Introduction

In recent years, the manufacture of composite materials from components with different macroscopic properties was studied extensively1,2,3,4,5. These composites have applications in electronic devices with novel distinct functionalities6,7,8. Oxide heterostructure thin films with electric and magnetic properties were prepared by various techniques, such as: sol-gel9, pulsed laser deposition10, rf sputtering11, tape-casting method12, etc. There are several reports on composites with electrical, ferroelectric and ferromagnetic behaviors, for example: ferroelectric-ferromagnetic composites (BiFeO3-CoFe2O413, nickel ferrite-PZT and manganite-PZT8, CoFe2O4-BaTiO314) and ferromagnetic-piezoelectric oxide heterostructures (La0.7Sr0.3MnO3-PbZr0.2Ti0.8O315, CoFe2O4–PZT10). There are few reports on lead free ferroelectric (Na0.5Bi0.5TiO3)–magnetostrictive (CoFe2O4) particulate composites16,17,18,19 but bilayer ferrite–piezoelectric composites of CoFe2O4 and BNT-BT0.08 have not been reported so far a heterostructure composed of a piezoelectric and a magnetic one.

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/CoFe2O4/BNT-BT0.08 in which BNT-BT0.08 and cobalt ferrite (CoFe2O4) 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 coefficient20. (Bi0.5Na0.5)TiO3 (BNT) doped with BaTiO3 (BT) is selected as ferroelectric layer because BNT-BT0.08 is considered a good candidate to replace the lead-based piezoelectric materials21,22. It has been shown previously that the (1-x)BNT-xBT (BNT-BTx) solid solution has in the compositional domain x = 0.06–0.10 a nearly morphotropic phase boundary (MPB)23,24 where rhombohedral Bi0.5Na0.5TiO3 and tetragonal BaTiO3 phases coexist. Compared with BNT ceramic, the BNT-BT0.08 composition placed in the MPB provides improved poling and piezoelectric properties23,25.

Results and Discussion

Structural characterization of Si-Pt/CoFe2O4/BNT-BT0.08 bilayer heterostructure

XRD spectra recorded in grazing incidence diffraction geometry of the Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film are shown in Fig. 1.

Figure 1
figure 1

(a) XRD patterns of the Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film and (b) detail between 38° and 48°.

As expected, it can be observed that the sample is biphasic (CoFe2O4 and BNT-BT0.08). All the diffraction peaks could be indexed with a spinel structure (Fd3m space group) for CoFe2O4 phase (JCPDS: 04-005-7078)26 and a perovskite rhombohedral structure for Bi0.5Na0.5TiO3 rhombohedral phase (JCPDS: 04-015-0482)27, respectively.The rhombohedral phase of BNT-BT0.08 layer at room temperature, is characterized by (003)/(021) peak splitting between 39° and 41° and, a single (202) peak between 45° and 48°, as indicates the JCPDS: 04-015-048227 while BNT-BT0.08 tetragonal phase shows splitting of the (200) and (220) peaks at 2θ ~ 46.5° (JCPDS: 04-011-3919)28. Figure 2(b) shows a not symmetrical peak at 45°, that can be deconvoluted in the (200) and (220) peaks, suggesting the existence of BNT-BT0.08 tetragonal phase. This result is in good agreement with the paper of Xu et al.29, which reported that the MPB of BNT-BTx exists for 0.06 < x < 0.10.

Figure 2
figure 2

(a) Cross-section SEM image of Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film and a schematic representation inset; (b) SEM image of the composite film surface.

Figure 2 shows the SEM images obtained by backscattering for Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film. The surface of BNT-BT0.08 top film of Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure shows pores formed as a result of the removal the organic species during the calcination of the film gel. (Fig. 2(b)).

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 CoFe2O4 and BNT-BT0.08 layers is clear, which suggests that both single-phase CoFe2O4 and BNT-BT0.08 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-BT0.08 layer is in the 150–180 nm range, while the thickness of CoFe2O4 bottom layer is about 480–490 nm.

Cross-sectional transmission electron microscopy shows BNT-BT0.08 top layer with thickness of 116 nm deposited on CoFe2O4 thin film of 568 nm thickness (Fig. 3(a)).

Figure 3
figure 3

(a) TEM image of Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film, (b) HR-TEM image of BNT-BT0.08 layer and (c) HR-TEM image of CoFe2O4 layer.

The high resolution TEM images (Fig. 3(b,c)) confirm the existence of CoFe2O4 cubic phase (d = 2.53 Å and (311) plane) and Na0.5Bi0.5TiO3 rhombohedral phase (d = 2.75 Å corresponding to the (104) crystallographic plane).

High-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy (STEM) was also used in order to further confirm the bilayered structure of Si-Pt/CoFe2O4/BNT-BT0.08 composite (Fig. 4).

Figure 4
figure 4

STEM of Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film. (a) High-angle annular dark-field imaging (HAADF). (ch) Elemental maps of Co (c), Fe (d), Bi (e), Na (f), Ti (g) and O (h) for CoFe2O4/BNT-BT0.08 bilayered composite deposited on Pt (b) substrate.

As can be seen in Fig. 4(a), the two chemically different layers (CoFe2O4 and BNT-BT0.08) 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/CoFe2O4/BNT-BT0.08 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-BT0.08 layer plane.

Figure 5
figure 5

Magnetic hysteresis curves recorded at (a) 5 K and (b) 295 K for Si-Pt/CoFe2O4/BNT-BT0.08 layered composite.

The composite CoFe2O4/BNT-BT0.08 exhibits magnetic hysteresis loops at 5 K and 295 K (Fig. 5(ab)), indicating an ordered magnetic structure. The hysteresis loops show a negative slope with value of −2·10–3 (emu/cm3)/Oe, due to the diamagnetic signal.

The values of remnant magnetization (Mr), saturation magnetization (Ms), and coercive magnetic field (Hc) of the CoFe2O4/BNT-BT0.08 thin film heterostructure, after diamagnetic corrections, are listed in Table 1.

Table 1 Magnetic parameters of the CoFe2O4/BNT-BT0.08 bilayered heterostructure measured at 5 K and 295 K (results from the literature are reported as well, for comparison).

As can see in Table 1, the magnetic parameters are higher for the CoFe2O4/BNT-BT0.08 composite thin film because the CoFe2O4 layer in this composite is thicker and has the crystallites larger than the CoFe2O4 layer in the BNT-BT0.08/CoFe2O4 composite structure30. These results are in good agreement with ref.31. In the CoFe2O4/BNT-BT0.08 bilayered heterostructure, the thickness of CoFe2O4 bottom layer is about 480–490 nm and crystallites size of ~65 nm while in the BNT-BT0.08/CoFe2O4 composite thin film, the thickness of CoFe2O4 layer is ~280 nm and crystallites size is ~20 nm30.

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 Bi0.5Na0.5TiO3 and BaTiO3 (BNTBT0.08) which are considered as diluted magnetic oxide32 and also, due of a very small contributions of the oxides from substrate. As we mentioned, the substrate is made of Si/SiO2(450 nm)/TiO2(15 nm)/Pt(100 nm); TiO2 being recognized as diluted magnetic oxide33, like also SiO234.

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 that reduce the magnetic parameters of our heterostructure compared with ferromagnetic–piezoelectric bilayer composites previously reported10,33. CoFe2O4 is a hard magnetic material with high coercivity and moderate magnetization. BNT-BT0.08 layer deposited on CoFe2O4 influences the magnetization of the heterostructure by the residual strain which appears at the CoFe2O4/BNT-BT0.08 interface, as has been reported for CoFe2O4-PZT bilayers10,35.

The bright and dark contrast of ferromagnetic domains indicates opposite direction of the magnetization, perpendicular to the surface of thin film. As can be seen in Fig. 6, Si-Pt/CoFe2O4/BNT-BT0.08 bilayered heterostructure shows magnetic domain at grains boundary and aggregates boundary region. The MFM images reveal magnetic properties for the composite film Si-Pt/CoFe2O4/BNT-BT0.08 and, a nanoscale magnetic domain configuration.

Figure 6
figure 6

(a) AFM image and (b,c) MFM images: (b) amplitude and (c) phase for Si-Pt/CoFe2O4/BNT-BT0.08 layered heterostructure.

Dielectric spectroscopy, tunability and leakage current measurements

Figure 7 shows results of the dielectric spectroscopy measurements obtained for the bilayered heterostructure CoFe2O4/BNT-BT0.08 (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.

Figure 7
figure 7

(a) Dielectric constant, (b) ac conductivity and (c) relative tunability for the Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin films.

One must take into account the heterostructure type of the samples, with a layer of CoFe2O4 deposited over the ferroelectric layer BNT-BT0.08. The dielectric measurements are performed on the capacitor between the bottom electrode (Pt) and the top electrode (Au foil) applied over the CoFe2O4 layer. Therefore the measurements are carried out over a very small capacitance (CoFe2O4) in series with a high capacitance (BNT-BT0.08) which results in an equivalent capacitance smaller that of CoFe2O4 layer. This explains the small dielectric constant of the heterostructure.

CoFe2O4/BNT-BT0.08 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 CoFe2O4 film thickness, the frequency dependence of dielectric constant becomes gradually stronger, due to Maxwell-Wagner polarization mechanism36,37. In multiphase composites, Maxwell-Wagner polarization at the interface of ferroelectric-ferromagnetic phases would lead to strong dependence of dielectric constant especially at low frequency38.

Measurements of the dielectric permittivity of samples under a dc bias electric field E yield the tunability factor (nr), according to the formula:

$${n}_{r}=\frac{\varepsilon (0)-\varepsilon (E)}{\varepsilon (0)}$$
(1)

where ε(E) is the dielectric constant under bias and ε(0) is the dielectric constant without bias electric field. The results for the CoFe2O4/BNT-BT0.08 sample are shown in Fig. 7(c). It can be observed that the CoFe2O4/BNT-BT0.08 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 CoFe2O4/BNT-BT0.08 bilayer composite have been made at ambient temperature. The results are shown in Fig. 8(a,b).

Figure 8
figure 8

(a) Leakage current density (J) dependence on electric field E for the Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin films; (b) leakage current density represented as a function of the square root (E1/2) of the field.

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/cm2 at low electric field and shows a strong increase with electric field. The highest value of the current density measured on Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film, at an electric field of 200 kV/cm, was 4.0·10–1 A/cm2 (Fig. 8(a)). The leakage current is mainly attributed to conduction through grain boundaries in the BNT-BT0.08 layer39.

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:

$$J\propto {E}^{2}$$
(2)

is characteristic for a space charge-limited conduction (SCLC) mechanism40,41. Alternatively, a dependence of the type:

$$\begin{array}{cc}J\propto {e}^{-\frac{1}{kT}({\rm{\Phi }}-a{E}^{1/2})} & 0\end{array}$$
(3)

is characteristic for a Schottky-type conduction mechanism40,42, 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 constant40,41. Thus by plotting the obtained data as a function of E2 or E1/2 (on a semilogarithmic scale) it is helpful to discern among the different possible conduction mechanisms. For the CoFe2O4/BNT-BT0.08 heterostructure the plot of J vs E2 (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 E1/2 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 CoFe2O4 and NBT-BT0.08 and between these layers and the hybrid electrodes in the leakage current measurements.

Piezoelectric properties

The ferroelectric behavior of the bilayered heterostructure CoFe2O4/BNT-BT0.08 was investigated by piezoelectric force microscopy (PFM) and their out-of-plane piezoelectric response is presented in Figs 9 and 10.

Figure 9
figure 9

(a) AFM image, (b,c) PFM images: (b) amplitude and (c) phase, (d) line profile analysis of the morphology and (e) of the piezoresponse signal amplitude for the Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film.

Figure 10
figure 10

Local PFM hysteresis loops: (a) amplitude signal and (b) phase signal of Si-Pt/CoFe2O4/BNT-BT0.08 heterostructure thin film.

In the piezoelectric images, domains with opposite polarities exhibit different phase contrast. The CoFe2O4/BNT-BT0.08 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/CoFe2O4/BNT-BT0.08 bilayer thin film is ferroelectric at nanoscale level.

In summary, bilayer composite CoFe2O4/BNT-BT0.08 was deposited on Si-Pt substrate by spin-coating technique. This heterostructure thin film presented CoFe2O4 cubic phase and BNT-BT0.08 phase crystallized on the lattices of rhombohedral and tetragonal Bi0.5Na0.5TiO3. The composite CoFe2O4/BNT-BT0.08 film shows ferromagnetic, ferroelectric and piezoelectric properties. The values of magnetic parameters for CoFe2O4/BNT-BT0.08 heterostructure, with nanoscaled magnetic domain, were: Ms = 0.23 emu/cm3, Mr = 0.086 emu/cm3 and Hc = 1.7 kOe, at 295 K. CoFe2O4/BNT-BT0.08 structure shows high dielectric constant ɛr = 548 at 1 kHz and room temperature and good tunability of 23% at 1 kHz. PFM investigations suggested a ferroelectric multidomains structure for the grains of this composite thin film. The ratio of magnetic and electric properties of this composite depends on the thickness ratio of magnetic and piezoelectric layers. These results demonstrate the possibility of manufacturing 2-D materials for sensors with multiple functionalities (ferromagnetic, ferroelectric and piezoelectric).

Methods

Si-Pt/CoFe2O4/BNT-BT0.08 ferromagnetic–piezoelectric bilayer composite was prepared on Si-Pt substrates, as follows. In the first step, 10 layers of CoFe2O4 were deposited on Si-Pt by spin-coating technique. The as-obtained thin film was calcined at 700 °C for 1 h, in air, in order to crystallize CoFe2O4. In the second step, 10 layers of BNT-BT0.08 sol-precursor were deposited by spin-coating on the cobalt ferrite thin film prepared earlier. The new structure was calcined at 800 °C for 5 min, in oxygen to crystallize the layer of BNT-BT0.08. The precursor sols of CoFe2O4 and BNT-BT0.08 were prepared by sol-gel technique as described in our previous works42,43, respectively. According to these reports, CoFe2O4 precursor sol was prepared from cobalt acetate (Co(CH3CO2)2·4H2O, 99.995%, Sigma-Aldrich), iron nitrate (Fe(NO3)3·9H2O, 99.99%, Sigma-Aldrich), citric acid (C6H8O7, 99%, Sigma-Aldrich), ethanol, acetic acid and distilled water. The precursor sol of BNT-BT0.08 was prepared from bismuth (III) acetate ((CH3COO)3Bi, 99.99%, Sigma-Aldrich), sodium acetate (CH3COONa, 99.995%, Sigma-Aldrich), barium acetate ((CH3COO)2Ba, 99%, Sigma-Aldrich), titanium (IV) isopropoxide (Ti{OCH(CH3)2}4) in isopropanol (Sigma-Aldrich), acetic acid, acetylacetone and formamide. In order to prepare CoFe2O4 precursor sol, the iron nitrate was dissolved in ethanol and separately, the amount of citric acid was dissolved in ethanol, at room temperature. The cobalt acetate was dissolved in a mixture of ethanol, acetic acid and water, at room temperature. The citric acid solution was added to the mixture solutions of Fe and Co, in a molar ratio of Co:Fe:citric of 1:2:3, and a cobalt ferrite precursor sol was obtained. The as obtained sol was used for deposition of CoFe2O4 thin film on Si-Pt substrate.

BNT-BT0.08 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-BT0.08 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-BT0.08 gel film during the thermal treatments of drying and crystallization. The films of CoFe2O4 and BNT-BT0.08 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 CoFe2O4 and BNT-BT0.08 were annealed at 700 °C for 1 h, in air, in order to crystallize CoFe2O4 and at 800 °C for 5 min, in oxygen to crystallize the layer of BNT-BT0.08. Thus, the final product is a Si-Pt/CoFe2O4/BNT-BT0.08 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 TecnaiTM G2 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.