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Growth mechanism and magnon excitation in NiO nanowalls.

Gandhi AC, Huang CY, Yang CC, Chan TS, Cheng CL, Ma YR, Wu SY - Nanoscale Res Lett (2011)

Bottom Line: The nanosized effects of short-range multimagnon excitation behavior and short-circuit diffusion in NiO nanowalls synthesized using the Ni grid thermal treatment method were observed.This study shows that short spin correlation leads to an exponential dependence of the growth temperatures and the existence of nickel vacancies during the magnon excitation.Four-magnon configurations were determined from the scattering factor, revealing a lowest state and monotonic change with the growth temperature.PACS: 75.47.Lx; 61.82.Rx; 75.50.Tt; 74.25.nd; 72.10.Di.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan. sywu@mail.ndhu.edu.tw.

ABSTRACT
The nanosized effects of short-range multimagnon excitation behavior and short-circuit diffusion in NiO nanowalls synthesized using the Ni grid thermal treatment method were observed. The energy dispersive spectroscopy mapping technique was used to characterize the growth mechanism, and confocal Raman scattering was used to probe the antiferromagnetic exchange energy J2 between next-nearest-neighboring Ni ions in NiO nanowalls at various growth temperatures below the Neel temperature. This study shows that short spin correlation leads to an exponential dependence of the growth temperatures and the existence of nickel vacancies during the magnon excitation. Four-magnon configurations were determined from the scattering factor, revealing a lowest state and monotonic change with the growth temperature.PACS: 75.47.Lx; 61.82.Rx; 75.50.Tt; 74.25.nd; 72.10.Di.

No MeSH data available.


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The X-ray diffraction patterns of the NiO nanowalls. (a-e) X-ray diffraction and Rietveld refinement at various TA; (f) ratio of the integrated intensity of [1 1 1] and [2 0 0] of NiO nanowalls as a function of TA. The ratio of the integrated intensity [1 1 1] and [2 0 0] is noticeably higher than the standard value of 0.74 in bulk NiO bulk (dashed line) at lower TA and closer to the standard value at TA = 800°C.
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Figure 4: The X-ray diffraction patterns of the NiO nanowalls. (a-e) X-ray diffraction and Rietveld refinement at various TA; (f) ratio of the integrated intensity of [1 1 1] and [2 0 0] of NiO nanowalls as a function of TA. The ratio of the integrated intensity [1 1 1] and [2 0 0] is noticeably higher than the standard value of 0.74 in bulk NiO bulk (dashed line) at lower TA and closer to the standard value at TA = 800°C.

Mentions: The high energy synchrotron radiation X-ray diffraction (SR-XRD) technique must be employed for detailed investigation of the microcrack-induced strain leading to the formation of NiO nanowalls because small changes in strain are undetectable by the usual XRD techniques. An analysis of the crystalline properties was carried out by XRD at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan (λ = 0.7749 Å) using SR-XRD with a BL01C2 beam line. Figure 3 shows the TA dependency of the X-ray diffraction patterns. Here, the different colors are used to differentiate the peak intensity of the diffraction patterns. At the bottom of Figure 3, there are two nuclear peaks at the [1 1 1] and [2 0 0] positions, indexed based on the space group of Fm-3m. It is worth noting that the contribution of NiO is very weak even at TA = 400°C. The intensity of the small NiO grains (as can be seen in the SEM image) is weak and undetectable in the SR-XRD images. These characteristics (i.e., the small NiO grains) can be examined by in situ confocal Raman scattering, suggesting by Mironova-Ulmane et al. [26] of previous results obtained for NiO nanoparticles. After increasing TA we observe a significant broader peak around 2θ = 21.3°, which is associated with the NiO structure of the Miller index [2 0 0], and indicates the coexistence of NiO nanowalls and Ni grids. This may be explained by assuming the existence of the NiO phase, with the oxidation contribution coming from the Ni atoms in the grid. The pattern in the upper part of Figure 3 should contain, in principle, contributions mainly from the NiO phase after an increase in TA near 800°C. The X-ray diffraction patterns of the NiO nanowalls are refined using Rietveld analysis [27,28]. The preferred orientation is taken into account, as shown in Figure 4a, b, c, d, e. The diffraction pattern (black crosses) taken at various TA are shown, where the solid curve (red curve) indicates the fitted pattern. The difference (blue curve) between the observed and the fitted patterns is plotted at the bottom of Figure 4a, b, c, d, e. The obtained refined lattice parameters are shown in Table 2. Figure 4f shows the ratio of integrated intensity of the [1 1 1] and [2 0 0] of the NiO nanowalls as a function of TA. The ratio between the [1 1 1] and [2 0 0] peaks is noticeably higher than the standard value of 0.74 in NiO bulk at a lower TA and close to the standard value when TA = 800°C. This reveals that the NiO nanowalls for the Miller index [1 1 1] are oxidized more rapidly than for the other index [2 0 0]. The oxidized faces grow at a rate dependent upon the preferred crystallographic orientation of the NiO [1 1 1] faces even at micrometer thicknesses, which is in good agreement with previous observations [20]. Varghese et al. reported lattice fringes with an interplanar spacing of 2.44 Å corresponding to the [1 1 1] planes in the HRTEM image of the NiO nanowalls. For a face-centered cubic structure, the general order of the surface energies associated with the crystallographic planes is γ{111} < γ{100} < γ{110}, so the [1 1 1] facets can be easily stabilized [29]. Low surface energy could enhance the growth rates along the [1 1 1] directions, further enhancing aggregation of the NiO grains, leading to selective induction of anisotropic growth on a specific facet to form nanowalls.


Growth mechanism and magnon excitation in NiO nanowalls.

Gandhi AC, Huang CY, Yang CC, Chan TS, Cheng CL, Ma YR, Wu SY - Nanoscale Res Lett (2011)

The X-ray diffraction patterns of the NiO nanowalls. (a-e) X-ray diffraction and Rietveld refinement at various TA; (f) ratio of the integrated intensity of [1 1 1] and [2 0 0] of NiO nanowalls as a function of TA. The ratio of the integrated intensity [1 1 1] and [2 0 0] is noticeably higher than the standard value of 0.74 in bulk NiO bulk (dashed line) at lower TA and closer to the standard value at TA = 800°C.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
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Figure 4: The X-ray diffraction patterns of the NiO nanowalls. (a-e) X-ray diffraction and Rietveld refinement at various TA; (f) ratio of the integrated intensity of [1 1 1] and [2 0 0] of NiO nanowalls as a function of TA. The ratio of the integrated intensity [1 1 1] and [2 0 0] is noticeably higher than the standard value of 0.74 in bulk NiO bulk (dashed line) at lower TA and closer to the standard value at TA = 800°C.
Mentions: The high energy synchrotron radiation X-ray diffraction (SR-XRD) technique must be employed for detailed investigation of the microcrack-induced strain leading to the formation of NiO nanowalls because small changes in strain are undetectable by the usual XRD techniques. An analysis of the crystalline properties was carried out by XRD at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan (λ = 0.7749 Å) using SR-XRD with a BL01C2 beam line. Figure 3 shows the TA dependency of the X-ray diffraction patterns. Here, the different colors are used to differentiate the peak intensity of the diffraction patterns. At the bottom of Figure 3, there are two nuclear peaks at the [1 1 1] and [2 0 0] positions, indexed based on the space group of Fm-3m. It is worth noting that the contribution of NiO is very weak even at TA = 400°C. The intensity of the small NiO grains (as can be seen in the SEM image) is weak and undetectable in the SR-XRD images. These characteristics (i.e., the small NiO grains) can be examined by in situ confocal Raman scattering, suggesting by Mironova-Ulmane et al. [26] of previous results obtained for NiO nanoparticles. After increasing TA we observe a significant broader peak around 2θ = 21.3°, which is associated with the NiO structure of the Miller index [2 0 0], and indicates the coexistence of NiO nanowalls and Ni grids. This may be explained by assuming the existence of the NiO phase, with the oxidation contribution coming from the Ni atoms in the grid. The pattern in the upper part of Figure 3 should contain, in principle, contributions mainly from the NiO phase after an increase in TA near 800°C. The X-ray diffraction patterns of the NiO nanowalls are refined using Rietveld analysis [27,28]. The preferred orientation is taken into account, as shown in Figure 4a, b, c, d, e. The diffraction pattern (black crosses) taken at various TA are shown, where the solid curve (red curve) indicates the fitted pattern. The difference (blue curve) between the observed and the fitted patterns is plotted at the bottom of Figure 4a, b, c, d, e. The obtained refined lattice parameters are shown in Table 2. Figure 4f shows the ratio of integrated intensity of the [1 1 1] and [2 0 0] of the NiO nanowalls as a function of TA. The ratio between the [1 1 1] and [2 0 0] peaks is noticeably higher than the standard value of 0.74 in NiO bulk at a lower TA and close to the standard value when TA = 800°C. This reveals that the NiO nanowalls for the Miller index [1 1 1] are oxidized more rapidly than for the other index [2 0 0]. The oxidized faces grow at a rate dependent upon the preferred crystallographic orientation of the NiO [1 1 1] faces even at micrometer thicknesses, which is in good agreement with previous observations [20]. Varghese et al. reported lattice fringes with an interplanar spacing of 2.44 Å corresponding to the [1 1 1] planes in the HRTEM image of the NiO nanowalls. For a face-centered cubic structure, the general order of the surface energies associated with the crystallographic planes is γ{111} < γ{100} < γ{110}, so the [1 1 1] facets can be easily stabilized [29]. Low surface energy could enhance the growth rates along the [1 1 1] directions, further enhancing aggregation of the NiO grains, leading to selective induction of anisotropic growth on a specific facet to form nanowalls.

Bottom Line: The nanosized effects of short-range multimagnon excitation behavior and short-circuit diffusion in NiO nanowalls synthesized using the Ni grid thermal treatment method were observed.This study shows that short spin correlation leads to an exponential dependence of the growth temperatures and the existence of nickel vacancies during the magnon excitation.Four-magnon configurations were determined from the scattering factor, revealing a lowest state and monotonic change with the growth temperature.PACS: 75.47.Lx; 61.82.Rx; 75.50.Tt; 74.25.nd; 72.10.Di.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan. sywu@mail.ndhu.edu.tw.

ABSTRACT
The nanosized effects of short-range multimagnon excitation behavior and short-circuit diffusion in NiO nanowalls synthesized using the Ni grid thermal treatment method were observed. The energy dispersive spectroscopy mapping technique was used to characterize the growth mechanism, and confocal Raman scattering was used to probe the antiferromagnetic exchange energy J2 between next-nearest-neighboring Ni ions in NiO nanowalls at various growth temperatures below the Neel temperature. This study shows that short spin correlation leads to an exponential dependence of the growth temperatures and the existence of nickel vacancies during the magnon excitation. Four-magnon configurations were determined from the scattering factor, revealing a lowest state and monotonic change with the growth temperature.PACS: 75.47.Lx; 61.82.Rx; 75.50.Tt; 74.25.nd; 72.10.Di.

No MeSH data available.


Related in: MedlinePlus