comprehensive understanding of the defect characteristics. Correlation between the interfacial defects and the ferromagnetism in these BTO nanocrystals was observed, and the origin of magnetization will be discussed.
2. Experiment Samples for this study were prepared from commercially available high-purity BTO (grain size ∼50 nm, purity ∼99.9%) nanopowders. The powders were hand milled in agate mortar with pestle for 2 h. After hand milling, the powders were pressed into pellets under a static pressure of about 6 MPa for 1 min at room temperature. The pellet samples were in disk shape having a diameter of 15 mm and a thickness of 1–2 mm. They were subsequently annealed in open air at 12 different temperatures ranging from 100 to 1200 ◦ C for 2 h in an electric furnace. The 22 Na positron source was prepared by depositing 22 NaCl onto a Ni thin film with a thickness of about 3 m, and was sandwiched between two identical plane-faced pellets. Positron lifetime measurements were performed using a conventional fast-fast coincidence system with a time resolution of about 280 ps in full width at half maximum (FWHM). The source activity was ∼5 Ci, and the count rate was about 40 cps. More than 106 total counts were accumulated in each measurement. The lifetime spectra were analyzed by the computer program PATFIT [28]. The contribution of positrons annihilating within the source and the foil was determined to be 530 ps with an intensity of 2% using a defect free silicon single crystal. Coincidence Doppler broadening spectra were measured using two high purity (HP)-Ge detectors with energy resolutions of about 1.76 and 1.64 keV (FWHM) at 1.33 MeV, respectively. At least 4 × 106 total counts were collected in each spectrum. The spectrum was characterized by the S and W parameter which is defined as the ratio of low momentum (|PL | < 0.68 keV) and high
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30
40
50
60
70
Fig. 1. XRD patterns for the 100, 400, 700, 1000 and 1200 ◦ C annealed BTO nanocrystals.
momentum (2.86 keV < | PL | < 5.73 keV) region to the total region of the spectrum, respectively. The positron lifetime and CDB spectra were measured simultaneously at room temperature. X-ray diffraction measurements were performed using Cu K˛ radiation (Bruker D8 Advance) with a Ni filter. The scanning rate was 1◦ /min with a step of 0.02◦ . Raman spectra excited with the 515 nm line of an Ar+ laser were recorded in the backscattering geometry with a power level of about 4.8 mW arriving at the sample surface using a Renishaw confocal micro-Raman spectroscopy RM-1000. The wavenumber of Raman shift ranged from 100 to 800 cm−1 . The magnetization behaviors of the BTO nanocrystals annealed at different temperatures were investigated by a physical properties measurement system (quantum design). All the above measurements were carried out at room temperature. 3. Results and Discussion 3.1. XRD characterization To study the grain size and the crystalline phase of the nanopowders, five samples were subjected to X-ray diffraction measurements. The samples selected were 100, 400, 700,1000 and 1200 ◦ C annealed BTO. The XRD patterns are shown in Fig. 1. For each sample, all the observed peaks can be indexed with the tetragonal phase of BaTiO3 (JCPDS cards No. 05-0626). The BTO samples exhibit good crystallinity with no obvious indication for the existence of amorphous domains. In the annealing temperature interval between 100 and 1200 ◦ C, no phase transition was observed. XRD patterns confirmed that all the samples were single phase with a tetragonal structure. In addition, the intensity of the peaks increases and the FWHM decreases with the increasing annealing temperature, which indicates a possible change in the grain size. The average grain size of the annealed BTO samples is calculated by Scherrer’s formula:[29] Dh k l =
K , ˇ cos
(1)
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Fig. 4. Average positron lifetime as a function of annealing temperature for BTO nanocrystals. Fig. 2. Variation in the average grain size with annealing temperature for BTO nanocrystals.
where Dh k l is the average grain size perpendicular to the (h k l) plane, K is the shape factor (usually taken as 0.89), is taken as the X-ray wavelength of Cu K˛1 radiation ( = 0.15418 nm), ˇ is the FWHM of the XRD peak (every observed peak in the spectra was fitted with a Gaussian function), and is the Bragg angle. Standard method to deduct the contribution of instrumental broadening in FWHM has been taken into account [29]. The calculated BTO grain size as a function of annealing temperature is shown in Fig. 2. It can be seen that obvious grain growth occurs only above 600 ◦ C and the fast growth starts above 900 ◦ C. The average grain size of the sample annealed below 600 ◦ C was estimated to be around 44 nm. With increasing annealing temperature, it increases continuously and reaches up to about 82 nm at 1200 ◦ C. 3.2. Raman scattering measurement Raman spectroscopy is one of the useful methods for investigating phase transformation behavior. There are Raman active lattice vibration modes in the tetragonal BaTiO3 (P 4mm), while there is no Raman active mode in the cubic BaTiO3 (Pm 3m) [30]. The Raman peaks in tetragonal BaTiO3 powder are at 720 cm−1 [E(4LO) + A1 (3LO)], 515 cm−1 [E(4TO) + A1 (3TO)], 305 cm−1 [E(3TO) + E(2LO) + B1 ], 260 cm−1 [A1 (2TO)], and 185 cm−1 [E(2TO) + E(1LO) +A1 (1TO) + A1 (1LO)],[31] where the lattice vibration modes are shown in parentheses. Using the present Raman spectroscopy apparatus, however, the peak at 185 cm−1 was not observed, because the Raman scattering light in the range from 0 to 250 cm−1 is attenuated owing to a notch filter which cuts off the Rayleigh scattering light.
Fig. 3. Evolution of Raman spectra as a function of annealing temperature for BTO nanocrystals.
Fig. 3 shows the evolution of the Raman spectra measured for BTO samples annealed at various temperatures. In the samples heated at 100, 400, 700, 1000 and 1200 ◦ C, the tetragonal peaks of 720, 515, 305 and 260 cm−1 were confirmed. Hayashi et al. [32] reported that a peak at around 310 cm−1 appeared below the Curie point and vanished above the Curie point in BaTiO3 ceramics, suggesting that the peak at 305 cm−1 is an intrinsic peak for tetragonal BaTiO3 . Our BTO samples annealed at various temperatures show a slight peak at 305 cm−1 , indicating that all the samples are single phase with a tetragonal structure. 3.3. Positron annihilation measurements A detailed study of the microstructural defects in the BTO nanocrystals was performed by positron lifetime measurements. Every lifetime spectrum was evaluated to two lifetime components. In the low temperature annealed sample, the short lifetime component 1 is around 182 ps, while the second lifetime 2 has a value of about 300 ps. Theoretical calculation predicted a bulk positron lifetime of 152 ps for BaTiO3 , 293 ps for VBa , 204 ps for VTi , and 162 ps for VO , respectively [33]. Oxygen vacancies are believed to be effective positron traps in other perovskites [34–36]. Therefore, 1 might be an average value of free positron lifetime and the lifetime in smaller cation vacancies like VTi or VO . It might be also due to the saturated trapping of positrons in some vacancies. The second lifetime component 2 is probably attributed to VBa –VO divacancies or larger vacancy clusters. All the defects revealed by 1 and 2 are most probably located in the grain boundary region, which are called interfacial defects in this study. Due to the complexity of the defects in our BTO nanocrystals, decomposition result of the positron lifetime spectra might be not so reliable. Therefore we need to rely on the average positron lifetime. The variation of average positron lifetime av as a function of the annealing temperature for BTO nanocrystals is presented in Fig. 4. It is seen that with the increase of annealing temperature, the average lifetime av keeps constant at around 245 ps up to annealing temperature of 400 ◦ C, then it begins to decrease slowly with temperature increasing up to 700 ◦ C, subsequently it decreases rapidly above 700 ◦ C. Thus it can be inferred that the interfacial defects in the grain boundary region begin to recover above 400 ◦ C, and they disappear rapidly above 700 ◦ C, and most of them may have been removed at 1200 ◦ C. However, even after annealing at 1200 ◦ C, the average positron lifetime is still higher than the bulk lifetime. This suggests that some interfacial defects still remain after high temperature annealing. In general, due to the tendency to form agglomerates, nanosize particles can lead to microstructures with defects that cannot be eliminated during sintering [37].
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Fig. 5. The Doppler broadening S-parameter as a function of annealing temperature for BTO nanocrystals.
The variation of S-parameter with the annealing temperature, shown in Fig. 5, further supports above arguments. It is clear from Fig. 5 that the S-parameter decreases slowly from the annealing temperature 400–700 ◦ C, and after that it decreases rapidly. The variation of S-parameter with annealing temperature gives a preliminary idea of the changing defective state of the sample. In general, the decrease of S-parameter is a signature of lower annihilation probability of positrons with the lower momentum valence electrons. This occurs when the relative concentrations of defects decrease in the material. Therefore the same conclusion about the defect recovery process can be drawn from the Doppler broadening measurements, i.e, the defects have a rapid recovery after annealing at above 700 ◦ C. To identify the different positron trapping sites, the correlation between S- and W-parameters for the sample during annealing process is also studied. This plot can provide complementary information on the defects, and is usually used to verify the change of defect species or the chemical surrounding of the defects. The Sparameter versus W-parameter graph is drawn in Fig. 6. All the data points were concentrated on a straight line, suggesting that the defect species do not change during annealing. There is only a variation of their respective concentrations. 3.4. Ferromagnetic behavior Fig. 7 illustrates the magnetization versus applied magnetic field M–H curves of the BTO nanocrystals annealed at 100, 700 and 1200 ◦ C. Hysteresis loops can be observed clearly in the M–H curves of the BTO nanocrystals annealed at 100 and 700 ◦ C. Since there are no magnetic elements involved in the preparation of BTO nanoparticles and there was no contamination of magnetic materials during characterization, the ferromagnetism is intrinsic to the nanoparticles. Considering that the bulk BTO is diamag-
Fig. 6. The variation of S-parameter versus W-parameter for the nanocrystalline BTO after annealing at different temperatures.
Fig. 7. M–H curves for the BTO nanocrystals annealed at different temperatures. The diamagnetic contribution was subtracted for the samples annealed at 100 and 700 ◦ C.
netic, the room-temperature ferromagnetism is likely associated with the surface state of the BTO nanocrystal which has high surface to volume ratio. After deducting the diamagnetic signal of the bulk region of BTO samples, the saturation magnetization of the annealed samples changes slightly with the annealing temperature in the interval between 100 and 700 ◦ C. However, for the BTO nanocrystals annealed at 1200 ◦ C, no apparent ferromagnetism can be observed. The variation in ferromagnetism in annealed BTO nanocrystals is consistent with the recovery process of defects located at the grain boundary region indicated by the above positron lifetime results. After lower temperature annealing (<700 ◦ C), the interfacial defects remain little change, while the magnetic moment also shows only slight change. With increasing annealing temperature, gradual disappearance of the surface defects takes place. Simultaneously, a decrease of the magnetization is observed. After annealing at 1200 ◦ C, most surface defects are annealed out. At the same time, hysteresis loop cannot be observed in the magnetization versus field data within the resolution of the measurements. Therefore, the change in the interfacial defects is consistent with the variation in the magnetic behavior. In other words, the experimental results support the interfacial defects playing a role in the ferromagnetism. From our results it can be also inferred that a critical concentration of defects is necessary to induce the ferromagnetism in BTO sample. Though the annealing induces an increasing of grain size from 44 to 82 nm, which causes a decrease in the ratio of surface to bulk region, this is not the sole reason for the reduction in magnetization. Even after annealing at 1200 ◦ C, the grain size increases to only 82 nm, and the volume fraction of the surface and interface region is still about 8%. On the other hand, the grain size of 82 nm is still smaller than the positron diffusion length (generally on the order of 100 nm), Thus, all positrons probably diffuse towards the grain boundaries. In other words, the decrease in positron lifetime
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or Doppler broadening S-parameter after annealing is primarily due to the recovery of vacancy defects in the interface region. Mangalam et al. [15] still observed ferromagnetism in BaTiO3 nanoparticles with a size of 40–300 nm. This may suggest that the room temperature ferromagnetism of the BaTiO3 nanocrystals originates mainly from the surface and interfacial defects, rather than the surface state. Vacancy-induced magnetism in nonmagnetic ferroelectric BaTiO3 has been studied by first principle calculation [38,39]. According to their calculation, the magnetism is induced by the partial spin-polarized O 2p states around Ti vacancies and the partially filled d-states Ti around the oxygen vacancies. 4. Conclusion In conclusion, we have studied the effect of annealing on the interfacial defects in high purity BTO nanocrystals. The grain size shows rapid increase after annealing at above 900 ◦ C and reaches around 82 nm at 1200 ◦ C. In the annealing temperature interval between 100 and 1200 ◦ C, no phase transition was observed. XRD patterns confirm that all the samples are single phase with a tetragonal structure. Raman scattering measurements confirm the tetragonal peaks at 720, 515, 305 and 260 cm−1 . The BTO nanocrystals annealed at various temperatures show a slight peak at 305 cm−1 in Raman spectra, indicating that all the samples were tetragonal structure. The interfacial defects evidenced by positrons in the grain boundary region do not change after annealing at 100–400 ◦ C, while recovery of the vacancies takes place above 400 ◦ C and begins to disappear rapidly above 700 ◦ C. The magnetic measurements indicate that the 100 and 700 ◦ C annealed BTO samples clearly show ferromagnetism at room temperature whereas the 1200 ◦ C annealed BTO shows diamagnetic. The disappearance of ferromagnetism in BTO nanocrystals coincides with the disappearance of interfacial defects, suggesting that the surface defects in BTO nanoparticles are the most probable origin of the ferromagnetism. Our results clearly show a correlation between positron response and magnetic behavior, but perhaps not directly. The mechanism through which defects may induce ferromagnetism still requires further study. Acknowledgments This work was supported by the Program for New Century Excellent Talents in University, the National Natural Science Foundation of China under grant nos. 10875088, 11075120, and the Natural Science Foundation of Xianning University under grant no. ZX1025.
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