External Magnetic Field Induced Synthesis And Growth Mechanism Of Ferromagnetic Metallic Materials

It has been generally accepted that the morphology of micro-/nanomaterials influences their properties significantly. So much interest has been attracted home and abroad to research the morphology control. During recent years, the ferromagnetic materials, especially Fe, Co, and Ni, have been studied widely because of their unique properties and potential applications. In this paper, we have investigated the influences of external magnetic fields on the morphology of the ferromagnetic materials such as nickel, cobalt and Ni-Co alloy crystals, and the morphological formation mechanism of ferromagnetic materials under an external magnetic field is investigated systematically.1. The influence of the distribution of magnetic lines of force on the morphology of the nickel crystalsResearch has been done to find the influence of external magnetic lines of force on the morphology of nickel crystals prepared by template-free aqueous solution reduction approach. The results show that one-dimensional nickel crystals formed at the regions where the magnetic lines of force are parallel to each other; and microspheres formed at the regions where the magnetic lines of force are not parallel to each other. According to these results, a multiple-step growth mechanism is suggested. The saturated magnetization and the coercivity of those samples are about 50 emu·g-1, and 100 Oe, which are close to that of bulk nickel. All these data confirm the one-dimensional structures of nickel are the assembly of isotropic nickel microspheres.2. Morphology control of cobalt crystals synthesized under external magnetic fields Cobalt crystals were prepared with the similar template-free aqueous solution reduction approach which was used for obtaining nickel crystals at 60℃. The results show that the morphology of the cobalt can be influenced by the saturated magnetization of the ferromagnetic materials, the distribution of the external magnetic field, the external magnetic field intensity, the self-generated magnetic field and the concentration of the cations. Owing to the much larger saturated magnetization (150.4 emu·g-1) than that of nickel crystals (50 emu·g-1), the cobalt crystals generate much larger magnetic field after being magnetized. The external magnetic fields have to overcome the magnetic field intensity generated by the cobalt crystals to influence the movement of the cobalt granular structures. The external magnetic field with strong magnetic field intensity would influence the morphology of the cobalt crystals if the synthetic progress carried out with low concentration of cobalt cations. The distribution of the external magnetic lines of force has different influences on the morphology of cobalt crystals. At the regions where the magnetic lines of force are parallel to each other, one-dimensional nickel crystals formed. At the regions where the magnetic lines of force are not parallel to each other, microspheres formed. Ferromagnetic materials with small saturated magnetization, preparing with low concentration cations under strong external magnetic field with parallel magnetic lines of force will be optimal conditions to obtain excellent one-dimensional structures.3. The Influence of the Concentration Ratio of Ni2+/Co2+ on the Morphology of Ni-Co Alloys Synthesized under an External Magnetic FieldDifferent morphologies of Ni-Co alloys were gained with varying the concentration ratios of Ni+/Co2+ under an external magnetic field by template-free aqueous solution reduction approach. The results show that same mixture of hcp and fcc metallic phases of Ni-Co alloys were gained with different concentration ratios of Ni2+/Co2+ 1:2,1:1, and 2:1. When the concentration ratio of Ni2+/Co2+ is small, the distribution of the external magnetic lines of force would have weak influence on the morphology of the Ni-Co alloys. When the concentration ratio of Ni2+/Co2+ is large, the distribution of the external magnetic lines of force would have strong influence on the morphology of the Ni-Co alloys. At the regions where the magnetic lines of force are parallel to each other, more and long one-dimensional structures would form. At the regions where the magnetic lines of force are unparallel to each other, larger Ni-Co particles would form mainly.