  The layered perovskite PrBaCo2O5.5+x demonstrates a strong negative thermal
expansion (NTE) which holds potential for being fabricated into composites with
zero thermal expansion. The NTE was found to be intimately associated with the
spontaneous magnetic ordering, known as magnetovolume effect (MVE). Here we
report with compelling evidences that the continuous-like MVE in PrBaCo2O5.5+x
is intrinsically of discontinuous character, originating from an
magnetoelectric transition from an antiferromagnetic insulating large-volume
(AFILV) phase to a ferromagnetic metallic small-volume (FMSV) phase.
Furthermore, the magnetoelectric effect (ME) shows high sensitivity to multiple
external stimuli such as temperature, carrier doping, hydrostatic pressure,
magnetic field etc. In contrast to the well-known ME such as colossal
magnetoresistance and multiferroic effect which involve symmetry breaking of
crystal structure, the ME in the cobaltite is purely isostructural. Our
discovery provides a new pathway to realizing the ME as well as the NTE, which
may find applications in new techniques.
