  To study spin flipping within the antiferromagnet IrMn, we extended prior
Current-Perpendicular-to-Plane (CPP) Giant Magnetoresistance (GMR) studies of
Py-based exchange-biased-spin-valves containing IrMn inserts to thicker IrMn
layers-5 nm less than or equal to t(IrMn) less than or equal to 30 nm.
Unexpectedly, A{\Delta}R = A[R(AP) - R(P)]--the difference in specific
resistance between the anti-parallel (AP) and parallel (P) magnetic states of
the two Py layers-did not decrease with increasing t(IrMn), for t(IrMn) greater
than 5 nm, but rather became constant to within our measuring uncertainty. This
constant looks to be due mostly to a new, small MR in thin Py layers. The
constant complicates isolating the spin-diffusion length, lsf(IrMn), in bulk
IrMn, but lsf(IrMn) is probably short, less than or equal to 1 nm. Similar
results were found with FeMn.
