  The latent heat (L) associated with the orbital order-disorder transition at
T_JT 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 d_c ~ 19 nm. The transition
temperature T_JT 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 T_JT 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.
