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Particle-size dependence of orbital order-disorder transition in LaMnO3

The latent heat (L) associated with the orbital order-disorder transition at TJT is found to depend significantly on the average particle size (d) of LaMnO3. It rises slowly with the decrease in d down to ∼100 nm and then jumps by more than an order of magnitude in between d∼100 nm and ∼30 nm. Finally, L falls sharply to zero at a critical particle size dc≈19 nm. The transition temperature TJT also exhibits an almost similar trend of variation with the particle size, near d∼30 nm and below, even though the extent of variation is relatively small. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature study over a temperature range 10–300 K reveals that the antiferromagnetic transition temperature decreases with d while the temperature range, over which the ZFC and FC data diverge, increases with the drop in d. The FC magnetization also is found to increase sharply with the drop in particle size. A conjecture of nonmonotonic variation in orbital domain structure with decrease in particle size—from smaller domains with large number of boundaries to larger domains with small number of boundaries due to lesser lattice defects and, finally, down to even finer domain structures with higher degree of metastability—along with increase in surface area in core-shell structure, could possibly rationalize the observed L versus d and TJT versus d patterns. Transmission electron microscopy data provide evidence for presence of core-shell structure as well as for increase in lattice defects in finer particles.