Bulk LawCoO3 particles with w = 1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La2O3 and Co3O4. The resulting particles are characterized as LaCoO3 crystals interfaced with a crystalline Co3O4 phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO3 phase resulting from the Co3O4 content, but magnetization data indicate that the amount of Co3O4 strongly affects the ferromagnetic ordering at the interfaces below TC ≈ 89 K. In addition to ferromagnetic long-range order, LaCoO3 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 Co3O4 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 Co3O4 interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.

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 coreinterface model is introduced, with the core region exhibiting the AFM crossover and with FM in the interface region near surfaces and impurity phases. PACS numbers:The unusual magnetic behavior of LaCoO 3 (LCO) has remained largely unexplained, despite the growing realization that structural distortion represents an important degree of freedom influencing the behavior of a large class of perovskites [1,2]. Recently, strain-switched ferromagnetism (FM) in LCO has been used to create a spintronic device. [3] Although the temperature at which that device operates is low (T < 90 K), understanding the mechanism behind strain-induced LCO magnetism should facilitate the search for similar perovskite materials that will allow switching of the ferromagnetic moment at higher temperatures. Finding such a material will allow construction of spintronic devices for widespread use. Recently, a model for the magnetism was developed [4] that explains the crucial role that the Co 3 O 4 impurity phase plays in the formation of long-range ferromagnetic order in LCO. The model involves two regions: the interface region near the boundaries between the LCO and Co 3 O 4 phases as well as near the LCO particle surfaces, and the core LCO region away from these interfaces and surfaces. In this work, we apply the model to explain the effects of the particle surfaces and Co 3 O 4 impurity phase on the LCO magnetism; these effects are more pronounced as the LCo particle size decreases to the nanoscale.Many earlier attempts to model LCO magnetism focused on local transitions between Co electron states. Such models do not provide a comprehensive description of the variety of phenomena observed in films, bulk and nanoparticle powders, and single crystals of LCO. More recent efforts recognize the importance of including collective behaviors of the correlated electrons. [5][6][7] By considering four samples with different sized particles and Co 3 O 4 impurity phase concentrations, we show that the disparate magnetic behaviors observed in various LCO systems fit well into the model developed [4] for the bulk particles. Synthesis and CharacterizationThe LCO bulk sample, A, was synthesized using a standard solid state reaction [8]. Stoichiometric amounts of La 2 O 3 and Co 3 O 4 were ground together thoroughly and fired for 8 hours. This process was repeated five times, with firing temperatures between 85...