| As an importment part of functional nanomaterials,magnetic nanocrystals have shown great application potential in many fields,such as data storage,magnetic fluid,biomedical research,magnetic resonance imaging,electrochemistry,catalysis and environment purification.In comparison with the traditional construction methods of magnetic thin-films(e.g.,chemical vapor deposition and magnetron sputtering),the solution-phase synthesis of magnetic nanocrystals has special advantages with simple method,strong operability and low cost.Our research interests are focused on the solution-phase construction of novel magnetic nanocrystals as an inheritance and development of tranditional magnetic thin-film system,which display anisotropic structure character and multi-magnetic integration character.Furthermore,we would explore their novel magnetic properties and understand their physical origin.In this article,we focus on the chemical solution-phase synthesis and magnetic exploration of anisotropic Fe3O4 magnetic nanocrystals and soft/hard bi-magnetic Fe3O4/Mn3O4 hybrid nanocrystals.The main innovation and achievements are as follows:(1)Thin triangular Fe3O4 nanoprisms had been prepared by a modified solvothermal method.After purification,the produced Fe3O4 nanocrystals displayed thin plate charater with an average thickness of 4.1 nm,and the length of triangle was approximately 27.5 nm.The crystalline structure of triangular Fe3O4 nanoprisms was systematically studied.We found that the triangular Fe3O4 nanoprisms were truncated by(111)crystal facet on the top and down surface with large surface area.Further study demonstrated that the Fe3O4(111)crystal facet displayed strong polar character,which could be protected well by the negative charge group,such as carboxyl and amidogen.In addition,we fould that stacking fault existed in the inner of Fe3O4 nanocrystal,which could result in the formation of groove and ridge structure.In comparison with the ridge structure,the groove structure displayed higher reaction activity,leading to the original crystal symmetry breaking and further formation of triangular nanoprisms.We studied the magnetic properties of triangular Fe3O4 nanoprisms.The samples displayed very weak magnetism at room temperature,and more robust ferrimagnetic character with lare saturation magnetization,coercivity and remanence at lower temperature.(2)A relatively green and mild synthetic procedure with seed-mediated method was successfully developed to produce Fe3O4/Mn3O4 hybrid nanocrystals.Briefly,a mixture of Fe3O4 nanoprisms,manganese decanoate,dodecanoic acid,and tetradecane was heated up to 240?;subsequently,a mixture of oleylamine and dodecanol was injected into the solution to facilitate decomposition of manganese decanoate for the epitaxial growth.The product were purified by several times of magnetic separation and centrifugation separation,and displayed a two-layer structure with Mn3O4 nanoprism grown onto Fe3O4 nanoprism,named as P-O-P(Prism-On-Prism)Fe3O4/Mn3O4 hybrid nanocrystals.The formation mechanism of P-O-P Fe3O4/Mn3O4 hybrid nanocrystals was briefly discussed,with consideration of heterogeneous interfacial analysis and Franck-Van der Merwe layer-by-layer growth mode.Furthermore,we studied the magnetic properties of P-O-P samples with the measurements of magnetization versus temperature and versus external applied fields.A series of novel magnetic behaviors were identified when temperature and external applied field were changed,such as compensatin point,spin flop from antiparallel exchange-coupling to twisted phase to parallel exchange-coupling,unusual hysteresis loop character(constricted at low field and expanded at high field),large positive or negative exchange bias,and unique coercivity versus temperature and remanence versus temperature curves.(3)When the synthetic procedure for producing P-O-P Fe3O4/Mn3O4 hybrid nanocrystals was carried out at a lower temperature,190?,the heterogeneous neculation and growth of Mn3O4 nanocrystals would experience a different route to form Mn3O4 nanohorns grown onto the vertexes of triangular Fe3O4 nanoprisms,called as H-O-P(Homs-On-Prism)hybrid nanocrystals.We discussed the formation mechanism of H-O-P Fe3O4/Mn3O4 hybrid nanocrystals,comparing with P-O-P Fe3O4/Mn3O4 hybrid nanocrystals.Also,we gave a detailed study of the magnetic properties of H-O-P samples with the measurements of magnetization versus temperature and versus external applied fields.We found that the novel magnetic behaviors observed in P-O-P samples could also be indentified in H-O-P samples but with differences quantificationally.The differences revealed different exchange-coupling strength for two types of Fe3O4/Mn3O4 hybrid nanocrystals. |
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