# Magnetoelectric, dielectric, and magnetic investigations of multiferroic NixCo1–xFe2O4–SryBa1–yNb2O6 composites

Roberto Köferstein, Maria-Sophie Wartmann, Stefan G Ebbinghaus

► To cite this version:

Roberto Köferstein, Maria-Sophie Wartmann, Stefan G Ebbinghaus. Magnetoelectric, dielectric, and magnetic investigations of multiferroic NixCo1–xFe2O4–SryBa1–yNb2O6 composites. Materials Research Bulletin, 2024, 177, pp.112860. 10.1016/j.materresbull.2024.112860. hal-04569738

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# Magnetoelectric, dielectric, and magnetic investigations of multiferroic NixCo1-xFe2O4-SryBa1-yNb2O6 composites

Roberto Köferstein, Maria-Sophie Wartmann, Stefan G Ebbinghaus

► To cite this version:

Roberto Köferstein, Maria-Sophie Wartmann, Stefan G Ebbinghaus. Magnetoelectric, dielectric, and magnetic investigations of multiferroic NixCo1-xFe2O4-SryBa1-yNb2O6 composites. Materials Research Bulletin, 2024, 177, pp.112860. 10.1016/j.materresbull.2024.112860. hal-04569738

HAL Id: hal-04569738

https://hal.science/hal-04569738v1

Submitted on 6 May 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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Distributed under a Creative Commons Attribution 4.0 International License
# Magnetoelectric, dielectric, and magnetic investigations of multiferroic NiₓCo₁₋ₓFe₂O₄–SrᵧBa₁₋ᵧNb₂O₆ composites

Roberto Köferstein *, Maria-Sophie Wartmann, Stefan G. Ebbinghaus

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

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

### Keywords:

- Composites

- Ceramics

- Phase transitions

- Magnetic properties

- Multiferroics

## ABSTRACT

Magnetoelectric (NiₓCo₁₋ₓFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇ composites with a 0–3 connectivity and different cation stoichiometries were synthesized via a classical mixed-oxide method. XRD patterns of all ceramics show reflections of the target phases SrᵧBa₁₋ᵧNb₂O₆ and NiₓCo₁₋ₓFe₂O₄. The influence of stoichiometry of the ferri-magnetic and ferroelectric phase on the magnetoelectric behavior was studied on composites with composition of (NiₓCo₁₋ₓFe₂O₄)₀.₃–(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ and (NiFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇. The magnetic Curie temperature increases with nickel content. However, the saturation magnetizations as well as the Curie temperatures of the composites are always lower than the ones of bulk NiₓCo₁₋ₓFe₂O₄ samples. The maximum magnetoelectric coefficient (α<sub>ME</sub>) rises with increasing nickel content from 40 (x = 0) to 180 μV Oe⁻¹ cm⁻¹ (x = 1) in accordance with the dynamic magnetostriction coefficient. The evolution of α<sub>ME</sub> of (NiFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇ composites shows an increase with strontium content up to y 0.5. Higher strontium content leads to a significant reducing of α<sub>ME</sub>.

## 1. Introduction

Multiferroic materials exhibit two or more ferroic orders. They are of special interest because of their potential applications as e.g. memories, spintronic, sensors, and in biomedical purposes [1,2]. Recently, Zi et al. [3] have reported on a low-frequency communication device based on the magnetoelectric effect. Coupling between ferro-/ferrimagnetism and ferroelectricity results in the co-called magnetoelectric (ME) effect in which the electrical polarization is affected by a magnetic field and vice versa. Most of the single-phase ME compounds (e. g. TbMn₂O₅, BiMnO₃) show a small ME effect and/or it appears at low temperatures [4,5]. However, Veena et al. [6] have recently reported on large magnetoelectric coupling at room temperature in Sr₂FeNbO₆. On the other hand, composite materials with separate ferro-/ferrimagnetic and ferroelectric phases often show a strong magnetoelectric response at room temperature, which is essential for technical applications [7,8]. Furthermore, in magnetoelectric composite materials, the ME effect can be tuned by changing the stoichiometry of each single phase and by varying the molar ratio between the ferro-/ferrimagnetic and ferroelectric phase [9, 10, 11, 12]. The coupling between those two phases is mediated by their interfaces and the magnetoelectric effect can be understood as a product property between those ferroic orders [13]. The two different phases

composite materials can be arranged in e.g. 0–3 (particles surrounded by a matrix), 2–2 (layers), and 1–3 (pillars in a matrix) dimensionalities [14]. Lead-free 0–3 magnetoelectric composites based on BaTiO₃ and MFe₂O₄ have been extensively investigated [9,15-22]. In comparison, composites with the relaxor ferroelectric SrᵧBa₁₋ᵧNb₂O₆ (0.25 ≤ y ≤ 0.75) have been only rarely studied [23–31]. In contrast to composites with BaTiO₃, the ferroelectric phase of SrᵧBa₁₋ᵧNb₂O₆ in composites is stable even after sintering at high temperatures [32–34]. Ferroelectric SrᵧBa₁₋ᵧNb₂O₆ crystallized in the open tungsten-bronze structure and shows a diffuse phase transition (DPT) between roughly 313–473 K, depending on y [35–39]. The piezoelectric coefficient also depends on the Sr/Ba ratio (y) [40]. The influence of the stoichiometry of the ferroelectric and ferro-/ferrimagnetic phases on the magnetoelectric output is less well studied [41,42].

The aim of this work is to investigate the influence of the cation stoichiometry of the ferrimagnetic and the ferroelectric phases of (NiₓCo₁₋ₓFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇ composites on the magnetoelectric output. The samples were synthesized by the conventional mixed-oxide method. Phase evolution and microstructure of the composite ceramics were monitored by XRD, SEM, and EDX. The magnetoelectric behavior was investigated depending on H<sub>DC</sub>, frequency of H<sub>AC</sub> and composition. The Curie temperatures were determined by

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: roberto.koeferstein@chemie.uni-halle.de (R. Köferstein).</footnote>
# Magnetoelectric, dielectric, and magnetic investigations of multiferroic NiₓCo₁₋ₓFe₂O₄–SrᵧBa₁₋ᵧNb₂O₆ composites

Roberto Köferstein *, Maria-Sophie Wartmann, Stefan G. Ebbinghaus

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

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

Keywords:

Composites

Ceramics

Phase transitions

Magnetic properties

Multiferroics

## ABSTRACT

Magnetoelectric (NiₓCo₁₋ₓFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇ composites with a 0–3 connectivity and different cation stoichiometries were synthesized via a classical mixed-oxide method. XRD patterns of all ceramics show reflections of the target phases SrᵧBa₁₋ᵧNb₂O₆ and NiₓCo₁₋ₓFe₂O₄. The influence of stoichiometry of the ferri-magnetic and ferroelectric phase on the magnetoelectric behavior was studied on composites with composition of (NiₓCo₁₋ₓFe₂O₄)₀.₃–(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ and (NiFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇. The magnetic Curie temperature increases with nickel content. However, the saturation magnetizations as well as the Curie temperatures of the composites are always lower than the ones of bulk NiₓCo₁₋ₓFe₂O₄ samples. The maximum magnetoelectric co-efficient (αₘₘ) rises with increasing nickel content from 40 (x = 0) to 180 μV Oe⁻¹ cm⁻¹ (x = 1) in accordance with the dynamic magnetostriction coefficient. The evolution of αₘₘ of (NiFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇ composites shows an increase with strontium content up to y 0.5. Higher strontium content leads to a significant reducing of αₘₘ.

## 1. Introduction

Multiferroic materials exhibit two or more ferroic orders. They are of special interest because of their potential applications as e.g. memories, spintronic, sensors, and in biomedical purposes [1,2]. Recently, Zi et al. [3] have reported on a low-frequency communication device based on the magnetoelectric effect. Coupling between ferro-/ferrimagnetism and ferroelectricity results in the co-called magnetoelectric (ME) effect in which the electrical polarization is affected by a magnetic field and vice versa. Most of the single-phase ME compounds (e. g. TbMn₂O₅, BiMnO₃) show a small ME effect and/or it appears at low temperatures [4,5]. However, Veena et al. [6] have recently reported on large magnetoelectric coupling at room temperature in Sr₂FeNbO₆. On the other hand, composite materials with separate ferro-/ferrimagnetic and ferroelectric phases often show a strong magnetoelectric response at room temperature, which is essential for technical applications [7,8]. Furthermore, in magnetoelectric composite materials, the ME effect can be tuned by changing the stoichiometry of each single phase and by varying the molar ratio between the ferro-/ferrimagnetic and ferroelectric phase [9, 10, 11, 12]. The coupling between those two phases is mediated by their interfaces and the magnetoelectric effect can be understood as a product property between those ferroic orders [13]. The two different phases

composite materials can be arranged in e.g. 0–3 (particles surrounded by a matrix), 2–2 (layers), and 1–3 (pillars in a matrix) dimensionalities [14]. Lead-free 0–3 magnetoelectric composites based on BaTiO₃ and MFe₂O₄ have been extensively investigated [9,15-22]. In comparison, composites with the relaxor ferroelectric SrᵧBa₁₋ᵧNb₂O₆ (0.25 ≤ y ≤ 0.75) have been only rarely studied [23–31]. In contrast to composites with BaTiO₃, the ferroelectric phase of SrᵧBa₁₋ᵧNb₂O₆ in composites is stable even after sintering at high temperatures [32–34]. Ferroelectric SrᵧBa₁₋ᵧNb₂O₆ crystallized in the open tungsten-bronze structure and shows a diffuse phase transition (DPT) between roughly 313–473 K, depending on y [35–39]. The piezoelectric coefficient also depends on the Sr/Ba ratio (y) [40]. The influence of the stoichiometry of the ferroelectric and ferro-/ferrimagnetic phases on the magnetoelectric output is less well studied [41,42].

The aim of this work is to investigate the influence of the cation stoichiometry of the ferrimagnetic and the ferroelectric phases of (NiₓCo₁₋ₓFe₂O₄)₀.₃–(SrᵧBa₁₋ᵧNb₂O₆)₀.₇ composites on the magnetoelectric output. The samples were synthesized by the conventional mixed-oxide method. Phase evolution and microstructure of the composite ceramics were monitored by XRD, SEM, and EDX. The magnetoelectric behavior was investigated depending on Hₐ₉, frequency of Hₐ₉ and composition. The Curie temperatures were determined by

<footnote>* Corresponding author.
E-mail address: roberto.koeferstein@chemie.uni-halle.de (R. Köferstein).</footnote>
thermomagnetometric measurements. Moreover, the samples were characterized by impedance spectroscopy and magnetic measurements.

## 2. Experimental

### 2.1. Material preparation

(NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites with varying Ni/Co contents x = 0, 0.25, 0.5, 0.75, and 1 (= 20.3 wt% NiₓCo₁₋ₓFe₂O₄) were prepared. Additionally, we also prepared (NiFe₂O₄)₀.₃-(SrᵧBa₁₋ᵧNb₂O₆)₀.₇ composites with different Sr/Ba ratios y = 0.3, 0.5, 0.6, and 0.7 (= 19.7 – 20.7 wt% NiFe₂O₄).

The composites were synthesized by a classical mixed oxide synthesis as described in our previous paper [31]. The oxides and carbonates were mixed in a planetary mill for 4 h using polyamide jars, ZrO₂-balls, and propan-2-ol. To prepare NiₓCo₁₋ₓFe₂O₄, NiO (Berlin—Chemie, puriss.), Co₂O₃ (Alfa Aesar, 99.7%) and Fe₂O₃ (Alfa Aesar, 99.9%) were mixed in the desired molar ratio. After filtering and drying the mixture was calcined in static air at 1423 K for 2 h (heating rate 10 K min⁻¹). SrᵧBa₁₋ᵧNb₂O₆ was synthesized by mixing stoichiometric amounts of BaCO₃ (Merck, > 98.5%), SrCO₃ (Sigma-Aldrich, 99.9%), and Nb₂O₅ (Alpha Aesar, 99.9%) and calcining for 5 h at 1473 K (y = 0.3 and 0.5), 1573 K (y = 0.6), and 1703 K for 30 h (y = 0.7) (heating rate 5 K min⁻¹) to get phase pure powders (Fig. S1, supporting information). Composite powders were prepared by mixing NiₓCo₁₋ₓFe₂O₄ and SrᵧBa₁₋ᵧNb₂O₆ powders to get the target composition of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(SrᵧBa₁₋ᵧNb₂O₆)₀.₇. The resulting composite powders were mixed with 10 wt% of a saturated aqueous polyvinyl alcohol (PVA) solution as a pressing aid and uniaxially pressed at about 110 MPa into pellets (green density: 3.0 g cm⁻³). These pellets were placed on a ZrO₂ fibre mat and sintered to ceramic bodies between 1273 and 1473 K (heating-/cooling rate 5 K min⁻¹) with a soaking time of 1 h.

### 2.2. Characterization

X-ray powder diffraction were carried out on a Bruker D8-Advance diffractometer at room temperature, equipped with a one-dimensional silicon strip detector (LynxEye™) using Cu-Kα radiation and a counting time of 1 s per data point. Scanning electron microscope images were collected with a Phenom ProX SEM in backscattered electron mode (BSE). 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 thermobalance (weighing precision 0.01 %). To determine the magnet transition temperature (Curie point), a bar magnet was placed underneath the balance [43]. A Quantum Design PPMS9 system was used for magnetic and magnetoelectric measurements. Magnetic hysteresis loops were taken at 300 K with magnetic DC field cycling between ±90 kOe. For magnetoelectric and impedance measurements, ceramic samples were sputtered on both sides with 100 nm thick gold electrodes using a Cressington Sputter Coater 108auto. For magnetoelectric measurements the samples were electrically poled along the thickness direction for 18 h at room temperature applying an electric field of about 7 kV cm⁻¹ with a current limit of 0.1 mA. Magnetoelectric measurements were performed using a self-made setup [9] 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<sub>ME</sub>) was recorded by a lock-in technique. The magnetoelectric coefficient (α<sub>ME</sub>) was calculated as α<sub>ME</sub> = U<sub>ME</sub>·(H<sub>AC</sub>·d)<sup>-1</sup> (d = sample thickness). The thickness of the ceramic samples was between 0.95 and 1.02 mm. The magnetoelectric performance was investigated at 300 K in a DC field cycling between ±15 kOe at v(H<sub>AC</sub>) = 900 Hz. Frequency-dependent measurements were done at the DC field at which the maximum of α<sub>ME</sub> was found. An Impedance Analyzer 4192A (Hewlett Packard) was used for impedance measurements.

## 3. Results and discussion

### 3.1. (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites

After pressing to pellets, the composite powders were sintered in static air at 1373 K for 1 h (heating-/ cooling rate: 5 K min⁻¹). The bulk densities of the black ceramic bodies were calculated from their weight and geometric dimension and the relative bulk densities were related to 5.37 g cm⁻³, calculated from the single crystal densities of Sr₀.₅Ba₀.₅Nb₂O₆ and NiₓCo₁₋ₓFe₂O₄ considering their nominal molar fractions [44]. The (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramic bodies reveal an increase in density with increasing cobalt content (Fig. 1), e.g. for x = 1 a relative density of 71(1)% was achieved, whereas composites with x = 0 reached 83(1)%.

Fig. 2 shows the XRD patterns after sintering of compacted (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite powders at 1373 K. The composite ceramics reveal reflections of cubic NiₓCo₁₋ₓFe₂O₄ (JCPDS #01-074-2081 [NiFe₂O₄], #00-022-1086 [CoFe₂O₄]) and tetragonal Sr₀.₅Ba₀.₅Nb₂O₆ (JCPDS #01-074-6520). Weak reflections at 24.7° and 30.5° indicate the formation of a secondary phase (2 – 4 wt %). According to EDX results and previous investigations [31], this secondary phase can be described as orthorhombic M₀.₄Fe₀.₆Nb₂O₆ (M = Ni/Co) (JCPDS #01-075-2158/ #01-076-2354) with columbite structure which change to a tetragonal rutile structure [JCPDS #01-077-1290/ #01-076-2355] after sintering at 1423 and 1473 K.

Fig. 3 shows the microstructure of selected composite ceramics. Due to the BSE mode of the SEM images, light grey grains represent Sr₀.₅Ba₀.₅Nb₂O₆ and the dark ones correspond to NiₓCo₁₋ₓFe₂O₄ and secondary phases, respectively. Representative EDX spectra are shown in Fig. S2 (supporting information) confirming the formation of Sr₀.₅Ba₀.₅Nb₂O₆ and NiₓCo₁₋ₓFe₂O₄ grains. Measurements on several large grains point to a possible iron doping (< 1 at.%) of the Sr₀.₅Ba₀.₅Nb₂O₆ phase, whereas traces of Sr and Nb in the NiₓCo₁₋ₓFe₂O₄ spectrum are most likely caused by the large interaction volume of the electron beam compared to the small grain size. The EDX spectra of the secondary phase reveal a composition of about M₀.₄Fe₀.₆Nb₂O₆ (M = Ni/Co), as aforementioned. The sporadic grains of the secondary phase show an irregular shape up to about 3 µm in all samples. After sintering at 1373 K, the NiₓCo₁₋ₓFe₂O₄ phase forms irregular and pyramidal-/ octahedral-like grains with dimensions between 0.5 – 3 µm for x = 1 – 0.75 and 0.8 – 5 µm for x = 0. The Sr₀.₅Ba₀.₅Nb₂O₆ grains show a globular-/irregular shape between 1 – 6 µm for x = 1 which slightly grows to 1 – 9 µm for x = 0. Besides these globular grains, we also observed the appearance of pillar-like Sr₀.₅Ba₀.₅Nb₂O₆ grains in samples with cobalt-substituted NiFe₂O₄. The pillars have dimensions of about 3 – 8×1.5 – 3 µm and the fraction of the pillars moderately increases with increasing cobalt substitution in NiₓCo₁₋ₓFe₂O₄. The formation of pillar-

<figure><img src="image_3.png" /><figcaption>Fig. 1. Bulk densities of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite ceramic bodies after firing at 1373 K for 1 h (heating-/cooling rate 5 K min⁻¹).</figcaption></figure>
thermomagnetometric measurements. Moreover, the samples were characterized by impedance spectroscopy and magnetic measurements.

## 2. Experimental

### 2.1. Material preparation

($Ni_xCo_{1-x}Fe_2O_4$)$_{0.3}$-($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{0.7}$ composites with varying $Ni/Co$ contents $x = 0, 0.25, 0.5, 0.75$, and $1$ (= 20.3 wt% $Ni_xCo_{1-x}Fe_2O_4$) were prepared. Additionally, we also prepared ($NiFe_2O_4$)$_{0.3}$-($Sr_yBa_{1-y}Nb_2O_6$)$_{0.7}$ composites with different $Sr/Ba$ ratios $y = 0.3, 0.5, 0.6$, and $0.7$ (= 19.7 - 20.7 wt% $NiFe_2O_4$).

The composites were synthesized by a classical mixed oxide synthesis as described in our previous paper [31]. The oxides and carbonates were mixed in a planetary mill for 4 h using polyamide jars, $ZrO_2$-balls, and propan-2-ol. To prepare $Ni_xCo_{1-x}Fe_2O_4$, $NiO$ (Berlin—Chemie, puriss.), $Co_2O_3$ (Alfa Aesar, 99.7%) and $Fe_2O_3$ (Alfa Aesar, 99.9%) were mixed in the desired molar ratio. After filtering and drying the mixture was calcined in static air at 1423 K for 2 h (heating rate 10 K min$^{-1}$). $Sr_yBa_{1-y}Nb_2O_6$ was synthesized by mixing stoichiometric amounts of $BaCO_3$ (Merck, > 98.5%), $SrCO_3$ (Sigma-Aldrich, 99.9%), and $Nb_2O_5$ (Alpha Aesar, 99.9%) and calcining for 5 h at 1473 K ($y = 0.3$ and $0.5$), 1573 K ($y = 0.6$), and 1703 K for 30 h ($y = 0.7$) (heating rate 5 K min$^{-1}$) to get phase pure powders (Fig. S1, supporting information). Composite powders were prepared by mixing $Ni_xCo_{1-x}Fe_2O_4$ and $Sr_yBa_{1-y}Nb_2O_6$ powders to get the target composition of ($Ni_xCo_{1-x}Fe_2O_4$)$_{0.3}$-($Sr_yBa_{1-y}Nb_2O_6$)$_{0.7}$. The resulting composite powders were mixed with 10 wt% of a saturated aqueous polyvinyl alcohol (PVA) solution as a pressing aid and uniaxially pressed at about 110 MPa into pellets (green density: 3.0 $g cm^{-3}$). These pellets were placed on a $ZrO_2$ fibre mat and sintered to ceramic bodies between 1273 and 1473 K (heating-/cooling rate 5 K min$^{-1}$) with a soaking time of 1 h.

### 2.2. Characterization

X-ray powder diffraction were carried out on a Bruker D8-Advance diffractometer at room temperature, equipped with a one-dimensional silicon strip detector (LynxEye™) using $Cu-Kα$ radiation and a counting time of 1 s per data point. Scanning electron microscope images were collected with a Phenom ProX SEM in backscattered electron mode (BSE). 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 thermobalance (weighing precision 0.01 %). To determine the magnet transition temperature (Curie point), a bar magnet was placed underneath the balance [43]. A Quantum Design PPMS9 system was used for magnetic and magnetoelectric measurements. Magnetic hysteresis loops were taken at 300 K with magnetic DC field cycling between ±90 kOe. For magnetoelectric and impedance measurements, ceramic samples were sputtered on both sides with 100 nm thick gold electrodes using a Cressington Sputter Coater 108auto. For magnetoelectric measurements the samples were electrically poled along the thickness direction for 18 h at room temperature applying an electric field of about 7 kV cm$^{-1}$ with a current limit of 0.1 mA. Magnetoelectric measurements were performed using a self-made setup [9] 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 by a lock-in technique. The magnetoelectric coefficient ($α_{ME}$) was calculated as $α_{ME} = U_{ME}·(H_{AC}·d)^{-1}$ ($d$ = sample thickness). The thickness of the ceramic samples was between 0.95 and 1.02 mm. The magnetoelectric performance was investigated at 300 K in a DC field cycling between ±15 kOe at $v(H_{AC}) = 900$ Hz. Frequency-dependent measurements were done at the DC field at which the maximum of $α_{ME}$ was found. An Impedance Analyzer 4192A (Hewlett Packard) was used for impedance measurements.

## 3. Results and discussion

### 3.1. ($Ni_xCo_{1-x}Fe_2O_4$)$_{0.3}$-($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{0.7}$ composites

After pressing to pellets, the composite powders were sintered in static air at 1373 K for 1 h (heating-/ cooling rate: 5 K min$^{-1}$). The bulk densities of the black ceramic bodies were calculated from their weight and geometric dimension and the relative bulk densities were related to 5.37 $g cm^{-3}$, calculated from the single crystal densities of $Sr_{0.5}Ba_{0.5}Nb_2O_6$ and $Ni_xCo_{1-x}Fe_2O_4$ considering their nominal molar fractions [44]. The ($Ni_xCo_{1-x}Fe_2O_4$)$_{0.3}$-($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{0.7}$ ceramic bodies reveal an increase in density with increasing cobalt content (Fig. 1), e.g. for $x = 1$ a relative density of 71(1)% was achieved, whereas composites with $x = 0$ reached 83(1)%.

Fig. 2 shows the XRD patterns after sintering of compacted ($Ni_xCo_{1-x}Fe_2O_4$)$_{0.3}$-($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{0.7}$ composite powders at 1373 K. The composite ceramics reveal reflections of cubic $Ni_xCo_{1-x}Fe_2O_4$ (JCPDS #01-074-2081 [NiFe$_2$O$_4$], #00-022-1086 [CoFe$_2$O$_4$]) and tetragonal $Sr_{0.5}Ba_{0.5}Nb_2O_6$ (JCPDS #01-074-6520). Weak reflections at 24.7° and 30.5° indicate the formation of a secondary phase (2 – 4 wt %). According to EDX results and previous investigations [31], this secondary phase can be described as orthorhombic $M_{0.4}Fe_{0.6}Nb_2O_6$ ($M = Ni/Co$) (JCPDS #01-075-2158/ #01-076-2354) with columbite structure which change to a tetragonal rutile structure [JCPDS #01-077-1290/ #01-076-2355] after sintering at 1423 and 1473 K.

Fig. 3 shows the microstructure of selected composite ceramics. Due to the BSE mode of the SEM images, light grey grains represent $Sr_{0.5}Ba_{0.5}Nb_2O_6$ and the dark ones correspond to $Ni_xCo_{1-x}Fe_2O_4$ and secondary phases, respectively. Representative EDX spectra are shown in Fig. S2 (supporting information) confirming the formation of $Sr_{0.5}Ba_{0.5}Nb_2O_6$ and $Ni_xCo_{1-x}Fe_2O_4$ grains. Measurements on several large grains point to a possible iron doping (< 1 at.%) of the $Sr_{0.5}Ba_{0.5}Nb_2O_6$ phase, whereas traces of Sr and Nb in the $Ni_xCo_{1-x}Fe_2O_4$ spectrum are most likely caused by the large interaction volume of the electron beam compared to the small grain size. The EDX spectra of the secondary phase reveal a composition of about $M_{0.4}Fe_{0.6}Nb_2O_6$ ($M = Ni/Co$), as aforementioned. The sporadic grains of the secondary phase show an irregular shape up to about 3 µm in all samples. After sintering at 1373 K, the $Ni_xCo_{1-x}Fe_2O_4$ phase forms irregular and pyramidal-/ octahedral-like grains with dimensions between 0.5 – 3 µm for $x = 1 – 0.75$ and 0.8 – 5 µm for $x = 0$. The $Sr_{0.5}Ba_{0.5}Nb_2O_6$ grains show a globular-/irregular shape between 1 – 6 µm for $x = 1$ which slightly grows to 1 – 9 µm for $x = 0$. Besides these globular grains, we also observed the appearance of pillar-like $Sr_{0.5}Ba_{0.5}Nb_2O_6$ grains in samples with cobalt-substituted $NiFe_2O_4$. The pillars have dimensions of about 3 – 8×1.5 – 3 µm and the fraction of the pillars moderately increases with increasing cobalt substitution in $Ni_xCo_{1-x}Fe_2O_4$. The formation of pillar-

<figure><img src="image_3.png" /><figcaption>Fig. 1. Bulk densities of ($Ni_xCo_{1-x}Fe_2O_4$)$_{0.3}$-($Sr_{0.5}Ba_{0.5}Nb_2O_6$)$_{0.7}$ composite ceramic bodies after firing at 1373 K for 1 h (heating-/cooling rate 5 K min$^{-1}$).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Room-temperature XRD patterns of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ powdered composite ceramics after sintering at 1373 K for 1 h (heating-/cooling rate 5 K min⁻¹).</figcaption></figure>

like grains was also observed in pure Sr₀.₅Ba₀.₅Nb₂O₆ ceramics sintered at high temperatures [45].

### 3.1.1. Magnetic and dielectric behavior

Fig. 4 shows the field-dependent magnetization curves at 300 K of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramics sintered at 1373 K. Data have been normalized with respect to the nominal NiₓCo₁₋ₓFe₂O₄ fraction. Magnetization data with respect to the sample mass are shown in Fig. S3 (supporting information). The occurrence of hystereses is due to the ferrimagnetic properties of the NiₓCo₁₋ₓFe₂O₄ phase. The coercivity values (inset in Fig. 4) increase with cobalt content from 29 to 260 Oe, because of the higher magnetocrystalline anisotropy constant of Co²⁺ compared to Ni²⁺ [46–48]. The saturation magnetizations (M_s) were calculated by extrapolating the magnetization at high fields to H = 0 because of the presence of small amounts of paramagnetic secondary phases (M₀.₄Fe₀.₆Nb₂O₆ (M = Ni/Co)) [33,49,50]. As expected, the saturation magnetization at 300 K of the composite ceramics raises with decreasing x from 46.1(2) to 76.7(3) emu g⁻¹ (1.93(1) – 3.22(1) µ_B f. u.⁻¹) (Fig. 5).

The increase of M_s with cobalt substitution is not only due to different magnetic moments of Ni²⁺ (2 µ_B) and Co²⁺ (3 µ_B), but it also reflects that NiFe₂O₄ is an inverse spinel, whereas CoFe₂O₄ crystallized in a partly inverse spinel structure [51]. However, the M_s values of the composites are slightly lower than the values of 49.7(2) – 89.4(2) emu g⁻¹ (2.09(1) – 3.76(1) µ_B f.u.⁻¹) for pure bulk NiₓCo₁₋ₓFe₂O₄ samples, which also increases with Co²⁺ content. The slightly lower M_s values of the composites are caused by the formation of the aforementioned secondary phases primarily leading to cation vacancies in the ferrite grains (Fe/M ≠ 2, M = Ni, Co) and to a small reduction of the NiₓCo₁₋ₓFe₂O₄ amount in the composites. In generally, the development of the saturation magnetization of spinel ferrites with Fe/M ≠ 2, compared to stoichiometric ferrites, is affected by the change of the inversions parameter (rearrangement of the metal cation over the tetragonal and octahedral sites) [52] and the weakening of the superexchange interaction is due the formation of metal vacancies and resulting oxygen vacancies. To investigate this effect, we prepared MFe₂O₄ (M = Ni/Co) ceramics with different Fe/M ratios from 1.90 to 2.22. Fig. S4 (supporting information) shows their evolution of the saturation magnetization. A ratio higher than 2 leads to smaller M_s values for CoFe₂O₄ samples, whereas for NiFe₂O₄ a slight increase can be obtained, in accordance with investigations by Shafer [53]. In contrast samples with

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** SEM-BSE images of selected (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramic bodies sintered at 1373 K for 1 h. a) x = 1, b) x = 0.5, c) x = 0.</figcaption></figure>

a Fe/M ratio lower than 2 show lower M_s values in both cases [54]. This effect is more pronounced for CoFe₂O₄ samples and reflects the M_s development between the composites and pure NiₓCo₁₋ₓFe₂O₄ ceramics (Fig. 5). These findings suggest an iron deficiency (Fe/M < 2) of the ferrite phase in the composites.

Not only the saturation magnetization but also the Curie
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Room-temperature XRD patterns of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ powdered composite ceramics after sintering at 1373 K for 1 h (heating-/cooling rate 5 K min⁻¹).</figcaption></figure>

like grains was also observed in pure Sr₀.₅Ba₀.₅Nb₂O₆ ceramics sintered at high temperatures [45].

### 3.1.1. Magnetic and dielectric behavior

Fig. 4 shows the field-dependent magnetization curves at 300 K of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramics sintered at 1373 K. Data have been normalized with respect to the nominal NiₓCo₁₋ₓFe₂O₄ fraction. Magnetization data with respect to the sample mass are shown in Fig. S3 (supporting information). The occurrence of hystereses is due to the ferrimagnetic properties of the NiₓCo₁₋ₓFe₂O₄ phase. The coercivity values (inset in Fig. 4) increase with cobalt content from 29 to 260 Oe, because of the higher magnetocrystalline anisotropy constant of Co²⁺ compared to Ni²⁺ [46–48]. The saturation magnetizations (M_s) were calculated by extrapolating the magnetization at high fields to H = 0 because of the presence of small amounts of paramagnetic secondary phases (M₀.₄Fe₀.₆Nb₂O₆ (M = Ni/Co)) [33,49,50]. As expected, the saturation magnetization at 300 K of the composite ceramics raises with decreasing x from 46.1(2) to 76.7(3) emu g⁻¹ (1.93(1) – 3.22(1) µ_B f. u.⁻¹) (Fig. 5).

The increase of M_s with cobalt substitution is not only due to different magnetic moments of Ni²⁺ (2 µ_B) and Co²⁺ (3 µ_B), but it also reflects that NiFe₂O₄ is an inverse spinel, whereas CoFe₂O₄ crystallized in a partly inverse spinel structure [51]. However, the M_s values of the composites are slightly lower than the values of 49.7(2) – 89.4(2) emu g⁻¹ (2.09(1) – 3.76(1) µ_B f.u.⁻¹) for pure bulk NiₓCo₁₋ₓFe₂O₄ samples, which also increases with Co²⁺ content. The slightly lower M_s values of the composites are caused by the formation of the aforementioned secondary phases primarily leading to cation vacancies in the ferrite grains (Fe/M ≠ 2, M = Ni, Co) and to a small reduction of the NiₓCo₁₋ₓFe₂O₄ amount in the composites. In generally, the development of the saturation magnetization of spinel ferrites with Fe/M ≠ 2, compared to stoichiometric ferrites, is affected by the change of the inversions parameter (rearrangement of the metal cation over the tetragonal and octahedral sites) [52] and the weakening of the superexchange interaction is due the formation of metal vacancies and resulting oxygen vacancies. To investigate this effect, we prepared MFe₂O₄ (M = Ni/Co) ceramics with different Fe/M ratios from 1.90 to 2.22. Fig. S4 (supporting information) shows their evolution of the saturation magnetization. A ratio higher than 2 leads to smaller M_s values for CoFe₂O₄ samples, whereas for NiFe₂O₄ a slight increase can be obtained, in accordance with investigations by Shafer [53]. In contrast samples with

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** SEM-BSE images of selected (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramic bodies sintered at 1373 K for 1 h. a) x = 1, b) x = 0.5, c) x = 0.</figcaption></figure>

a Fe/M ratio lower than 2 show lower M_s values in both cases [54]. This effect is more pronounced for CoFe₂O₄ samples and reflects the M_s development between the composites and pure NiₓCo₁₋ₓFe₂O₄ ceramics (Fig. 5). These findings suggest an iron deficiency (Fe/M < 2) of the ferrite phase in the composites.

Not only the saturation magnetization but also the Curie
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Magnetization at 300 K for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite ceramics sintered at 1373 K for 1 h. The inset shows M versus H in a small field range. The magnetization values are given with respect to the nominal NiₓCo₁₋ₓFe₂O₄ content.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Dependence of M_s on the nickel content (x) of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites and NiₓCo₁₋ₓFe₂O₄ ceramic samples sintered at 1373 K for 1 h.</figcaption></figure>

temperatures of spinel ferrites strongly depend on their composition. The development of the Curie temperature (T_c) (paramagnetic $\rightleftharpoons$ ferromagnetic transition) of the NiₓCo₁₋ₓFe₂O₄ phase in the composites sintered at 1373 K was measured in a thermobalance in which a magnet was placed underneath the furnace. In the ferrimagnetic state of the sample the magnet applied an extra attractive force on the hanging sample, resulting in an apparently higher weight. During cooling from the paramagnetic to the ferrimagnetic state a sudden rising weight can be observed at T_c (see inset in Fig. 6) [30]. The Curie temperature was determined from the onset of this weight increase. As shown in Fig. 6, T_c increases almost linearly with increasing nickel substitution (x) from 773(1) for CoFe₂O₄ to 860(1) K for NiFe₂O₄ in the composite ceramics. The Curie temperatures of Co- and Ni-ferrites are in good agreement with literature data [55,56]. According to the two sub-lattice model by Néel, the Curie temperature in spinels is mainly determined by the superexchange interaction (via an O²⁻ ion) between the A and B sub-lattices, representing the cations on the tetrahedral (A) and octahedral (B) sites, respectively [57,58]. A weakening of the strength of the A – B exchange interaction results in a lower Curie temperature. The reported value for the exchange integral $J_{AB}$ for NiFe₂O₄ (–27.4 K) is higher than for CoFe₂O₄ (–22.7 K) [59,60], meaning that the A – B interaction is lower in CoFe₂O₄. The weakening of the A – B interaction in CoFe₂O₄ is due to a change of the cation distribution between the A and B sites. NiFe₂O₄ is a complete inverse spinel, whereas CoFe₂O₄ crystallized in a partly invers structure [61]. Additionally, the larger

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Evolution of the Curie temperature depending on the nickel content (x) of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites and pure NiₓCo₁₋ₓFe₂O₄ ceramic samples sintered at 1373 K for 1 h. The inset shows the thermogravimetric measurements in flowing nitrogen (cooling rate 10 K min⁻¹) of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite bodies in a magnetic field. x = 1 (a), 0.75 (b), 0.5 (c), 0.25 (d), 0 (e).</figcaption></figure>

cation radius of Co²⁺ and thus an increase in the cubic lattice parameter for cobalt ferrite leads to larger interatomic distances, which also cause a reduction of the A – B exchange interaction strength [57,62,63].

It can further be seen that the Curie temperature of the ferrite phase in the composites is slightly lower than T_c for pure NiₓCo₁₋ₓFe₂O₄ ceramics sintered under the same conditions, which was also observed by Ciomaga et al. [64]. This effect increases slightly with raising Co-substitution. As mentioned above, EDX analysis of the ferrite grains did not show any significant doping. Thus, we suppose that the slightly lower Curie temperatures can be explained by the formation of the secondary phase (M₀.₄Fe₀.₆Nb₂O₆) and the a small migration of iron into the Sr₀.₅Ba₀.₅Nb₂O₆ grains in the composite ceramics which causes cation vacancies in the ferrite grains. Cation vacancies influence the cation distribution over the tetrahedral and octahedral sites and the strength of the A – B exchange interaction [54,65]. The Curie temperatures and the XRD patterns of the above-mentioned single phase NiFe₂O₄ and CoFe₂O₄ ceramics, sintered at 1373 K for 1 h, with different Fe/M ratios (1.90–2.22, M = Ni, Co) are shown in Fig. S5 and S6 (supporting information). It can be seen that a Fe/M ratio lower than 2 leads to a decreasing Curie temperature by 17(2) K for Fe/M = 1.90 in cobalt ferrite, whereas in nickel ferrite the Curie point does not significantly change. On the other hand, a Fe/M ratio higher than 2 causes an increase of T_c by 16(2) and 5(2) K (Fe/M = 2.22) for cobalt ferrite and nickel ferrite, respectively. These findings suggest that the ferrite phase in the composites exhibit an iron deficiency (Fe/M ratio < 2) which leads to slightly lower Curie temperatures of the composites compared to pure NiₓCo₁₋ₓFe₂O₄ samples.

Dielectric properties of the composites were investigated by impedance spectroscopy. The temperature dependence of ε<sub>r</sub>' and tan δ at a frequency of 1 kHz is shown in Fig. 7. The permittivity values increase with temperature. Sr₀.₅Ba₀.₅Nb₂O₆ is a relaxor ferroelectric and shows a diffuse phase transition around 373–423 K (Curie-range [45,66]). In contrast to ceramics of pure Sr₀.₅Ba₀.₅Nb₂O₆, for which a broad maximum of the permittivity occurs (see also Fig. S7, supporting information) no such maximum were found for the composite ceramic samples. However, the permittivity curves reveal a weak shoulder or at least a significant change in the slope at around 373 K, which is more visible by plotting ε<sub>r</sub>'⁻¹ vs. temperature (Fig. S8, supporting information). Rising tan δ values at high temperatures (inset in Fig. 7) are most
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Magnetization at 300 K for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite ceramics sintered at 1373 K for 1 h. The inset shows M versus H in a small field range. The magnetization values are given with respect to the nominal NiₓCo₁₋ₓFe₂O₄ content.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Dependence of M_s on the nickel content (x) of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites and NiₓCo₁₋ₓFe₂O₄ ceramic samples sintered at 1373 K for 1 h.</figcaption></figure>

temperatures of spinel ferrites strongly depend on their composition. The development of the Curie temperature (T_c) (paramagnetic $\rightleftharpoons$ ferromagnetic transition) of the NiₓCo₁₋ₓFe₂O₄ phase in the composites sintered at 1373 K was measured in a thermobalance in which a magnet was placed underneath the furnace. In the ferrimagnetic state of the sample the magnet applied an extra attractive force on the hanging sample, resulting in an apparently higher weight. During cooling from the paramagnetic to the ferrimagnetic state a sudden rising weight can be observed at T_c (see inset in Fig. 6) [30]. The Curie temperature was determined from the onset of this weight increase. As shown in Fig. 6, T_c increases almost linearly with increasing nickel substitution (x) from 773(1) for CoFe₂O₄ to 860(1) K for NiFe₂O₄ in the composite ceramics. The Curie temperatures of Co- and Ni-ferrites are in good agreement with literature data [55,56]. According to the two sub-lattice model by Néel, the Curie temperature in spinels is mainly determined by the superexchange interaction (via an O²⁻ ion) between the A and B sub-lattices, representing the cations on the tetrahedral (A) and octahedral (B) sites, respectively [57,58]. A weakening of the strength of the A – B exchange interaction results in a lower Curie temperature. The reported value for the exchange integral $J_{AB}$ for NiFe₂O₄ (–27.4 K) is higher than for CoFe₂O₄ (–22.7 K) [59,60], meaning that the A – B interaction is lower in CoFe₂O₄. The weakening of the A – B interaction in CoFe₂O₄ is due to a change of the cation distribution between the A and B sites. NiFe₂O₄ is a complete inverse spinel, whereas CoFe₂O₄ crystallized in a partly invers structure [61]. Additionally, the larger

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Evolution of the Curie temperature depending on the nickel content (x) of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites and pure NiₓCo₁₋ₓFe₂O₄ ceramic samples sintered at 1373 K for 1 h. The inset shows the thermogravimetric measurements in flowing nitrogen (cooling rate 10 K min⁻¹) of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite bodies in a magnetic field. x = 1 (a), 0.75 (b), 0.5 (c), 0.25 (d), 0 (e).</figcaption></figure>

cation radius of Co²⁺ and thus an increase in the cubic lattice parameter for cobalt ferrite leads to larger interatomic distances, which also cause a reduction of the A – B exchange interaction strength [57,62,63].

It can further be seen that the Curie temperature of the ferrite phase in the composites is slightly lower than T_c for pure NiₓCo₁₋ₓFe₂O₄ ceramics sintered under the same conditions, which was also observed by Ciomaga et al. [64]. This effect increases slightly with raising Co-substitution. As mentioned above, EDX analysis of the ferrite grains did not show any significant doping. Thus, we suppose that the slightly lower Curie temperatures can be explained by the formation of the secondary phase (M₀.₄Fe₀.₆Nb₂O₆) and the a small migration of iron into the Sr₀.₅Ba₀.₅Nb₂O₆ grains in the composite ceramics which causes cation vacancies in the ferrite grains. Cation vacancies influence the cation distribution over the tetrahedral and octahedral sites and the strength of the A – B exchange interaction [54,65]. The Curie temperatures and the XRD patterns of the above-mentioned single phase NiFe₂O₄ and CoFe₂O₄ ceramics, sintered at 1373 K for 1 h, with different Fe/M ratios (1.90–2.22, M = Ni, Co) are shown in Fig. S5 and S6 (supporting information). It can be seen that a Fe/M ratio lower than 2 leads to a decreasing Curie temperature by 17(2) K for Fe/M = 1.90 in cobalt ferrite, whereas in nickel ferrite the Curie point does not significantly change. On the other hand, a Fe/M ratio higher than 2 causes an increase of T_c by 16(2) and 5(2) K (Fe/M = 2.22) for cobalt ferrite and nickel ferrite, respectively. These findings suggest that the ferrite phase in the composites exhibit an iron deficiency (Fe/M ratio < 2) which leads to slightly lower Curie temperatures of the composites compared to pure NiₓCo₁₋ₓFe₂O₄ samples.

Dielectric properties of the composites were investigated by impedance spectroscopy. The temperature dependence of $\epsilon_r'$ and tan δ at a frequency of 1 kHz is shown in Fig. 7. The permittivity values increase with temperature. Sr₀.₅Ba₀.₅Nb₂O₆ is a relaxor ferroelectric and shows a diffuse phase transition around 373–423 K (Curie-range [45,66]). In contrast to ceramics of pure Sr₀.₅Ba₀.₅Nb₂O₆, for which a broad maximum of the permittivity occurs (see also Fig. S7, supporting information) no such maximum were found for the composite ceramic samples. However, the permittivity curves reveal a weak shoulder or at least a significant change in the slope at around 373 K, which is more visible by plotting $\epsilon_r'^{-1}$ vs. temperature (Fig. S8, supporting information). Rising tan δ values at high temperatures (inset in Fig. 7) are most
<figure><img src="image_9.png" /><figcaption>Fig. 7. Dependence of the real part of ε' and tan δ (inset) on temperature at 1 kHz for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramic bodies sintered at 1373 K for 1 h. For the sake of clarity every 4th data point is represented by a symbol.</figcaption></figure>

likely due to a decreasing resistivity of the samples.

Because of the low conductivities of the samples at room temperature (σDC << 10⁻⁷ S cm⁻¹), the high-temperature impedance data were fitted by an equivalent circuit consisting of one resistance-capacitor (RC) element including a constant phase-shift element. The specific complex impedance (Z*spec) for a single RC element is described by [67]:

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

where, β is the constant phase-shift (CPE) coefficient and τ = ρDCε₀. Cole-Cole plots of the specific impedance of the (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ magnetoelectric composites can be well described by one RC element (inset in Fig. 8). The calculated specific DC resistivities (ρDC) of the (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramics are similar for all samples. ρDC decreases slightly with increasing nickel content from 8.30(2)·10² kΩcm (x = 0) to 5.38 (3)·10² kΩcm (x = 0.75) and finally rises to 15.56(3)·10² kΩcm for x = 1

<figure><img src="image_10.png" /><figcaption>Fig. 8. Development of the specific DC resistivity with composition of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramic sintered at 1373 K for 1 h. The uncertainties of the data are smaller than the symbol size. The inset shows the Cole-Cole plots of the samples.</figcaption></figure>

(Fig. 8). Due to the electrically leaky nature of ferrites, the DC resistivities of the composites are significantly lower compared to pure Sr₀.₅Ba₀.₅Nb₂O₆ (83.71(7)·10² kΩcm).

### 3.1.2. Magnetoelectric properties

The magnetoelectric coefficient (αME) dependence on the static magnetic field (HDC) at 300 K for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite ceramics sintered at 1373 K is shown in Fig. 9. The evolution of αME reveals a hysteretic behavior with coercive fields smaller than 100 Oe. αME increases with rising HDC field up to a maximum/minimum (αMEmax) and then continuously decreases to zero at higher fields.

As seen in Fig. 10, αMEmax growths with increasing nickel content from 40(2) µV cm⁻¹ Oe⁻¹ (x = 0) to 180(10) µV cm⁻¹ Oe⁻¹ (x = 1). Analogous tendencies were also reported in magnetoelectric composites consisting of Ni/Co-ferrites and BaTiO₃, Pb(Zr/Ti)O₃, or PbFe₀.₅Nb₀.₅O₃ [41,68,69,70]. The value of the magnetoelectric coefficient is influenced by resistivity, density, and grain size of the ceramics, as well as by the magnetostrictive-/ piezoelectric output of the individual composite phases. In generally, considerably decreasing resistivities (at least one order of magnitude) of the samples can reduced the magnetoelectric effect [30,31,71,72]. However, the resistivity values of the samples are in the same order of magnitude and also the grain sizes are in a comparable range. The density of the ceramics decreases with nickel substitution, but this would rather degrade the magnetoelectric coupling. Therefore, we conclude that the magnetoelectric output of the samples is mainly determined by the magnetostrictive behaviour of the NiₓCo₁₋ₓFe₂O₄ phase. On the other hand, the magnetostriction induced by a static magnetic field (HDC) is reduced from CoFe₂O₄ to NiFe₂O₄ which would expected lower αME values for composite samples with increasing nickel content [73,74]. However, to measure the magnetoelectric performance, an alternating magnetic field (HAC) is superimposed to the HDC field. As pointed out by Aubert et al. [70,75], the applied HAC field leads to a dynamic deformation (piezomagnetic effect) of the ferrite grains. The resulting dynamic magnetostriction coefficient of CoFe₂O₄ is lower than the one of NiFe₂O₄ for low HAC fields [70,75], which explains the observed αME evolution of our (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites. Higher αME values for composites with NiFe₂O₄ compared to CoFe₂O₄ were also found for other sintering temperatures (Fig. 11). The NiFe₂O₄ based composites always show higher αME values, although the density of the CoFe₂O₄-Sr₀.₅Ba₀.₅Nb₂O₆ ceramics is higher which should lead to a better connectivity between the ferroelectric and ferromagnetic grains. After sintering at 1473 K in both composite

<figure><img src="image_11.png" /><figcaption>Fig. 9. Magnetoelectric coefficient (αME) vs. magnetic DC field for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites sintered at 1373 K for 1 h. The inset shows the frequency dependence (HAC) of αME at HDC(αmax). For the sake of clarity, in the inset every 10th data point is represented by a symbol.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. Dependence of the real part of ε' and tan δ (inset) on temperature at 1 kHz for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramic bodies sintered at 1373 K for 1 h. For the sake of clarity every 4th data point is represented by a symbol.</figcaption></figure>

likely due to a decreasing resistivity of the samples.

Because of the low conductivities of the samples at room temperature (σDC << 10⁻⁷ S cm⁻¹), the high-temperature impedance data were fitted by an equivalent circuit consisting of one resistance-capacitor (RC) element including a constant phase-shift element. The specific complex impedance (Z*spec) for a single RC element is described by [67]:

$$Z_{spec}^* = \frac{\rho_{DC}}{1 + (i\omega\tau)^\beta} \quad (1)$$

where, β is the constant phase-shift (CPE) coefficient and τ = ρDCε₀. Cole-Cole plots of the specific impedance of the (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ magnetoelectric composites can be well described by one RC element (inset in Fig. 8). The calculated specific DC resistivities (ρDC) of the (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramics are similar for all samples. ρDC decreases slightly with increasing nickel content from 8.30(2)·10² kΩcm (x = 0) to 5.38 (3)·10² kΩcm (x = 0.75) and finally rises to 15.56(3)·10² kΩcm for x = 1

<figure><img src="image_10.png" /><figcaption>Fig. 8. Development of the specific DC resistivity with composition of (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ ceramic sintered at 1373 K for 1 h. The uncertainties of the data are smaller than the symbol size. The inset shows the Cole-Cole plots of the samples.</figcaption></figure>

(Fig. 8). Due to the electrically leaky nature of ferrites, the DC resistivities of the composites are significantly lower compared to pure Sr₀.₅Ba₀.₅Nb₂O₆ (83.71(7)·10² kΩcm).

### 3.1.2. Magnetoelectric properties

The magnetoelectric coefficient (αME) dependence on the static magnetic field (HDC) at 300 K for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composite ceramics sintered at 1373 K is shown in Fig. 9. The evolution of αME reveals a hysteretic behavior with coercive fields smaller than 100 Oe. αME increases with rising HDC field up to a maximum/minimum (αMEmax) and then continuously decreases to zero at higher fields.

As seen in Fig. 10, αMEmax growths with increasing nickel content from 40(2) µV cm⁻¹ Oe⁻¹ (x = 0) to 180(10) µV cm⁻¹ Oe⁻¹ (x = 1). Analogous tendencies were also reported in magnetoelectric composites consisting of Ni/Co-ferrites and BaTiO₃, Pb(Zr/Ti)O₃, or PbFe₀.₅Nb₀.₅O₃ [41,68,69,70]. The value of the magnetoelectric coefficient is influenced by resistivity, density, and grain size of the ceramics, as well as by the magnetostrictive-/ piezoelectric output of the individual composite phases. In generally, considerably decreasing resistivities (at least one order of magnitude) of the samples can reduced the magnetoelectric effect [30,31,71,72]. However, the resistivity values of the samples are in the same order of magnitude and also the grain sizes are in a comparable range. The density of the ceramics decreases with nickel substitution, but this would rather degrade the magnetoelectric coupling. Therefore, we conclude that the magnetoelectric output of the samples is mainly determined by the magnetostrictive behaviour of the NiₓCo₁₋ₓFe₂O₄ phase. On the other hand, the magnetostriction induced by a static magnetic field (HDC) is reduced from CoFe₂O₄ to NiFe₂O₄ which would expected lower αME values for composite samples with increasing nickel content [73,74]. However, to measure the magnetoelectric performance, an alternating magnetic field (HAC) is superimposed to the HDC field. As pointed out by Aubert et al. [70,75], the applied HAC field leads to a dynamic deformation (piezomagnetic effect) of the ferrite grains. The resulting dynamic magnetostriction coefficient of CoFe₂O₄ is lower than the one of NiFe₂O₄ for low HAC fields [70,75], which explains the observed αME evolution of our (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites. Higher αME values for composites with NiFe₂O₄ compared to CoFe₂O₄ were also found for other sintering temperatures (Fig. 11). The NiFe₂O₄ based composites always show higher αME values, although the density of the CoFe₂O₄-Sr₀.₅Ba₀.₅Nb₂O₆ ceramics is higher which should lead to a better connectivity between the ferroelectric and ferromagnetic grains. After sintering at 1473 K in both composite

<figure><img src="image_11.png" /><figcaption>Fig. 9. Magnetoelectric coefficient (αME) vs. magnetic DC field for (NiₓCo₁₋ₓFe₂O₄)₀.₃-(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites sintered at 1373 K for 1 h. The inset shows the frequency dependence (HAC) of αME at HDC(αmax). For the sake of clarity, in the inset every 10th data point is represented by a symbol.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Maximum of the magnetoelectric coefficient ($\alpha_{ME,max}$) and the $H_{DC}$ field at which the maximum of $\alpha_{ME}$ appeared ($H_{DC}(\alpha_{max})$) depending on the composition x for $(Ni_xCo_{1-x}Fe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Relative bulk density (A) and maximum magnetoelectric coefficient ($\alpha_{ME,max}$) (B) of $(NiFe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ (a) and $(CoFe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ (b) composites versus sintering temperature (soaking time 1 h, heating-/cooling rate 5 K min$^{-1}$). The uncertainties of the data are sometimes smaller than the symbol size.</figcaption></figure>

samples $\alpha_{ME}$ drops down because of a considerable increase in conductivity of more than one order of magnitude [31].

The $H_{DC}$ field at which $\alpha_{ME}$ reaches its maximum ($H_{DC}(\alpha_{max})$) almost linearity decreases with x from 5.0 to 1.0 kOe (Fig. 10) as a result of the change in the magnetostrictive behaviour of $Ni_xCo_{1-x}Fe_2O_4$. The magnetoelectric effect in composites is widely accepted as a product property between a magnetostrictive ($Ni_xCo_{1-x}Fe_2O_4}$) and a piezoelectric material ($Sr_{0.5}Ba_{0.5}Nb_2O_6$) [76,77]. Therefore, the field-dependence development of the magnetoelectric coefficient reflects the magnetostriction evolution of $Ni_xCo_{1-x}Fe_2O_4$. The maximum slope of the magnetostriction (strain derivative) and the maximum of the piezomagnetic coefficient of $NiFe_2O_4$ appears at a lower $H_{DC}$ field in contrast to $CoFe_2O_4$ [70,75].

### 3.2. $(NiFe_2O_4)_{0.3}-(Sr_yBa_{1-y}Nb_2O_6)_{0.7}$ composites

In a second set of samples with $NiFe_2O_4$ as the ferrimagnetic component, the $Sr/Ba$ ratio of the ferroelectric component was varied and its effect on the dielectric and magnetoelectric properties was studied. The $(NiFe_2O_4)_{0.3}-(Sr_yBa_{1-y}Nb_2O_6)_{0.7}$ composites with $y = 0.3, 0.5, 0.6$, and $0.7$ were sintered at 1373 K for 1 h and reached relative

densities of 71(1)–72(1)% ($y = 0.5, 0.6, 0.7$) and 76(1)% ($y = 0.3$), respectively. The relative densities were related to $5.30–5.39 \text{ g cm}^{-3}$, depending on y [44] (see Tab. S1, supporting information). Up to a sintering temperature of 1373 K, the XRD patterns (Fig. S9, supporting information) show reflections of $Sr_yBa_{1-y}Nb_2O_6$, $NiFe_2O_4$ and small amounts (2–4 wt%) of orthorhombic or tetragonal ($NiFe)Nb_2O_6$ [JCPDS #01-077-1290/ #01-076-2355] as secondary phase [31], as aforementioned. Additionally, for the sintered composites with $y = 0.7$ reflections at 17.6, 22.9, 28.2, 29.1, and $31.8^\circ$ in the XRD pattern indicate the formation of $SrNb_2O_6$ (JCPDS #01-072-2088) with a fraction of about 20 wt%. Sintering at 1423 K leads in the case of composites with $y = 0.3$ to the formation of small amounts (ca. 6 wt%) of $Ba_3(Fe,Ni)Nb_6O_{21}$ (JCPDS #01-079-0074, #01-079-0075), as confirmed by EDX analysis. The phase composition for $y = 0.5, 0.6$ and $0.7$ does not change after firing at 1423 K. However, in all samples, the orthorhombic ($NiFe)Nb_2O_6$ phase completely transforms to the tetragonal one with rutile structure [31] (Fig. S10, supporting information) after sintering above 1373 K.

The saturation magnetization values, related to the nominal $NiFe_2O_4$ content, are between 46.1(2)–47.0(2) emu $g^{-1}$ for composites sintered at 1373 K and thus independent on the strontium content (y) as expected. The same is true for other sintering temperatures (Fig. S11, supporting information). As mentioned above, these values are slightly lower than the $M_s$ values of pure $NiFe_2O_4$.

The evolution of the permittivity with temperature at 1 kHz depending on the $Sr$ content (y) is depicted in Fig. 12. All samples show increasing permittivity values with rising temperatures and a decrease of $\epsilon_r'$ above 473 K. The permittivity curves of the ceramic bodies do not show a clear maximum reflecting the transition from the ferroelectric to the paraelectric state. In contrast, measurements on phase-pure $Sr_yBa_1-yNb_2O_6$ ceramics show dielectric maxima at 324(2), 353(2), 375(2), and 506(2) K (Fig. S7, supporting information), in accordance with literature data [36,39]. As shown in the inset of Fig. 12, the DC resistivity values, calculated from the impedance data, of the $(NiFe_2O_4)_{0.3}-(Sr_yBa_1-yNb_2O_6)_{0.7}$ composites lie in the same order of magnitude. Composites with $y = 0.5$ and $0.6$ reveal only slightly lower DC resistivity values than samples with $y = 0.3$ and $0.7$.

The maximal magnetoelectric response ($\alpha_{MEmax}$) of $(NiFe_2O_4)_{0.3}-(Sr_yBa_1-yNb_2O_6)_{0.7}$ samples depending on y is shown in Fig. 13. $\alpha_{MEmax}$ rises from $y = 0.3$ to $y = 0.5$ reflecting the increasing piezoelectric coefficient of $Sr_yBa_1-yNb_2O_6$ with rising strontium content (y) [40]. However, the significantly lower $\alpha_{MEmax}$ values for $y = 0.6$ and especially for $0.7$ is unexpected. The resistivities of the samples (inset in

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Dielectric behavior ($\epsilon_r'$/closed symbols, tan $\delta$/open symbols) at 1 kHz for $(NiFe_2O_4)_{0.3}-(Sr_yBa_1-yNb_2O_6)_{0.7}$ ceramic bodies sintered at 1373 K for 1 h. For the sake of clarity every 4th data point is represented by a symbol. The inset shows the specific DC resistivity in dependence of the strontium content (y). The uncertainties of the data are smaller than the symbol size.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Maximum of the magnetoelectric coefficient ($\alpha_{ME,max}$) and the $H_{DC}$ field at which the maximum of $\alpha_{ME}$ appeared ($H_{DC}(\alpha_{max})$) depending on the composition x for $(Ni_xCo_{1-x}Fe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Relative bulk density (A) and maximum magnetoelectric coefficient ($\alpha_{ME,max}$) (B) of $(NiFe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ (a) and $(CoFe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ (b) composites versus sintering temperature (soaking time 1 h, heating-/cooling rate 5 K min$^{-1}$). The uncertainties of the data are sometimes smaller than the symbol size.</figcaption></figure>

samples $\alpha_{ME}$ drops down because of a considerable increase in conductivity of more than one order of magnitude [31].

The $H_{DC}$ field at which $\alpha_{ME}$ reaches its maximum ($H_{DC}(\alpha_{max})$) almost linearity decreases with x from 5.0 to 1.0 kOe (Fig. 10) as a result of the change in the magnetostrictive behaviour of $Ni_xCo_{1-x}Fe_2O_4$. The magnetoelectric effect in composites is widely accepted as a product property between a magnetostrictive ($Ni_xCo_{1-x}Fe_2O_4}$) and a piezoelectric material ($Sr_{0.5}Ba_{0.5}Nb_2O_6$) [76,77]. Therefore, the field-dependence development of the magnetoelectric coefficient reflects the magnetostriction evolution of $Ni_xCo_{1-x}Fe_2O_4$. The maximum slope of the magnetostriction (strain derivative) and the maximum of the piezomagnetic coefficient of $NiFe_2O_4$ appears at a lower $H_{DC}$ field in contrast to $CoFe_2O_4$ [70,75].

### 3.2. $(NiFe_2O_4)_{0.3}-(Sr_yBa_{1-y}Nb_2O_6)_{0.7}$ composites

In a second set of samples with $NiFe_2O_4$ as the ferrimagnetic component, the Sr/Ba ratio of the ferroelectric component was varied and its effect on the dielectric and magnetoelectric properties was studied. The $(NiFe_2O_4)_{0.3}-(Sr_yBa_{1-y}Nb_2O_6)_{0.7}$ composites with y = 0.3, 0.5, 0.6, and 0.7 were sintered at 1373 K for 1 h and reached relative

densities of 71(1)–72(1)% (y = 0.5, 0.6, 0.7) and 76(1)% (y = 0.3), respectively. The relative densities were related to 5.30–5.39 g cm$^{-3}$, depending on y [44] (see Tab. S1, supporting information). Up to a sintering temperature of 1373 K, the XRD patterns (Fig. S9, supporting information) show reflections of $Sr_yBa_{1-y}Nb_2O_6$, $NiFe_2O_4$ and small amounts (2 – 4 wt%) of orthorhombic or tetragonal ($NiFe_2Nb_2O_6$) [JCPDS #01-077-1290/ #01-076-2355] as secondary phase [31], as aforementioned. Additionally, for the sintered composites with y = 0.7 reflections at 17.6, 22.9, 28.2, 29.1, and 31.8° in the XRD pattern indicate the formation of $SrNb_2O_6$ (JCPDS #01-072-2088) with a fraction of about 20 wt%. Sintering at 1423 K leads in the case of composites with y = 0.3 to the formation of small amounts (ca. 6 wt%) of $Ba_3(Fe,Ni)Nb_6O_{21}$ (JCPDS #01-079-0074, #01-079-0075), as confirmed by EDX analysis. The phase composition for y = 0.5, 0.6 and 0.7 does not change after firing at 1423 K. However, in all samples, the orthorhombic ($NiFe_2Nb_2O_6$) phase completely transforms to the tetragonal one with rutile structure [31] (Fig. S10, supporting information) after sintering above 1373 K.

The saturation magnetization values, related to the nominal $NiFe_2O_4$ content, are between 46.1(2) – 47.0(2) emu g$^{-1}$ for composites sintered at 1373 K and thus independent on the strontium content (y) as expected. The same is true for other sintering temperatures (Fig. S11, supporting information). As mentioned above, these values are slightly lower than the $M_s$ values of pure $NiFe_2O_4$.

The evolution of the permittivity with temperature at 1 kHz depending on the Sr content (y) is depicted in Fig. 12. All samples show increasing permittivity values with rising temperatures and a decrease of $\epsilon_r'$ above 473 K. The permittivity curves of the ceramic bodies do not show a clear maximum reflecting the transition from the ferroelectric to the paraelectric state. In contrast, measurements on phase-pure $Sr_yBa_1-yNb_2O_6$ ceramics show dielectric maxima at 324(2), 353(2), 375(2), and 506(2) K (Fig. S7, supporting information), in accordance with literature data [36,39]. As shown in the inset of Fig. 12, the DC resistivity values, calculated from the impedance data, of the $(NiFe_2O_4)_{0.3}-(Sr_yBa_1-yNb_2O_6)_{0.7}$ composites lie in the same order of magnitude. Composites with y = 0.5 and 0.6 reveal only slightly lower DC resistivity values than samples with y = 0.3 and 0.7.

The maximal magnetoelectric response ($\alpha_{MEmax}$) of $(NiFe_2O_4)_{0.3}-(Sr_yBa_1-yNb_2O_6)_{0.7}$ samples depending on y is shown in Fig. 13. $\alpha_{MEmax}$ rises from y = 0.3 to y = 0.5 reflecting the increasing piezoelectric coefficient of $Sr_yBa_1-yNb_2O_6$ with rising strontium content (y) [40]. However, the significantly lower $\alpha_{MEmax}$ values for y = 0.6 and especially for 0.7 is unexpected. The resistivities of the samples (inset in

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Dielectric behavior ($\epsilon_r'$/closed symbols, tan $\delta$/open symbols) at 1 kHz for $(NiFe_2O_4)_{0.3}-(Sr_yBa_1-yNb_2O_6)_{0.7}$ ceramic bodies sintered at 1373 K for 1 h. For the sake of clarity every 4th data point is represented by a symbol. The inset shows the specific DC resistivity in dependence of the strontium content (y). The uncertainties of the data are smaller than the symbol size.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Maximum magnetoelectric coefficient ($\alpha_{ME}$) depending on the sintering temperature for (NiFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ composites.</figcaption></figure>

Fig. 12), which also influenced the magnetoelectric output, differ only slightly from each other (1.6 – 4.5·10³ kΩcm) and can therefore not explain the reduced $\alpha_{MEmax}$ values for y > 0.5. We suppose that the reason for the decline of $\alpha_{MEmax}$ lie in the microstructure and the formation of non-ferroelectric SrNb₂O₆ [78] for y = 0.7. Due to the higher calcining temperatures necessary for the formation of phase-pure Sr_yBa₁-yNb₂O₆ (y = 0.6, 0.7) the ferroelectric phase in the resulting composites reveal considerably larger grain sizes. For example, after sintering at 1373 K, the size of the Sr_yBa₁-yNb₂O₆ grains are between 1 – 6 µm (y = 0.3, 0.5), 1 – 12 µm (y = 0.6), and 2 – 33 µm (y = 0.7), whereas the size of the NiFe₂O₄ grains (0.5 – 3 µm) do not differ significantly with y (Fig. S12, supporting information). We suppose that the large grain size of the ferroelectric phase for y > 0.5 leads to a poorer contact between the ferrimagnetic and ferroelectric grains. The strong reduction of $\alpha_{ME}$ for composites with y = 0.7 may additionally caused by the formation of considerable amounts of SrNb₂O₆ of about 21 vol%, which appears as cuboid-like grains (1 – 4 µm) near the Sr₀.₇Ba₀.₃Nb₂O₆ grains and hinders an effective mechanical coupling between nickel ferrite and strontium barium niobate grains (Fig. S13, supporting information). The complete development of $\alpha_{ME}$ depending on HDC for all (NiFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ samples is shown in Fig. S14 (supporting information).

The highest magnetoelectric coefficient was found for y = 0.5, whereas Sr_yBa₁-yNb₂O₆ with significantly higher strontium content are less suitable as ferroelectric components for 0 – 3 magnetoelectric composite ceramics. Additionally, during the sintering process, composites with a Sr_yBa₁-yNb₂O₆ composition near the limit of the ferroelectric range (y = 0.3, 0.7), tend to segregate into barium- and strontium-rich phases, respectively.

## 4. Conclusion

Magnetoelectric (NiₓCo₁-xFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ composite ceramics were prepared by the conventional mixed-oxide method. The influence of the nickel content on the magnetoelectric output was investigated on samples containing Sr₀.₅Ba₀.₅Nb₂O₆ as ferroelectric component. After sintering at 1373 K for 1 h, densities of the ceramics in the range 70–85% were achieved, which slightly decrease with rising nickel content (x). SEM investigations confirm a 0–3 connectivity of the samples with NiₓCo₁-xFe₂O₄ grains up to 5 µm and Sr₀.₅Ba₀.₅Nb₂O₆ grains up to 9 µm. DC conductivities of the ceramic samples differs only slightly with x. The magnetoelectric coefficient ($\alpha_{ME}$) shows hystereses with maxima/minima at HDC between ±1.0 and ±5.0 kOe depending on x. The maximum magnetoelectric coefficient ($\alpha_{MEmax}$) considerably rises with nickel content from 40 to 180 µV Oe⁻¹ cm⁻¹ (@ 900 Hz). The development of $\alpha_{MEmax}$ with x is similar to the evolution of the dynamic

magnetostriction of the ferrite phase. The magnetic Curie temperature also increases with nickel content, however the Curie temperatures of the (NiₓCo₁-xFe₂O₄)₀.₃–(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites are lower than for pure NiₓCo₁-xFe₂O₄ ceramics most likely because of cation vacancies in the ferrite phase. The influence of the Sr/Ba ratio in the ferroelectric phase was investigated on (NiFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ ceramic specimens. $\alpha_{ME,max}$ increases with the strontium content, however for y ≫ 0.5 $\alpha_{ME,max}$ considerably decreases due to the formation of a larger amount of secondary phases and/or large grain sizes of Sr_yBa₁-yNb₂O₆ (y > 0.5) which hinders an effective coupling between the ferrimagnetic and ferroelectric phase.

The investigations show that the magnetoelectric performance of the system (NiₓCo₁-xFe₂O₄)–(Sr_yBa₁-yNb₂O₆) is influenced by the stoichiometries of both composite phases. Additionally, the study emphasized the importance of the dynamic magnetostriction for the magnetoelectric output.

## CRediT authorship contribution statement

**Roberto Köferstein:** Writing – review & editing, Writing – original draft, Validation, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. **Maria-Sophie Wartmann:** Valiation, Investigation. **Stefan G. Ebbinghaus:** Writing – review & editing, Validation, Supervision, Resources.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.materresbull.2024.112860.

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<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Maximum magnetoelectric coefficient ($\alpha_{ME}$) depending on the sintering temperature for (NiFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ composites.</figcaption></figure>

Fig. 12), which also influenced the magnetoelectric output, differ only slightly from each other (1.6 – 4.5·10³ kΩcm) and can therefore not explain the reduced $\alpha_{MEmax}$ values for $y > 0.5$. We suppose that the reason for the decline of $\alpha_{MEmax}$ lie in the microstructure and the formation of non-ferroelectric SrNb₂O₆ [78] for $y = 0.7$. Due to the higher calcining temperatures necessary for the formation of phase-pure Sr_yBa₁-yNb₂O₆ ($y = 0.6, 0.7$) the ferroelectric phase in the resulting composites reveal considerably larger grain sizes. For example, after sintering at 1373 K, the size of the Sr_yBa₁-yNb₂O₆ grains are between 1 – 6 µm ($y = 0.3, 0.5$), 1 – 12 µm ($y = 0.6$), and 2 – 33 µm ($y = 0.7$), whereas the size of the NiFe₂O₄ grains (0.5 – 3 µm) do not differ significantly with $y$ (Fig. S12, supporting information). We suppose that the large grain size of the ferroelectric phase for $y > 0.5$ leads to a poorer contact between the ferrimagnetic and ferroelectric grains. The strong reduction of $\alpha_{ME}$ for composites with $y = 0.7$ may additionally caused by the formation of considerable amounts of SrNb₂O₆ of about 21 vol%, which appears as cuboid-like grains (1 – 4 µm) near the Sr₀.₇Ba₀.₃Nb₂O₆ grains and hinders an effective mechanical coupling between nickel ferrite and strontium barium niobate grains (Fig. S13, supporting information). The complete development of $\alpha_{ME}$ depending on HDC for all (NiFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ samples is shown in Fig. S14 (supporting information).

The highest magnetoelectric coefficient was found for $y = 0.5$, whereas Sr_yBa₁-yNb₂O₆ with significantly higher strontium content are less suitable as ferroelectric components for 0 – 3 magnetoelectric composite ceramics. Additionally, during the sintering process, composites with a Sr_yBa₁-yNb₂O₆ composition near the limit of the ferroelectric range ($y = 0.3, 0.7$), tend to segregate into barium- and strontium-rich phases, respectively.

## 4. Conclusion

Magnetoelectric (NiₓCo₁-xFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ composite ceramics were prepared by the conventional mixed-oxide method. The influence of the nickel content on the magnetoelectric output was investigated on samples containing Sr₀.₅Ba₀.₅Nb₂O₆ as ferroelectric component. After sintering at 1373 K for 1 h, densities of the ceramics in the range 70–85% were achieved, which slightly decrease with rising nickel content ($x$). SEM investigations confirm a 0–3 connectivity of the samples with NiₓCo₁-xFe₂O₄ grains up to 5 µm and Sr₀.₅Ba₀.₅Nb₂O₆ grains up to 9 µm. DC conductivities of the ceramic samples differs only slightly with $x$. The magnetoelectric coefficient ($\alpha_{ME}$) shows hystereses with maxima/minima at HDC between ±1.0 and ±5.0 kOe depending on $x$. The maximum magnetoelectric coefficient ($\alpha_{MEmax}$) considerably rises with nickel content from 40 to 180 µV Oe⁻¹ cm⁻¹ (@ 900 Hz). The development of $\alpha_{MEmax}$ with $x$ is similar to the evolution of the dynamic

magnetostriction of the ferrite phase. The magnetic Curie temperature also increases with nickel content, however the Curie temperatures of the (NiₓCo₁-xFe₂O₄)₀.₃–(Sr₀.₅Ba₀.₅Nb₂O₆)₀.₇ composites are lower than for pure NiₓCo₁-xFe₂O₄ ceramics most likely because of cation vacancies in the ferrite phase. The influence of the Sr/Ba ratio in the ferroelectric phase was investigated on (NiFe₂O₄)₀.₃–(Sr_yBa₁-yNb₂O₆)₀.₇ ceramic specimens. $\alpha_{ME,max}$ increases with the strontium content, however for $y \gg 0.5$ $\alpha_{ME,max}$ considerably decreases due to the formation of a larger amount of secondary phases and/or large grain sizes of Sr_yBa₁-yNb₂O₆ ($y > 0.5$) which hinders an effective coupling between the ferrimagnetic and ferroelectric phase.

The investigations show that the magnetoelectric performance of the system (NiₓCo₁-xFe₂O₄)–(Sr_yBa₁-yNb₂O₆) is influenced by the stoichiometries of both composite phases. Additionally, the study emphasized the importance of the dynamic magnetostriction for the magnetoelectric output.

## CRediT authorship contribution statement

**Roberto Köferstein:** Writing – review & editing, Writing – original draft, Validation, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. **Maria-Sophie Wartmann:** Valuation, Investigation. **Stefan G. Ebbinghaus:** Writing – review & editing, Validation, Supervision, Resources.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.materresbull.2024.112860.

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Magnetoelectric, dielectric, and magnetic investigations of multiferroic

NiₓCo₁₋ₓFe₂O₄–SrᵧBa₁₋ᵧNb₂O₆ composites

Roberto Köferstein*, Maria-Sophie Wartmann, and Stefan G. Ebbinghaus

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

* Corresponding author

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

E-mail address: roberto.koeferstein@chemie.uni-halle.de
Magnetoelectric, dielectric, and magnetic investigations of multiferroic

NiₓCo₁₋ₓFe₂O₄–SrᵧBa₁₋ᵧNb₂O₆ composites

Roberto Köferstein*, Maria-Sophie Wartmann, and Stefan G. Ebbinghaus

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

* Corresponding author

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

E-mail address: roberto.koeferstein@chemie.uni-halle.de
<table><caption>**Table 1:** Theoretical bulk densities of (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻_yNb₂O₆)₀.₇ composites</caption><thead><tr><th rowspan="2">Composites<br>(NiFe<sub>2</sub>O<sub>4</sub>)<sub>0.3</sub>-(Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.7</sub></th><th colspan="2">Single crystal densities of the single<br>composite compounds (g cm<sup>-3</sup>)</th><th rowspan="2">Calculated theoretical bulk densities<br>of the composites according to [2]*<br>(g cm<sup>-3</sup>)</th></tr><tr><th>Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub></th><th>NiFe<sub>2</sub>O<sub>4</sub> [1]</th></tr></thead><tbody><tr><td>y = 0.3</td><td>5.40<sup>a</sup></td><td>5.38</td><td>5.39</td></tr><tr><td>y = 0.5</td><td>5.37<sup>b</sup></td><td>5.38</td><td>5.37</td></tr><tr><td>y = 0.6</td><td>5.28<sup>c</sup></td><td>5.38</td><td>5.30</td></tr><tr><td>y = 0.7</td><td>5.28<sup>d</sup></td><td>5.38</td><td>5.30</td></tr></tbody></table>

[1] M. A. Gabal and Y. M. Al Angari, Mater. Chem. Phys. 118 (2009) 153–160; ICSD #165448

[2] G. W. Marks and L. A. Monson, Ind. Eng. Chem. 47 (1955) 1611–1620.

* The theoretical bulk densities represent the maximum achievable densities.

The used X-ray densities were taken from available database entries near to the sample composition.

a) $Sr_{0.33}Ba_{0.67}Nb_2O_6$ (S. Podlozhenov, et al., Acta Cryst. B 62 (2006) 960–965; ICSD #240387)

b) $Sr_{0.50}Ba_{0.50}Nb_2O_6$ (S. S. Rathore and S. Vitta, AIP Conf. Proc. 1591 (2014) 133–135; ICSD #291175)

c) $Sr_{0.61}Ba_{0.39}Nb_2O_6$ (S. Podlozhenov, et al., Acta Cryst. B 62 (2006) 960–965; ICSD #240389)

d) $Sr_{0.75}Ba_{0.27}Nb_2O_6$ (P. B. Jamieson, et al., J. Chem. Phys. 48 (1968) 5048–5047; ICSD #15614)

<figure><img src="image_16.png" /></figure>

**Fig. S1:** Room-temperature XRD patterns of (a) $Sr_{0.3}Ba_{0.7}Nb_2O_6$, calcined at 1473 K for 5 h, b) $Sr_{0.5}Ba_{0.5}Nb_2O_6$, calcined at 1473 K for 5 h, (c) $Sr_{0.6}Ba_{0.4}Nb_2O_6$, calcined at 1573 K for 5 h and (d) $Sr_{0.7}Ba_{0.3}Nb_2O_6$, calcined at 1703 K for 30 h; (heating-/cooling rate 5 K min⁻¹). The vertical bars show peak positions according to the available database entries close to the sample composition: (a) ICSD #240387 (Sr<sub>0.33</sub>Ba<sub>0.67</sub>Nb<sub>2</sub>O<sub>6</sub>), (b) ICSD #240388 (Sr<sub>0.48</sub>Ba<sub>0.52</sub>Nb<sub>2</sub>O<sub>6</sub>), (c) ICSD #240389 (Sr<sub>0.61</sub>Ba<sub>0.39</sub>Nb<sub>2</sub>O<sub>6</sub>) (d) ICSD #15614 (Sr<sub>0.75</sub>Ba<sub>0.27</sub>Nb<sub>2</sub>O<sub>6</sub>).
<table><caption>**Table 1:** Theoretical bulk densities of (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻_yNb₂O₆)₀.₇ composites</caption><thead><tr><th rowspan="2">Composites<br>(NiFe<sub>2</sub>O<sub>4</sub>)<sub>0.3</sub>-(Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub>)<sub>0.7</sub></th><th colspan="2">Single crystal densities of the single<br>composite compounds (g cm<sup>-3</sup>)</th><th rowspan="2">Calculated theoretical bulk densities<br>of the composites according to [2]*<br>(g cm<sup>-3</sup>)</th></tr><tr><th>Sr<sub>y</sub>Ba<sub>1-y</sub>Nb<sub>2</sub>O<sub>6</sub></th><th>NiFe<sub>2</sub>O<sub>4</sub> [1]</th></tr></thead><tbody><tr><td>y = 0.3</td><td>5.40<sup>a</sup></td><td>5.38</td><td>5.39</td></tr><tr><td>y = 0.5</td><td>5.37<sup>b</sup></td><td>5.38</td><td>5.37</td></tr><tr><td>y = 0.6</td><td>5.28<sup>c</sup></td><td>5.38</td><td>5.30</td></tr><tr><td>y = 0.7</td><td>5.28<sup>d</sup></td><td>5.38</td><td>5.30</td></tr></tbody></table>

[1] M. A. Gabal and Y. M. Al Angari, Mater. Chem. Phys. 118 (2009) 153–160; ICSD #165448

[2] G. W. Marks and L. A. Monson, Ind. Eng. Chem. 47 (1955) 1611–1620.

* The theoretical bulk densities represent the maximum achievable densities.

The used X-ray densities were taken from available database entries near to the sample composition.

a) $Sr_{0.33}Ba_{0.67}Nb_2O_6$ (S. Podlozhenov, et al., Acta Cryst. B 62 (2006) 960–965; ICSD #240387)

b) $Sr_{0.50}Ba_{0.50}Nb_2O_6$ (S. S. Rathore and S. Vitta, AIP Conf. Proc. 1591 (2014) 133–135; ICSD #291175)

c) $Sr_{0.61}Ba_{0.39}Nb_2O_6$ (S. Podlozhenov, et al., Acta Cryst. B 62 (2006) 960–965; ICSD #240389)

d) $Sr_{0.75}Ba_{0.27}Nb_2O_6$ (P. B. Jamieson, et al., J. Chem. Phys. 48 (1968) 5048–5047; ICSD #15614)

<figure><img src="image_16.png" /></figure>

**Fig. S1:** Room-temperature XRD patterns of (a) $Sr_{0.3}Ba_{0.7}Nb_2O_6$, calcined at 1473 K for 5 h, b) $Sr_{0.5}Ba_{0.5}Nb_2O_6$, calcined at 1473 K for 5 h, (c) $Sr_{0.6}Ba_{0.4}Nb_2O_6$, calcined at 1573 K for 5 h and (d) $Sr_{0.7}Ba_{0.3}Nb_2O_6$, calcined at 1703 K for 30 h; (heating-/cooling rate 5 K min⁻¹). The vertical bars show peak positions according to the available database entries close to the sample composition: (a) ICSD #240387 ($Sr_{0.33}Ba_{0.67}Nb_2O_6$), (b) ICSD #240388 ($Sr_{0.48}Ba_{0.52}Nb_2O_6$), (c) ICSD #240389 ($Sr_{0.61}Ba_{0.39}Nb_2O_6$) (d) ICSD #15614 ($Sr_{0.75}Ba_{0.27}Nb_2O_6$).
<figure><img src="image_17.png" /><figcaption>**Fig. S2:** EDX spectra of selected $(Ni_xCo_{1-x}Fe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ composite ceramics sintered at 1373 K for 1 h with (a) $x = 1$, (b) $x = 0.5$, and (c) $x = 0$ at the positions marked in the pictures below. (1) $Sr_{0.5}Ba_{0.5}Nb_2O_6$ grain, (2) $Ni_xCo_{1-x}Fe_2O_4$ grain, and (3) grain of the secondary phase ($MFe_2O_6$, M = Ni/Co).</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. S2:** EDX spectra of selected $(Ni_xCo_{1-x}Fe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ composite ceramics sintered at 1373 K for 1 h with (a) $x = 1$, (b) $x = 0.5$, and (c) $x = 0$ at the positions marked in the pictures below. (1) $Sr_{0.5}Ba_{0.5}Nb_2O_6$ grain, (2) $Ni_xCo_{1-x}Fe_2O_4$ grain, and (3) grain of the secondary phase ($MFe_2O_6$, M = Ni/Co).</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. S3:** Magnetization at 300 K for (Ni$_{x}$Co$_{1-x}$Fe$_{2}$O$_{4}$)$_{0.3}$-(Sr$_{0.5}$Ba$_{0.5}$Nb$_{2}$O$_{6}$)$_{0.7}$ composite ceramics sintered at 1373 K for 1 h. The magnetization values are given with respect to the sample mass.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. S3:** Magnetization at 300 K for (Ni$_{x}$Co$_{1-x}$Fe$_{2}$O$_{4}$)$_{0.3}$-(Sr$_{0.5}$Ba$_{0.5}$Nb$_{2}$O$_{6}$)$_{0.7}$ composite ceramics sintered at 1373 K for 1 h. The magnetization values are given with respect to the sample mass.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. S4:** Saturation magnetization (Ms) depending on the Fe/M ratio in NiFe₂O₄ (1) and CoFe₂O₄ (2) ceramics sintered at 1373 K for 1 h. a) MFe₁.₉O₃.₈₅, b) MFe₁.₉₅O₃.₉₃, c) MFe₂O₄, d) M₀.₉₅Fe₂O₃.₉₅, e) M₀.₉₀Fe₂O₃.₉ (M = Ni, Co).</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. S4:** Saturation magnetization (Ms) depending on the Fe/M ratio in NiFe₂O₄ (1) and CoFe₂O₄ (2) ceramics sintered at 1373 K for 1 h. a) MFe₁.₉O₃.₈₅, b) MFe₁.₉₅O₃.₉₃, c) MFe₂O₄, d) M₀.₉₅Fe₂O₃.₉₅, e) M₀.₉₀Fe₂O₃.₉ (M = Ni, Co).</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. S5:** Curie temperature of NiFe₂O₄ (1) and CoFe₂O₄ (2) ceramics sintered at 1373 K for 1 h with different Fe/M ratios. a) MFe₁.₉O₃.₈₅, b) MFe₁.₉₅O₃.₉₃, c) MFe₂O₄, d) M₀.₉₅Fe₂O₃.₉₅, e) M₀.₉₀Fe₂O₃.₉₃ (M = Ni, Co). The Curie temperatures were determined by thermogravimetric measurements in a magnetic field from the cooling curve.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. S5:** Curie temperature of NiFe₂O₄ (1) and CoFe₂O₄ (2) ceramics sintered at 1373 K for 1 h with different Fe/M ratios. a) MFe₁.₉O₃.₈₅, b) MFe₁.₉₅O₃.₉₃, c) MFe₂O₄, d) M₀.₉₅Fe₂O₃.₉₅, e) M₀.₉₀Fe₂O₃.₉₃ (M = Ni, Co). The Curie temperatures were determined by thermogravimetric measurements in a magnetic field from the cooling curve.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. S6:** Room-temperature XRD patterns of NiFe₂O₄ (1) and CoFe₂O₄ (2) ceramics sintered at 1373 K for 1 h with different Fe/M ratios (M = Ni, Co). a) MFe₁.₉O₃.₈₅ (Fe/M = 1.90), b) MFe₂O₄ (Fe/M = 2.00), c) M₀.₉₀Fe₂O₃.₉ (Fe/M = 2.22). The patterns were recorded with a prolonged counting time of 5 s per data point.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Fig. S6:** Room-temperature XRD patterns of NiFe₂O₄ (1) and CoFe₂O₄ (2) ceramics sintered at 1373 K for 1 h with different Fe/M ratios (M = Ni, Co). a) MFe₁.₉O₃.₈₅ (Fe/M = 1.90), b) MFe₂O₄ (Fe/M = 2.00), c) M₀.₉₀Fe₂O₃.₉ (Fe/M = 2.22). The patterns were recorded with a prolonged counting time of 5 s per data point.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. S7:** Dependence of the real part of the permittivity ($\epsilon_r'$) on temperature at 1 kHz for various $Sr_yBa_{1-y}Nb_2O_6$ ceramic bodies sintered between 1473 and 1703 K for 1 h. For the sake of clarity every second data point is represented by a symbol. The inset shows the specific DC resistivity ($\rho_{DC}$) depending in the strontium content (y) of the $Sr_yBa_{1-y}Nb_2O_6$ ceramics. The DC resistivity values were calculated by fitting the impedance data by an equivalent circuit consisting of one resistance-capacitor (RC) element including a constant phase-shift element. The uncertainties of the data are smaller than the symbol size.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Fig. S7:** Dependence of the real part of the permittivity ($\epsilon_r'$) on temperature at 1 kHz for various $Sr_yBa_{1-y}Nb_2O_6$ ceramic bodies sintered between 1473 and 1703 K for 1 h. For the sake of clarity every second data point is represented by a symbol. The inset shows the specific DC resistivity ($\rho_{DC}$) depending in the strontium content (y) of the $Sr_yBa_{1-y}Nb_2O_6$ ceramics. The DC resistivity values were calculated by fitting the impedance data by an equivalent circuit consisting of one resistance-capacitor (RC) element including a constant phase-shift element. The uncertainties of the data are smaller than the symbol size.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. S8:** Reciprocal real part of the permittivity ($\epsilon_r^{-1}$) versus temperature at 1 kHz of $(Ni_xCo_{1-x}Fe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ ceramic bodies sintered for 1 h at 1373 K.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Fig. S8:** Reciprocal real part of the permittivity ($\epsilon_r^{-1}$) versus temperature at 1 kHz of $(Ni_xCo_{1-x}Fe_2O_4)_{0.3}-(Sr_{0.5}Ba_{0.5}Nb_2O_6)_{0.7}$ ceramic bodies sintered for 1 h at 1373 K.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Fig. S9:** Room-temperature XRD patterns of (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻*_y*Nb₂O₆)₀.₇ crushed composite ceramics after sintering at 1373 K for 1 h (heating-/cooling rate 5 K min⁻¹).</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Fig. S9:** Room-temperature XRD patterns of (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻*_y*Nb₂O₆)₀.₇ crushed composite ceramics after sintering at 1373 K for 1 h (heating-/cooling rate 5 K min⁻¹).</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>**Fig. S10:** Room-temperature XRD patterns of (NiFe₂O₄)₀.₃-(Sr_yBa₁-yNb₂O₆)₀.₇ crushed composite ceramics after sintering at 1423 K for 1 h (heating-/cooling rate 5 K min⁻¹). For the sample with y = 0.3, EDX measurements of Ba₃(Fe,Ni)Nb₂O₂₁ grains suggest a Fe/Ni ratio of about 2.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>**Fig. S10:** Room-temperature XRD patterns of (NiFe₂O₄)₀.₃-(Sr_yBa₁-yNb₂O₆)₀.₇ crushed composite ceramics after sintering at 1423 K for 1 h (heating-/cooling rate 5 K min⁻¹). For the sample with y = 0.3, EDX measurements of Ba₃(Fe,Ni)Nb₂O₂₁ grains suggest a Fe/Ni ratio of about 2.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Fig. S11:** Magnetization at 300 K for (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻*_yNb₂O₆)₀.₇ composite ceramics sintered at 1373 K (a) and 1423 K (b) for 1 h. The magnetization values are given with respect to the nominal NiFe₂O₄ content. In the inset the magnetization values are given with respect to the sample mass.</figcaption></figure>
<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /><figcaption>**Fig. S11:** Magnetization at 300 K for (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻*_yNb₂O₆)₀.₇ composite ceramics sintered at 1373 K (a) and 1423 K (b) for 1 h. The magnetization values are given with respect to the nominal NiFe₂O₄ content. In the inset the magnetization values are given with respect to the sample mass.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Fig. S12:** SEM-BSE images of (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻*_y*Nb₂O₆)₀.₇ ceramics sintered at 1373 K for 1 h.</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Fig. S12:** SEM-BSE images of (NiFe₂O₄)₀.₃-(Sr_yBa₁⁻*_yNb₂O₆)₀.₇ ceramics sintered at 1373 K for 1 h.</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Fig. S13:** SEM-BSE images of $(NiFe_2O_4)_{0.3}-(Sr_{0.7}Ba_{0.3}Nb_2O_6)_{0.7}$ ceramics sintered at 1373 K for 1 h. SBN70 = $Sr_{0.7}Ba_{0.3}Nb_2O_6$, SN = $SrNb_2O_6$, NF = $NiFe_2O_4$.</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>**Fig. S13:** SEM-BSE images of $(NiFe_2O_4)_{0.3}-(Sr_{0.7}Ba_{0.3}Nb_2O_6)_{0.7}$ ceramics sintered at 1373 K for 1 h. SBN70 = $Sr_{0.7}Ba_{0.3}Nb_2O_6$, SN = $SrNb_2O_6$, NF = $NiFe_2O_4$.</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>**Fig. S14:** Magnetoelectric coefficient ($\alpha_{ME}$) vs. magnetic DC field for $(NiFe_2O_4)_{0.3}- (Sr_yBa_{1-y}Nb_2O_6)_{0.7}$ composites sintered at 1373 K for 1 h.</figcaption></figure>
<figure><img src="image_30.png" /><figcaption>**Fig. S14:** Magnetoelectric coefficient ($\alpha_{ME}$) vs. magnetic DC field for $(NiFe_2O_4)_{0.3}- (Sr_yBa_{1-y}Nb_2O_6)_{0.7}$ composites sintered at 1373 K for 1 h.</figcaption></figure>