  The exchange interaction determines the ferromagnetic (FM) or
antiferromagnetic (AFM) ordering of atomic spins. When ferromagnets and
antiferromagnets are coupled together, they often exhibit the exchange bias
effect, a unidirectional interface exchange field causing a shift of the
magnetic hysteresis loop. The effective magnitude of this interface exchange
field is at most a few percent of the bulk exchange, arising from pinned
interfacial spins in the antiferromagnet. The pinned spins are known to
comprise a small fraction of the total number of interface spins, yet their
exact nature and physical origin has so far been elusive. Here we show that in
the technologically important $\gamma - IrMn_3/CoFe$ structure the pinned
interface spins are in fact delocalised over the whole interface layer. The
pinned spins arise from the small imbalance of the number of spins in each
magnetic sublattice in the antiferromagnet due to the natural atomic disorder.
These pinned spins are strongly coupled to the bulk antiferromagnet explaining
their remarkable stability. Moreover, we find that the ferromagnet strongly
distorts the interface spin structure of the antiferromagnet, causing a large
reversible interface magnetisation that does not contribute to exchange bias.
The unexpected delocalised nature of the pinned interface spins explains both
their small number and their stability, uncovering the mysterious microscopic
origin of the exchange bias effect.
