γ-Fe_2O_3/NiO核-壳纳米花的合成、微结构与磁性

Synthesis, microstructure, and magnetic properties of γ-Fe_2O_3/NiO core/shell nanoflowers

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).

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, Fe2O3 nanospheres are synthesized by the solvothermal reaction of Fe Cl3 dissolved in ethylene glycol and Na Ac. Secondly, Fe2O3/Ni（OH）2core/shell precursor is fabricated by solvothermal method through using the early Fe2O3 spheres and Ni（NO3）2·6H2O in an ethanol solution. Finally, the precursor Fe2O3/Ni（OH）2is calcined in air at 300？C 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 Fe2O3/Ni（OH）2precursor has significant influences on the grain growth, the NiO content and the compactness of NiO shells in theγ-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 Ms and smaller HC） 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（HE = 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（HC（FC） = 252 and 288 Oe, respectively）. Further, the values of HE and HC 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.