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A series of multiferroic (1− x )BiFeO 3 − x (Bi 0.5 Na 0.5 )TiO 3 (BF–BNT) ( x = 0 − 0.6) solid solution ceramics were prepared by a sol–gel method. The XRD results show that increasing BNT content induce a gradual phase transformation from rhombohedral to pseudocubic structure near x = 0.4. Compared with pure BiFeO 3 , superior multiferroic properties are obtained for x = 0.3 with remnant polarization P r = 1.49 μC/cm 2 and saturated magnetization M s = 0.51 emu/g. Importantly, the paramagnetic (PM) to ferromagnetic (FM) transition is observed for the solutions, and the Curie temperature ( T C ) can be tuned by varying the content of BNT. This observed FM ordering is discussed in terms of the possible existence of the long-range superexchange interaction of Fe 3+ –O–Ti–O–Fe 3+ in the chemically ordered regions.

The structural, magnetic and ferroelectric properties of the (1-x)BiFeO3-xSrTiO3 (0 ≤ x ≤ 1.0) solid solutions synthesized by a modified Pechini method are investigated. Structure phase transition from rhombohedral to cubic phase was found with x increasing from the X-ray diffraction pattern. Compared with antiferromagnetic BiFeO3 compound, BFST-x (0.1 ≤ x ≤ 0.4) ceramics show ferromagnetism at room temperature, with their remnant magnetization (Mr) and coercive field (HC) strongly dependent on the content of SrTiO3. Moreover, appropriate SrTiO3 solution with BiFeO3 significantly reduces electric leakage and enhances electrical polarization. Among all the samples, the x = 0.2 samples shows the optimal ferromagnetism with Mr ～ 0.10 emu/g and the x = 0.3 samples the optimal ferroelectricity with Pr ～ 7.3 μC/cm2 at room temperature.

A granular system composed of ferromagnetic NiFe 2 O 4 nanoparticles embedded in the Cu diluted NiO matrix has been synthesized by the chemical concentration precipitation method. Phase composition analysis shows that Cu ions have been successfully incorporated into NiO matrix substituting for Ni ions. The magnetic properties of all the bulk samples were investigated by measuring their magnetization as a function of temperature and magnetic field. The field-cooling hysteresis loop shift was observed after cooling the nanocomposites. With the increase of the Cu dilution ratio, the saturation magnetization first increases, and then decreases after reaching a maximum. The exchange bias field gradually decreases with the Cu dilution ratio, reaching a plateau when x ≥ 0.05. These results are tentatively interpretated by the misalignment of magnetic moments between the ferromagnetic and the antiferromagnetic spins. The decrease of the coupling area between the ferromagnetic phase and the antiferromagnetic phase at the interface is another possible reason.

La 0.4 Sr 0.6 Ti 1− x Ni x O 3−δ powders with x =0 and 2% were synthesized by the sol–gel method and characterized by a variety of techniques. We mainly investigate the effect of annealing conditions on the structural and magnetic properties of La 0.4 Sr 0.6 Ti 1− x Ni x O 3−δ powders. X-ray diffraction and transmission electron microscopy studies indicate all samples annealed at high temperature ( T >873 K) are single phase and no secondary magnetic phase in this study. By the commercial physical properties measurements system (PPMS), the sample with x =0 exhibits diamagnetic behavior, whereas all the samples with x =2% are found to be ferromagnetic even at room temperature. With the increase of annealing temperature, the ferromagnetism of the samples increases initially, and then it decreases after reaching the maximum value. The structural defects such as oxygen vacancies mediated mechanism is reasonable explanation of the magnetic properties in the present results.

The training effect has been systematically studied in exchange coupled NiFe2O4/NiO nanogranular system. Both exchange bias field (HEB) and vertical magnetization shifts (MShift) can be observed after the system field cooled from 350 K to low temperatures, which decrease monotonically through consecutive loop cycles. During this procedure, linear dependence between HEB and MShift is found for this system, revealing the critical role of the pinned uncompensated spins. With the increase of cooling field, the relative change of HEB becomes more pronounced, which shows that the rapid reduction of the pinned uncompensated spins for high cooling field. Moreover, the reduction of HEB becomes weakened with decreasing measured temperatures, which indicated the spin configuration at low temperatures possesses higher dynamic stability. The cooling field and temperature dependence on training effect is discussed in terms of the evolution of the metastable spin configurations at the interfaces and fitted by a recent t...

Structural, magnetic, and ferroelectric properties have been investigated for Bi2Fe4(1−x)Ti4xO9 (0≤x≤0.2) bulk ceramics, which were synthesized by a modified Pechini method. X-ray diffraction reveals that all samples are single phase with no impurities detected. Compared with antiferromagnetic Bi2Fe4O9 compound, doping with Ti ions induces the appearance of weak ferromagnetism at room temperature, which is discussed in terms of the collapse of the frustrated antiferromagnetic spin structure. Moreover, appropriate Ti doping also significantly reduces electric leakage and leads to the enhancement of electrical polarization. Among all samples, the optimal multiferroics with Mr∼0.0188 emu/g and Pr∼0.262 μC/cm2 at room temperature is found for x=0.15 ceramics. It is thus shown that Ti-doped Bi2Fe4O9 is a promising candidate for preparing multiferroic materials.

A series of Cu1−xFexO (x=0.10, 0.15, 0.20, and 0.30) powder samples were synthesized by a coprecipitation method. The exchange bias field (HEB) accompanying vertical magnetization shift is observed in the system at low temperatures, after the sample is cooled from 300 to 10 K under 10 kOe magnetic field. The exchange bias effect has been investigated for Cu1−xFexO with different doping concentration. Although the magnetic properties increases with the increasing doping concentration, the HEB and vertical magnetization shift vary nonmonotonously. The significant difference is indicated the exchange bias effect can be controlled by tuning the doping concentration for altering coupling interaction at interface layers. Furthermore, the exchange bias field shows a linear dependence on the vertical shift. The exchange coupling at the interface between the ferromagnetic phase and the spin-glass-like phase (or antiferromagnetic) can explain these phenomenon.

Effect of particle size on exchange bias in Fe-doped CuO nanoparticles is investigated, which are sintered at different temperatures from 350 to 650 °C, respectively. The structure and magnetic properties for different particle size samples were probed. It is found that the system shows magnetic properties transition from paramagnetic to ferromagnetic with increasing grain size, and exhibits the variations in exchange bias field (HEB) and coercivity (HC) at low temperature after field-cooled from 300 K. With the increase in the particles size, HEB decreases monotonously. Furthermore, vertical magnetization shift was also observed for the small particles. Exchange bias is attributed to the exchange coupling interactions between ferromagnetic and spin-glass-like (or antiferromagnetic) phase interface layers.