Improvement and stabilization of exchange bias in ferromagnet/antiferromagnet/ferromagnet trilayers

Abstract

Exchange bias (EB) in ferromagnet/antiferromagnet bilayers, which has been extensively studied and applied for several decades, is sensitive to many factors such as layer thickness, texture and crystallization. Various factors in an antiferromagnet may counterbalance each other to limit and deteriorate EB. We used an unbiased Monte-Carlo method based on a modified Metropolis algorithm to predict that dependence of EB properties on antiferromagnetic anisotropy (KAF) are highly improved by attaching a soft ferromagnet on the other side of the antiferromagnet. On one hand, target ferromagnet in trilayers displays a pronounced and stabilized EB plateau at high KAF, on the contrary, EB is completely removed and instead a high coercivity is observed at low KAF, exhibiting a roughly KAF-modulated EB switching effect. On the other hand, EB is identified with no training in trilayers and well axially symmetric about antiferromagnetic easy axis. Meanwhile, in trilayers EB versus angle (θ) which is between antiferromagnetic easy axis and the direction of magnetic field is a roughly linear relationship in the intermediate θ range. Microscopic explorations found that a fully uncompensated magnetization in antiferromagnet may appear in trilayers and its rotatability is precisely controlled by KAF, designating that the antiferromagnet/seed-ferromagnet bilayers resemble a ferromagnet with changeable hardness to induce a maximized coercivity of target ferromagnet at low KAF to a maximized EB at high KAF. Finally, a phenomenological model reveals that antiferromagnetic spins change from a fluctuated state to a blocked state due to the existence of seed ferromagnet, and thus in this work we conceived an artificial pinning layer to establish and regulate EB.