  LaCoO3 (LCO) nanoparticles were synthesized and their magnetic and structural
properties were examined using SQUID magnetometery and neutron diffraction. The
nanoparticles exhibit ferromagnetic long-range order beginning at T_C
approximately 87K that persists to low temperatures. This behavior is
contrasted with the ferromagnetism of bulk LCO, which also starts at T_C
approximately 87K but is suppressed below a second transition at T_o
approximately 37K, due to a structural phase transition. The ferromagnetism in
both systems is attributed to the tensile stress from particle surfaces and
impurity phase interfaces. This stress locally increases the Co-O-Co bond angle
gamma, and competes with the thermal contraction of the lattice. It has
recently been shown that LCO loses long-range ferromagnetic order when gamma
decreases below the critical value gamma_c = 162.8 degrees. Consistent with
this model, we show that gamma in nanoparticles remains larger than gamma_c at
low temperatures, likely a consequence of all spins being in close proximity to
surfaces or interfaces.
