[en] Manganites have been known to exhibit giant anisotropic magnetoresistance (GAMR) near metal-insulator transition temperatures. Interestingly, we observed a second GAMR peak at lower temperatures in manganite strips fabricated from epitaxial thin films. The second low-temperature GAMR peak is highly sensitive to magnetic field and vanishes quickly upon increasing of magnetic field. We attribute the emergent GAMR behavior to spatial confinement effect on electronic phase separation in manganite strips.

[en] Despite low resistivity (∼1 mΩ cm), metallic electrical transport has not been commonly observed in inverse spinel NiCo₂O₄, except in certain epitaxial thin films. Previous studies have stressed the effect of valence mixing and the degree of spinel inversion on the electrical conduction of NiCo₂O₄ films. In this work, we studied the effect of nanostructural disorder by comparing the NiCo₂O₄ epitaxial films grown on MgAl₂O₄ (1 1 1) and on Al₂O₃ (0 0 1) substrates. Although the optimal growth conditions are similar for the NiCo₂O₄ (1 1 1)/MgAl₂O₄ (1 1 1) and the NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, they show metallic and semiconducting electrical transport, respectively. Post-growth annealing decreases the resistivity of NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, but the annealed films are still semiconducting. While the semiconductivity and the large magnetoresistance in NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films cannot be accounted for in terms of non-optimal valence mixing and spinel inversion, the presence of anti-phase boundaries between nano-sized crystallites, generated by the structural mismatch between NiCo₂O₄ and Al₂O₃, may explain all the experimental observations in this work. These results reveal nanostructural disorder as being another key factor for controlling the electrical transport of NiCo₂O₄, with potentially large magnetoresistance for spintronics applications. (paper)