Bulk and nanoparticle powders of LaCoO3 (LCO) were synthesized and their magnetic and structural properties were studied using SQUID magnetometry and neutron diffraction. The bulk and large nanoparticles exhibit weak ferromagnetism (FM) below T ≈ 85 K and a crossover from strong to weak antiferromagnetic (AFM) correlations near a transition expressed in the lattice parameters, To≈40 K. This crossover does not occur in the smallest nanoparticles; instead, the magnetic behavior is predominantly ferromagnetic. The amount of FM in the nanoparticles depends on the amount of Co3O4 impurity phase, which induces tensile strain on the LCO lattice. A core-interface model is introduced, with the core region exhibiting the AFM crossover and with FM in the interface region near surfaces and impurity phases.

Bulk La(w)CoO(3) particles with w = 1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La(2)O(3) and Co(3)O(4). The resulting particles are characterized as LaCoO(3) crystals interfaced with a crystalline Co(3)O(4) phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO(3) phase resulting from the Co(3)O(4) content, but magnetization data indicate that the amount of Co(3)O(4) strongly affects the ferromagnetic ordering at the interfaces below TC ≈ 89 K. In addition to ferromagnetic long-range order, LaCoO(3) exhibits antiferromagnetic behavior with an unusual temperature dependence. The magnetization for fields 20 Oe ⩽ H ⩽ 5 kOe is fit to a combination of a power law ((T - TC)/TC)(β) behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie-Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent β = 0.63 ± 0.02 is consistent with 2D (surface) ordering. Increased Co(3)O(4) correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below T ≈ 40 K is a consequence of the lattice reaching a critical rhombahedral distortion as T is decreased for core regions far from the Co(3)O(4) interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.