Abstract
Neutron scattering and magnetometry measurements have been used to study phase transitions
in LaCoO3 (LCO). For H 100 Oe, evidence for a ferromagnetic (FM) transition is observed
at Tc 87 K. For 1 kOe H 60 kOe, no transition is apparent. For all H, Curie–Weiss
analysis shows predominantly antiferromagnetic (AFM) interactions for T > Tc, but the lack
of long-range AFM order indicates magnetic frustration. We argue that the weak
ferromagnetism in bulk LCO is induced by lattice strain, as is the case with thin films and
nanoparticles. The lattice strain is present at the bulk surfaces and at the interfaces between the
LCO and a trace cobalt oxide phase. The ferromagnetic ordering in the LCO bulk is strongly
affected by the Co–O–Co angle (
), in agreement with recent band calculations which predict
that ferromagnetic long-range order can only take place above a critical value,
C. Consistent
with recent thin film estimations, we find
C D 162:8. For
>
C, we observe power-law
behavior in the structural parameters.
decreases with T until the critical temperature,
To 37 K; below To the rate of change becomes very small. For T < To, FM order appears to
be confined to regions close to the surfaces, likely due to the lattice strain keeping the local
Co–O–Co angle above
C.
