Z.M. Tian

Size effect on magnetic and ferroelectric properties in Bi2Fe4O9 multiferroic ceramics

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 Neel 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).

Enhanced multiferroic properties in Ti-doped Bi2Fe4O9 ceramics

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.

Exchange bias in Fe and Ni codoped CuO nanocomposites

Exchange bias nanocomposites were obtained by the chemical concentration precipitation method, in which the ferrimagnetic MFe2O4 (M=Cu,Ni) particles were embedded in the antiferromagnetic (AFM) CuO matrix. The dependence of magnetization on temperature measurements show that the exchange bias effect in these composites is ascribed to the exchange coupling at the interface between the ferrimagnetic particles and spin-glass-like phase. With continuous introduction of magnetic Ni ions, the existence of domain state structure and the formation of soft magnetic phase in AFM matrix are responsible for the different behaviors of the exchange bias field and coercivity in these nanocomposites.

Spin-glasslike behavior and exchange bias in multiferroic Bi1/3Sr2/3FeO3 ceramics

Spin-glasslike (SGL) behavior and exchange bias (EB) effect have been reported in multiferroic Bi1/3Sr2/3FeO3 ceramics. Temperature dependence of magnetization and high field relaxation properties reveal the existence of SGL phases. After field cooling the sample from 350 to 10 K, exchange bias field (HEB), vertical magnetization shifts (MShift) and increment of saturation magnetization (MS) are observed, and exhibit a strong dependence on the strength of cooling fields. Furthermore, HEB shows a linear dependence on MShift. This observed EB effect is discussed in terms of the exchange coupling between ferromagnetic clusters and the SGL phases at interface.

Exchange bias training effect in NiFe2O4/NiO nanocomposites

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.

Suppression of charge order and exchange bias effect in Nd0.5Ca0.5MnO3 nanocrystalline

An Nd0.5Ca0.5MnO3 (NCMO) sample (average diameter ~45?nm) is synthesized by the sol?gel method. The temperature dependence of magnetization indicates that the charge order state is suppressed and a ferromagnetic (FM) transition occurs at ~100?K. In addition, the magnetic hysteresis loop at 10?K under a cooling field of 10?kOe shifts to both the horizontal and the vertical directions when the measure field is 10?kOe. With an increase in the measure field, both the horizontal and the vertical shifts decrease. When the measure field is 50?kOe, the vertical shift vanishes but the horizontal shift still exists. The observed exchange bias effect is attributed to the exchange coupling between the antiferromagnetic core and the FM shell which embodies spin glass-like surface layers.

Effect of particle size on the exchange bias of Fe-doped CuO nanoparticles

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.

Exchange bias and the origin of room-temperature ferromagnetism in Fe-doped NiO bulk samples

A series of Ni(1−x)FexO (x=0, 0.015, 0.03, 0.05, and 0.1) bulk samples was synthesized by the chemical concentration-precipitation method. Phase composition analysis was carried out, which showed that trace amounts of ferromagnetic phase NiFe2O4 could not be detected by x-ray diffraction in these bulk samples with x≤0.03. When x>0.03, NiFe2O4 ferrite is detected easily. The magnetic properties of all the bulk samples were investigated by measuring their magnetization as a function of temperature and magnetic field. The results indicated that all the bulk samples sintered in air exhibited large room-temperature ferromagnetic behavior ascribed to a ferromagnetic impurity phase. Simultaneously, an exchange bias and training effect were also observed in all the bulk samples, suggesting the possibility of the existence of a strong ferromagnetic/antiferromagnetic exchange coupling in this kind of compound. Specifically, the exchange bias field could be tuned by changing the concentration of the Fe dopant.

Magnetic studies on Mn-doped TiO2 bulk samples

Mn-doped TiO2 (Ti1−xMnxO2) bulk samples with nominal composition x = 0.02, 0.04, 0.08 have been prepared by a solid-state reaction and sintered at different temperatures ranging from 450 °C to 900 °C. For samples with x = 0.02, magnetic investigations show that room temperature ferromagnetism can be obtained and the magnetization of samples decreases monotonically with the increase in the sintering temperature. For the samples sintered at 600 °C with different doping content, the temperature dependence of magnetic susceptibility shows that a magnetic transition appears near room temperature, besides a magnetic transition at 43 K perhaps caused by Mn3O4. From the extrapolation of the inverse magnetic susceptibility curves at high temperatures, a positive Curie–Weiss temperature is obtained and it shifts to a low temperature with Mn doping content, revealing that ferromagnetic coupling decreases monotonically with the increase in Mn doping content. In addition, an exchange bias is clearly observed below 60 K, which also provides strong evidence that Ti1−xMnxO2 is ferromagnetic.

Negative magnetization and zero-field cooled exchange bias effect in Co0.8Cu0.2Cr2O4 ceramics

The negative magnetization and zero-field cooled exchange bias (ZFC EB) effect are observed in Co0.8Cu0.2Cr2O4 polycrystalline ceramics. 20% Cu substitution for Co in CoCr2O4 leads to the evident magnetization reversal at the compensation temperature (Tcomp ∼ 50 K) with applied magnetic field of 500 Oe. Besides, Tcomp decreases monotonously with increasing applied field, and the negative magnetization finally disappears when the field increases to 9000 Oe. Different temperature dependence of sublattice magnetization at different crystallographic sites is proved to induce the magnetization reversal. In addition, ZFC EB effect can be tuned by measuring temperature and presents the maximum of exchange bias field (HEB) with ∼2300 Oe at 50 K. This unconventional EB effect can be attributed to the coupling interaction between the two sublattices.