Coherent magnetization reversal and high magnetic coercivity in Co nanowire assemblies

Nanowires (NWs) of single crystalline hcp Co with length from 200 to 530 nm and diameter from 8 to 20 nm (corresponding to the aspect ratio from 10 to 66) are synthesized via a solvothermal method by controlling the Co-precursor to amine mole concentration. The increased aspect ratio leads to enhanced coercivity of randomly oriented Co NWs up to an optimum value of 6.7 kOe, for the NWs of average length 200 nm and average diameter 15 nm (aspect ratio ∼13). Alignment of the NWs in a magnetic field leads to further enhanced coercivity up to a doubled value of 12.5 kOe at 300 K. The high magnetic coercivity achieved in the random and aligned assemblies is due to both the magnetocrystalline anisotropy and the shape anisotropy. For a better understanding of the coercivity mechanism of the NWs, angular dependence of the coercivity has been experimentally investigated for the aligned NW assemblies and the corresponding magnetization reversal mode is determined to be a coherent reversal mode according to an analytical simulation based on the Stoner-Wohlfarth model.