Figure 2 Series of Raman spectra taken at various temperatures of

Figure 2 Series of Raman spectra taken at various temperatures of CuO nanowires with a mean average diameter < d > = 120 ± 8 nm. Two main phonon modes corresponding to the A g and B g 1 symmetries, respectively, are revealed. As the temperature was reduced to143 K, a well-defined peak at 238 cm−1 developed, signifying the spin-phonon coupling. Figure 3 C188-9 shows the temperature dependence of the spin-phonon

mode for in-plane CuO nanowires of various diameters. Typical examples for bulk CuO are shown in Figure 3, indicated by open and solid squares [8]. It has been suggested in previous reports that the temperature dependence of the spin-phonon mode (the origin of the peak at 228 cm−1) might be associated

with magnetic ordering, the frequency shift corresponding to the spin-correlation function times a spin-phonon coupling coefficient λ sp. The temperature dependence of the spin-phonon peak can be represented as , where is the Raman shift in the absence of spin-phonon PARP inhibitor coupling at T N and ϕ(T) is the order parameter estimated from the mean field theory [24]. The order parameter can be described as ϕ(T) = 1 − (T/T N ) γ , where the order parameter γ varied from 3.4 ± 0.2 to 20 ± 5. The solid curves indicate the theoretical fitting, and the corresponding parameters are presented in Table 1. The size effect acts to confine the spin-phonon coupling by increasing the T N from 210 to 88 K, as shown in Figure 4a, when the size is reduced from bulk to 15 ± 1 nm (see

for comparison T N = 213 K for CuO single crystal and powder [8, 16]). The obtained spin-phonon coupling coefficient λ sp also tends to decrease with decreased phonon amplitudes as the diameter decreased, as shown in Figure 4b, revealing the existence of not short-range coupling. This result is consistent with past reports which state that the magnetic transition temperature of Cr2O3[25, 26] and CuO nanoparticles (open square) is reduced [12], which can be attributed to the fact that the ground state fails to Selleck DMXAA develop long-range antiferromagnetic ordering. This occurs because of quantum lattice fluctuations and being energetically favorable to some kinds of short-range order state, resulting in a lower spin-phonon coefficient with reduced size [27, 28]. The magnitudes of these obtained λ sp values are intermediate compared to approximately 1 cm−1 for FeF2 and MnF2[24], and approximately 50 cm−1 for bulk CuO [8], indicating that the size effects could result in a tendency to weaken the strong spin-phonon coupling. A minimum spin-phonon coefficient of λ sp = 10 cm−1 was obtained in = 15 ± 1 nm in-plane CuO nanowires, which was found to be weaker by a factor of 0.018 than the nearest neighbor spin-spin coupling strength of J = 552 cm−1 for one-dimensional antiferromagnetic Heisenberg chain [29].

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