S.Y. Yin

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.

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.

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.

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.

Giant magnetoresistance and unusual hysteresis behavior in La0.67Ca0.33MnO3∕xCuO (x=20%) composite

La0.67Ca0.33MnO3(LCMO)∕xCuO (x=0 and 20mole%) composites were fabricated and investigated in detail for their electrical and magnetic transport properties. Compared with pure LCMO, the magnetoresistance (MR) effect for x=20% sample is substantially enhanced especially at the vicinity of TIM, where TIM is the transition temperature from insulator to metallic state. Application of even a low magnetic field of H=0.5T leads to a MR value as large as ∼98%. It is interesting to note that at the same temperature region where substantial MR effect is obtained, considerable thermal and magnetic hystereses in resistivity (ρ) appear. The measurement of magnetization (M) versus magnetic field (H) also exhibits an unusual magnetic hysteresis. Due to the absence of these unusual hysteresis behaviors in pure LCMO, it is discussed that the abnormal experimental observations are attributed to the spin disorder especially at grain boundaries induced by the interaction product layer between LCMO and CuO.

Multiferroic properties in Ba0.93Bi0.07Ti1−xMnxO3 ceramics

Nominal composition of Ba0.93Bi0.07Ti1−xMnxO3 (x=0, 0.02, and 0.04) ceramics have been prepared by a modified Pechini method. X-ray diffraction analysis reveals that the samples are pure perovskite BaTiO3 (BTO) structure with no trace of impurity phase. The cell volume of the composites increases monotonously with the increase in Mn content, which indicates that Mn ions have been incorporated into the lattice of Ba0.93Bi0.07TiO3. The samples are experimentally confirmed to show ferromagnetic (FM) and ferroelectric behaviors simultaneously at room temperature. The temperature dependent magnetic behaviors show complex magnetic interactions including FM, antiferromagnetic, and paramagnetic. These results suggest that the dopant Bi and Mn help BTO transit from pure ferroelectric to multiferroic materials and the magnetic behaviors can be explained by the bound magnetic polaron model with inhomogeneity of Mn dopant distribution.Nominal composition of Ba{sub 0.93}Bi{sub 0.07}Ti{sub 1-x}Mn{sub x}O{sub 3} (x=0, 0.02, and 0.04) ceramics have been prepared by a modified Pechini method. X-ray diffraction analysis reveals that the samples are pure perovskite BaTiO{sub 3} (BTO) structure with no trace of impurity phase. The cell volume of the composites increases monotonously with the increase in Mn content, which indicates that Mn ions have been incorporated into the lattice of Ba{sub 0.93}Bi{sub 0.07}TiO{sub 3}. The samples are experimentally confirmed to show ferromagnetic (FM) and ferroelectric behaviors simultaneously at room temperature. The temperature dependent magnetic behaviors show complex magnetic interactions including FM, antiferromagnetic, and paramagnetic. These results suggest that the dopant Bi and Mn help BTO transit from pure ferroelectric to multiferroic materials and the magnetic behaviors can be explained by the bound magnetic polaron model with inhomogeneity of Mn dopant distribution.

Exchange bias effect in Cu1−xFexO (0<x≤0.30) composites

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.