  Magnetism in lanthanum cobaltite (LCO, LaCoO$_3$) appears to be strongly
dependent on strain, defects, and nanostructuring. LCO on strontium titanate
(STO, SrTiO$_3$) is a ferromagnet with an interesting strain relaxation
mechanism that yields a lattice modulation. However, the driving force of the
ferromagnetism is still controversial. Experiments debate between a
vacancy-driven or strain-driven mechanism for the ferromagnetism of epitaxial
LCO. We found that a weak lateral modulation of the superstructure is
sufficient to promote ferromagnetism. We find that ferromagnetism appears under
uniaxial compression and expansion. Although earlier experiments suggest that
bulk LCO is nonmagnetic, we find an antiferromagnetic ground state for bulk
LCO. We discuss the recent experiments which indicate a more complicated
picture for bulk magnetism and a closer agreement with our calculations. Role
of defects are also discussed through excited state calculations.
