S.L. Yuan

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).

Exchange bias effect in a granular system of NiFe2O4 nanoparticles embedded in an antiferromagnetic NiO matrix

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.

Effects of temperature and atmosphere on the magnetism properties of Mn-doped ZnO

Magnetic properties of Mn-doped ZnO (Zn0.98Mn0.02O) bulk materials prepared by the solid-state reaction method were investigated by measuring magnetization as functions of temperature and magnetic field. The special feature of our sample preparation was the low-temperature processing. When high-temperature (T>700°C) was used, secondary phase was found. The results indicate that the samples sintered in Ar gas show ferromagnetic behavior at room temperature, but it disappears in samples sintered in air. Even for ferromagnetic samples, the obtained saturation value of magnetization is much smaller than the theoretical value, suggesting the possibility that there is a strong antiferromagnetic exchange coupling in this kind of compound.

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.

Origins of both insulator–metal transition and colossal magnetoresistance in doped manganese perovskites

Based on experiments of both transport and paramagnetic resonance for (La1−xYx)2/3Ca1/3MnO3, the ground state above Tc and possible origins of both insulator–metal transition and colossal magnetoresistance near Tc are discussed. Modeling the system as a network of junctions, each consisting of a paramagnetic region sandwiched between two ferromagnetic domains, a phenomenological expression is proposed for resistance as a function of temperature and magnetic field. We show that the observed transport and magnetotransport phenomena can be quantitatively explained by the present model for the whole temperature range studied.

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.

Percolation model of the temperature dependence of resistance in doped manganese perovskites

Assuming the sample to consist of ferromagnetic metallic (FMM) particles with a volume fraction (f ) randomly distributed over the paramagnetic insulating background, Monte Carlo simulations of electrical conductivity show excellent fits to the data measured in (La1−xYx)2/3Ca1/3MnO3 (x=0, 0.1 and 0.2). We find that the transition to metallic state occurs as the f reaches a percolation threshold, suggesting that the percolation of FMM domains is responsible for the observed insulator–metal transition.

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.