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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.


Related in: MedlinePlus

Growth temperature TA dependence of the E2 M peak. The inset shows a possible model for the two-magnon configuration and minimum energy as predicted by an Ising cluster model calculation, where the linkages denote the exchange interactions and numbers give the number of spin deviations on each site.
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Figure 8: Growth temperature TA dependence of the E2 M peak. The inset shows a possible model for the two-magnon configuration and minimum energy as predicted by an Ising cluster model calculation, where the linkages denote the exchange interactions and numbers give the number of spin deviations on each site.

Mentions: It can be inferred that the additional peak visible just below the phonon modes (labeled as 2 M) is magnetic in origin, since it vanishes above the Neel temperature [15]. The extrapolation of the TA dependence of the intensity of the two-magnon frequency to I2 M to approximately 0 gives the growth temperature TA approximately 400°C. This signals the start of a spin correlation which is much weaker than that of the Bulk NiO. The growth temperature TA dependence of the E2 M peak is shown in Figure 8. The solid curve describes an exponential growth function for the magnetic short-range correlation, namely , where and τ = 131(4)°C represent the final energy and fitted parameters, respectively. Furthermore, the deviation of the two-magnon energy with growth temperature, defined by G = dE2M/dTA, can be used to probe the two-magnon correlation growth rate. Thus, when TA = 400°C to 600°C, the peak shift rate would be 1.4 meV/°C. The increase in energy of the two-magnon growth rate with increasing TA can be explained by the increase in the spin correlation length, while the smaller intensity of the Raman response caused by the Ni2+-O2--Ni2+ superexchange mechanism is associated with decreased amounts of NiO nanowalls due to the finite size effect. The reduced coordination of surface spin and the incomplete compensation between the antiferromagnetic sublattices will also cause a fundamental decrease in the magnetic ordering in the NiO nanowalls. These characteristics (i.e., the lower intensity) agree with results previously reported for NiO nanoparticles by Mironova-Ulmane et al. [26]. In their comprehensive analysis, they found that the intensity of the two-magnon peak decreases with particle size, but would, upon heating subsequently shift to lower energies and broaden. Furthermore, it is well-known that the lattice strain will introduce a slight rhombohedral distortion. The small distortion will create a difference in the anisotropic energy gap between and which is associated with the rhombohedral contraction occurring at lower TA. This will contribute to line broadening at the magnon peak. The inset to Figure 8 shows a possible model for the two-magnon configuration and minimum energy as predicted by the Ising cluster model calculation, where the linkages denote the exchange interactions and numbers give the number of spin deviations on each site. Chinn et al. reported the two-magnon models, the simple cubic lattice, for bulk KNiF3 [38]. There are three transformations of the , , and states from two-magnon excitations, but only the states have a nonzero spectral density for a scattering element matrix of the form , where the sum includes the next-nearest-neighbors in the case of NiO. Utilizing Chinn's Green's function, Dietz et al. [12] reported a value of J2 approximately 18 meV for bulk NiO. The strong two-magnon peak at E2 M = 183.2(2) meV with <d > = 416(18) nm is due to the two-magnon excitation, which is nearly ten times that of J2. This is in agreement with the values previously reported for bulk NiO [8,37], but smaller than the predicted value of 11 from the Ising cluster model. The mean number of N2 M obtained from N2 M = E2 M/J2 with the next-nearest-neighbor exchange J2 = 18 meV, shown in Table 5 may be associated with the concentration of nickel vacancies. According to previous NicO doping nonmagnetic ion models [39], the two-magnon peak can be roughly expressed as , where c is the chemical composition of nickel and x = (1-c)% is defined as the concentration of nickel vacancies; z = 6 is the number of next-nearest-neighbor; J2 = 18 meV is the superexchange interaction energy within 180° of the Ni2+-O2--Ni2+ atomic chain; and S = 1 is the antiferromagnetic spin. The values obtained for x for the NicO nanowalls are listed in Table 5. The results are consistent with the Ni1-xMgxO system [40,41]. Lowering the local symmetry at the Ni2+ sites caused by the chemical substitution and vacancies will result in shifting of the two-magnon peaks. It is worth noting that the one-magnon Raman frequency has a very weak reported value of 38 cm-1, which is undetectable in the study of NiO nanowalls [40].


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)

Growth temperature TA dependence of the E2 M peak. The inset shows a possible model for the two-magnon configuration and minimum energy as predicted by an Ising cluster model calculation, where the linkages denote the exchange interactions and numbers give the number of spin deviations on each site.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211999&req=5

Figure 8: Growth temperature TA dependence of the E2 M peak. The inset shows a possible model for the two-magnon configuration and minimum energy as predicted by an Ising cluster model calculation, where the linkages denote the exchange interactions and numbers give the number of spin deviations on each site.
Mentions: It can be inferred that the additional peak visible just below the phonon modes (labeled as 2 M) is magnetic in origin, since it vanishes above the Neel temperature [15]. The extrapolation of the TA dependence of the intensity of the two-magnon frequency to I2 M to approximately 0 gives the growth temperature TA approximately 400°C. This signals the start of a spin correlation which is much weaker than that of the Bulk NiO. The growth temperature TA dependence of the E2 M peak is shown in Figure 8. The solid curve describes an exponential growth function for the magnetic short-range correlation, namely , where and τ = 131(4)°C represent the final energy and fitted parameters, respectively. Furthermore, the deviation of the two-magnon energy with growth temperature, defined by G = dE2M/dTA, can be used to probe the two-magnon correlation growth rate. Thus, when TA = 400°C to 600°C, the peak shift rate would be 1.4 meV/°C. The increase in energy of the two-magnon growth rate with increasing TA can be explained by the increase in the spin correlation length, while the smaller intensity of the Raman response caused by the Ni2+-O2--Ni2+ superexchange mechanism is associated with decreased amounts of NiO nanowalls due to the finite size effect. The reduced coordination of surface spin and the incomplete compensation between the antiferromagnetic sublattices will also cause a fundamental decrease in the magnetic ordering in the NiO nanowalls. These characteristics (i.e., the lower intensity) agree with results previously reported for NiO nanoparticles by Mironova-Ulmane et al. [26]. In their comprehensive analysis, they found that the intensity of the two-magnon peak decreases with particle size, but would, upon heating subsequently shift to lower energies and broaden. Furthermore, it is well-known that the lattice strain will introduce a slight rhombohedral distortion. The small distortion will create a difference in the anisotropic energy gap between and which is associated with the rhombohedral contraction occurring at lower TA. This will contribute to line broadening at the magnon peak. The inset to Figure 8 shows a possible model for the two-magnon configuration and minimum energy as predicted by the Ising cluster model calculation, where the linkages denote the exchange interactions and numbers give the number of spin deviations on each site. Chinn et al. reported the two-magnon models, the simple cubic lattice, for bulk KNiF3 [38]. There are three transformations of the , , and states from two-magnon excitations, but only the states have a nonzero spectral density for a scattering element matrix of the form , where the sum includes the next-nearest-neighbors in the case of NiO. Utilizing Chinn's Green's function, Dietz et al. [12] reported a value of J2 approximately 18 meV for bulk NiO. The strong two-magnon peak at E2 M = 183.2(2) meV with <d > = 416(18) nm is due to the two-magnon excitation, which is nearly ten times that of J2. This is in agreement with the values previously reported for bulk NiO [8,37], but smaller than the predicted value of 11 from the Ising cluster model. The mean number of N2 M obtained from N2 M = E2 M/J2 with the next-nearest-neighbor exchange J2 = 18 meV, shown in Table 5 may be associated with the concentration of nickel vacancies. According to previous NicO doping nonmagnetic ion models [39], the two-magnon peak can be roughly expressed as , where c is the chemical composition of nickel and x = (1-c)% is defined as the concentration of nickel vacancies; z = 6 is the number of next-nearest-neighbor; J2 = 18 meV is the superexchange interaction energy within 180° of the Ni2+-O2--Ni2+ atomic chain; and S = 1 is the antiferromagnetic spin. The values obtained for x for the NicO nanowalls are listed in Table 5. The results are consistent with the Ni1-xMgxO system [40,41]. Lowering the local symmetry at the Ni2+ sites caused by the chemical substitution and vacancies will result in shifting of the two-magnon peaks. It is worth noting that the one-magnon Raman frequency has a very weak reported value of 38 cm-1, which is undetectable in the study of NiO nanowalls [40].

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