High-Coercivity Cobalt Nanowire Arrays: Synthesis and Heat Treatment

Abstract

An anodic aluminum oxide (AAO) template with a highly ordered structure is prepared and used for the synthesis of Co nanowire arrays. The surface of Co nanowires is smooth and uniform, the diameter is 50 nm, the length is about 20 μm, and the aspect ratio of a Co nanowire is about 400. A large aspect ratio represents a strong shape anisotropy and dipolar interactions, resulting in high coercivity. In addition, the small diameter of the nanowires also contributes to the increased coercivity because the small size hinders the propagation mechanism of the domain walls during magnetic inversion. The magnetic curve in the parallel configuration shows a higher coercivity and remanence than that in the perpendicular configuration, and the rectangle is also more obvious. The shape anisotropy of nanowires tends to be magnetized along the axis, so an easy magnetization axis is parallel to nanowires. In order to investigate the effect of heat treatment on the magnetic properties of nanowires, we treated the samples for different times. The Co 3 O 4 (shell)−Co (core) structure was formed after the surface oxidation of the annealed Co nanowires. The grain size and diameter after heat treatment are twice as large as those without treatment; the thickness of the shell is about 25 nm, and the diameter of the core is about 50 nm. The core−shell interface is clearly distinguishable with increasing roughness. Due to the coexistence of two phases and the increase in grain size, the saturated magnetic moments decrease, and the coercivity and remanence ratio increase with the increase of heat treatment time. There is an exchange interaction between the core and shell magnetic phases, and the magnetic signal is a result of the competing ferromagnetic core and antiferromagnetic shell. The Co 3 O 4 (shell)−Co (core) nanowires are basically in the ferromagnetic state, the coercivity is twice that of Co showing a large magnetocrystalline anisotropy and energy, and the direction parallel to the nanowire axis is easier to be magnetized. The exchange bias effect can persist up to above room temperature, which is directed by the interfacial spin pinning effect, and the coercivity and the shift of the hysteresis loop depend on the contribution of the interfacial surface spins to the magnetization reversal.