The effects of Co₃O₄on the structure and unusual magnetism of LaCoO₃

Abstract

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.

Origin of the net magnetic moment in LaCoO3

“…The suppression of the moment in the magnetization experiments [59] upon cooling in very small fields, H ≤ 3 Oe, is also consistent with the ZFC calculations that show no significant net moment. For the polarized neutron scattering experiments, the fields at the sample were H z ≥ 16 Oe, so the sample was in the metastable state for all the measurements.…”

“…Experiments [53,29,59,70] and calculations [14] indicate that antiferromagnetic correlations are supported only for γ greater than the critical value γ C ≈ 163…”

“…[57,15,58] There appears to be a relation between magnetic moment and diameter M ∝ 1/D. [30,15] Impurity contributions from Co 3 O 4 also appear to enhance the net moment [59]. The mechanism might be an increased strain on the Co 3 O 4 and LCO interface.…”

“…Then, there is a power-law type transition [59] and the measured volume of the unit cell (as well as the average angle) increases. Similar behavior is seen in LCO single polycrystals.…”

“…[59] In a single polycrystal of LCO, both the surface effects and impurity concentrations are negligible. Magnetic susceptibility measurements have been done on an LCO polycrystal, [1] confirming the presence of a net moment in the bulk.…”