Study On The Fabrication And Magnetic Properties Of Oriented Spinel Type Films Derived From Layered Double Hydroxide Precursors

Spinel ferrite films are an important class of inorganic functional materials. They have potentially been used in many fields such as high density storage, electromagnetic wave absorbing, and magnetic resonance, because of their excellent structural and physicochemical properties. In order to improve the magnetic properties of ferrite films, increasing interest has been paid to regulate the composition, structure and morphologies of ferrite films with simple preparation methods. In the present thesis, our research mainly focuses on the construction of spinel ferrite film materials and the enhancement of corresponding magnetic properties. Oriented spinel type ferrite films have been fabricated through high temperature calcinations process using layered double hydroxides (LDHs) films as precursors. With changing the preparation parameter of LDHs films precursors and controlling the high temperature calcinations process, we have realized controllable preparation of a series of ferrite film materials with different composition, morphology and orientation. Meanwhile, we have also investigated the solvent evaporation process of LDHs nanoparticles and discussed possible driving force in the LDHs film formation process. Finally, magnetic properties of oriented spinel type ferrite films have been investigated in detail. We think our research work may be regarded as some experimental basis for the further study on structural design, building process control and application performance of spinel type ferrite films.The innovation of this thesis and results are as follows:First, in order to beat "superparamagnetic limit" of ferromagnetic nanoparticles in high-density storage field, cobalt-iron layered double hydroxides (CoFe-LDHs) film with (00l) orientation has been fabricated through a solvent evaporation method, and (111) oriented CoFe2O4/CoO nanocomposite film formed in a subsequent thermal treatment process in He atmosphere. The resulting product is composed of nano-sized ferromagnetic (FM) CoFe2O4 particles embedded in antiferromagnetic (AFM) CoO matrix. A topotactic transformation from the LDHs precursor film to the CoFe2O4/CoO nanocomposite film was proposed to cause an exchange coupling between the FM CoFe2O4 and AFM CoO phases in the nanocomposite, leading to an enhanced magnetic stability of the CoFe2O4. The blocking temperature of CoFe2O4 nanoparticles in CoFe2O4/CoO nanocomposite film increased more than 100 k than that for pure CoFe2O4 nanoparticles with similar size. Furthermore, the orientation of the CoFe2O4/CoO nanocomposite film is found to be attributed to the magnetic anisotropy when the magnetic field was applied on different directions.Then, porous oriented NiFe2O4 films have also been prepared using LDH films as precursors. The process mainly involves the formation of (00l) oriented NiFe-LDH film through solvent evaporation, subsequently topotactic transformation from (00l) oriented NiFe-LDH film to (111) oriented NiFe2O4/NiO composite films induced by high-temperature calcination, followed by selective leaching of NiO sacrificial phase from NiFe2O4/NiO composite films. The surface roughness can be tuned by altering calcined temperature or acid treatment time. With hydrophobic treatment, the as-prepared NiFe2O4 film shows stable superhydrophobicity. Furthermore, surface wettability of NiFe2O4 film can be tuned in a wide range through alteration of the surface roughness. The controllable regulations of magnetism and surface wettability make porous oriented NiFe2O4 films could potentially be used in harsh external environments.Besides, we discussed the possible driving force for film formation through investigating the assembly process of LDHs nanoparticles during solvent evaporation. LDHs nanoparticles were prepared at different aging temperatures conditions. TEM, AFM were used to characterize the products. With increasing the aging temperature, the morphology of LDHs nanoparticles changes from spherelike to hexagonal platelike. The increased aspect ratio can be used as a direct evidence for the enhancement of anisotropic degree of LDHs nanoparticles. The film formation results exhibit the aspect ratio of LDHs nanoparticles can greatly influence the continuity of LDHs film and orientation of (00l) crystal plane. The high aspect ratios is favor to the enhancement of Surface-to-surface interaction of LDHs nanoparticles. In NiFe-LDHs system, it is easy to obtain large continuous LDHs film when aspect ratio is greater than 3.Finally, we attempt to prepare LDHs films precursors by coating techniques. MgFe2O4/MgO, NiFe2O4/MgO composite films and pure MgFe2O4 film were obtained after calcinations of corresponding LDHs film precursors. The analysis results show the two phases in the composite films are uniformly interdispersed. At the same time, the magnetization of composite films can be tuned by alterating the M(â…¡)/Fe(â…¢) molar ratio of LDHS precursors. Moreover, the pure MgFe2O4 film exhibits a promising superparamagnetic behavior under room temperature condition.