Preparation Of Spinel Ferrite Nanocrystals And Their Magnetic Properties

Due to its unique properties,ferrite nanomaterials have attracted more and more attentions in the fields of magnetic hyperthermia,microwave absorbing materials,magnetic storage and so on.Among them,the magnetic properties of spinel ferrite are closely related to the cation occupation.At the nanoscale,the tendency of cation occupation is quite different from that of the bulk materials,when the size is decrease.This situation provides an idea for the regulation and optimization of magnetic properties.Meanwhile,the corresponding inducement of magnetic change becomes complicated.Therefore,it is of great significance to study the structure-activity relationships among the size,cation occupation and magnetism of ferrite nanomaterials with small size,which is important for understanding the origin mechanism of magnetism and expanding their applications.Based on this,this paper carries out the following aspects:1.Effect of size on the cation occupation and magnetic properties of inverse spinel ferrite nanomaterials.Taking CoFe₂O₄ with inverse spinel structure as an example,single-domain CoFe₂O₄ nanoparticles with controllable size smaller than 10 nm were prepared by thermal decomposition method.As the particle size increases,the saturation magnetization of CoFe₂O₄ nanoparticles increases.However,the coercivity showed an abnormal size-dependent tendency.The coercivity decreases with the increase of particle size.M(?)ssbauer spectra show that more Co²⁺ ions transfer into the tetrahedral site as the particle size increases,which can result in reduction of magnetic anisotropy of CoFe₂O₄ nanoparticles.Besides,we found that Co impurity with fcc or hcp phase can be precipitated in the CoFe₂O₄ nanoparticles by prolonging the reaction time,which can further improve the magnetic property of CoFe₂O₄ nanoparticles.This work provides a new insight to establish the relationship between size,cation distribution,secondary phase precipitation and magnetism.2.Effect of cation occupation on the magnetic properties of mixed spinel ferrite nanomaterials.Taking Fe₃O₄ doped by Mg²⁺ ions with zero magnetic moment as an example,Mg_xFe_3-xO₄(0?x?0.4)nanoparticles were fabricated by thermal decomposition method.By studying the changes of Fe²⁺,Fe³⁺ occupancy sites and net magnetic moments in the nanoparticles,the relationship between cationic occupancy and magnetism was constructed.By increasing the concentration of Mg²⁺,the particle size has no obvious change,and the saturation magnetization shows a trend of decreasing first and then increasing.M(?)ssbauer spectra show that the change of the net magnetic moments is caused by the occupancy of Mg²⁺,Fe²⁺ and Fe³⁺ at the nanometer size,which is accordant with the change of saturation magnetization.These results show that the cation occupancy can be changed by doping elements in spinel ferrite,leading to the adjustment of the magnetic properties of nanoparticles.3.Effect of Zn²⁺ dopants with zero magnetic moment on the cation occupation and magnetic properties of reverse spinel ferrite.We use Zn²⁺ ions as the dopants,which tend to occupy the tetrahedral sites.The cation distribution in CoFe₂O₄ nanoparticles change from the reverse type of CoFe₂O₄ to the positive type of Zn Fe₂O₄.The results show that the saturation magnetization decreases with the increase of Zn²⁺ concentration.This is due to that the A-O-B interactions are weaken and the net magnetic moments are decreased by doping nonmagnetic Zn²⁺.These results further prove that the magnetic properties of spinel ferrite nanostructures depend on the cation occupation and the superexchange interaction between oxygen ions.4.Effect of Cd²⁺ dopants on the magnetic properties of ferrite.Based on the above research,Fe₃O₄ nanoparticles were further doped by Cd²⁺ ions with zero magnetic moment.And the influence of particle size and cation occupation on the magnetic properties of spinel nanoparticles is further investigated.The results show that the non-magnetic Cd²⁺ions can reduce the magnetic properties of the nanoparticles.The magnetic properties of the nanoparticles are consistent with the changes of net magnetic moment and particle size.