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Zhang Y.,Wuhan University | Zhang Y.,Hubei Key Laboratory on Organic and Polymeric Opto electronic Materials | Fei C.,Wuhan University | Liu Y.,Wuhan University | And 7 more authors.
Journal of Nanoscience and Nanotechnology | Year: 2010

Cobalt ferrite (CoFe 2O 4) nano-particles were synthesized by the hydrothermal method with the addition of a surfactant sodium bis(2-ethylhexyl) sulphosuccinate (AOT). Characterization measurements including X-ray diffraction, transmission electron microscopy and fourier transform infrared spectroscopy showed that all the final products were single-phase CoFe 2O 4 nano-crystals with AOT molecules bonding to the surfaces, the average crystallite sizes were all near 25 nm, and the lattice constant increased with the increasing mass of AOT. The magnetic hysteresis loops measured at room temperature indicated that the bonding of the AOT to the surfaces led to an increase of the saturation magnetization (Ms), the coercivity (Hc) and the remanence ratio (Mr/Ms). Furthermore, as the concentration of AOT reached the critical micelle concentration (CMC), turning points were observed in the the curves of Hc, Mr/Ms and K eff (effective magnetic anisotropy constants) versus, the mass of AOT due to the formation of the AOT micelles. Copyright © 2010 American Scientific Publishers. Source


Gao R.-R.,Wuhan University | Zhang Y.,Wuhan University | Zhang Y.,Huazhong University of Science and Technology | Yu W.,Wuhan University | And 5 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2012

MnFe 2O 4 nano-particles with an average size of about 7 nm were synthesized by the thermal decomposition method. Based on the magnetic hysteresis loops measured at different temperatures the temperature-dependent saturation magnetization (M S) and coercivity (H C) are determined. It is shown that above 20 K the temperature-dependence of the M S and H C indicates the magnetic behaviors in the single-domain nano-particles, while below 20 K, the change of the M S and H C indicates the freezing of the spin-glass like state on the surfaces. By measuring the magnetization-temperature (M-T) curves under the zero-field-cooling (ZFC) and field-cooling procedures at different applied fields, superparamagnetism behavior is also studied. Even though in the ZFC M-T curves peaks can be observed below 160 K, superparamagnetism does not appear until the temperature goes above 300 K, which is related with the strong inter-particle interaction. © 2012 Elsevier B.V. All rights reserved. Source


Zhang Y.,Wuhan University | Liu Y.,Wuhan University | Fei C.,Wuhan University | Yang Z.,Wuhan University | And 7 more authors.
Journal of Applied Physics | Year: 2010

Cobalt ferrite nanoparticles were synthesized via the hydrothermal route with the addition of trisodium citrate dihydrate (Na3 CA2 H 2 O). The characterizations including x-ray diffraction (XRD) and transmission electron microscope showed that the products of this hydrothermal reaction are composed of cobalt ferrite nanocrystallite and a small amount of FeOOH, and the average crystallite size of the nanoparticles is 7.6±0.3 nm by XRD. The magnetic measurements revealed the temperature-dependent magnetic properties: The superparamagnetism occurs above 380 K due to the overcoming of energy barrier for the flip of spins, which arises from the magnetocrystalline anisotropy energy and the interparticle interactions due to the aggregation of the nanoparticles; a frozen spin-glasslike state was observed below 20 K, which is accompanied with the decrease in coercivity and high-field paramagnetic susceptibility, as well as the enhancement of saturated magnetization and the effective magnetic anisotropy constant. © 2010 American Institute of Physics. Source


Zhang Y.,Wuhan University | Dong F.,Wuhan University | Liu Y.,Wuhan University | Fei C.,Wuhan University | And 7 more authors.
Materials Chemistry and Physics | Year: 2010

Composite ceramics composed of the ferrimagnetic (FM) CoFe 2O4 and the antiferromagnetic (AFM) CoO were synthesized by using the chemical combustion method. The characterization measurements including X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis show that CoFe2O4 nano-particles are embedded in CoO matrix. The magnetic measurements show that the hysteresis loops display a small negative exchange bias field (He) of 106 Oe as the ceramic is cooled to 10 K in an applied field of 10,000 Oe. Furthermore, the magnetic parameters including coercivity, remanence, and He show clear dependence on temperature and the applied magnetic field in the cooling procedures. This exchange bias effect is ascribed to the exchange coupling at the FM/AFM interfaces. © 2010 Elsevier B.V. All rights reserved. Source


Zhang Y.,Wuhan University | Liu Y.,Wuhan University | Yang Z.,Wuhan University | Xiong R.,Wuhan University | And 4 more authors.
Journal of Nanoparticle Research | Year: 2011

Cobalt ferrite (CoFe2O4) nanoparticles were synthesized by using the hydrothermal route with the addition of trisodium citrate dihydrate (Na3CA•2H2O). The formation of CoFe2O4 nanoparticles with size ranging from 13 to 19 nm was confirmed by X-ray diffraction, energy dispersive X-ray analysis, and Fourier transform infrared spectroscopy; the clear-cut sharp of the nanoparticles was observed by transmission electron microscopy. By these characterization methods, the evolution of lattice constant and morphologies of the nanoparticles with the addition of Na3CA•2H2O is observed. Furthermore, the magnetic hysteresis loops measured at room temperature indicate that the magnetic properties of the products also show clear relationship with the masses of Na3CA•2H2O. For example, coercivity and high-field paramagnetic susceptibility increase with the increasing masses of Na3CA•2H2O, whereas the saturation magnetization and the effective magnetic anisotropy constant have the maximum values as the mass of Na3CA•2H2O is 1 g. This change of magnetic properties is related with the expanded lattice and the varied size and shape because of the addition of Na3CA•2H 2O. © 2011 Springer Science+Business Media B.V. Source

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