The effects of Co3O4 on the Structure and Unusual Magnetism of LaCoO3

TL;DR

This study investigates how Co3O4 influences the structure and magnetic properties of LaCoO3 particles, revealing that Co3O4 enhances ferromagnetism at interfaces while LaCoO3 exhibits complex magnetic behavior including unusual antiferromagnetism.

Contribution

The paper introduces a model describing the distinct ferromagnetic interface behavior and unusual antiferromagnetism in LaCoO3 with Co3O4, highlighting the role of interfaces and lattice distortions.

Findings

Co3O4 content increases ferromagnetism at interfaces

LaCoO3 shows unusual temperature-dependent antiferromagnetism

A model explains the magnetic behavior of interface and core regions

Abstract

Bulk La_wCoO3 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 T_C ~89K. 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-T_C)/T_C)^beta behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie-Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent beta=0.63 +- 0.02 is consistent with 2D (surface) ordering. Increased Co3O4 correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below T ~ 40K 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.