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Abstract
We show how nanocrystalline Co films formed by separated oblique nano-sheets display anisotropy in their resistivity, magnetization process, surface nano-morphology and optical transmission. After performing a heat treatment at 270 °C, these anisotropies decrease. This loss has been monitored measuring the resistivity as a function of temperature. The resistivity measured parallel to the direction of the nano-sheets has been constant up to 270 °C, but it decreases when measured perpendicular to the nano-sheets. This suggests the existence of a structural relaxation, which produces the change of the Co nano-sheets during annealing. The changes in the nano-morphology and the local chemical composition of the films at the nanoscale after heating above 270 °C have been analysed by scanning transmission electron microscopy (STEM). Thus, an approach and coalescence of the nano-sheets have been directly visualized. The spectrum of activation energies of this structural relaxation has indicated that the coalescence of the nano-sheets has taken place between 1.2 and 1.7 eV. In addition, an increase in the size of the nano-crystals has occurred in the samples annealed at 400 °C. This study may be relevant for the application in devices working, for example, in the GHz range and to achieve the retention of the anisotropy of these films at higher temperatures.
Highlights
The deposition of films at oblique angles of incidence has become an object of interest since novel applications were observed in them because of their particular nano-morphology based on nano-columns for some cases [1]
The well-known magnetic anisotropy of these samples has been extended in this work to include the resistivity, surface nano-morphology and optical transmission anisotropies, as well as their simultaneous vanishing
After annealing the Co samples to 270 ◦ C, the anisotropy in these physical properties has disappeared, even though no growth of the nano-crystals or crystallization processes of the as-deposited film have occurred at that temperature
Summary
The deposition of films at oblique angles of incidence has become an object of interest since novel applications were observed in them because of their particular nano-morphology based on nano-columns for some cases [1]. The particular geometry of the nano-columnar films was tailored to different shapes such as vertical [7], zigzag [8] or nano-stripes [9], depending on the deposition characteristics The presence of these nano-columns generated the anisotropic behaviour of the magnetic [10,11,12,13], optical [14,15], and transport properties [16,17,18]. No uniaxial magnetic anisotropy in the film plane was shown
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