Pinned magnetic moments in exchange bias: Role of the antiferromagnetic bulk spin structure

Abstract

Magneto-optical Kerr effect measurements of epitaxial AFM/FM bilayers and FM/AFM/FM trilayers on Cu 3 Au(001), where "AFM" stands for a Ni 25 Mn 75 antiferromagnetic layer, and "FM" for ferromagnetic layers that are either Ni or Ni/Co with out-of-plane or in-plane easy axis of magnetization, show that trilayers with collinear magnetization directions of both FM layers exhibit always a much lower exchange bias field H eb at a fixed temperature compared to bilayers of the same Ni 25 Mn 75 thickness. At the same time, the blocking temperature for exchange bias T b is strongly reduced. In trilayers with orthogonal easy axes of the two FM layers (in-plane and out-of-plane), in contrast, both H eb and T b are nearly identical to that of the corresponding bilayers. Such a behavior can be explained by pinned magnetic moments inside the bulk of the AFM layer that coexist independently for orthogonal spin directions, but have to be equally shared between both interfaces in the case of collinear spin directions. This result thus also confirms a 3Q-like noncollinear spin structure of Ni 25 Mn 75 .

Role of the antiferromagnetic bulk spins in exchange bias

“…Similar conclusions are drawn from a recent experiment with Ni/NiMn/Ni trilayers which combine in-plane and out-of-plane anisotropies [28]. Fig.…”

Exchange bias in FeNi/FeMn/FeNi multilayers

“…This is due to the fact that the two FM layers with collinear magnetization direction are competing for the pinned uncompensated AF moments, even being magnetized in parallel. During field cooling, the pinned moments separate into two parts, each one providing EB only for one of the FM layers [33]. According to this point of view, in our case, the values of H EB should be the same both for parallel and antiparallel configurations.…”

“…Therefore, the suggestion about the existence of magnetic coupling between the FM layers through the AF layer is quite rare [28,29]. In most of the published articles, the authors study the hypothesis of possible interaction or its absence between two exchange bias systems through a common antiferromagnetic spacer [11,13,[21][22][23][30][31][32][33]. Malinowski et al found for FeNi/IrMn/FeNi trilayers that the magnetic coherence of the antiferromagnetic layer at the bottom interface does not extend up to the top, the upper side of the IrMn layer has no chance to get biased from the bottom FeNi layer [11].…”

Exchange-Bias Phenomenon: The Role of the Ferromagnetic Spin Structure

“…However, recent experiments [15,16] and theories [17] have emerged which show that EB is affected by long range interactions in the AF. The spin structure in the bulk of the AF has shown to be crucial determining the magnitude of H EB .…”