Orbital ordering-driven ferromagnetism in LaCoO₃ nanowires

The structure and magnetic properties of LaCoO₃ nanowires are investigated as a function of the diameter in the temperature range of 5-300 K. Ferromagnetism below 85 K is observed in these nanowires, in agreement with the recent observations in LaCoO₃ epitaxial thin films and nanoparticles. With the diameter of nanowires decreasing, the unit-cell volume increases, while both the global and local structural distortions lessen, accompanied by the gradual enhancement of ferromagnetism. The structure analysis reveals that LaCoO₃ nanowires exhibit a monoclinic distorted structure with I2/a space group in the entire investigated temperature range. Different from bulks, there is no clear spin-state transition occurring with temperature in LaCoO ₃ nanowires. There exists a noticeable Jahn-Teller (JT) distortion in the nanowires even at the lowest temperature, namely, orbital-ordered JT active Co³⁺ ions with intermediate-spin (IS) state persist at low temperatures, which is not observed in bulk LaCoO₃. These results indicate that the ferromagnetism in the nanowires is driven by the orbital ordering of IS Co³⁺.