# Fine-grained magnetoelectric Sr0.5Ba0.5Nb2O6–CoFe2O4 composites synthesized by a straightforward one-pot method

Roberto Köferstein, Florian Oehler, Stefan G Ebbinghaus

► To cite this version:

Roberto Köferstein, Florian Oehler, Stefan G Ebbinghaus. Fine-grained magnetoelectric Sr0.5Ba0.5Nb2O6–CoFe2O4 composites synthesized by a straightforward one-pot method. Materials Chemistry and Physics, 2022, 278, pp.125616. 10.1016/j.matchemph10.1016/j.matchemphys.2021.125616ys.2021.125616. hal-03502572

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# Fine-grained magnetoelectric Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>–CoFe<sub>2</sub>O<sub>4</sub> composites synthesized by a straightforward one-pot method

Roberto Köferstein *, Florian Oehler, Stefan G. Ebbinghaus

Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Straße 2, 06120, Halle, Germany

## HIGHLIGHTS

* Straightforward synthesis for 0–3 Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>–CoFe<sub>2</sub>O<sub>4</sub> composites.

* Monitoring the phase evolution during sintering to ceramics bodies.

* Determination of Curie temperature.

* Impedance spectroscopy up to 200 °C and 10 MHz.

* Investigations of the magnetoelectric coefficient in dependence on magnetic DC field, frequency, and temperature.

## GRAPHICAL ABSTRACT

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Magnetoelectric effect

0–3 composites

Ferrite

Strontium barium niobate

Nano-particle

## ABSTRACT

Magnetoelectric (Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>1-x</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>x</sub> (x = 0.2–0.6) composites were prepared by a one-pot soft-chemistry synthesis using PEG400. Calcining at 700 °C resulted in nanocrystalline composite powders (d<sub>cryst.</sub> = 24–30 nm) which were sintered between 1050 and 1200 °C to ceramic bodies with relative densities up to 98%. SEM investigations confirm the formation of composite ceramics with a 0–3 connectivity and variable grain sizes from 0.2 to 3.6 µm for sintering up to 1150 °C, while sintering at 1200 °C leads both to a change in the microstructure and a considerable grain growth. Magnetic measurements at 300 K reveal ferrimagnetic behaviour with saturation magnetization values smaller than bulk CoFe<sub>2</sub>O<sub>4</sub> and coercivities between 790 and 160 Oe. Temperature-dependent impedance spectroscopy showed that the relative permittivities decrease both with rising frequency and CoFe<sub>2</sub>O<sub>4</sub> fraction. The frequency dependence of the impedance can be well described using a single RC circuit. Magnetoelectric measurements show the presence of pronounced field hystereses. The maximum magnetoelectric coefficient (α<sub>ME</sub>) depends both on the CoFe<sub>2</sub>O<sub>4</sub> fraction (x) and sintering temperature. The composite with x = 0.3 exhibits the largest α<sub>ME</sub> value of 37 µV Oe<sup>-1</sup> cm<sup>-1</sup> (@ 900 Hz). With rising frequency of the AC driving field α<sub>ME</sub> increases up to 300–400 Hz and is nearly constant until 1 kHz.

## 1. Introduction

Multiferroics, i.e. materials, which exhibit at least two ferroic orders (e.g. ferromagnetism, ferroelectricity, and ferroelasticity) are of special

interest because of their potential applications as e.g. memories, actuators, spintronics, and sensors [1,2], because the coupling of order parameters results in new functionalities. In particular, the coupling of ferro-/ferrimagnetic and ferroelectric phases leads to the so-called

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: roberto.koeferstein@chemie.uni-halle.de (R. Köferstein).</footnote>
magnetoelectric (ME) effect. The ME effect, first experimentally verified by Astrov in 1960 [3], allows to change the electrical polarization by applying a magnetic field (direct ME effect), or the magnetization by an electric field (converse ME effect). Single-phase ME materials like Cr₂O₃, BiFeO₃, or BiMnO₃ often show a rather small ME effect and/or it appears only at low temperatures [4–6]. In contrast, multiferroic composites, consisting of a ferroelectric and a ferrimagnetic compound show strong ME effects at room temperatures which is mandatory for practical applications [7]. In such composite materials the coupling between the ferroic phases is mediated through the interface. Magnetoelectric composites can be assembled in various connectivities such as 0–3, 2–2, 1–3, and 3–3 geometries, in which the numbers reflect the dimension in which each phase is self-connected [8]. Composites based on e.g. BaTiO₃ or PbTiO₃ as ferroelectric phase and ferrites as ferrimagnetic one have been extensively investigated [9–15]. In contrast, magnetoelectric composites containing Sr₀.₅Ba₀.₅Nb₂O₆ (SBN) are by far less well examined [16–26]. SrₓBa₁ₓNb₂O₆ (0.25 ≤ x ≤ 0.75) is a lead-free relaxor ferroelectric with useful piezoelectric properties and crystallizes in the open tungsten-bronze structure [27–31]. The Curie temperature strongly depends on the Sr/Ba ratio and is about 120 °C for the composition Sr₀.₅Ba₀.₅Nb₂O₆ [28]. To prepare Sr₀.₅Ba₀.₅Nb₂O₆–MFe₂O₄ composites mainly the conventional mixed oxide method has been used [16–25], whereas Hoyos et al. [26] synthesized Sr₀.₆₁Ba₀.₃₉Nb₂O₆–NiFe₂O₄ composites by a Pechini-like soft-chemistry route. Usually, soft-chemistry routes, e.g. one-pot syntheses, lead to fine-grained powders which enable lower sintering temperatures resulting in ceramic bodies with tunable grain sizes [32,33].

In this paper, we describe a straightforward one-pot soft-chemistry synthesis route using PEG400 to synthesize Sr₀.₅Ba₀.₅Nb₂O₆–CoFe₂O₄ composites with a 0–3 connectivity and with different CoFe₂O₄ molar fractions. The first step of the synthesis leads to nanoscaled powders, which are sintered to fine-grained composite ceramics. Phase formation during calcination and sintering were monitored by XRD. Measurements of the magnetoelectric coupling were carried out depending on the magnetic DC field, frequency of the superimposed AC driving field, and temperature. Furthermore, the composite samples were characterized by magnetic measurements, impedance spectroscopy, and SEM.

## 2. Experimental

### 2.1. Material preparation

($Sr_{0.5}Ba_{0.5}Nb_{2}O_{6}$)$_{1-x}$–($CoFe_{2}O_{4}$)$_{x}$ ($x$ = molar fraction) composites with $x = 0.2, 0.3, 0.4, 0.5$, and 0.6 were synthesized according to a polymerization method as described elsewhere [28].

For each composition 0.01 mol NbCl₅ (Alfa Aesar, 99%) was dissolved in 15 ml 2-methoxyethanol. Afterwards, 30 ml polyethylene glycol (PEG400) and 26 g citric acid were added and the mixture was stirred on a heating plate at 150 °C until it became a clear solution. SrCl₂·6H₂O (Merck, ≥ 99%) and BaCl₂·2H₂O (Fluka, ≥ 99%) (0.0025 mol each) were dissolved together in 7 ml 1,2-ethanediol and added to the hot Nb-solution. The resulting solution was stirred on the heating plate at 180 °C until it turned into a white suspension. According to the desired composition, stoichiometric amounts of Co(NO₃)·6H₂O (Sigma-Aldrich, ACS) and Fe(NO₃)·9H₂O (Merck, ACS) ($n_{Co}/n_{Fe} = 1/2$) were dissolved together in 5–10 ml 1,2-ethanediol and added to the (Sr,Ba, Nb)-suspension. The resulting brown reaction mixture was further stirred on the heating plate at 180 °C for about 3 h until it turned into a green, highly viscous suspension. The resulting (Sr,Ba,Nb,Co,Fe)-mixtures were calcined at 700 °C for 2 h (heating rate 10 K min⁻¹) in a muffle furnace in static air leading to (Sr₀.₅Ba₀.₅Nb₂O₆)₁-x–(CoFe₂O₄)ₓ nano-powders. Ceramic bodies were obtained from these powders after mixing with 10 wt% of a saturated aqueous polyvinyl alcohol (PVA) solution as pressing aid and uniaxial pressing into pellets at about 85 MPa (green density: 2.2 g cm⁻³). These pellets were placed on a ZrO₂ fibre mat and sintered to ceramic bodies between 1050 and 1200 °C

(heating-/cooling rate 5 K min⁻¹) with a soaking time of 1 h.

The chemical compositions of the composite powders, checked by XRF, agreed well with the expected values (Tab. S1, supporting information). In all samples, we found traces of 0.12–0.16 wt% CaO, which stem mainly from the strontium source.

For comparative purposes, pure CoFe₂O₄ was also prepared according to the described synthesis route.

### 2.2. Characterization

X-ray powder diffraction patterns were recorded at room temperature on a Bruker D8-Advance diffractometer, equipped with a one-dimensional silicon strip detector (LynxEye™) using Cu-Kα radiation and a counting time of 1 s per data point. Crystallite sizes and strain parameters were calculated from the XRD line broadening (integral peak breadth) using the software suite WinXPOW [34] applying the Scherrer and Wilson equations. Thermal analyses were performed with a heating-/cooling rate of 10 K min⁻¹ in flowing nitrogen (75 ml min⁻¹) using a TA Instruments TGA 550 (weighing precision 0.01%). To determine the magnet transition temperature of CoFe₂O₄, a bar magnet was placed underneath the balance [35]. The specific surface area (BET) was determined by nitrogen five-point gasphysisorption (Nova touch 2LX, Quantachrome Corporation). The equivalent BET particle diameter was calculated assuming a spherical or cubic particle shape. XRF measurements were carried out with a Malvern Panalytical Epsilon 4. Scanning electron microscope images were recorded with a Phenom ProX SEM in the backscattered electron mode (BSE). Magnetic and magnetoelectric measurements were carried out using a Quantum Design PPMS9. Magnetic hysteresis loops were taken at 300 K with magnetic DC field cycling between –90 and +90 kOe. The samples were enclosed in gel capsules whose very small diamagnetic contribution was subtracted before data evaluation. For magnetoelectric and impedance investigations, ceramic bodies with a thickness of 1.2–1.4 mm were sputtered on both sides with 100 nm thick gold electrodes using a Cressington Sputter Coater 108auto. The samples for magnetoelectric measurements were electrically poled for 18 h at room temperature applying an electric field of about 6 kV cm⁻¹ with a current limit of 0.1 mA [36]. Afterwards, the poled samples were short-circuited for 5 min. The magnetoelectric measurements were performed using a self-made setup [37] with the magnetic DC field parallel to the electrical polarization and a small AC driving field of about 8 Oe was superimposed collinear to the static field by a solenoid. The in-phase voltage ($U_{ME}$) was recorded using a lock-in technique. The magnetoelectric coefficient ($\alpha_{ME}$) was calculated as $\alpha_{ME} = U_{ME}·(H_{AC}·d)^{-1}$ with d being the sample thickness. The magnetoelectric behavior was investigated at 300 K using a DC field cycling between ±15 kOe and $\nu(H_{AC}) = 900$ Hz. The dependence of the magnetoelectric coefficient on the AC field was measured between 20 and 1000 Hz. The temperature dependence of $\alpha_{ME}$ was measured between 50 and 300 K at $\nu(H_{AC}) = 900$ Hz. Both the frequency- and temperature-dependent measurements were done at the DC field at which the maximum of $\alpha_{ME}$ was found. An Impedance Analyzer 4192A (Hewlett Packard) was used for frequency- and temperature-dependent impedance measurements in ranges of 25–220 °C and 5 Hz – 10 MHz applying a voltage of 1 V.

## 3. Results and discussion

### 3.1. Synthesis and powder characterization

(Sr₀.₅Ba₀.₅Nb₂O₆)₁-x–(CoFe₂O₄)ₓ composites with $x = 0.2, 0.3, 0.4, 0.5$, and 0.6, which corresponds to a CoFe₂O₄ weight fraction between 13.0 and 47.1%, were successfully prepared by the one-pot synthesis as described above. To obtain brown nanoscaled composite powders, the green viscous (Sr,Ba,Nb,Co,Fe)-solutions were calcined at 700 °C in a muffle furnace in static air (heating rate 5 K min⁻¹, soaking time 2 h). The XRD patterns (Fig. 1) of all samples show reflections corresponding
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Room-temperature XRD patterns of selected (Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>1-x</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>x</sub> composite powders after calcining at 700 °C for 2 h (heating rate 10 K min<sup>-1</sup>): a) x = 0.2, b) x = 0.4, c) x = 0.6.</figcaption></figure>

to tetragonal Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> (JCPDS #01-074-6520). XRDs for x = 0.2, 0.3, and 0.4 show only a weak reflection of cubic CoFe<sub>2</sub>O<sub>4</sub> (JCPDS #00-022-1086), whereas for x = 0.5 and 0.6 the formation of crystalline CoFe<sub>2</sub>O<sub>4</sub> can be clearly seen. Additionally, an increasing peak intensity at 2θ ≈ 28.5° for composite powders with x = 0.5 and 0.6 points to the formation of orthorhombic BaFe<sub>2</sub>O<sub>4</sub> (JCPDS #00-025-1191) as an intermediate phase, which disappears after calcining at 800 °C (Fig. S1, supporting information). A slightly increasing background level primarily between 2θ = 20–30° suggests an amorphous fraction in all samples.

The specific surface areas of the (Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>1-x</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>x</sub> powders, derived from BET measurements, decrease with increasing CoFe<sub>2</sub>O<sub>4</sub> content from 31(3) to 21(2) m² g<sup>-1</sup> corresponding to equivalent primary particle sizes of 36(3) and 53(2) nm for x = 0.2 and x = 0.6, respectively (see Fig. 2). The volume-weighted average crystallite size

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Development of the crystallite size of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> and specific surface area depending on the fraction of CoFe<sub>2</sub>O<sub>4</sub> of (Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>1-x</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>x</sub> composite powders calcined at 700 °C for 2 h (heating rate 10 K min<sup>-1</sup>).</figcaption></figure>

(size of a coherent scattering domain) obtained from the XRD peaks of the Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> phase is between 30(3) and 24(3) nm. The difference between the crystallite size and the primary particle size is most likely caused by the formation of closely joined crystallites with surface areas unavailable for nitrogen adsorption [38]. Furthermore, each primary particle may consist of more than one crystallite domain.

## 3.2. Sintering, microstructure, and phase composition of ceramic bodies

After pressing the nanocrystalline composite powders into pellets and sintering at temperatures between 1050 and 1200 °C for 1 h (heating-/cooling rate: 5 K min<sup>-1</sup>) black-brown ceramic bodies were obtained. Their bulk densities were calculated from the weight and geometric dimension and related to the single crystal densities of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> and CoFe<sub>2</sub>O<sub>4</sub> considering their nominal molar fractions [39]. As shown in Fig. 3, the densities increase between 1050 and 1150 °C. Dense ceramic bodies (relative density ≥90%) were obtained after sintering above 1075 °C. Firing up to 1150 °C leads to ceramic bodies with relative densities of 98(1) %. Upon further increasing the sintering temperature to 1200 °C the densities of most samples did not change, however, for x = 0.5 and 0.6 the densities slightly decreased to 92(1) % due to the change in the microstructure as described below.

Representative microstructures of the surface of the composite ceramics are shown in Fig. 4. In the applied BSE mode, the dark grains correspond to CoFe<sub>2</sub>O<sub>4</sub> while the light grey ones are Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>. Upon sintering at 1150 °C the Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> phase forms globular- or irregular-shaped particles, while the CoFe<sub>2</sub>O<sub>4</sub> phase consists of pyramidal- and octahedral-like grains as shown in Fig. 4a and b for x = 0.3 and 0.6, respectively. Sintering at 1200 °C leads to a change of the microstructure as can be seen in Fig. 4c and d. At this temperature Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> (SBN) forms predominantly small pillars besides some globular-like grains. The fraction of the globular-like grains of SBN decreases with increasing CoFe<sub>2</sub>O<sub>4</sub> fraction (x) which is most-likely the reason for reduced bulk densities at x = 0.5 and 0.6. Similar observations were done in pure SBN ceramics [28]. The CoFe<sub>2</sub>O<sub>4</sub> (CFO) particles retain their octahedron-/pyramidal-like shape. The grain diameters range between about 0.2 and 1.3 µm (SBN) and 0.3–1.1 µm (CFO) after sintering at 1075 °C and increase to 0.3–1.6 µm (SBN)/0.6–2.6 µm (CFO) and 0.5–3.6 µm (SBN)/0.6–3.4 µm (CFO) after 1100 °C and 1150 °C,

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Relative bulk densities of (Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>1-x</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>x</sub> composite ceramic bodies versus sintering temperatures (soaking time 1 h, heating rate 5 K min<sup>-1</sup>).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** SEM-BSE images of selected (Sr~0.5~Ba~0.5~Nb~2~O~6)~1~_x-(CoFe~2~O~4)~x composite ceramics sintered at different temperatures (T~s~) for 1 h. (a) x = 0.3, T~s~ = 1150 °C, (b) x = 0.6, T~s~ = 1150 °C, (c) x = 0.3, T~s~ = 1200 °C, (d) x = 0.6, T~s~ = 1200 °C.</figcaption></figure>

respectively. Firing at 1200 °C SBN forms pillars with lengths in the range of about 2–14 µm and widths between 1.5 and 3 µm corresponding to an aspect ratio between 2 and 6. Besides the pillars, the sizes of the globular-like grains are between 1 and 5 µm and the octahedral-like CoFe~2~O~4~ grains are between 1.5 and 6 µm. The grain sizes of all samples are listed in Tab. S2 (supporting information). The grain sizes of both components considerably rise with sintering temperature. With increasing CoFe~2~O~4~ fraction (x) we observed a tendency of increasing sizes of the CoFe~2~O~4~ grains while the SBN particles are not affected by x.

Powder XRD patterns of selected sintered composite ceramics are shown in Fig. 5. All patterns show strong and sharp reflections of the target phases Sr~0.5~Ba~0.5~Nb~2~O~6 and CoFe~2~O~4. Sintering between 1050 and 1150 °C leads to small amounts of monoclinic FeNbO~4~ (JCPDS #01-071-1849) with a fraction of about 3–4 wt%. In contrast, after sintering at 1200 °C, the FeNbO~4~ phase vanished, while the formation of 2–3 wt% tetragonal CoNb~2~O~6~ (JCPDS #01-075-2340) was observed. The appearance of secondary phases is caused by interdiffusion processes between Sr~0.5~Ba~0.5~Nb~2~O~6 and CoFe~2~O~4. The amounts of both secondary phases do not change significantly with the fraction of CoFe~2~O~4~. In contrast, a Pechini-like synthesis by Hoyos et al. [26] with 1,2-ethane-diol at pH = 9 led to the formation of secondary phases up to 17 wt%. Thus, our synthesis route leads to composite ceramics with much higher purity.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Room-temperature powder XRD patterns of crushed (Sr~0.5~Ba~0.5~Nb~2~O~6)~1~_x-(CoFe~2~O~4)~x composite ceramics after sintering at different temperatures (T~s~) for 1 h (heating-/cooling rate 5 K min⁻¹): a) x = 0.2, T~s~ = 1150 °C, b) x = 0.6, T~s~ = 1150 °C, c) x = 0.2, T~s~ = 1200 °C, d) x = 0.6, T~s~ = 1200 °C.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Field dependence of the magnetization at 300 K for composite ceramics sintered at 1100 °C for 1 h. The inset shows the saturation magnetization ($M_s$) depending on the CoFe₂O₄ fraction (x). The horizontal line represents the value of $M_s$ measured for pure CoFe₂O₄. The uncertainties of $M_s$ ($\le 0.06$ emu g⁻¹) are smaller than the symbol sizes. The magnetization is given with respect to the nominal CoFe₂O₄ mass.</figcaption></figure>

### 3.3. Magnetic behavior

Fig. 6 exemplarily shows the field-dependent magnetization at 300 K of ceramics sintered at 1100 °C. Data have been normalized with respect to the CoFe₂O₄ fraction. All samples show hystereses due to the ferromagnetic nature of CoFe₂O₄. The saturation magnetizations ($M_s$) for the composite samples were calculated by extrapolating the magnetization at high field to $H = 0$ because of the presence of small amounts of paramagnetic secondary phases (FeNbO₄, CoNb₂O₆), as aforementioned [40–43]. The $M_s$ values of the ($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{1-x}$–($CoFe_2O_4$)$_x$ ceramics sintered at 1100 °C increase with rising CoFe₂O₄ content from 66.8(6) for $x = 0.2$ to 80.0(2) emu g⁻¹ for $x = 0.6$ (inset in Fig. 6). The same trend was found for composite ceramics sintered at other temperatures (see Fig. S2, supporting information).

The difference between the calculated $M_s$ values of the composites and the $M_s$ value of 87.4(1) emu g⁻¹ (3.67(1) µB f.u.⁻¹) of pure CoFe₂O₄ (sintered at 1100 °C) is most likely due to the formation of nonferro-/nonterrimagnetic FeNbO₄ and CoN₂O₆ secondary phases [41–43] and the observed grain growth of the CoFe₂O₄ grains with increasing x. Additionally, the shape of the grains may affect $M_s$. As mentioned above, the CoFe₂O₄ grains in the composites have a pyramidal-/octahedral-like shape, while the grains in the CoFe₂O₄ bulk ceramic consists of globular-like grains. Shen et al [44] observed in micro-sized NiFe₂O₄ with octahedral grains a reduced saturation magnetization compared to the bulk value. The decrease of $M_s$ with decreasing CoFe₂O₄ content can be explained on basis of the mentioned impurities, because the fraction of these paramagnetic, secondary phases is nearly independent on the nominal CoFe₂O₄ content. Its relative effect is larger for lower values of x. The $M_s$ values (Fig. 7) of the composites vary slightly with sintering temperature due to increasing CoFe₂O₄ grain sizes as demonstrated for the sample with $x = 0.3$ (inset I in Fig. 7). In contrast, the coercivities ($H_c$) strongly decrease with sintering temperature, because of grain growth of the ferrite phase [45–47], whereas the dependence on the CoFe₂O₄ content is marginal (inset II in Fig. 7).

To determine the paramagnetic ≡ ferrimagnetic transition temperature $T_C$ (Curie-temperature) of the CoFe₂O₄ phase in the composites we performed thermogravimetric measurements in which a magnet was attached under the furnace of the thermobalance. In the ferrimagnetic temperature region, the magnet applies an extra attractive force on the hanging sample, resulting in an apparent higher weight. Thus, during cooling down from 650 °C (paramagnetic state) the weight of the samples remains almost constant until $T_C$ is reached, where a sudden

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Magnetization (M) versus applied field ($H_{DC}$) at 300 K of ($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{0.7}$–($CoFe_2O_4$)$_{0.3}$ composite bodies sintered at the indicated temperatures for 1 h. Inset (I) shows the saturation magnetization ($M_s$) depending on the sintering temperature ($T_s$). Inset (II) shows the coercivities as a function of the CoFe₂O₄ fraction (x) for different sintering temperatures. Uncertainties of the coercivities values ($\le 20$ Oe) are smaller than the symbol sizes. The magnetization is given with respect to the nominal CoFe₂O₄ mass.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Thermogravimetric investigations in flowing nitrogen (cooling rate 10 K min⁻¹) in a magnetic field for selected ($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{1-x}$–($CoFe_2O_4$)$_x$ and pure CoFe₂O₄ ceramic bodies sintered at 1100 °C for 1 h. For the sake of clarity, only every 20th data point is shown as a symbol. The inset shows $T_C$ depending on the CoFe₂O₄ fraction (x).</figcaption></figure>

increase in weight is observed. $T_C$ was determined from the onset of the weight increase. As demonstrated for selected samples in Fig. 8, the Curie temperature increases slightly with increasing CoFe₂O₄ content and amount to 488(1), 493(1), 502(1), 506(1), and 505(1) °C for $x = 0.2, 0.3, 0.4, 0.5, 0.6$, respectively, while for pure CoFe₂O₄ ceramics $T_C$ was determined as 507(1) °C. We suppose, the decrease of $T_C$ at lower CoFe₂O₄ fraction is due to interdiffusion processes resulting in the formation of CoFe₂O₄ grain boundaries containing Sr/Ba/Nb-ions and/or vacancies [48,49]. Gashemi [48] found a reduced Curie temperature in Sr-doped CoFe₂O₄ with rising doping level. As mentioned above, related to the whole CoFe₂O₄ fraction the amount of such doped grain boundaries increases with decreasing ferrite content.
<figure><img src="image_12.png" /><figcaption>Fig. 9. Frequency dependence of the real part of the permittivity (εr') and tan δ at room temperature of (Sr0.5Ba0.5Nb2O6)1-x-(CoFe2O4)x ceramic bodies sintered at 1100 °C.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. Temperature dependence of the real part of the permittivity (εr', closed symbols) and tan δ (open symbols) at 1 kHz of (Sr0.5Ba0.5Nb2O6)1-x-(CoFe2O4)x ceramic bodies sintered at 1100 °C. (a) x = 0.2 and 0.3, (b) x = 0.4, 0.5, 0.6. The insets show the temperature dependence of εr' at 10 kHz. For the sake of clarity only every third data point is represented by a symbol.</figcaption></figure>

### 3.4. Impedance spectroscopy

Impedance spectroscopy was carried out on composite ceramics sintered at 1100 °C. The development of the real part of the relative permittivity (εr') and dissipation factor (tan δ) with the applied frequency at room temperature is shown in Fig. 9.

All composites show slightly decreasing permittivities and tan δ values with rising frequency, with a strong decrease of εr' above about 2 MHz. This behavior is most likely due to the Maxwell-Wagner interface polarization caused by electrically conductive particles (CoFe2O4) embedded in an insulating matrix (Sr0.5Ba0.5Nb2O6) [50,51]. Additionally, εr' becomes smaller with rising CoFe2O4 fraction which is typical for ferrite-ferroelectric composites [52-55]. Simultaneously, tan δ increases with rising CoFe2O4 content reflecting the electrically leaky nature of ferrites (see Fig. 9). In Fig. 10 the temperature dependence of εr' and tan δ at a frequency of 1 kHz are shown. The permittivity values of (Sr0.5Ba0.5Nb2O6)0.8-(CoFe2O4)0.2, which is the most insulating sample even at high temperatures, vary only slightly up to a temperature of 200 °C. The samples with CoFe2O4 contents of x = 0.3 and 0.4 show a broad shoulder roughly between 80 and 130 °C, whereas the permittivity values of composites with x = 0.5 and 0.6 increase continuously with temperature and reach values of 3300 and 3900 at 200 °C, respectively. Rising permittivity values at higher temperatures go along with an increase in the loss tangents, most likely due to an increasing conductivity of these samples. A clear ferroelectric ↔ paraelectric phase transition maxima of the Sr0.5Ba0.5Nb2O6 phase could not be observed. However, at 1 kHz the samples with ferrite contents of x = 0.2, 0.3, and 0.4 show the formation of a weak shoulder below 130 °C, which may be attributed to the ferroelectric ↔ paraelectric phase transition. The shoulder is more pronounced at higher frequencies (see insets in Fig. 10). In contrast, single Sr0.5Ba0.5Nb2O6 shows a clear phase transition at about 120 °C (see Fig. S3, supporting information) [28,56]. However, Chen et al. [17] and Lin et al. [18] found a broad phase transition temperature below 25 °C in Sr0.5Ba0.5Nb2O6-CoFe2O4 composites prepared by the conventional mixed-oxide method. We suppose that the considerable broadening of the phase transition maxima is caused by doping of Sr0.5Ba0.5Nb2O6 with Co2+ and/or Fe3+. Kim et al. [57] and Kshirsagar et al. [58] observed a broadening of the phase transition in iron/cobalt-doped Sr0.5Ba0.5Nb2O6. Our findings are in accordance with investigations by Rathore and Vitta [25] on Sr0.5Ba0.5Nb2O6-ferrite composites who also observed a broadening and reduction of the phase transition temperature.

The conductivity of the (Sr0.5Ba0.5Nb2O6)1-x-(CoFe2O4)x ceramics is rather low (σDC ≪ 10⁻⁷ S cm⁻¹) at room temperatures. While the ceramic with x = 0.2 remains insulating even at high temperatures, the composite ceramics with x = 0.3-0.6 exhibit considerable DC conductivities at elevated temperatures suggesting a semiconducting nature. Therefore, the high-temperature impedance data were modelled using an equivalent circuit consisting of one resistance-capacitor (RC) element including a constant phase shift element. The specific complex impedance (ρ*) for a single RC element is described by:

$$ \rho^* = \frac{\rho_{DC}}{1 + (i\omega\tau)^\beta} \qquad (1) $$

where, β is the constant phase shift (CPE) coefficient and τ = ρDCεε₀. Details of this approach have been reported elsewhere [59]. The resulting Cole-Cole plots (Fig. 11) reveal single semicircular arcs up to 220 °C, indicating that the impedance data can be well described by the applied model. Hence, bulk and grain boundary contributions on impedance cannot be distinguished, which may be the result of the high density and large grain sizes of our ceramics, reducing the effect of grain boundaries. The calculated DC resistivities (ρDC) decrease continuously with rising CoFe2O4 content, e.g. from 1.63(5)·10⁻⁵ kΩ cm (x = 0.3) to 83 (3) kΩ cm (x = 0.6) at 220 °C.
<figure><img src="image_14.png" /><figcaption>Fig. 11. Cole–Cole plots of (Sr~0.5~Ba~0.5~Nb~2~O~6)~1-x~(CoFe~2~O~4)~x~ ceramics sintered at 1100 °C. a) x = 0.3, b) x = 0.4, c) x = 0.5, d) x = 0.6.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Magnetoelectric coefficient (α~ME~) vs. magnetic DC field for various composite ceramics sintered at 1100 °C for 1 h. The inset shows the maximum magnetoelectric coefficient (α~MEmax~) depending on the CoFe~2~O~4~ fraction (x). The uncertainty of the data (1 μV Oe~−1~ cm~−1) is smaller than the symbol size.</figcaption></figure>

### 3.5. Magnetoelectric properties

Fig. 12 exhibits the dependence of the magnetoelectric coefficient (α~ME~) at 300 K on the static magnetic field (H~DC~) for composite ceramics sintered at 1100 °C. Composites sintered at other temperatures show analogous behaviors (Fig. S4 and S5, supporting information).

Starting from the maximum field of 15 kOe, α~ME~ increases with decreasing DC field up to a maximum (α~MEmax~) and almost vanishes at zero field. For negative fields an inverse behavior is observed. Upon reversing sweep direction of the H~DC~ field small hystereses are found with coercivities up to 250 Oe. The magnetic DC field at which α~ME~

reaches its maximum (H~αmax~) increases with the CoFe~2~O~4~ fraction in the composites, e.g. for x = 0.2 the maximum for α~ME~ appears at 3.0 kOe, whereas for x = 0.6 the maximum α~ME~ value is found at 5.5 kOe (Fig. S6, supporting information). Also, the magnetoelectric coefficient depends on the CoFe~2~O~4~ content and shows a maximum of α~ME~ = 37(1) μV Oe~−1~ cm~−1~ (ν(H~AC~) = 900 Hz) at x = 0.3 while further increasing of the CoFe~2~O~4~ content leads to decreasing α~ME~ values (inset in Fig. 12). This behaviour is somewhat different from our results for BaTiO~3~–CoFe~2~O~4~ or BaTiO~3~–Ni for which the maxima were found around x = 0.4 [36,37,60]. We assume that the decrease of α~MEmax~ with higher ferrite content is due to an increase of the conductivity leading to an inner discharging (see impedance section). The maximum α~ME~ value was further found to depend on the sintering temperature, e.g.

<figure><img src="image_16.png" /><figcaption>Fig. 13. Maximum magnetoelectric coefficient (α~MEmax~) depending on the sintering temperature for (Sr~0.5~Ba~0.5~Nb~2~O~6)~0.7~–(CoFe~2~O~4)~0.3~ composite ceramics.</figcaption></figure>
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Fine-grained magnetoelectric Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>–CoFe<sub>2</sub>O<sub>4</sub>
composites synthesized by a straightforward one-pot method

Roberto Köferstein*, Florian Oehler, and Stefan G. Ebbinghaus

Institute of Chemistry, Martin Luther University Halle-Wittenberg,
Kurt-Mothes-Straße 2, 06120 Halle, Germany.

Tel.: +49-345-5525630; Fax: +49-345-5527028.

E-mail address: roberto.koeferstein@chemie.uni-halle.de

<footnote>* Corresponding author</footnote>
$(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{1-x}-(CoFe_2O_4)_x$ composites ($x$ = molar fraction)

<table><caption>**Table S1:** XRF results of $(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{1-x}-(CoFe_2O_4)_x$ composite powders</caption><thead><tr><th>molar ratio</th><th>Fe/Co</th><th>Sr/Ba</th><th>Nb/(Sr+Ba)</th><th>Nb/Fe</th><th>(Sr+Ba+Nb)/(Co+Fe)</th></tr></thead><tbody><tr><td colspan="6">(Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.8</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>0.2</sub></td></tr><tr><td>expected</td><td>2</td><td>1</td><td>2</td><td>4</td><td>4</td></tr><tr><td>found</td><td>2.083(3)</td><td>1.071(4)</td><td>1.996(4)</td><td>3.961(4)</td><td>4.017(6)</td></tr><tr><td colspan="6">CaO: 0.158(1) wt%</td></tr><tr><td colspan="6">(Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.7</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>0.3</sub></td></tr><tr><td>expected</td><td>2</td><td>1</td><td>2</td><td>2.333</td><td>2.333</td></tr><tr><td>found</td><td>2.074(1)</td><td>1.053(4)</td><td>1.983(4)</td><td>2.282(4)</td><td>2.316(4)</td></tr><tr><td colspan="6">CaO: 0.143(3) wt%</td></tr><tr><td colspan="6">(Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.6</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>0.4</sub></td></tr><tr><td>expected</td><td>2</td><td>1</td><td>2</td><td>1.5</td><td>1.5</td></tr><tr><td>found</td><td>2.102(6)</td><td>1.050(4)</td><td>1.989(5)</td><td>1.503(2)</td><td>1.530(4)</td></tr><tr><td colspan="6">CaO: 0.138(2) wt%</td></tr><tr><td colspan="6">(Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.5</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>0.5</sub></td></tr><tr><td>expected</td><td>2</td><td>1</td><td>2</td><td>1</td><td>1</td></tr><tr><td>found</td><td>2.087(2)</td><td>1.029(2)</td><td>1.967(3)</td><td>0.983(2)</td><td>1.003(2)</td></tr><tr><td colspan="6">CaO: 0.128(7) wt%</td></tr><tr><td colspan="6">(Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.4</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>0.6</sub></td></tr><tr><td>expected</td><td>2</td><td>1</td><td>2</td><td>0.667</td><td>0.667</td></tr><tr><td>found</td><td>2.106(4)</td><td>1.035(1)</td><td>2.003(3)</td><td>0.664(1)</td><td>0.675(1)</td></tr><tr><td colspan="6">CaO: 0.116(2) wt%</td></tr></tbody></table>
<table><caption>**Table S2:** Grain sizes of $(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{1-x}-(CoFe_2O_4)_x$ ceramic bodies</caption><thead><tr><th>Composition</th><th>Sintering temperature (°C)</th><th colspan="2">Grain size range (µm)</th></tr></thead><tbody><tr><td rowspan="6">x = 0.2</td><td>1075</td><td>CFO: octahedral-like: 0.3–1.1<br>SBN: globular-like: 0.2–1.3</td><td></td></tr><tr><td>1100</td><td>CFO: octahedral-like: 0.6–1.3<br>SBN: globular-like: 0.3–1.3</td><td></td></tr><tr><td>1150</td><td>CFO: octahedral-like: 0.6–2.5<br>SBN: globular-like: 0.5–3.2</td><td></td></tr><tr><td>1200</td><td>CFO: octahedral-like: 1.1–4.0<br>SBN: globular-like: 0.9–4.5/ pillar: 1.5–3.0 × 2.0–9.0</td><td></td></tr><tr><td>1075</td><td>CFO: octahedral-like: 0.3–1.2<br>SBN: globular-like: 0.2–1.2</td><td></td></tr><tr><td>1100</td><td>CFO: octahedral-like: 0.4–1.6<br>SBN: globular-like: 0.3–1.3</td><td></td></tr><tr><td rowspan="6">x = 0.3</td><td>1150</td><td>CFO: octahedral-like: 0.5–2.7<br>SBN: globular-like: 0.5–3.3</td><td></td></tr><tr><td>1200</td><td>CFO: octahedral-like: 1.0–4.2<br>SBN: globular-like: 1–4.5/ pillar: 1.5–3.0 × 2.0–13</td><td></td></tr><tr><td>1075</td><td>CFO: octahedral-like: 0.5–1.1<br>SBN: globular-like: 0.4–1.2</td><td></td></tr><tr><td>1100</td><td>CFO: octahedral-like: 0.7–2.1<br>SBN: globular-like: 0.5–1.6</td><td></td></tr><tr><td>1150</td><td>CFO: octahedral-like: 0.5–3.2<br>SBN: globular-like: 0.7–3.3</td><td></td></tr><tr><td>1200</td><td>CFO: octahedral-like: 1.4–5.1<br>SBN: globular-like: 1.0–5.0/ pillar: 1.2–2.9 × 2.0–14</td><td></td></tr><tr><td rowspan="6">x = 0.4</td><td>1075</td><td>CFO: octahedral-like: 0.4–1.1<br>SBN: globular-like: 0.4–1.3</td><td></td></tr><tr><td>1100</td><td>CFO: octahedral-like: 0.6–2.3<br>SBN: globular-like: 0.4–1.5</td><td></td></tr><tr><td>1150</td><td>CFO: octahedral-like: 0.6–3.4<br>SBN: globular-like: 0.5–3.6</td><td></td></tr><tr><td>1200</td><td>CFO: octahedral-like: 1.5–6.0<br>SBN: globular-like: 1.1–5.0/ pillar: 1.4–3.0 × 2.1–14</td><td></td></tr><tr><td>1075</td><td>CFO: octahedral-like: 0.3–1.1<br>SBN: globular-like: 0.3–1.3</td><td></td></tr><tr><td>1100</td><td>CFO: octahedral-like: 0.7–2.6<br>SBN: globular-like: 0.5–1.6</td><td></td></tr><tr><td rowspan="6">x = 0.5</td><td>1150</td><td>CFO: octahedral-like: 0.9–3.4<br>SBN: globular-like: 0.6–3.6</td><td></td></tr><tr><td>1200</td><td>CFO: octahedral-like: 1.4–6.0<br>SBN: globular-like: 1.3–5.0/ pillar: 1.5–3.0 × 2.0–14</td><td></td></tr><tr><td>1075</td><td>CFO: octahedral-like: 0.3–1.1<br>SBN: globular-like: 0.3–1.3</td><td></td></tr><tr><td>1100</td><td>CFO: octahedral-like: 0.7–2.6<br>SBN: globular-like: 0.5–1.6</td><td></td></tr><tr><td>1150</td><td>CFO: octahedral-like: 0.9–3.4<br>SBN: globular-like: 0.6–3.6</td><td></td></tr><tr><td>1200</td><td>CFO: octahedral-like: 1.4–6.0<br>SBN: globular-like: 1.3–5.0/ pillar: 1.5–3.0 × 2.0–14</td><td></td></tr></tbody></table>

CFO = CoFe₂O₄; SBN = Sr₀.₅Ba₀.₅Nb₂O₆

The uncertainties of the data is about 10 % (at least 0.1 µm)
<figure><img src="image_17.png" /><figcaption>**Fig. S1:** Room-temperature XRD pattern of (Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.4</sub>-(CoFe<sub>2</sub>O<sub>4</sub>)<sub>0.6</sub> composite powder after calcining at 800 °C for 2 h (heating rate 10 K min<sup>-1</sup>).</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. S2:** Saturation magnetization ($M_s$) depending on the $CoFe_2O_4$ fraction ($x$) in $(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{1-x}-(CoFe_2O_4)_x$ composite ceramics sintered at the indicated temperatures for 1 h. $M_s$ values for the composite samples were calculated by extrapolating of the magnetization at high field to $H = 0$ [1] and are given with respect to the nominal $CoFe_2O_4$ content. The black dotted line represents the $M_s$ value of pure bulk $CoFe_2O_4$, synthesized under the same conditions.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. S3:** Dielectric behavior of Sr<sub>0.5</sub>Ba<sub>0.5</sub>Nb<sub>2</sub>O<sub>6</sub> ceramic bodies sintered at different temperatures for 10 h. a) 1200 °C, b) 1250 °C, c) 1300 °C, d) 1325 °C, e) 1350 °C (from ref. [2])</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. S4:** Magnetoelectric coefficient ($\alpha_{ME}$) vs. magnetic DC field for various $(Sr_{0.5}Ba_{0.5}Nb_{2}O_{6})_{1-x}-(CoFe_{2}O_{4})_{x}$ composite ceramics sintered at 1150 °C for 1 h.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Fig. S5:** Magnetoelectric coefficient ($\alpha_{ME}$) vs. magnetic DC field for $(Sr_{0.5}Ba_{0.5}Nb_{2}O_{6})_{1-x}-(CoFe_{2}O_{4})_{x}$ ($x$ = 0.3 and 0.5) composites sintered at the indicated temperatures ($T_{s}$) for 1 h.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. S6:** Magnetic DC field ($H_{αmax}$) at which the magnetoelectric coefficient ($α_{ME,max}$) reaches its maximum with respect to the $CoFe_2O_4$ fraction ($x$) in $(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{1-x}-(CoFe_2O_4)_x$ composites sintered at 1100 °C for 1 h.</figcaption></figure>

[1] R. Köferstein, T. Walther, D. Hesse, S. G. Ebbinghaus, Fine-grained $BaTiO_3$–$MgFe_2O_4$ Composites Prepared by a Pechini-like Process, J. Alloys Compd. 638 (2015) 141–147.

[2] R. Köferstein, F. Oehler, S. G. Ebbinghaus, Investigations of Nano-crystalline $Sr_{0.5}Ba_{0.5}Nb_2O_6$ and Bulk Ceramics Synthesized by a Polymerization Method Using PEG400, J. Eur. Ceram. Soc. 39 (2019) 1156–1163.