γ-Fe_2O_3/NiO核-壳纳米花的合成、微结构与磁性

利用溶剂热/热分解的方法合成出微结构可控的γ-Fe_2O_3/NiO核-壳结构纳米花.分析表明NiO壳层是由单晶结构的纳米片构成,这些纳米片不规则地镶嵌在γ-Fe_2O_3核心的表面.Fe3O4/Ni(OH)_2前驱体的煅烧时间对γ-Fe_2O_3/NiO核-壳体系的晶粒生长、NiO相含量和壳层致密度均有很大的影响.振动样品磁强计和超导量子干涉仪的测试分析表明,尺寸效应、NiO相含量和铁磁-反铁磁界面耦合效应是决定γ-Fe_2O_3/NiO核-壳纳米花磁性能的重要因素.随着NiO相含量的增加,磁化强度减小,矫顽力增大.在5 K下,γ-Fe_2O_3/NiO核-壳纳米花表现出一定的交换偏置效应(H_E=46 Oe),这来自于(亚)铁磁性γ-Fe_2O_3和反铁磁性NiO之间的耦合相互作用.与此同时,这种交换耦合效应也进一步提高了样品的矫顽力(H_C=288 Oe).

Synthesis,microstructure, and magnetic properties of γ-Fe2O3/NiO core/shell nanoflowers

The main purpose of this work is to explore the influences of microstructures on the magnetic properties, as well as the formation mechanism of γ-Fe₂O₃/NiO core/shell nanoflowers. The synthesis of nanoflower-like samples includes three processes. Firstly, Fe₃O₄ nanospheres are synthesized by the solvothermal reaction of FeCl₃ dissolved in ethylene glycol and NaAc. Secondly, Fe₃O₄/Ni(OH)₂ core/shell precursor is fabricated by solvothermal method through using the early Fe₃O₄ spheres and Ni(NO₃)₂·6H₂O in an ethanol solution. Finally, the precursor Fe₃O₄/Ni(OH)₂ is calcined in air at 300 ℃ for 3-6 h, and therefore resulting in γ-Fe₂O₃/NiO core/shell nanoflowers. Their microstructures are characterized by using XRD, XPS, SEM, HRTEM and SAED techniques. The results show that the final powder samples are γ-Fe₂O₃/NiO with typical core/shell structure. In this core/shell system, the γ-Fe₂O₃ sphere acts as core and the NiO acts as shell, which are comprised of many irregular flake-like nanosheets with monocrystalline structure, and these nanosheets are packed together on the surfaces of γ-Fe₂O₃ spheres. The calcination time of Fe₃O₄/Ni(OH)₂ precursor has significant influences on the grain growth, the NiO content and the compactness of NiO shells in the γ-Fe₂O₃/NiO core/shell system. VSM and SQUID are used to characterize the magnetic properties of γ-Fe₂O₃/NiO core/shell nanoflowers. The results indicate that the 3 h-calcined sample displays better ferromagnetic properties (such as higher m_s and smaller H_C) because of their high γ-Fe₂O₃ content. In addition, as the coupling interaction between the FM γ-Fe₂O₃ and AFM NiO components, we observe that the γ-Fe₂O₃/NiO samples formed in 3 h and 6 h display certain exchange bias (H_E=20 and 46 Oe, respectively). Such a coupling effect allows a variety of reversal paths for the spins upon cycling the applied field, and thereby resulting in the enhancement of coercivity (H_C(FC)=252 and 288 Oe, respectively). Further, the values of H_E and H_C for the former are smaller than those of the latter, this is because of the AFM NiO content in 6 h-calcined sample much higher than that in 3 h-calcined sample. Especially, the temperature dependences of the magnetization M of the two samples under both ZFC and FC conditions indicate that an extra anisotropy is induced. In a word, the size effect, NiO phase content, and FM-AFM (where FM denotes the ferromagnetic γ-Fe₂O₃ component, while AFM is the antiferromagnetic NiO component) interface coupling effect have significant influence on the magnetic properties of γ-Fe₂O₃/NiO core/shell nanoflowers.