Structural, morphological and magnetic properties of compositionally modulated CoNi nanowires

Abstract

Ferromagnetic nanowires (NWs) are novel materials that offer unique magnetic properties, as the geometrical dimensions become comparable to key length scales in magnetism, such as the exchange length or the domain wall width. In this work, compositionally modulated Co1−xNix nanowires (diameter 10–15 nm) with high coercivity have been synthesized via a thermal decomposition method. The structural analysis demonstrates that the hexagonal close-packed (hcp) crystal structure and the wire-shape morphology are maintained up to Ni content of x = 0.3. Based on the shape anisotropy and orientation, the aligned Co1−xNix nanowire assemblies show that the coercivity at room temperature decreases from 11.4 to 5.4 kOe with increasing x from 0 to 0.3. The monotonous decrease in coercivity with Ni content is related to the effective magnetic anisotropy and nanowire diameter which are found to be strongly varied with Ni addition. In addition, it is found that the increase of Ni content in the nanowires brings more resistance to oxidation than the pristine Co nanowires. The exchange bias study indicates that the Ni addition leads to lower blocking temperature of the CoNiO grains and consequently the switch-on temperature for the exchange bias field shifts to low temperatures with the increase of Ni content. Further, the exchange bias behavior that is associated with the existence of antiferromagnetic and spin-glass-like states are confirmed by temperature-dependent magnetization measurements.