The structure and magnetic properties of orthorhombic DyMnO3 nanoparticles with different particle sizes are investigated in this paper. With decreasing particle size, all the lattice parameters a, b, and c gradually decrease, whereas the orthorhombic distortion increases. Magnetic measurements reveal that the antiferromagnetic interaction of Mn ions is weakened due to the decrease in Mn-O-Mn bond angle. Above a critical field H*, DyMnO3 undergoes a field-induced metamagnetic transition at 4 K, which is related to the spin reversal of Dy moments. The critical field H* increases monotonically with size reduction, indicating an enhancement of the antiferromagnetic interaction of Dy ions due to the decreased distance between rare earth ions. The magnetization at 4 K and 5 T, i.e., M(4 K, 5 T) shows a non-monotonic variation with particle size d, i.e., M(4 K, 5 T) initially increases with size reduction but decreases again for d < 68 nm. A modified core-shell model, in which the ferromagnetic ordering (Dy magnetic structure) and antiferromagnetic ordering (Mn magnetic structure) coexist in the core, is proposed to explain this behavior.

Electric current induced Joule heating effects have been investigated in La0.8Ba0.2MnO3 ultrathin films deposited on a LaAlO3(001) single crystal substrate with a smaller lattice constant by using the sol–gel method. By applying moderate bias currents (∼10 mA), it is found that Joule self-heating simply gives rise to a temperature deviation between the thermostat and the test sample, but the intrinsic ρ(T) relationship measured at a low current (0.1 mA) changes a little. However, it is noteworthy that the low-temperature transport behavior degrades from the metallic to the insulating state after applying higher bias currents (>31 mA) in vacuum. Furthermore, the metallic transport can be recovered by placing the degraded film in air. The results clearly suggest that the oxygen vacancy in the La0.8Ba0.2MnO3 films is controllable in different atmospheres, particularly with the aid of the Joule self-heating. According to the SEM images, we attribute the controlled oxygen vacancy to the nano-sized labyrinth pattern of the films, where the large surface-to-volume ratio plays a crucial role.

Optical spectra are very sensitive to investigate strong charge correlations in transition-metal oxides. Recent studies on nanosized half-doped manganites frequently reported that the robust charge ordering present in the bulk is strongly weakened or suppressed by reducing the sample size to nanoscale; however, the origin of the novel phenomenon is not clearly understood until now. Here, we study this nanosize effect through infrared spectra on Nd0.5Ca0.5MnO3 nanoparticles with different particle sizes. Optical phonon modes demonstrate that the cooperative Jahn–Teller distortion associated with charge ordering is not only clearly observed in the large nanoparticles with a long-range charge-ordered transition but also visible in the small nanoparticles where such the transition is completely suppressed. Lattice distortion is interestingly found to be weakened at low temperatures but enhanced at the high-temperature paramagnetic regime by the size reduction, which is responsible for the change in magnetization with particle size. Detailed analysis on the optical density further reveals that size-induced suppression of the charge-ordered transition does not mean a close of the charge gap, which still persists with just a slight decrease in the energy and exhibits no obvious change in the dependence on the temperature. Localization of charge carriers increases with decreasing particle size, evidenced by the decrease in the effective number of carriers. Taken together, our findings give good evidence that a short-range charge-ordered state exists in the half-doped manganite nanoparticles when the long-range one is destroyed by the size reduction. More importantly, these features can be well ascribed to originate from the surface disorder effects. Our optical study provides deep insight into the nanosize effects on charge ordering in the transition-metal oxides.