Growth mechanism and magnon excitation in NiO nanowalls
© Gandhi et al; licensee Springer. 2011
Received: 31 March 2011
Accepted: 8 August 2011
Published: 8 August 2011
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 J 2 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
Keywordsmagnetic oxides nanocrystalline materials confocal Raman scattering short-circuit diffusion magnon excitation
Nanomaterials aroused considerable interest in the twentieth century because it was found that nanoparticles (0D, 1D, and 2D) ranging in size from 1 to 100 nm exhibit novel properties which are significantly different from properties of the bulk material. These properties arise from different nanoscale effects such as the quantum size effect, surface effect, finite size effect, and macroscopic quantum tunnel effect [1–4]. Most of the anomalous behaviors are observed in the nano-region of antiferromagnetic- and ferromagnetic-amorphous and crystalline nanoparticles [4, 5]. For example, bulk NiO has a two-sublattice antiferromagnetic crystalline structure, but on the nanoscale, it has a many-sublattice antiferromagnetic crystalline structure  along with novel mechanical, optical, electronic, magnetic, and thermal properties. It has potential applications for catalysts, battery electrodes, gas sensors, electrochemical films, and photoelectronic devices [7–11]. There have been numerous qualitative and quantitative theoretical and experimental investigations of NiO materials using Raman and neutron scattering techniques [12–16]. Raman spectroscopy has been utilized to study magnon and phonon excitation in bulk NiO and NiO nanoparticles below the Neel temperature [2–4]. The dominant superexchange interactions  between the next-nearest-neighboring (NNN) Ni ions in the linear atomic chain Ni2+-O2--Ni2+ in a multisublattice magnet have been investigated. Analysis of the short range multimagnon interaction and even determination of the configuration of the magnetic structure is becoming possible. Vertical and interconnected two-dimensional nanostructures, such as NiO nanowalls have been produced with various fabrication methods [18, 19]. Such structures have recently aroused a great deal of interest due to their application in high performance Lithium ion batteries . Two-dimensional NiO nanowalls are considered to be ideal for the study of short-range magnetic ordering due to their high surface to volume ratio, interconnecting behavior and open-edge geometry [19, 21].
In this study, we report on the successful synthesis of well-aligned and interconnected 2D nanowalls of NiO by a simple thermal treatment method for oxidizing the Ni grid. The energy dispersive spectroscopy (EDS) mapping technique is used to characterize the formation of NiO nanowalls at various annealing temperatures. This process can also be well described using a short-circuit diffusion simulation. The interaction of light with the spin degrees of freedom gives two-magnon (2 M) and four-magnon (4 M) shifts in the Raman energy spectra. The energy shift in the two-magnon peaks is due to the lowering of local symmetry at the Ni2+ sites caused by chemical substitution and vacancies. The magnon configurations can be determined because of the dependence of the two- and four-magnon peaks on the growth temperatures.
T A (°C)
Width <d> (nm)
Growth mechanism of NiO nanowalls
Analysis of morphology by FE-SEM
Annealing temperature dependence of X-ray diffraction
Summary of X-ray refinement results for NiO nanowalls
Lattice parameters a(Å)
R p (%)
R wp (%)
T A (K)
Q S/Q D
Q S (kcal/mol)
ΔL cal (μm)
Short-circuit diffusion in NiO nanowalls
The oxidation of metals in an ambient atmosphere results in the formation of a protective oxide layer on the surface of the metal, which will not allow any further diffusion of oxygen from the surface. In the case of Ni, where the diffusion coefficient of Ni is higher than that of oxygen , surface diffusion takes place, resulting in an increase in the NiO layer with annealing temperature. It is well-known that annealing of the Ni grid at a temperature above 500°C results in the parabolic growth of nickel oxide nanowalls. The contribution from boundary diffusion at the grain boundaries decreases with increasing nanowall size . This oxidation rate is also strongly affected by line defects. This rate will be more pronounced with increased densities of line defects, where incoherent crystalline boundaries are available for short-circuit diffusion [32, 33]. It is therefore important to consider the influence of the scale on Ni transport when metal facets are oxidized at temperatures less than one half the melting point of NiO. At these temperatures, recrystallization and grain growth proceed slowly; with polycrystalline oxide boundaries serving as effective short-circuit diffusion paths . A wide range of growth rates may, therefore, be expected for different metal faces. The degree of short-circuit diffusion is determined by the density and crystallographic orientation of boundaries within the oriented oxide layer . A simple diffusion model is employed to interpret the oxidation kinetics. In this model, Ni transport proceeds in NiO both by short-circuit diffusion at the grain boundaries and by lattice diffusion at lower annealing temperatures. The theory of lattice diffusion fully explains the diffusion mechanism . The diffusion length can be obtained from the lattice diffusivity (square centimeter per second) according to following formula :
Phonon and magnon study of NiO nanowalls
Superexchange of Ni-O-Ni
In the study of the system of NiO nanowalls, it is of interest to directly observe the influence of the nanowall size on the coupling strength from the phonon vibration and magnon excitation. Confocal Raman spectroscopy has the high spatial resolution and sensitivity necessary for probing the local atomic vibration and multimagnon interaction below the Neel temperature. The multimagnon properties of NiO nanowalls were investigated using a confocal Raman spectrometer (Alpha 300, WiTec Pte. Ltd., Germany) equipped with a piezo scanner and 9100 microscope objectives (n.a. = 0.90; Nikon Imaging Japan Inc., Japan). The samples were excited with a 488-nm Ar ion laser (CVI Melles Griot, Carlsbad, USA) (5 mW laser power), to form a spot 0.3 μm in diameter, giving a power density of 100 mW cm-2. Here configurations of two and higher order magnon excitation in NiO nanowalls are mainly determined by the dominating superexchange interaction in the Ni2+-O2--Ni2+ linear atom chains and nano-sized effects. The incident photon is virtually absorbed in an electric dipole transition process that results in the magnon excitation. The exchange mechanism through the oxygen p-orbital then produces a spin flip in the excited state, and a second photon is virtually emitted leaving the system with a magnon excitation. The resultant momentum transfer can exceed 1 cm-1 for second-order Raman scattering. The two-magnon excitation is mainly dominated by the NNN Ni ions along the [1 0 0] direction, and is presented in terms of the antiferromagnetic exchange energy J 2 (approximately 221 K), which is much stronger than the nearest-neighboring ion exchange energy (approximately 15.7 K) in the same [1 1 1] plane of ferromagnetic coupling. Furthermore the (approximately 16.1 K) between the nearest neighbor in adjacent [1 1 1] planes (normally antiferromagnetically aligned) which can thus be ignored and canceled out in an ideal structure . The experimental values of J 2 as presented in previous reports by Dietz's group [8, 37] (Raman scattering) below T N are 18.5 meV in bulk NiO. Multimagnon Raman scattering in NiO is well described by the spin-wave theory utilizing the Green function formalism for an S = 1 antiferromagnet. The frequency of the two-magnon Raman line is estimated to be proportional to the superexchange integral J 2. We measured the annealing temperature dependence of the two- and four-magnon Raman frequencies. This allows us to find out the magnetic exchange coupling and determine the possible configuration of short range four-magnon models.
Confocal Raman scattering of NiO nanowalls
Summary of the Voigt fitting parameters for one- and two-phonon mode of NiO nanowalls
TO + LO (meV)
Magnon excitation of NiO nanowalls
Summary of the Voigt fitting parameters for two- and four-magnon mode of NiO nanowalls
T A (°C)
2 M (meV)
4 M (meV)
R= E 4 M/E 2 M
The chemical vapor deposition technique was successfully utilized to grow NiO nanowalls on a Ni grid with various mean widths without using any catalyst. The growth temperature for the NiO nanowalls was confined from 400°C to 800°C, which is 0.275 and 0.55 times the Ni melting point, following the parabolic rate law of Wagner's scaling theory. X-ray refinement reveals that the NiO nanowalls with the Miller index [1 1 1] oxidized more rapidly than with the other index [2 0 0], and the oxidized faces grow at a rate dependent upon the crystallographic preferred orientation of the NiO [1 1 1] faces. The length of diffusion of nickel along the [1 1 1] plane at various growth temperatures can be obtained from EDS mapping. The results agreed with the short-circuit diffusion mechanism simulation. Confocal Raman scattering was utilized to study the phonon and magnon configurations for these samples. The appearance of integrated intensity for the one- and two-phonon modes reflects the existence of the finite size effect and nickel vacancies. Two- and four-magnon excitations generated in NiO nanowalls may help to identify the Ni2+-O2--Ni2+ superexchange mechanism associated with the short-range magnetic interactions and magnon configurations.
aThe vibrational frequency of Ni atom is defined as follows: , where α = 0.2412 nm is the d-spacing of the [1 1 1] plane, m = 74.7 g/mol is the Ni molar weight, and Q D = 59(1) kcal/mol is the activation energy of Ni.
We would like to thank the National Science Council of the Republic of China for their financial support through project numbers NSC 97-2112-M-259-004-MY3 and NSC 100-2112-M-259-003-MY3. We would also like to thank Prof. W.-H. Li and Mr. S.B. Liu of the National Central University for their valuable discussions and contributions in this work.
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