Microstructure And Magnetic Properties Of Low-dimensional Nickel-based Magnetic Nano-Metals

Low-dimensional nickel-based magnetic nano-metals are considered as one of the most important research topics in condensed matter physics and material science because of their unique magnetic and electrical properties,and also an important component of a new generation of magnetoelectric multifunctional devices.Current studies show that they own not only novel size,but also obvious interface effects.The ability of conquering magnetic limits makes them possible to be widely used in hard disk drives,high density magnetic recording memory,magnetic random access memory,magnetic field sensors,catalysis and biomedical fields.The feature sizes of magnetoelectronic devices have reached nanometer scale with the development of semiconductor manufacturing technology,the size-effect has begun to significantly affect their physical properties in all aspects;It is significant and essential to study the magnetic properties and magnetic modulation mechanisms of low-dimensional magnetic hybrids at the atomic level.In the thesis,Au-Ni,Cu-Ni heterogeneous nanoparticles and Au/Ni,Cu/Ni multilayered films were prepared by chemical and physical methods respectively.At the atomic scale,their magnetic properties were studied by high resolution spherical aberration corrected transmission electron microscopy(Cs-corrected TEM).The main results are summarized as follows:(1)The exchange bias mechanism of Au-Ni heterogeneous nanoparticles at atomic scale was investigated for the first time by spherical aberration corrected scanning transmission electron microscopy(Cs-corrected STEM).Monodisperse Au-Ni nanoheterojunctions were prepared by high temperature organic liquid method.Morphological characterization showed that individual nanoparticles formed a heterostructure.A 31 Oe weak exchange bias field of the Au-Ni nano-heterojunction was measured by SQUID at 2 K and 2.5 T applied field.In order to figure out the intrinsic mechanism of exchange bias effect,we characterized the interface at atomic scale by Cs-corrected STEM.It was found that there was mutual diffusion at the interface.At 2 K,the dilute magnetic alloy formed by Ni diffused into the Au portion had a spin-glass-like state and the exchange bias effect was produced by the coupling of the spin-glass-like with the ferromagnetic layer Ni.(2)The modulation effect of Cu on the magnetic properties of ferromagnetic Ni in Cu-Ni alloy nanoparticles at atomic scale was studied.Similar with above Au-Ni nanoheterojunctions,the preparation of monodisperse Cu-Ni alloy nanoparticles ws also prepared by a high temperature organic liquid phase.Morphological characterization showed that the Cu-Ni alloy nanoparticles formed a core-shell structure,and both the core and the shell are Cu-Ni alloys.Exchange bias effect was not observed in these CuNi alloy nanoparticles at 2 K measured by SQUID.Component analysis showed that both the core and shell were rich in Ni and showed ferromagnetism at 2 K,and no ferromagnetic/antiferromagnetic interface or similar interface could be formed,which was the reason for the failure of exchange bias effect.(3)In the magnetic multilayered film systems,the modulation effects of Au and Cu on the magnetic properties of ferromagnetic material Ni were also studied at the atomic scale for a comparison.Au/Ni and Cu/Ni multilayered films with various thicknesses of Ni layers were deposited by magnetron sputtering system.The magnetic properties of Au/Ni multilayers and Cu/Ni multilayers were measured by VSM and SQUID at 70 K and room temperature respectively.The experimental results showed that when the temperature was 70 K,the interface effect between Au and Ni layers failed to produce exchange bias effect in Au/Ni multilayer film,the interface effect between Cu and Ni layers also failed to produce exchange bias effect in Cu/Ni multilayer film.