Magnetic and ferroelectric properties are investigated for the polycrystalline Bi2Fe4O9 ceramics with different grain sizes (60–2000 nm) synthesized by a modified Pechini method. It shows that magnetic and ferroelectric properties are strongly dependent on the grain size. For the 60 nm samples, the magnetization curves exhibit a superimposed behavior of antiferromagnetic (AFM) with ferromagnetic (FM) component. As the grain size increases, FM component is suppressed and AFM interaction becomes dominant. Simultaneously, the Néel temperature (TN) shifts to high temperatures as the grain size increases. Compared with the 60 nm sample, ferroelectric hysteresis loops at room temperature are observed for the samples with large grain sizes (>200 nm) due to the reduced leakage currents. Among all samples, the 900 nm sample is found to have the smallest leakage current density (<10−6) and the largest remnant polarization (0.21 μC/cm2).

A granular system composed of ferrimagnetic NiFe2O4 nanoparticles, about 8 nm in size, embedded in an antiferromagnetic NiO matrix has been synthesized by a high-temperature phase precipitation method from Fe-doped NiO matrix. Both the exchange bias field and vertical magnetization shift can be observed in this system below 250 K after field cooling, above which the exchange bias disappears. Furthermore, the exchange bias field shows a linear dependence on the magnetization shift. This observed exchange bias effect is explained in terms of the exchange interaction between the ferrimagnetic phase and the spin-glass-like phase at the interface.

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Discrete, magnetically recyclable, and oxidation-resistant nanoparticles with nickel/nickel phosphide core-shell structure were synthesized through surface-phosphatizing Ni nanoparticles using triphenylphosphine as the phosphorus source. The Ni 2 P shell thickness was tunable by changing the reaction time in the mild temperature organic solution. And it was shown that the chemical architecture of core-shell can be useful to prepare Ni 3 P nanogranular films through the chemical combination of Ni 2 P shell with inner Ni during annealing. The core-shell magnetic architecture can also be helpful in improving the magnetically thermal stability through the effective influence of surface anisotropy after magnetic surface modification. The mechanism could be used to guide the synthesis and application of similar core-shell structured nanomaterials.

Exchange bias field (HEB) accompanying vertical magnetization shift (ΔM) is observed in a granular system composed of ferrimagnetic (Ferri) NiFe2O4 nanoparticles embedded in an antiferromagnetic NiO matrix, after the sample is cooled from 350 to 10 K under a 40 kOe magnetic field. Consecutive hysteresis loops show that both HEB and ΔM decrease with magnetic field cycling, which is referred to as the training effect. Furthermore, HEB shows a linear dependence on ΔM throughout the training procedure, and HEB originates mainly from the cycle-dependent shift of the left coercivity (HC1) while the right coercivity (HC2) remains almost constant. This observed training effect is interpreted in the framework of the spin configurational relaxation model.