Atomistic origin of exchange anisotropy in noncollinear γ-IrMn3 -CoFe bilayers

Antiferromagnetic spintronic devices could offer ultrafast dynamics and a higher data density than conventional ferromagnetic devices. One of the challenges in designing such devices is the control and detection of the magnetization of the antiferromagnet due to its lack of stray fields, and this is often achieved through the exchange bias effect. In exchange biased systems, 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 disordered γ-IrMn3-CoFe structure, the pinned spins arise from the small imbalance in the number of spins in each magnetic sublattice in the antiferromagnet due to the naturally occurring atomic disorder. These pinned spins are strongly coupled to the bulk antiferromagnet, explaining their stability. Moreover, we find that the ferromagnet strongly distorts the interface spin structure of the antiferromagnet, causing a large reversible interface magnetization that does not contribute to exchange bias but does increase the coercivity. We find that the uncompensated spins are not localized spins which occur due to point defects or domain walls but instead constitute a small motion of every antiferromagnet spin at the interface. This unexpected finding resolves one of the long-standing puzzles of exchange bias and provides a route to developing optimized nanoscale antiferromagnetic spintronic devices.