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The Effect of Gelation on the Apparent Magnetism of ZnFe2O4 Sol-Gel Systems  [cached]
Yueqiang Lin,Jian Li,Xiaodong Liu,Jun Fu
Applied Physics Research , 2013, DOI: 10.5539/apr.v5n1p19
Abstract: Experiments have shown that for thixotropic sol-gel systems consisting of ZnFe2O4 nanoparticles without any matrix material, the measured magnetization, or susceptibility of gels, are greater than those of sols. For the reduced susceptibility, a system with a volume fraction of particles of fv=2.0% is lower than a system with fv=1.5%. These results have been interpreted in terms of a magnetization mechanism based on the Brownian rotation of the moments fixed inside the colloidal particles, which would be dramatically affected by the non-magnetic hydrodynamic interaction. For weakly cross-linked gels, the translational degree of freedom is “frozen” while the rotational degree of freedom remains unchanged, so that their hydrodynamic interaction effect is weaker, and they are more easily magnetized than the sols with both rotational and translational degrees of freedom. The action of gelation preventing the hydrodynamic interaction effect on the magnetization process can be referred to as the “gelation decoupling”. Correspondingly, such behavior of the hydrodynamic interaction in affecting the apparent magnetism can be referred to as a “viscomagnetic effect”.
An Overview on Nanocrystalline ZnFe2O4, MnFe2O4, and CoFe2O4 Synthesized by a Thermal Treatment Method  [PDF]
Mahmoud Goodarz Naseri,Elias B. Saion,Ahmad Kamali
ISRN Nanotechnology , 2012, DOI: 10.5402/2012/604241
Abstract: This study reports the simple synthesis of MFe2O4 (where M = Z n , Mn, and Co) nanoparticles by a thermal treatment method, followed by calcination at various temperatures from 723 to 873?K. Poly(vinyl pyrrolidone) (PVP) was used as a capping agent to stabilize the particles and prevent them from agglomeration. The characterization studies were conducted by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average particle sizes were obtained by TEM images, which were in good agreement with the XRD results. Fourier transform infrared spectroscopy (FT-IR) confirmed the presence of metal oxide bands for all the calcined samples. Magnetic properties were demonstrated by a vibrating sample magnetometer (VSM), which displayed that the calcined samples exhibited superparamagnetic and ferromagnetic behaviors. 1. Introduction Nanoscience and nanotechnology are involved in the manipulation of materials and the creation of structures and systems at the nanometer scale. As a result, nanomaterials have attracted much attention because of their surface effect (large surface-to-volume ratio) and quantum confinement effects (size-dependent properties). These factors affect their physical and chemical properties, which differ from the properties of their molecular and bulk counterparts. Nanoparticles with zero-dimensional nanostructures are generally classified according to their compositions, that is, metal oxides, noble metals, transition metals, magnetic metals, and semiconductor nanomaterials or quantum dots. Like all nanostructures, magnetic metals nanoparticles are dependent on their size and shape. Currently, magnetic oxide nanoparticles are attracting significant interest due to their extensive applications, ranging from fundamental research to industrial use. Spinel nanocrystals are regarded as two of the most important inorganic nanomaterials because of their electronic, optical, electrical, magnetic, and catalytic properties. Spinel has the structure AB2O4 in which A and B display tetrahedral and octahedral cation sites, respectively, and O indicates the oxygen anion site. Metal spinel ferrite nanoparticles have the general molecular formula MFe2O4 (e.g., M = Ni, Zn, Mn, Co, or Mg), and they have a face-centered-cubic (fcc) close packing structure. Among the spinel ferrites compounds, zinc ferrite, manganese ferrite, and nickel ferrite have been studied extensively due to their different structures composed of inverse, normal, and mixed spinel structures, respectively, and their high electromagnetic performance, excellent chemical
Muon diffusion and electronic magnetism in Y$_2$Ti$_2$O$_7$  [PDF]
J. A. Rodriguez,A. Yaouanc,B. Barbara,E. Pomjakushina,P. Quémerais,Z. Salman
Physics , 2013, DOI: 10.1103/PhysRevB.87.184427
Abstract: We report a $\mu$SR study in a Y$_2$Ti$_2$O$_7$ single crystal. We observe slow local field fluctuations at low temperature which become faster as the temperature is increased. Our analysis suggests that muon diffusion is present in this system and becomes small below 40 K and therefore incoherent. A surprisingly strong electronic magnetic signal is observed with features typical for muons thermally diffusing towards magnetic traps below $\approx 100$ K and released from them above this temperature. We attribute the traps to Ti$^{3+}$ defects in the diluted limit. Our observations are highly relevant to the persistent spin dynamics debate on $R_2$Ti$_2$O$_7$ pyrochlores and their crystal quality.
Electronic structure and magnetism in X_xW_{1-x}O_3 (X=Nb,V,Re) from supercell calculations  [PDF]
T. Jarlborg
Physics , 2003, DOI: 10.1016/S0304-8853(03)00379-2
Abstract: Some doped semiconductors have recently been shown to display superconductivity or weak ferromagnetism. Here we investigate the electronic structure and conditions for magnetism in a supercells of cubic XW_{26}O_{81}, where X=Nb,V and Re. The undoped material is an insulator, and although the slightly doped material is a metal, it is far from the Stoner criterion of magnetism. The conditions of a localized density-of-states which varies rapidly with the energy, resemble those of doped hexaborides. The virtual crystal approximation is used to vary the doping level. A small moment appears if the Fermi energy coincides with a large derivative of the DOS.
Synthesis and Characterization of Multifunctional Chitosan- MnFe2O4 Nanoparticles for Magnetic Hyperthermia and Drug Delivery  [PDF]
Dong-Hyun Kim,David E. Nikles,Christopher S. Brazel
Materials , 2010, DOI: 10.3390/ma3074051
Abstract: Multifunctional nanoparticles composed of MnFe 2O 4 were encapsulated in chitosan for investigation of system to combine magnetically-triggered drug delivery and localized hyperthermia for cancer treatment with the previously published capacity of MnFe 2O 4 to be used as an efficient MRI contrast agent for cancer diagnosis. This paper focuses on the synthesis and characterization of magnetic MnFe 2O 4 nanoparticles, their dispersion in water and their incorporation in chitosan, which serves as a drug carrier. The surface of the MnFe 2O 4 nanoparticles was modified with meso-2,3-di-mercaptosuccinic acid (DMSA) to develop stable aqueous dispersions. The nanoparticles were coated with chitosan, and the magnetic properties, heat generation and hydrodynamic size of chitosan-coated MnFe 2O 4 were evaluated for various linker concentrations and in a range of pH conditions.
Evidence of defect-induced ferromagnetism in ZnFe$_{2}$O$_{4}$ thin films  [PDF]
C. E. Rodríguez Torres,F. Golmar,M. Ziese,P. Esquinazi,S. P. Heluani
Physics , 2011, DOI: 10.1103/PhysRevB.84.064404
Abstract: X-ray absorption near-edge and grazing incidence X-ray fluorescence spectroscopy are employed to investigate the electronic structure of ZnFe$_{2}$O$_{4}$ thin films. The spectroscopy techniques are used to determine the non-equilibrium cation site occupancy as a function of depth and oxygen pressure during deposition and its effects on the magnetic properties. It is found that low deposition pressures below 10$^{-3}$ mbar cause iron superoccupation of tetrahedral sites without Zn$^{2+}$ inversion, resulting in an ordered magnetic phase with high room temperature magnetic moment.
Mixed Magnetism for Refrigeration and Energy Conversion  [PDF]
N. H. Dung,Z. Q. Ou,L. Caron,L. Zhang,D. T. Cam Thanh,G. A. de Wijs,R. A. de Groot,K. H. J. Buschow,E. Brück
Physics , 2012, DOI: 10.1002/aenm.201100252
Abstract: The efficient coupling between lattice degrees of freedom and spin degrees of freedom in magnetic materials can be used for refrigeration and energy conversion. This coupling is enhanced in materials exhibiting the giant magnetocaloric effect. First principle electronic structure calculations on hexagonal MnFe(P, Si) reveal a new form of magnetism: the coexistence of strong and weak magnetism in alternate atomic layers. The weak magnetism of Fe layers (disappearance of local magnetic moments at the Curie temperature) is responsible for a strong coupling with the crystal lattice while the strong magnetism in adjacent Mn-layers ensures Curie temperatures high enough to enable operation at and above room temperature. Varying the composition on these magnetic sublattices gives a handle to tune the working temperature and to achieve a strong reduction of the undesired thermal hysteresis. In this way we design novel materials based on abundantly available elements with properties matched to the requirements of an efficient refrigeration or energy-conversion cycle.
回流法制备MnFe2O4纳米晶及磁性能  [PDF]
四川师范大学学报(自然科学版) , 2014,
Abstract: 以NaOH为沉淀剂,通过一种简单的低温回流法制备了MnFe2O4纳米晶.采用XRD、SEM、FESEM、FTIR和VSM等手段对MnFe2O4纳米晶尺寸、晶相结构和磁性能进行了分析表征.结果表明,所制备的MnFe2O4纳米晶为尖晶石相结构,晶粒尺寸随着反应温度的升高而增大,从50℃的10nm增大到90℃的22nm;样品的饱和磁化度随温度的升高而增大.
Modelling magnetism of C at O and B monovacancies in graphene  [PDF]
T. P. Kaloni,M. Upadhyay Kahaly,R. Faccio,U. Schwingenschl?gl
Physics , 2013, DOI: 10.1016/j.carbon.2013.07.062
Abstract: The presence of defects can introduce important changes in the electronic structure of graphene, leading to phenomena such as C magnetism. In addition, vacancies are reactive and permit the incorporation of dopants. This paper discusses the electronic properties of defective graphene for O and B decoration. Phonon calculations allow us to address directly the stability of the systems under study. We show that it is possible to obtain magnetic solutions with and without dangling bonds, demonstrating that C magnetism can be achieved in the presence of B and O.
Electronic structure and magnetism of transition metal doped Zn12O12 clusters: Role of defects  [PDF]
Nirmal Ganguli,Indra Dasgupta,Biplab Sanyal
Physics , 2010, DOI: 10.1063/1.3525649
Abstract: We present a comprehensive study of the energetics and magnetic properties of ZnO clusters doped with 3d transition metals (TM) using ab initio density functional calculations in the framework of generalized gradient approximation + Hubbard U (GGA+U) method. Our results within GGA+U for all 3d dopants except Ti indicate that antiferromagnetic interaction dominates in a neutral, defect-free cluster. Formation energies are calculated to identify the stable defects in the ZnO cluster. We have analyzed in details the role of these defects to stabilize ferromagnetism when the cluster is doped with Mn, Fe, and Co. Our calculations reveal that in the presence of charged defects the transition metal atoms residing at the surface of the cluster may have an unusual oxidation state, that plays an important role to render the cluster ferromagnetic. Defect induced magnetism in ZnO clusters without any TM dopants is also analyzed. These results on ZnO clusters may have significant contributions in the nanoengineering of defects to achieve desired ferromagnetic properties for spintronic applications.
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