  Exchange bias phenomenon is generally ascribed to the exchange coupling at
the interfaces between ferromagnetic and antiferromagnetic layers. Here, we
propose a bulk form of exchange bias in a single-phase magnet where the
coupling between two magnetic sublattices induces a significant shift of the
coercive field after a field cooling. Our experiments in a complicated magnet
YbFe2O4 demonstrate a giant exchange bias at low temperature when the coupling
between the Yb3+ and Fe2+/Fe3+ sublattices take places. The cooling magnetic
field dependence and the training effect of exchange bias are consistent with
our model. In strong contrast to conventional interfacial exchange bias, this
bulk form of exchange bias can be huge, reaching the order of a few Tesla.
