We have investigated normal and resonant Raman scattering in Me-doped ZnO nanorods (Me = Mn, Co, Cu and Ni) prepared by thermal diffusion. Experimental results show that the normal Raman spectra consist of the conventional modes associated with wurtzite ZnO and impurity-related additional modes. Under resonant conditions, only longitudinal optical (LO) phonon scattering and its overtones are observed. The number of LO phonon lines and their relative intensity depend on the doping element and level. For the nanorods doped with Cu and Ni, we have observed LO phonon overtones up to eleventh order. This situation does not happen for the Mn-doped nanorods, which show only five LO phonon modes. By co-doping Mn and Co into the ZnO host lattice, however, the LO phonon overtones up to eleventh order are observed again. The nature of this phenomenon is explained by means of the study of XRD, TEM and photoluminescence.

Understanding the impact of reduced dimensionality on the magnetic and magnetocaloric responses of a material is vital in incorporating it as an active magnetic refrigerant in cooling devices. By contrasting the magnetic and magnetocaloric behaviors of bulk polycrystalline, sol-gel derived nanocrystalline, and pulsed laser deposited thin film forms of the La 0.7 Ca 0.3 MnO 3 system, we show that reducing the dimensionality of a ferromagnetic material tends to broaden and shift the paramagnetic to ferromagnetic transition to lower temperatures, while decreasing the saturation magnetization and the magnitude of the magnetic entropy change. Relative to its bulk counterpart, a pronounced broadening of the magnetic entropy change peak in the thin film leads to enhanced refrigerant capacity-an important figure-of-merit for active magnetic refrigeration technology. With reduced dimensionality, universal curves based on re-scaled entropy change curves tend toward collapse, indicating a weakening of the first order nature of the transition in the nanocrystalline samples and a crossover to second order in the thin film. V

We have studied the magnetic and spin-glass (SG) properties of La0.7Ca0.3MnO3 single-crystalline nanoparticles, which were prepared by the mechanical milling method with different milling times (tm). Analyzing the susceptibility data in the paramagnetic region indicates both ferromagnetic (FM) and anti-FM interactions coexisting in nanoparticles. Additionally, there is a peak associated with the freezing temperature (Tf) appearing on the real part curve of the ac susceptibility, χ′(T). The Tf value increases with increasing frequency as expected for SG systems. The SG behavior was also checked by using the criterion parameter c = ΔTf/TfΔ(log10f), and the power law τ = τ0(T/Tg − 1)−zν. The obtained values of c ≈ 5 × 10−2, τ0 ≈ 10−5 s and zν ≈ 2–3 are consistent with those expected for SG-like systems, suggesting an existence of a SG phase transition at Tg below Tf, which decreases with decreasing ⟨D⟩. Basing on ln(f) versus Tf data, and the Néel-Arrhenius model [ln(f) = ln(f0) - Ea/kBT] and Vogel–Fulcher law [ln(f) = ln(f0) - Ea/kB(T - T0)], the Larmor frequency (f0), activation energy (Ea) and effective temperature (T0) for the samples with different ⟨D⟩ were determined. Obtained results indicate the existence a strong interaction between nanoparticles.

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