  Antiferromagnetic spintronic devices have the potential to outperform
conventional ferromagnetic devices due to their ultrafast dynamics and high
data density. A challenge in designing these devices is the control and
detection of the orientation of the anti-ferromagnet. One of the most promising
ways to achieve this is through the exchange bias effect. This is of particular
importance in large scale multigranular devices. Due to the large system sizes,
previously, only micromagnetic simulations have been possible, these have an
assumed distribution of antiferromagnetic anisotropy directions. Here, we use
an atomistic model where the distribution of antiferromagnetic anisotropy
directions occurs naturally and the exchange bias occurs due to the intrinsic
disorder in the antiferromagnet. We perform large scale simulations, generating
realistic values of exchange bias. We find a strong temperature dependance of
the exchange bias, which approaches zero at the blocking temperature while the
coercivity has a peak at the blocking temeprature due to the superparamagnetic
flipping of the antiferromagnet during the hysteresis loop. We find a large
discrepancy between the exchange bias predicted from a geometric model of the
antiferromagnetic interface indicating the importance of grain edge effects in
multigranular exchange biased systems. The grain size dependence shows the
expected peak due to a competition between the superparamagnetic nature of
small grains and reduction in the statistical imbalance in the number of
interfacial spins for larger grain sizes. Our simulations confirm the existence
of single antiferromagnetic domains within each grain. The model gives insights
into the physical origin of exchange bias and provides a route to developing
optimised nanoscale antiferromagnetic spintronic devices.
