Element-specific study of the coupled magneto-structural and magneto-electronic properties of CoNi nanoarrays
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The magneto-structural (MS) and magneto-electronic (ME) effects, as well as their coupling relationship, were investigated in electroless-plated (EL) Co0.5Ni0.5 arrays treated by post N2 annealing and in situ field plating. Separately and combined, these two treatments have been widely employed to improve the properties of magnetic nanostructures. This work aimed to discriminate between treatments with respect to electronic and structural properties, and magnetic degrees of freedom of Co0.5Ni0.5 nanostructures. The field-plated sample exhibited a strong MS–ME coupling due to magneto-crystalline anisotropy (MCA), arising from a FCC (111) preferred orientation with lattice planes stacking orthogonally to the long axial direction of the arrays. A large coercivity was observed in this structure, arising from high magnetic stability. X-ray magnetic circular dichroism revealed that magnetization was enhanced primarily by Co magnetism, while the field-plated sample underwent a MS/ME transition with corresponding increase of the plating field. Conversely, the heat-treated sample comprised isotropically oriented nanocrystals approximately 20 ± 3 nm in diameter, coated with an oxidation layer (approximately 5 ± 2 nm thick). The absence of MCA in these samples ensured a weak MS–ME coupling. Although the Ni magnetization of heat-treated samples remained close to that of the field-plated sample, the Co constituent exhibited CoO and Co3O4 phases in addition to the metallic state. By contrast, the Co constituent of the field-plated sample was mainly metallic. The lack of MCA, combined with a complex Co magnetic state, appears responsible for the divergent macroscopic magnetic behaviors of the heat-treated and the field-plated samples. By isolating changes in local magnetic moments of Ni and Co, we gained a fundamental understanding of the effects of post-N2 annealing and field plating on CoNi. Such knowledge may assist researches in improving the magnetic properties of bimetallic nanostructures.
KeywordsNanoarrays CoNi XMCD Electroless Magneto-crystalline anisotropy
The authors appreciate the great help of TEM analyses from Mr. Cheng-Yu Hsieh. This work is supported by the National Science Council of Taiwan, under Grant No. NSC 98-2112-M-009 022-MY3.
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