PHOSPHATE BONDED CoFe₂O₄–BaTiO₃ LAYERED STRUCTURES: DIELECTRIC RELAXATIONS AND MAGNETOELECTRIC COUPLING

1. Introduction

Multiferroics are single- or multi-phase materials that demonstrate more than one ferroic order: ferroelastic, ferromagnetic or ferroelectric. Magnetoelectric (ME) composites comprising piezoelectric and ferrite phases exhibit unique properties, in particular, the coupling effect between the components allows one to polarize samples with an external magnetic field and vice versa.

According to different spatial distributions of phases or connectivity, composites are divided into several groups. The whole variety of connectivities is not limited to only 0–3 and 2–2 [1, 2]; however, the most studied are the two mentioned. The 0-3 type composites are particulate composites with high sintering temperatures. Due to this, some unpredictable phases are produced easily at the interfaces. As a result, the performance of the 0–3 composites degrades [3, 4]. Layered composites possess higher values of the  magnetoelectric response in comparison to those of bulk ME materials [5–8]. Yang et al. demonstrated that the magnetoelectric coupling coefficient of layered 2–2 structures is four times larger than that of the bulk ones [7]. Together with that, the  2–2 structures offer a  wider range of preparation techniques: bonding of previously sintered layers [9], tape casting [10, 11], chemical solution deposition  [12], sputtering and physical vapour deposition [13]. Any of the mentioned methods have pros and cons: casted structures induce inner mechanical stresses upon annealing due to shrinkage mismatch [14], bonded with polymer tablets [9] degrade at higher temperatures due to the  degradation of epoxy, deposition techniques are time and resource consuming.

Previously, it has been demonstrated that phosphate bonding is a  promising approach for the  preparation of composite bulk 0–3 magnetoelectric ceramics. It provides a  combination of the  simplicity of the  preparation procedure with a relatively high coupling coefficient [15]. Samples are uniaxially pressed, which means that such an approach may be applied for the 2–2 connectivity.

This research aims to synthesize the  layered BaTiO₃ and CoFe₂O₄ structures, using a  phosphate bonding of powders, into a ceramic material. The impact of the 2–2 connectivity on the dielectric properties and magnetoelectric coupling coefficient is measured and discussed.

2. Materials and methods

An aluminium phosphate binder (APB) was synthesized by the dissolution method. At the first step, an aqueous suspension of aluminium hydroxide Al(OH)₃ was prepared. Then a concentrated solution (85 wt.%) of orthophosphoric acid was added in portions to Al(OH)₃ dispersion under continuous stirring and heating of the reaction mixture up to 363–373 K for 2.5–3 h until a viscous transparent solution was obtained. The  molar ratio of H₃PO₄ and Al(OH)₃ was equal to 3:1. Being prepared, the obtained transparent solution of APB was diluted with distilled water to a density of 1.42 g/cm³.

Commercially available BaTiO₃ (Sigma-Aldrich, 208108, grain size <3 µm, designated as BTO) and CoFe₂O₄ (Sigma-Aldrich, 773352, mean grain size of 30 nm, designated as CFO) powders were used for the  preparation of the  multilayer phosphate bonded structures. Two different mixtures of barium titanate with diluted Al(H₂PO₄)₃ and cobalt ferrite with the binder were prepared separately by carefully grinding the components in an agate mortar for 10–15 min. The content of the binder in both mixtures was 5 wt.%. The prepared mixtures were used to form a layered structure. On each step, an amount of 0.1 g BTO/APB or CFO/APB mixture was poured into a  pressing mould and the  plain surface was levelled by manually pressing. After the  3-layer structure was formed it was pressed into tablets of 1 cm in diameter under 6 tons. As a result, layered tablets CoFe₂O₄–BaTiO₃–CoFe₂O₄ and BaTiO₃–CoFe₂O₄–BaTiO₃ were prepared (see Fig. 1). Further in the text, the samples are labelled as CBC and BCB, respectively. The average thickness of a single layer is 0.3 mm, and the total thickness is 0.9–0.91 mm.

Scanning electron microscopy was performed with a  Helios NanoLab 650 microscope. The  broadband dielectric spectra were investigated in a  frequency range of 20  Hz  –  1  GHz. For the quasi-static range up to 1 MHz, an LCR HP4284A was used. In a  frequency range of 1 MHz – 1 GHz, ε was studied with a coaxial line spectrometer with a vector network analyzer Agilent 8714ET. For both frequency ranges, custom-made heaters and liquid nitrogen cryostats were used for temperature measurements. The temperature was measured with a  Keithley 2700 multimeter. The  measurements were done on cooling with a rate of 1 K/min. The samples with an area of 2–3 mm² were prepared for measurements. Silver paste was applied as an electric contact.

The direct magnetoelectric coupling coefficient was measured with a custom-made set-up [16] based on the  dynamic lock-in detection technique  [17]. The  polarized sample (5  kV/cm) was placed in the system of four electromagnets with the configuration of the magnetic field of H = H₀ + H_ac. Bruker electromagnets were used to generate the static field µ₀H₀ in a range of –1 to 1 T. A low amplitude µ₀H_ac field was generated with Helmholtz coils connected to an ac source (Brul and Kjaer, Naerum, Denmark). Both magnetic fields were applied in parallel. The ME-induced voltage was measured by a lock-in amplifier SR830. Silver electrodes were sputtered to as-prepared pellets for measurements.

3. Results and discussion

Scanning electron microscopy of the CBC sample is presented in Fig. 2(a). Bigger grains on the right correspond to the BTO/APB layer, and CFO/APB is on the  left. According to the  EDX mapping (Fig. 2(b)), the interface is clear, interpenetrations were not detected.

3.1. Dielectric properties

Temperature dependences of the  real and imaginary parts of dielectric permittivity are presented in Fig. 3. Below 250 K, both samples demonstrate frequency- and temperature-independent dielectric permittivity of 15–0.17i and 22–0.04i for CBC and BCB, respectively. Above 250 K, a strong frequency dispersion is observed in the real part accompanied by frequency-dependent pronounced maxima in ε''. At higher temperatures, the  dielectric permittivity of CBC is twice higher than the one of BCB and reaches 700. The dielectric losses of CBC are 3 times higher in comparison with those of BCB, up to 250 at peak. In contrast to the bulk samples [15], anomalies related to the phase transition of BaTiO₃ were not detected.

Such behaviour of ε(T) is determined by very different dielectric properties of BaTiO₃ and CoFe₂O₄. Cobalt ferrite demonstrates a low dielectric permittivity at room temperature and below it; however, both real and imaginary ε increase rapidly upon heating [18]. In the ferroelectric phase, barium titanate has a higher permittivity but it decreases according to the Curie’s law above the phase transition [19].

The frequency spectra of dielectric permittivity demonstrate several relaxation maxima of the imaginary part for both of the samples under investigation, as presented in Fig. 4. The Havriliak–Negami function with N = 2 or 3 relaxation terms was used to describe the frequency spectra of ε,

$$\epsilon ={\epsilon }_{\infty }+{\displaystyle \sum _{i=1}^N\frac{\Delta {\epsilon }_i}{{\left[1+{\left(\text{j}\omega {\tau }_i\right)}^{1-{\alpha }_i}\right]}^{{\beta }_i}}},\left(1\right)$$

where ε_∞ = lim_ν→∞ ε, τ_i is the relaxation time of the ith process, ω  =  2πν is the  angular frequency, α_i and β_i (0 < α, β ≤ 1) describe the broadness and symmetry of the maximum of the imaginary part, and j² = –1. The function of two relaxation terms was used for BCB and 3 for CBC spectra. The relaxation time τ depends on the  temperature following the Arrhenius law (see Fig. 5). The activation energies are 0.60  eV for BCB and 0.58 for CBC structures which is close to the values obtained for the bulk composites [15].

The Maxwell–Wagner relaxations are related with non-homogeneous media. The  difference in the  dielectric properties of components leads to the  polarization at the  interfaces in the  external electric field. Such relaxation is typical of the bulk BaTiO₃–CoFe₂O₄ composites [15]. In the previous case, the polarization occurred on BT/CF and BT/phosphate grain boundaries. But in the  studied case, the layered sandwich structure of the sample plays the main role. That can be proved as follows. The  weight concentration of cobalt ferrite in the CBC sample is 66 wt.% and 33 wt.% in BCB. For comparison, the relaxation of the bulk sample of similar compositions BaTiO₃–0.3CoFe₂O₄ or BaTiO₃–0.6CoFe₂O₄ is weak and can be observed only in the reciprocal permittivity (electric modulus) spectra. The sample can be considered as a circuit of impedances connected in series and the polarization at the interface between the BT and CF layers is much stronger in comparison to the one at grain boundaries in the bulk composite.

3.2. Magnetization and magnetoelectric coupling

Magnetic hysteresis loops are presented in Fig. 6. The coercive field of the studied samples is as high as 2256 Oe for CBC and 2332 Oe for BCB. That is attributed to the 30 nm size of CoFe₂O₄ grains [20]. The  saturation magnetization depends on the concentration of CoFe₂O₄ and increases from 17 Am²/kg for BCB to 31 A m²/kg for CBC.

The amplitude of the  direct magnetoelectric coupling coefficient is presented in Fig. 7. The frequency of the applied H_ac field is 90 Hz. The magnetoelectric coefficient of BCB structure reaches 0.2  mVOe^–1cm^–1, and 0.12  mVOe^–1cm^–1 for CBC. Similarly to the  bulk composites, the  magnetic field dependence reaches a maximum of the coupling coefficient at a field of 0.5 T. That is the size-related effect of the CoFe₂O₄ grains of 30 nm [20]. The magnetoelectric coupling of 0.2 mVOe^–1cm^–1 for the BCB structure is higher than that of CBC. The response of both structures is lower in comparison to 1.1 mVOe^–1cm^–1 of bulk phosphate bonded composites [15]. Several factors are responsible for such drawbacks. The  main one is the  porosity of the samples. Due to this, the magnetostriction effect of the whole CFO layer is lower in comparison to that of sintered ceramics of high density. The mechanical contact between CFO and BTO also loses quality, and finally, being porous, the  BTO layer provides a lower piezo voltage.

The phosphate binder partially absorbs mechanical stresses and influences the  properties of composites [21]. Another factor is that the ferroelectric layer is insulated or separated with CFO. Due to this, it is difficult to polarize it fully.

However, the  presence of magnetoelectric coupling evidences the  direct interface contact between the  phases. That makes the  phosphate bonded ceramic-based approach promising for the  layered 2–2 structures. The  comparison with data presented in the literature (see Table 1) supports the conclusion.

Table 1. Comparison of the  direct magnetoelectric coupling coefficients of layered BT-CF structures.

α, mVOe–1cm–1	Method	Reference
12	pulse laser deposition	[22]
14	casting + spark plasma sintering	[23]
14 × 10–3	spin coating	[24]
8.1 × 10–3	tape casting	[11]
36 × 10–3	tape casting	[25]
3.9	electrophoretic deposition	[26]

4. Conclusions

The barium titanate and cobalt ferrite powders were bonded with a  small amount of aluminium phosphate binder into layered BaTiO₃–CoFe₂O₄–BaTiO₃ and CoFe₂O₄–BaTiO₃–CoFe₂O₄ structures. The dielectric properties were studied in wide temperature and frequency ranges. The behaviour of ε is mostly determined by the layered structure of samples. Huge Maxwell–Wagner relaxations resulting from the polarization at the interface of layers were observed. Experimentally measured direct magnetoelectric coupling coefficients are 0.2 mVOe^–1cm^–1 for the BCB sample and 0.12 mVOe^–1cm^–1 for CBC. These moderate values are attributed to porosity and difficulties with the polarization of BaTiO₃.

The obtained results demonstrate that the proposed approach is promising for the  synthesis of the layered structures and may successfully compete with others due to its simplicity. The method is eco-friendly and cost-effective.

Acknowledgements

This work was funded by the Lithuanian Academy of Sciences, Grant No. CERN-VU-2021-2022.

References

FOSFATAIS SURIŠTI CoFe₂O₄ – BaTiO₃ SLUOKSNINIAI DARINIAI: DIELEKTRINĖS RELAKSACIJOS IR MAGNETOELEKTRINĖ SĄVEIKA

^a  Vilniaus universiteto Fizikos fakultetas, Vilnius, Lietuva

^b  Duisburgo-Eseno universiteto Medžiagų mokslo institutas ir Nanointegracijos centras, Esenas, Vokietija

^c  Nepriklausomi nuo afiliacijos tyrėjai

^d  Duisburgo-Eseno universiteto Fizikos fakultetas ir Nanointegracijos centras, Duisburgas, Vokietija

^e Fizinių ir technologijos mokslų centro Medžiagų struktūrinės analizės skyrius, Vilnius, Lietuva

Santrauka

Daugiasluoksniai fosfatais surišti CoFe₂O₄  – BaTiO₃  –  CoFe₂O₄ (CBC) ir BaTiO₃  –  CoFe₂O₄  – BaTiO₃ (BCB) dariniai buvo pagaminti presavimo būdu. Dielektrinės savybės ištirtos 20  Hz  –  1  GHz dažnių ir 120–500  K temperatūrų intervaluose. CBC ir BCB dariniuose buvo išmatuotos dielektrinės skvarbos vertės, 15–0,17i ir 22–0,04i atitinkamai, kurios nepriklauso nuo dažnio ir temperatūros žemiau 250  K. Aukštesnėse temperatūrose atsiranda stiprioji dispersija, būdinga Maksvelo–Vagnerio relaksacijai. Toks elgesys yra susijęs su 2–2 fazių erdviniu pasiskirstymu. BaTiO₃ – CoFe₂O₄ – BaTiO₃ darinyje buvo išmatuotas magnetoelektrinės sąveikos koeficientas, kurio gauta vertė yra 0,2 mVOe^–1cm^–1.