Origin of magnetovolume effect in a cobaltite

The layered perovskite ${\mathrm{PrBaCo}}_{2}{\mathrm{O}}_{5.5+x}$ 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 evidence that the continuouslike MVE in ${\mathrm{PrBaCo}}_{2}{\mathrm{O}}_{5.5+x}$ is intrinsically of discontinuous character, originating from a magnetoelectric transition from an antiferromagnetic insulating large-volume (AFILV) phase to a ferromagnetic less-insulating small-volume (FLISV) 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 pathway to realizing the ME as well as the NTE, which may find applications in new techniques.