Short-range spin-phonon coupling in in-plane CuO nanowires: a low-temperature Raman investigation
© Shih et al.; licensee Springer. 2013
Received: 30 July 2013
Accepted: 11 September 2013
Published: 25 September 2013
We report an application of low-temperature Raman scattering on in-plane CuO nanowires, in which an overview of the characteristic parameter of spin-phonon coefficient, the interaction of incident light with the spin degrees of freedom, and size effects will be given. The appearance of spin-phonon coefficient decrease reflects the existence of finite size effect.
Low-dimensional nanosized effects in CuO systems, especially their different physical properties such as spin-spin [1, 2], electron–phonon , spin-phonon interactions , and giant negative thermal expansion have recently received a lot of attention . The spin-spin superexchange interaction occurs via the oxygen orbital [4, 6]. The magnetic interactions and Néel transition temperature (TN) of the CuO system are strongly dependent on the exchange interaction and the number of neighboring atoms. A transition from a first-order transition to a commensurate antiferromagnetic state near TN ~ 213 K reported for bulk CuO from neutron scattering experiments [7, 8] is well understood. Controlling the size of CuO nanocrystals resulted in short-range correlation and commensurate antiferromagnetic (AFM) ordering, where the TN decreased from the bulk value of 213 K [9–11], with decreasing particle size, down to 40 K for 6.6-nm nanoparticles [1, 2] and 13 K for 2- to 3-nm nanorods . It is known that spin-phonon coupling is usually weak and undetectable because symmetric vibrations of relevant atoms will cancel the contributions from negative and positive displacements. The main feature of cupric oxide is the low-symmetry monoclinic lattice, which differs from the other transition metal monoxides, e.g., MnO, FeO, CoO, and NiO with rock salt structure . The low symmetry of the CuO lattice and the anisotropic dispersion curves indicated lattice vibration which caused a modulation of the spin-phonon interaction. This originated from slight changes in the inter-ionic distances and bond angles, leading to spin-phonon coupling that can be detected in the Raman spectrum, to produce a weak feature at about 230 cm−1 below TN[14, 15]. The discovery of spin-phonon coupling in CuO nanocrystals has led to renewed interest in this phenomenon. Up to now, there have been few experimental alternatives for the determination of the size effect of spin-phonon coupling of CuO nanowires. In this study, low-temperature Raman spectroscopy is employed to investigate the size effects of spin-phonon coupling in in-plane CuO nanowires. Low-temperature Raman spectroscopy has the high spatial resolution and sensitivity necessary for probing the local atomic vibrations of nanowires. Our results reveal that below Néel temperature there is a ready shift of the spin-phonon coefficient λsp decreases as the mean diameter of in-plane CuO nanowire decreases, exhibiting a long- to short-range spin-phonon coupling that can be nicely described with the expected theoretical order parameter as due to antiferromagnetic ordering in in-plane CuO nanowires.
Results and discussion
Summary of the fitting results of the in-plane CuO nanowires
3.4 ± 0.2
210 ± 15
4.5 ± 0.5
120 ± 8
5.1 ± 0.2
52 ± 3
8 ± 1
15 ± 1
20 ± 5
In conclusion, we investigate the size dependence of CuO nanowires and the nanosized spin-phonon effects. Raising the temperature and decreasing the diameter of CuO nanowires result in the weakening of spin-phonon coupling. The temperature variations of the spin-phonon mode at various diameters are in good agreement with the theoretical results. We found that the spin-phonon mode varies with the size of the CuO nanowires and in corroboration with the strength of spin-phonon coupling. Our result reveals that low-temperature Raman scattering techniques are a useful tool to probe the short-range spin-phonon coupling and exchange energy between antiferromagnetic next-nearest neighboring magnons in nanocrystals below the Néel temperature. The application of low-temperature Raman spectroscopy on magnetic nanostructures represents an extremely active and exciting field for the benefit of science and technology at the nanoscale. The rising new phenomena and technical possibilities open new avenues in the characterization of short-range spin-phonon interactions but also for the understanding of the fundamental process of magnetic correlation growth in nanomaterials.
a The log-normal distribution is defined as follows: , where <d> is the mean value and σ is the standard deviation of the function.
This research was supported by a grant from the National Science Council of Taiwan, the Republic of China, under grant number NSC-100-2112-M-259-003-MY3.
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