Flipping anisotropy and changing magnetization reversal modes in nano-confined Cobalt structures

We investigate the temperature dependent magnetic properties of arrays of Cobalt nanowires created by sputtering Co on two separate nanoporous anodized alumina (AAO) templates each with pores of diameter 50 nm and 10 nm. We observe differences in the easy axis of magnetization for the two nanowire arrays and a flip in the easy axis at low temperatures. The easy axis of 50 nm nanowire array at room temperature is found to be perpendicular to the nanowire long axis, whereas for 10 nm arrays the easy axis is parallel to the wire axis. With lowering of temperature, however the easy axes of both arrays orient away from that at room temperature. Along with that the saturation magnetic field and coercive field show sudden increase in their values at low temperatures, accompanied by a crossing over seen in the saturation magnetic fields. We propose changes in magneto-crystalline anisotropy of cobalt present in the nanowires as a result of the stress developed in the nanowire array due to the alumina template, with lowering of temperature. We support our experimental results with micro-magnetic simulations done to generate the M(H) curves and study the magnetization reversal processes in the nanowire arrays with varying anisotropies. With lowering of anisotropy which we believe is triggered by stress at low temperature, our simulations show magnetic vortex like instabilities originating in the nanowires. We suggest the possible modes of domain reversals: curling in 50 nm and either coherent rotation or buckling in 10 nm nanowires through our simulations. Our results suggest that temperature can serve as an effective tool to tune magnetic anisotropy in Co nanowires embedded in AAO and generate unusual magnetic textures.