Multiferroic materials, which exhibit two or more primary iron orders (such as ferroelectricity and ferromagnetism), hold a significant potential in sensing and storage applications. BiFeO₃ stands as the most representative room-temperature single-phase multiferroic material. Its ferroelectricity originates from lattice distortion caused by the 6s² lone-pair electrons of Bi³⁺hybridizing with O 2p, while magnetism arises from Fe—O—Fe super-exchange. However, its narrow bandgap results in high leakage current and coercive field, making ferroelectric characterization challenging. Also, the periodic arrangement of G-type antiferromagnetic structures renders it macroscopically non-magnetic with a weak magnetoelectric coupling. Researchers develop some methods such as ion substitution and process optimization, among which composites with magnetic materials are effective in inducing magnetism and enhancing magnetoelectric coupling. M-type strontium ferrite (i.e., SrFe₁₂O₁₉, abbreviated as SrM) as a typical permanent magnet with high saturation magnetization has attracted attention due to its complex magnetic structure that can induce ferroelectric polarization, but its high leakage current limits its application in electrically controlled magnetism. To address this issue, this study was to introduce SrM into the BiFeO₃ matrix to enhance magnetism and magnetoelectric coupling. To overcome the high leakage current of pure BiFeO₃, we selected 0.75BiFeO₃-0.25BaTiO₃ (BFO-BTO) with superior iron-voltage properties as a ferroelectric matrix.We prepared a series of (1-x)(BFO-BTO)/xSrM composite multiferroic ceramics via solid-state reaction and systematically investigated the evolution of their structural, ferroelectric, magnetic, and magnetoelectric coupling properties.

In this study, (1-x)(BFO-BTO)/xSrM composite ceramics were synthesized by a solid-state reaction method. Bi₂O₃, Fe₂O₃, SrCO₃, BaCO₃, and TiO₂ (all with purity ≥99.9%) were used as raw materials. The materials were weighed in stoichiometric proportions after drying and ground in deionized water for 24 h. The slurries were dried and sieved, then subjected to pre-sintering for BFO-BTO at 870 ℃ for 3 h and SrM at 1100 ℃ for 3 h. The resulting powders were mixed proportionally, ground again for 24 h, dried, granulated, and pressed into green bodies (10 mm diameter, 1-2 mm thickness), followed by sintering at 940 ℃ for 3 h. The sintered samples were polished and coated with silver paste, then fired at 700 ℃ for electrodes.

The crystal structure was analyzed by X-ray diffraction (XRD). The surface morphology was determined by scanning electron microscopy (SEM), and the elemental distribution was analyzed by energy dispersive spectroscopy (EDS). After 30 min polarization in a DC electric field of 40 kV/cm, the samples underwent performance testing. The dielectric properties were obtained by an impedance analyzer with a high-temperature testing system. The room-temperature ferroelectric properties were measured by a ferroelectric analyzer. The room-temperature magnetic properties before and after polarization were determined by a vibrating sample magnetometer (±20 kOe). The linear magnetoelectric coupling coefficient was determined by a magnetoelectric effect measurement system (at a test frequency of 2 kHz, DC bias magnetic fields of 0-3 kOe, and an AC excitation magnetic field of 1.07 Oe. The coupling coefficient was calculated based on α_ME = δV/(δHt).

The structure, electrical, magnetic and magnetoelectric coupling properties of BFO-BTO/SrM composite ceramics are systematically investigated. The XRD patterns indicate that the composite consists of coexisting perovskite-type BFO-BTO and hexagonal SrM phases. The proportion of rhombohedral R3c phase in BFO-BTO gradually increases, while the tetragonal P4mm phase decreases as the SrM content increases, demonstrating that SrM addition affects the phase composition of BFO-BTO. The Rietveld refinement results are in reasonable agreement with the XRD patterns. The backscattered electron imaging reveals that the SrM phase (dark region) is uniformly distributed within the BFO-BTO matrix (bright region), exhibiting a point-like dispersion at a low doping and aggregation at a high doping. The surface scanning confirms a minor interphase element diffusion between the two phases. The average grain size of BFO-BTO increases from 0.80 μm to 1.53 μm with increasing the SrM content. The XPS spectra reveal that Fe in all the samples exists in mixed valence states of Fe²⁺/Fe³⁺, with the valence ratio showing a minimal variation upon SrM addition. The oxygen vacancy concentration has a slight increase as the SrM content increases, remaining in the range of 45%-56%.

The dielectric properties show that the dielectric constant and loss increase gradually with the increase of SrM content, which is attributed to the Maxwell-Wagner interface polarization effect. Two dielectric anomaly peaksappear in the high temperature dielectric spectra at 510 ℃ and 600 ℃, corresponding to the Curie temperature of SrM and the BFO-BTO ferroelectric phase transition temperature, respectively. The ferroelectric measurements demonstrate that a moderate content of SrM (i.e., x=0.015) optimizes the ferroelectric properties, achieving a remanent polarization of 27.33 μC·cm^-2. However, the excessive content of SrM causes a sharp increase in leakage current and ferroelectric deterioration. The PFM analysis confirms that SrM addition weakens the polarization response. The results of magnetic property tests reveal that the magnetization strength of the composite material increases significantly with the increase of SrM content. As x=0.015, the saturation magnetization reaches 4.15 emu·g^-1, which exceeds that of pure BFO-BTO (i.e., 0.4 emu·g^-1). The sample exhibits a pronounced magnetic anisotropy, with a parallel magnetization outperforming perpendicular magnetization. After poling treatment, the remanence of the sample changes markedly, indicating the presence of stress/charge-mediated magnetoelectric coupling. The results of magnetoelectric coupling tests demonstrate that the coupling coefficient exhibits an initial increase and a subsequent decrease with increasing SrM content, reaching a maximum of 0.72 mV·cm^-1·Oe^-1 as x=0.035. An appropriate incorporation of SrM enhances strain-mediated magnetoelectric coupling, whereas the excessive addition degrades the piezoelectric performance and weakens the coupling effect. Compared with the similar studies, this system demonstrates a superior comprehensive performance optimization.

This study prepared (1-x)BFO-BTO/xSrM composite multiferroic ceramics. The results showed that the introduction of SrM could modulate the phase composition, with the optimized ferroelectric performance (i.e., P_r=27.33 μC·cm^-2) as x=0.015and the maximum magnetoelectric coupling coefficient (i.e., 0.72 mV·cm^-1·Oe^-1) as x=0.035. The material could demonstrate potential applications in storage, sensing and related fields.