II. General Backgrounds
2.2.3 Electric Properties
BFO also has interisting electric properties, especially its dielectric properties. To understand dielectric properties, polarization should be defined.
When a dielectric is placed in an electric field, electric charge do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium position causing dielectric polarization.
Figure 11. Types of electric polarization [19].
Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric it self. If
17
a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axis aligns to the field. There are four kind of polarization, electronic polarization, ionic polarization, dipolar polarization and space charge polarization as depicted schematically in Fig. 11. A dielectric material is an electrical insulator that can be polarized by an applied field. In BFO we could find two kinds of polarization. Namely, electronic polarization, due to lone pair of electrons in bismuth atom and ionic polarization originated from primitive rombohedral structure in perovskite structure crystal of BFO.
Dielectric properties also related to capacitance measurement.
Capacitance can be calculated if the geometry of the conductors and the dielectric properties of the insulator between the conductors are known. A qualitative explanation for this can be given as follows. As a quantitative example consider the capacitance of a parallel plate capacitor constructed by two parallel plates both of area and separated by a distance d:
(3)
Here εr is the relative static permitivity and ε0 is the permitivity of vacuum (ε0 ≈ 8.854×10−12 F m–1). BFO is an electrical insulator, the electric polarization of BFO, can be evaluated by dielectric and capacitance measurements. Because BFO is electrical insulator, so that electric resistance for BFO is very high.
2.2.4 BFO and CFO Properties in Below Temperaure
Generally, antiferromagnetic order may exist at suffeciently low temperature, vanishing at and above a certain temperature, or the Nẻel temperature TN. Above the Nẻel temperature, the material is typically paramagnetic. Neel temperature or magnetic ordering temperature is the temperature above which an antiferromagnetic or ferrimagnetic material become paramagnetic, that is the thermal energy beocmes large enough to destroy
18
macroscopic magnetic ordering within the material. The Nẻel temperature is analogous to the Curie temperature for magnetic material.
Figure 12. M-T measurement for BFO powder single crystal [7].
For BFO, although TN=653 K has been clearly indentified, however, as shown in Fig. 12, there is magnetic phase transition observed at low temperatures. The magnetic phase transition is related to the antiferromagnetic domain pinning effect. Increasing magnetic field will increase allignment of momen magnetic, so that the magnetic phase transition was shifted to lower temperatures because of the antiferromagnetic domain pinning effect was changed.
In addition to those observed in the M-T curve, magnon measurement also found anomalies at usual temperature. Figure 13 shows the Raman measurement of BFO which clearly displays phase transitions at ~140 K and ~200 K. The origin of which is as yet unclear but has been tentatively attributed to spin reorientations. The above right figure indicates that magnon linewidth narrows near 140 K, suggesting the features of "critical slowing down" of spin fluctuations. The cross section divergence cannot come from impurities.
Moreover, as shows in the lower right figure of Fig. 13, preliminary electron paramagnetic resonance measurements also shows clear anomalies at 140 K and 200 K.
0.1T
1T
19
Figure 13. Intensity of magnon peaks in the Raman spectra as a function of temperature [20].
Dielectric measurement also displays similar low temperature anomalies in BFO. BFO is piezoelectric at all temperatures below 1100 K, any magnetoelastic phenomena at its magnetic-phase transitions are apt to create responses in the dielectric response. In Figure 14, the subtle low-temperature anomalies at 215 K, 140 K and 55 K coincide with the temperatures where magnetic, magneto-optic and elastic anomalies have been seen. Nevertheles, none of the dielectric anomalies is strong and, curiously, none seems to affect the dielectric loss.
Figure 14. dielectric measumerent from BFO [4].
Anomalies in the relative dielectric constant (ε), is possibly due to coupling to magnetic (or magnetoelastic) transitions at low temperature. The anomalies
20
do not seem to affect the dielectric loss (tan δ) [4]. Their weakness shows that they do not correspond to ferroelectric phase transitions, but arises instead from weak coupling to another order parameter, most likely magnetic. Additional dielectric and conductivity anomalies ware reported [21] at TN=643 K (370 oC), clearly related to magnetoelectric coupling, and magnetodielectric coupling is also responsible for the reported anomaly in the birefringence of BFO at TN
[22]. Another anomaly was reported at mysterious transition at 458 K, although this dielectric anomaly may it self be an artifact caused by the change in resistivity [9, 22-24].
Resistance measurement in the temperature regime also reveals some aspects of magnetoelectric properties. As shown in Figure 15, the resistance for BFO displays a clear insulating beharvior as function of temperature. However, under external applied magnetic field, magnetoresistance at low temperatures is clearly revealed, indicating possible magnetoelectric coupling at low temperature.
Figure 15. Magnetotransport study on BFO 109o domain Wall. Resistance - temperature curves at two different external magnetic fields, 8 T (red) and 0 T (blue) and the corresponding magnetoresistance (green) [25].
More detailed analysis on the data shown in Fig. 15, indicated that the MR is directly related to the preferential transport parallel to the ferroelectric domain
21
walls. The temperature - dependent resistance and I-V behavior of similar devices were also measured under two different magnetic fields, 0 T and 8 T (blue and red curves in Figure 15), respectively, the inset shows linear I-V plots with and without the magnetic field. Negative MR is only observed for temperatures below the transition temperature (~200 K), which suggests that magnetic interactions are likely to play a key role in influencing the observed transport behavior. Moreover, below 40 K, the magnitude of MR gets significantly larger, from ~20% at ~100 K to ~60% at ~10 K [25].
From the above discussion it is apparent that substential magnetoelectric coupling exist in BFO at low temperatures. Thus, combination BFO with ferromagnetic CFO interisting to observed, especially when both are in nanoscale. However, before we explore the possible coupling between antiferromagnetic-ferroelectric BFO and ferromagnetic CFO, we first introduce briefly properties of CFO it self.
Figure 16, shows the zero-field-cooled and field-cooled (ZFC/FC) magnetization curves of the as-prepared CoFe2O4 samples measured at temperatures between 10 K and 330 K with an applied field of 100 Oe.
Figure 16. Zero-field-cooled (ZFC) and field-cooled (FC) curves for the as-synthesized CoFe2O4 nanoparticles under an applied magnetic field of 100 Oe [26].
22
As the temperature rises from 10 K to 330 K, the ZFC magnetization increases first and then decreases after reaching a maximum at 240 K, which is correspond to the blocking temperature (TB). This result further proves that the CoFe2O4 displays a superparamagnetic behavior at room temperature [26].
Whereas the FC magnetization decreased endlessly as the temperature increased. It is argued that the difference between ZFC magnetization and FC magnetization below TB is caused by energy barriers of the magnetic anisotropy [27].
From the interisting properties observed in BFO and CFO in the below room temperature regime, it will be the key to explore the possible of magnetoelectric and ferromagnetic-antiferromagnetic coupling that might be occur when combining the two materials with designed structures.
23
III. Experiment
3.1 Solid State Reaction
In this research, solid state reaction was used to make BFO and CFO bulk target and these bulk target ware used to make BFO-CFO thin film. To make BFO and CFO target from BFO and CFO powder by solid state reaction, precise calculation of chemical reaction of each basic material should be done. sintered at 600oC for 4h thus continued by 880oC for 480 second. Do grinding process again, thus sample ware pressed uniaxially (500 Mpa) into pellets and annealed in furnace at 600oC for 12 hours. For CFO, calculation was done similarly as that used to get BiFeO3 powder. However, in this case CoO and Fe2O3 powder ware used to prepare CoFe2O4 powder, the powder was dried and
24
ground, then calcined at 1000oC for 12 hours. Finally, it was pressed into pellets and annealed at 1000oC for 12 hours.
The XRD result shown in Fig. 17 (a) and 17 (b) evidently confirmed that the target obtained by the processes describe above were indeed of single phase materials with correct stoichiometry.
Figure17. BiFeO3 and CoFe2O4 XRD pattern.
It is noted that heat treatment process is very important in this experiment.
Although the temperatures were chosen according to the phase diagram and
25
parameters reported in the literature, we note that in order to get good quality samples, environmental conditions should be considered as well.
3.2 Pulse Laser Depositition
The set up of the PLD system is schematically illustrated in Fig. 18.
Briefly, it contains a laser generation system and a vacuum chamber with substrate and target holders.
Figure18. Schematic of PLD experiment set up.
The vacuum chamber should be able to reach a base preseure of 10-7 mT which is acheive by sequential pumping using scroll pump and turbo pump. A gauges and oksigen pipe were equiped to control pressure inside the chamber. In this study, the BFO-CFO film on STO substrate were deposited at an oxigen pressure 50 mT to 200 mT depending on what surface desired and how thick the film will be both substrate temperature and deposition time were further optimazed. The chamber also has heater and thermocouple to control the temperature used to make samples.
26
In further controls on the deposition process were accomplished by tuning the laser pulse energy, repetition rate direction and focus of the laser beam with lenses direct to chamber. Beside the main conditions in PLD system, we can also control the rotation of target and the distance between substrate and target to optimize the conditions for making good quality samples. Distance between target-substrate will affect the incident adatom from target coming to substrate.
If closer distance was predicted the incident adatoms are more abundant than that obtained if larger target-substrate distance was used within the same deposition time is rich than far distance in the same deposition time.
For BFO-CFO vertically alligned nanocomposite fabrication, two targets were used simultaneously. There are several ways to make BFO-CFO vertically alligned nancomposite. The popular way is to use single target made BCFO mixtures. BCFO target is a combination of BFO powder and CFO powder, where there is no chemical reaction between mixed BFO and CFO. Such that, BFO and CFO particles will be separated during deposition process and self assembled into nanocomposite. Howerver, in order to gain more control on the microstructure, in this reseach we used two targets to make BFO-CFO nanocomposites.
The deposition time for each target is important in fabrication of BFO-CFO vertically alligned nanocomposite by using two targets. In this case,we need to know how much deposition time is needed to build single three-dimensional islands first. Because, further growth of island will result in nanopillars embedded in matrix until nanocomposite pattern is accomplished. For BFO-CFO on STO (001), BFO-CFO would be pillar, so in that case we need to consider the duration of deposition time to make the first single CFO islands. After CFO was deposited, the BFO deposited on the STO (001) substrate will form a continous matrix because of the smaller lattice mismatch between them.
27
Fig. 19. Deposition step and time duration selected for forming BFO-CFO vertically aligned nanocomposite.
In this case, a 5-15 second deposition time combination is appropriate with total a deposition time of 30 minute. It was found that if the deposition time of CFO step is longer than 15 second, pillars cannot be constructed and film tend to collapse into layer. The 30 minutes total deposition time correspond to 180 time cycle for each target if 10 second for each target is used.
3.3 Characterization
The microstructure and physical properties of the obtained samples were characterized by XRD, scanning electron microscope (SEM), superconducting quantum interference device (SQUID), and impedance analyzer or (LCR meter), respectively.
3.3.1 X-ray Diffraction (XRD)
XRD characterization is due essential to identify the phases formed in the obtained films. The Bragg’s law gives the angles for coherent and incoherent scattering from a crystal lattice, which describes the relations between the wavelength (λ),diffraction angles (θ), and the lattice spacing between certain planes of the crystalline phases inside the film by the following relation :
2𝑑 𝑠𝑖𝑛𝜃 = 𝑛 𝜆 (4)
28
Figure 20. Schematic of the Bragg’s law.
Thus, from the diffraction peak, one can confirm the crystal structure of the material in study. In this research, XRD also used to investigate strain effect in the sample. BFO, CFO and STO have different lattice constants, resulting in strain effect among them when they are in proximity. Especially in the case of BFO-CFO on STO VAN, the BFO has a compressive in-plane strain due to the lattice mismatch with STO substrate. In addition, BFO also has a compressive strain along out-of plane direction due to the lattice mismatch with the CFO.
Strain effect in BFO due to lattice mismatch with STO and CFO makes the XRD peaks of BFO in BFO-CFO/STO VAN thin film shift as compared to that of the BFO powder.
Lattice constant of BFO was change because those strain effect which make X-ray diffraction also change. According to Bragg’s law, angle of reflection beam (θ) will change if spacing between diffracting planes (d) was changed. where d is also indicated as a lattice constant in the sample. So, when lattice constant of BFO was changed to higher or lower value caused lattice mismatch among BFO, CFO and STO, so angle of reflection beam (θ) also will be changed which will be evidence by shift of the BFO peak in diffraction pattern of XRD result.
29
3.3.2 Scannng Electron Microscope (SEM)
SEM was used to identify sample microstructure. In this research SEM was also used to reveal both the surface and cross sectional structures, especially for BFO-CFO on STO VAN to indicate nanocomposite structure. SEM is a type of electron microscope to take an images from the sample surface by scanning it with a high-energy beam of electrons. In SEM, electrons are emitted from a tungsten filament cathode and accelerated towards an anode. There are lenses to focus the beam to have spot radius from 0.4 µm to 5 µm. The electron beam will interact with the sample and its interaction volume can extend from less than 100 nm to around 5 μm into the surface. Pairs of scanning coils deflect the beam horizontally and vertically make a rectangular area scan in the sample surface.
Figure 21. Schematic illustration of a SEM (rotated by 90o counter-clockwise) [28].
Imaging can be achieved through the detection of low energy (< 50 eV) secondary electrons, or backscattered electrons. The latter consist of high-energy electrons originating in the electron beam, that are reflected or back-scattered out of the specimen interaction volume. Backback-scattered electron imaging is useful for distinguishing one material from another, since the yield of the collected backscattered electrons increases monotonically with the specimens atomic number. Backscatter imaging can distinguish elements with atomic number differences of at least three, i.e., materials with atomic number
30
differences of at least three would appear with good contrast on the image.
Because these electrons are emitted from a depth in the sample, the resolution in the image is not as good as that obtained from the secondary electrons, with a beam resolution ranging from 10 to 20 nm [28].
3.3.3 Atomic Force Microscopy (AFM)
AFM is a scanning probe microscope tool to image and measure a material in nanoscale. In AFM consist of a microscale cantilever with an atomically sharp tip, used to scan material surface. The tip could produce a Coulomb repulsive force from deflection of the cantilever according to Hooke’s law.
Then, it will effect to laser spot reflecting in photodiodes. To maintain a constant tip-to-sample distance, a feedback loop is implemented between the photodetector and the cantilever. The cantilever is maintained fixed in a tip holder, and the sample placed on top of a piezoelectric column that can move in the z direction for height adjusting and in the x, y directions for surface scanning [28].
Figure 22. Schematic of an AFM.
Two scanning modes, namely contact mode and resonace mode are commonly used in AFM operation. In the contact mode, the force between the tip and the sample is kept constant by the feedback loop by maintaining a
31
constant cantilever deflection. While in resonance mode, the cantilever is driven to oscillate near its resonance frequency by a small piezoelectric element mounted in the tip holder; the oscillation amplitude is modified as a result of tip-sample forces, so that the reflected laser beam is deflected in a regular pattern over the photodiode array, generating a sinusoidal electronic signal which is modified by the oscillation amplitude variation.
Depending on the height of the tip above the sample, different forces such as electrostatic interaction, magnetic forces, Van Der Waals attraction, water adhesion and Coulomb repulsion can play a role. If the cantilever enter intermittently into contact with the surface, the technique is known as tapping mode; otherwise, it is referred to as non-contact mode.
3.3.4 Superconducting Quantum Interference Device (SQUID)
SQUID has been widely used to measure magnetic properties of material.
The SQUID devices is due to the Josephson effect exhibiting in two parallel Josephson junctions consisting of two superconductors separated by a thin insulating layer. The device allows a change of magnetic field associated by one flux quantum to be measured. The flux which associated by Josephson junctions is quantized in units of flux quantum ϕo=2.07x10-15 T.m2.
Figure 23. Schematic illustration of a SQUID [28].
32
SQUID is operated with a constant bias current, thus the measured voltage oscillates with the phase changes between the two junctions, which depends upon the change in the magnetic flux, as illustrated in Figure 23. Detecting this circulating current enables the use of the SQUID as a magnetometer.
3.3.5 Impedance analyzer (LCR meter)
LCR meter is an equipment to measure the inductance (L), capacitance (C) and resistance (R) in material. In this research, LCR meter was used to measure and analyze electric properties of BFO-CFO on STO substrate especially capacitance and resistance dependence on temperature. For capacitance measurement, two probe is enough to get capacitance value of the sample. DC volatge drop will build across the sample, thus temperature dependent capacitance of the sample could be obtained. For resistance measurements, both two probe and four probe methode were used.
33
IV. Results and Discussion
4.1 Structure Analysis
In this research, three kinds of BFO-CFO vertically align nanocomposite were fabricated according to substrate direction. As shown in the Fig. 24, the temperature suitable fo growing the BFO-CFO nanocomposite film are 600oC, 650oC, and 700oC. Below 600oC, BFO growth was not good, i.e. only amorphous film were obtained at T=550oC (Fig. 24). On the other hand, for temperature higher than 700oC, bismuth was very easy to evaporate, making the stochiometry of the sample unstable and may even leading the formation of impurity phases, such as Fe2O3 and Bi2O3.
Figure 24. XRD pattern of BFO thin film on STO (001) substrate with different temperature growth conditions.
Deposition condition, such as pressure, repetition rate, and pulse energy are also dependent on the temperature used. In general, with the same growth
34
pressure. The thickness of BFO also depend on repititon rate. In present experiment, pressure at around 100 mT-250 mT and repitition rate at 5 Hz-10
pressure. The thickness of BFO also depend on repititon rate. In present experiment, pressure at around 100 mT-250 mT and repitition rate at 5 Hz-10