Probing the dielectric, piezoelectric and magnetic behavior of CoFe2O4/BNT-BT0.08 composite thin film fabricated by sol-gel and spin-coating methods

We investigated in this paper a novel bilayer composite obtained by sol-gel and spin coating of the ferroelectric 0.92Na0.5Bi0.5TiO3–0.08BaTiO3 (abbreviated as BNT-BT0.08) and ferromagnetic CoFe2O4 phases, for miniature low-frequency magnetic sensors and piezoelectric sensors. This heterostructure, deposited on Si-Pt substrate (Si-Pt/CoFe2O4/BNT-BT0.08), was characterized using selected method such as: X-ray diffraction, dielectric spectroscopy, piezoelectric force microscopy, SQUID magnetometry, atomic force microscopy/magnetic force microscopy, and advanced methods of transmission electron microscopy. CoFe2O4/BNT-BT0.08 ferromagnetic–piezoelectric thin films show good magnetization, dielectric constant and piezoelectric response. The results of analyses and measurements reveal that this heterostructure can have applications in high-performance magnetoelectric devices at room temperature.

In this work we have created a heterostructure thin film composed from piezoelectric and magnetic materials. We have investigated the composite thin film Si-Pt/CoFe 2 O 4 /BNT-BT 0.08 in which BNT-BT 0.08 and cobalt ferrite (CoFe 2 O 4 ) films were deposited in two subsequent steps on Si-Pt buffer, by sol-gel and spin-coating techniques. Cobalt ferrite was chosen because is an important component for multiferroic heterostructure thin films or composites due to its high coercivity, moderate magnetization and highest magnetostriction coefficient 20 . (Bi 0.5 Na 0.5 )TiO 3 (BNT) doped with BaTiO 3 (BT) is selected as ferroelectric layer because BNT-BT 0.08 is considered a good candidate to replace the lead-based piezoelectric materials 21,22 . It has been shown previously that the (1-x)BNT-xBT (BNT-BT x ) solid solution has in the compositional domain x = 0.06-0.10 a nearly morphotropic phase boundary (MPB) 23 with BNT ceramic, the BNT-BT 0.08 composition placed in the MPB provides improved poling and piezoelectric properties 23,25 .

Results and Discussion
Structural characterization of Si-Pt/CoFe 2 O 4 /BNT-BT 0.08 bilayer heterostructure. XRD spectra recorded in grazing incidence diffraction geometry of the Si-Pt/CoFe 2 O 4 /BNT-BT 0.08 heterostructure thin film are shown in Fig. 1.
The cross-section and plan-view images of the heterostructure thin film indicate a granular structure, specific for the thin films prepared by the sol-gel method, with grain of polyhedral shape and various sizes. As can be seen on Fig. 2(a,b), some pores on the surface are connected while the internal pores are in small quantities and not connected. Therefore, the porosity of the surface is much higher than internal porosity. No intermediate layer is observed and the interface between CoFe 2 O 4 and BNT-BT 0.08 layers is clear, which suggests that both single-phase CoFe 2 O 4 and BNT-BT 0.08 films can coexist without interface diffusion. Thus results are agreement with the XRD patterns.
As it results from BSE/SEM image in Fig. 2(a), the thickness of BNT-BT 0.08 layer is in the 150-180 nm range, while the thickness of CoFe 2 O 4 bottom layer is about 480-490 nm.
Cross-sectional transmission electron microscopy shows BNT-BT 0.08 top layer with thickness of 116 nm deposited on CoFe 2 O 4 thin film of 568 nm thickness ( Fig. 3(a)).
As can be seen in Fig. 4(a), the two chemically different layers (CoFe 2 O 4 and BNT-BT 0.08 ) are clearly distinguished from each other. Elemental mapping clearly shows the homogenous dispersion of Co, Fe, Bi, Na and Ti elements into their corresponding layers ( Fig. 4(c-g)).
Magnetic properties. The magnetic hysteresis loops of Si-Pt/CoFe 2 O 4 /BNT-BT 0.08 bilayer heterostructure ( Fig. 5(a,b)) were recorded at 5 K and 295 K, under a magnetic field of ≤ 70 kOe, perpendicular to the BNT-BT 0.08 layer plane.
The values of remnant magnetization (M r ), saturation magnetization (M s ), and coercive magnetic field (H c ) of the CoFe 2 O 4 /BNT-BT 0.08 thin film heterostructure, after diamagnetic corrections, are listed in Table 1.
As can see in Table 1, the magnetic parameters are higher for the CoFe 2 O 4 /BNT-BT 0.08 composite thin film because the CoFe 2 O 4 layer in this composite is thicker and has the crystallites larger than the CoFe 2 O 4 layer in the BNT-BT 0.08 /CoFe 2 O 4 composite structure 30 . These results are in good agreement with ref. 31 . In the CoFe 2 O 4 / BNT-BT 0.08 bilayered heterostructure, the thickness of CoFe 2 O 4 bottom layer is about 480-490 nm and crystallites size of ~65 nm while in the BNT-BT 0.08 /CoFe 2 O 4 composite thin film, the thickness of CoFe 2 O 4 layer is ~280 nm and crystallites size is ~20 nm 30 .
The low values of magnetization and coercive field, as well as the diamagnetic component of magnetization, indicate the behavior of a diluted magnetic oxide which may be due to Bi 0.5 Na 0.5 TiO 3 and BaTiO 3 (BNT-BT 0.08 ) which are considered as diluted magnetic oxide 32 and also, due of a very small contributions of the oxides from substrate. As we mentioned, the substrate is made of Si/SiO 2 (450 nm)/TiO 2 (15 nm)/Pt(100 nm); TiO 2 being recognized as diluted magnetic oxide 33 , like also SiO 2 34 .
The different component phases of the heterostructures, their crystallographic parameters, the thickness ratio between the magnetic and piezoelectric phases, the stress condition and the nature of the substrate are factors Dielectric spectroscopy, tunability and leakage current measurements. Figure 7 shows results of the dielectric spectroscopy measurements obtained for the bilayered heterostructure CoFe 2 O 4 /BNT-BT 0.08 ( Fig. 7(a,b)). The two different curves correspond to different capacitors (A and B). As can be seen in Fig. 7(a), the dielectric constant increases at low frequency and remains constant at higher frequency.
One must take into account the heterostructure type of the samples, with a layer of CoFe 2 O 4 deposited over the ferroelectric layer BNT-BT 0.08 . The dielectric measurements are performed on the capacitor between the bottom electrode (Pt) and the top electrode (Au foil) applied over the CoFe 2 O 4 layer. Therefore the measurements are carried out over a very small capacitance (CoFe 2 O 4 ) in series with a high capacitance (BNT-BT 0.08 ) which results in an equivalent capacitance smaller that of CoFe 2 O 4 layer. This explains the small dielectric constant of the heterostructure. CoFe 2 O 4 /BNT-BT 0.08 structure shows high dielectric constant and high ac conductivity at low frequency (below 1 kHz) which indicates a strong contribution of relaxation processes associated with conductivity (due to oxygen vacancies and other defects) in this sample. Variation of the dielectric properties with frequency of the heterostructure thin film can be due to an extrinsic behavior resulting from the microstructure deficiency.  The deviation from flat conductivity at low-frequency is also due to the interface between the layers and the electrodes. In ref. 8 , it has been reported that with increasing CoFe 2 O 4 film thickness, the frequency dependence of dielectric constant becomes gradually stronger, due to Maxwell-Wagner polarization mechanism 36,37 . In multiphase composites, Maxwell-Wagner polarization at the interface of ferroelectric-ferromagnetic phases would lead to strong dependence of dielectric constant especially at low frequency 38 .
Measurements of the dielectric permittivity of samples under a dc bias electric field E yield the tunability factor (n r ), according to the formula:  Fig. 7(c). It can be observed that the CoFe 2 O 4 / BNT-BT 0.08 heterostructure displays a tunability of 23% at 1 kHz and 100 kV/cm. This value decreases strongly when the frequency increases. This decreasing is associated with the strong dependence of dielectric constant on frequency.
Leakage current density is a key factor for multiphase composite thick film and reflects the quality and reliability of a film. Therefore, it is important to investigate the leakage property and conduction mechanisms of the films. Leakage current measurements for the CoFe 2 O 4 /BNT-BT 0.08 bilayer composite have been made at ambient temperature. The results are shown in Fig. 8(a,b).
It can be observed from Fig. 8(a) that the leakage current density shows symmetrical positive and negative arms. Leakage current density has the lowest value of about 1.5·10 -4 A/cm 2 at low electric field and shows a strong increase with electric field. The highest value of the current density measured on Si-Pt/CoFe 2 O 4 /BNT-BT 0.08 heterostructure thin film, at an electric field of 200 kV/cm, was 4.0·10 -1 A/cm 2 (Fig. 8(a)). The leakage current is mainly attributed to conduction through grain boundaries in the BNT-BT 0.08 layer 39 .
The parameters of the J (E) dependence law can give information about the possible conduction mechanisms in the composite structure. Thus a dependence of the type: 2 is characteristic for a space charge-limited conduction (SCLC) mechanism 40,41 . Alternatively, a dependence of the type: is characteristic for a Schottky-type conduction mechanism 40,42 , related to the potential barrier created by the different Fermi levels of dielectric and metal. In equation (3), Φ is the height of the Schottky barrier height, a is a material constant and k is the Boltzmann constant 40,41 . Thus by plotting the obtained data as a function of E 2 or E 1/2 (on a semilogarithmic scale) it is helpful to discern among the different possible conduction mechanisms. For the CoFe 2 O 4 /BNT-BT 0.08 heterostructure the plot of J vs E 2 (not shown here) clearly showed that this law is not obeyed for any electric field range, therefore a SCLC mechanism was ruled out. Instead, as can be observed from Fig. 8(b), a plot of J (in log scale) vs E 1/2 clearly shows that the leakage current is controlled by a Schottky barrier-type mechanism in almost all the field range. It is concluded that the leakage mechanism found for the investigated hetereostructure points to the dominant role of the interfaces between the CoFe 2 O 4 and NBT-BT 0.08 and between these layers and the hybrid electrodes in the leakage current measurements.
Piezoelectric properties. The ferroelectric behavior of the bilayered heterostructure CoFe 2 O 4 /BNT-BT 0.08 was investigated by piezoelectric force microscopy (PFM) and their out-of-plane piezoelectric response is presented in Figs 9 and 10.
In the piezoelectric images, domains with opposite polarities exhibit different phase contrast. The CoFe 2 O 4 / BNT-BT 0.08 composite thin film exhibits good piezoresponse over a large area ( Fig. 9(b,c)). From the cross-section analysis, which simultaneously investigates the surface morphology (grain size) (Fig. 9(d)) and domain populations (domain size and response signal) (Fig. 9(e)), it can be seen that a majority of the domains, randomly distributed in the composite, have dimensions smaller than the grain size, suggesting a ferroelectric multidomains structure for the grains. Figure 10(a,b) reveal typical remnant piezoresponse amplitude and phase measurements, respectively, recorded as a function of applied dc voltage. The polarization switching and hysteresis show clearly that the Si-Pt/CoFe 2 O 4 /BNT-BT 0.08 bilayer thin film is ferroelectric at nanoscale level.
In summary, bilayer composite CoFe 2 O 4 /BNT-BT 0.08 was deposited on Si-Pt substrate by spin-coating technique. This heterostructure thin film presented CoFe 2 O 4 cubic phase and BNT-BT 0.08 phase crystallized on the lattices of rhombohedral and tetragonal Bi 0.5 Na 0.   BNT-BT 0.08 precursor gel was prepared using the following strategy: first, was prepared a mixture of sodium, bismuth and barium acetic solutions and separately, a mixed solution of titanium isopropoxide: isopropanol: acetylacetone in the volume ration 1:2:0.5. The complex solution of Na, Bi and Ba was added to the titanium isopropoxide solution under continuous magnetic stirring and heating at 85 °C, and a sol precursor of BNT-BT 0.08 was prepared. Acetylacetone is used as stabilizer for the sol while formamide (N, N-dimethylformamide, Aldrich) was used in order to to avoid formation of the cracks in BNT-BT 0.08 gel film during the thermal treatments of drying and crystallization. The films of CoFe 2 O 4 and BNT-BT 0.08 were deposited by spin casting at 3000 rpm for 20 seconds. After spin coating, the thin films were heated at 200 °C for 2 min to evaporate the solvent, and then at 400 °C for 4 min in air to eliminate organic components. The films were prepared by repeating (10 times) the deposition and pyrolysis cycle. The coated films of CoFe 2 O 4 and BNT-BT 0.08 were annealed at 700 °C for 1 h, in air, in order to crystallize CoFe 2 O 4 and at 800 °C for 5 min, in oxygen to crystallize the layer of BNT-BT 0.08 . Thus, the final product is a Si-Pt/CoFe 2 O 4 /BNT-BT 0.08 heterostructure thin film. The structure of the heterostructure thin film was investigated by using a Bruker-AXS tip D8 ADVANCE diffractometer while the microstructure was investigated with a FEI QUANTA INSPECT F scanning electron microscope (SEM) with field emission gun and a Tecnai TM G 2 F30 S-TWIN transmission electron microscope (TEM) with a line-resolution of 1 Å, in high resolution transmission electron microscopy (HRTEM) mode. The crystalline structure of the samples was investigated also the selected area electron diffraction (SAED) method. A Titan Themis 200, transmission and scanning transmission electron microscope (S/TEM) was used for analysis of the chemical elements present in the samples. The magnetic hysteresis loops of the bilayer composite were recorded using a Quantum Design MPMS-5S SQUID magnetometer, in the temperature range of 5-355 K and an applied magnetic field of ≤3580 kA/m. The out-of-plane magnetic characteristics of the samples were observed with an atomic force microscope/magnetic force microscope (AFM/MFM), Model 5400, from Agilent Technology. Measurements of the capacitance and dielectric loss have been carried out with an HP 4194 A impedance bridge under an electric bias field with variable amplitude. A Radiant Technologies RT-66A system has been employed for measurements of leakage currents on the same heterostructure samples. PFM measurement of local piezoresponse was performed using a sequence of DC switching bias from −3 V to +3 V, superimposed on a AC modulation bias with a frequency of 14 kHz, using a PFM tip directly on the heterostructure surface.