Atomistic origin of exchange anisotropy in noncollinear γ−IrMn3 –CoFe bilayers

Abstract

Anti-ferromagnetic spintronic devices could offer ultra fast dynamics and a higher data density than conventional ferromagnetic devices. One of the challenges designing such devices is the control and detection of the magnetisation of the anti-ferromagnet 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 IrMn 3 /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 magnetisation that does not contribute to exchange bias but does increase the coercivity. We find that the uncompensated spins are not localised spins which occur due to point defects or domain walls but instead constitute a small motion of every AFM 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.

Exchange bias in multigranular noncollinear IrMn3/CoFe thin films

“…The exchange bias is similar to the exchange bias found by Jenkins et al [11] for a single grain system. In the multigrain system the exchange bias is an average of the individual grains explaining the similarity to the single grain result.…”

“…simulations, as plotted in Fig. 9 with the experimental results from O'Grady et al The exchange bias has a maximum for smaller grain sizes, because the smaller the grain size the larger the statistical imbalance between the number of spins in each sublattice [11]. In reality, with temperature the small grains would become superparamgnetic and not contribute to the exchange bias as in equation 3.…”

“…The setting procedure forces the AFM to set with the direction of the net interface magnetisation along the direction of the setting field. From Jenkins et al [11], we know that the interface moment is caused by an imbalance of Mn atoms in each of the four sublattices, causing one sublattice to have a larger moment than the other four sublattices. In each grain the magnetisation of the AFM sublattices can lie along four possible directions.…”

“…Most of these models were based on the idea that the uncompensated spins occurred due to AFM domains or impurities. Recently, a new model has been proposed by Jenkins et al [11]. They proposed a natural model of exchange bias for γ-IrMn 3 / CoFe bilayers which included a realistic 3Q tetrahedral spin structure in the antiferromagnet.…”

“…Although many models of exchange bias in polycrystalline thin films have been developed, all of these models assumed a distribution of set directions for the AFM grains [9,[13][14][15][16]. In this paper we continue with the natural model of Jenkins et al [11] and instead of assuming an arbitrary distribution of set directions and anisotropies these will occur due to the naturally occurring distribution of the specific atomic configuration at the interface. We then perform large scale simulations of a multigranular γ-IrMn 3 / CoFe bilayer system investigating the setting of granular and continuous ferromagnetic layers, the computation of the exchange bias field and coercivity as a function of the system temperature and their dependence on the grain size dependence.…”