Synthesis Mechanism And Magnetic Properties Of Mn-based Alloys And Compounds Nanoparticles

With the rapid development of the information technology,mass data processing and cloud stroage technique promise to address challenging issues with freshness and rigour.It is universally recognised that any achievement of new technique is based on the corresponding physical medium.The recording techniques and stroage medium have been rapidly developed,while the stroage medium are far behind the step of recoring techniques.Even though,there is a golden time of storage medium in 1990's and the areal density of stroage medium had a leap forward.All the conventional hard disk stroage medium(mainly CoPtCr)is faced with the superparamagnetic limit which is a fatal threat to data storage.New materials is urged for the new page of mass data age.There are two way to solve the challenge of data storage:on one hand,to enhance the sensitivity of read head;on the other hand,to improve the magnetic properties of storage materials.As for the storage materials,the research is stuck in the conventional solidication reaction and is left far behind the research of recording techniques.In order to ensure the information can be stored for a long time,thermal stability of storage materials is required,which means that product of the volume of recording unit(V)and anisotropy value of recording materials(Ku)is large than 45KBT(KB is the Boltzmann constant and T is the Kelvin temperature).To further increase the areal density,the size of recording unit must be reduced,which inevitably leads to reduce the number of material grains and eventually affects the singal to noise ratio(SNR).Therefore,combined with areal density and SNR,materials with high anisotropy values is the key role in the progress of recording materials.Manganese bismuth alloy is a promising magnetic storage medium for its high magnetocrystalline anisotropy values and relatively low saturation magnetization.All the magnetic atoms are from the manganese atoms,which originates from the configuration of extra-nuclear electron of manganese atom.The 3d layer of Mn element is in the state of half full,and according to the Hund rules,Mn is the most potential magnetic element in the 3d transition elements.It is a pity that the study on manganese is confined to the colossal magnetoresistance effect of perovskite manganites and manganese-nickel based Heusler alloys.For the method,it is stagnated at the solidication reaction not even the nano-scale chemical synthesis.The motivation of this thesis is to synthesize Mn-based alloys and compounds with nano-scale chemical reaction,in order to investage the mangetic properties and explore the growth meachinisms,preparation conditions and applications.The main work of this thesis includes the following aspects:1.Systematically study of the possibility of manganese bismuth alloy nanoparticles synthesis by organic chemical method.First of all,air-stable manganese nanoparticles and bismuth particles with variable sizes are synthesized.Based on the previous reports,a series of comparative experiment were carried out to optimize the synthesis conditions of manganese particles.Based on the priciple of electronegativity and the effect on nanocrystallization of halogen ions,one conclusion is proposed:the antioxidation of synthesized manganese particles can be enhanced with the increase of halogen number.Secondly,through the attempt of co-reduction and two-step reduction,limitation of organic chemical syntheis of manganese bismuth alloy nanoparticles were explored and discussed.It is found that the large reduciton potential gradient((Bi3+/Bi=-0.31V,Mn2+/Mn=-1.185V)and the large lattice mismatch(>24%)limit the organic chemical synthesis method,resulting in phase separation phenomenon.In order to ensure the reduction of manganese element,strong reductants are require,which leads to the spontaneous nucleation of manganese particles.All above reasons astrict the organic synthesis of manganese bismuth nanoparticles.2.Synthesis of manganese-nickel core-shell heterogeneous nanoparticles.On the basis of the organic synthesis of manganese bismuth alloy nanoparticles,manganese and nickel are selected as heterogeneous materials to prepare manganese-nickel core-shell heterostructure for smaller lattice mismatch(<10%).After the thermal annealing treatment,pure phase manganese nickel alloy is obtained.Nickel is chosen as the shell element considering the oxophilicity of manganese element.Otherwise,if manganese is chosen as the shell element,unexpected manganese nickel oxide is observed in the XRD results.Furthermore,(NiO)0.5(MnO)0.5 particles can be synthesized through adjusting the Mn:Ni ratio and atmosphere,which is a candidate for anode material of Li-ion battery.And the mangeic properties is measured,which is different from the magnetic shape memory effect of Heusler alloy.3.Synthesis of room temperature ferromagnetic manganese selenide particles.Organic synthesis method of manganese-based alloys and compounds is limited due to the high reduction potential of manganese element:reduce agent must ensure the manganese can be reduced,and not let the manganese to nucleat spontaneously.The rate of particles can be well controlled through hydrothermal method,thereby inhibiting the phase separation in the synthesis process.By adjustin the pH values,nanoscale to microscale manganese selenides/diselenides are synthesized.With the change of particles sizes,the color of manganese selenide/diselenides gradually varies from gray black to yellow green,which can be designed for color/optical coding.The difference between the dangling bonds of manganese and selenide may result in distinct chemical activity of those terminations,which may further lead to their diverse growth rates.Acetic acid is services as a reagent to adjust the pH,and acetic ions are seletively bonded to the manganese termination face,which blocks the growth of absorption face.Above two reasons lead to the quasi-two-dimensional morphology of manganes selendie/diselenide particles.The reason for the transformation of manganese selenide between manganese diselenide is proposed and growth mechanism of seletively synthesis for transition metal selenide is proposed.4.Hydrothermal synthesis of manganese bismuth alloy magnetic nanoparticles.By inhibiting the spontaneous nucleation of manganese particles,the synthesis rate can be well controlled with hydrothermal method.Despite of the melting and fusion of bismuth particles,the magnetic properties of synthesized manganese bismuth nanoparticles are comparable with the reported works using conventional method,which means our report provides a method for mass production and easy manufacture of manganese bismuth alloys nanoparticles,which provides a new idea for the application of magnetic storage.The mechanism of the formation of manganese bismuth particles is proposed for further optimization.In order to synthesize manganese bismuth nanoparticles with chemical method,organic synthesis is commonly used and on the top of the method list.According to the experiment results,organic synthesis method can not effectively take control of the nucleation stage and the growth stage,due to the instinct properties of manganese and bismuth.Thus,large reduction potential difference and large lattice mismatch lead to phase separation,which fails to synthesize manganese bismuth nanoparticles.By controlling the reduction rate and deposition rate of nickel,manganese-nickel core-shell heterogeneous structure is prepared,which can be used to prove that controlling the growth stage can effectively controll the synthesis of heterogeneous alloys.In order to further control the nucleation stage and growth stage,hydrothermal synthesis method is applied.Through the control of acidity,temperature,time and so on,the room temperature ferromagnetic manganese selenide/diselenide were synthesized,whose size can be varied from nanoscale to microscale.Based on the above experiment results,hydrothermal synthesis of manganese bismuth nanoparticles are firstly achieved.By controlling the nucleation stage and growth stage,a facile and economical method is proposed,which brings a great application prospect.