Despite its technological importance for the development of advanced magnetic data storage devices and extensive studies, the details of the magnetic interface coupling between antiferromagnets and ferromagnets have remained concealed. This is in part due to the incomplete characterization of the interface in the sputtered polycrystalline samples that are typically used. Characterization and control of the interface structure and morphology at the atomic level is indeed the important issue, because the atomic spins in an antiferromagnet change their direction on the lengthscale of nearest atomic distances.
The behavior of the exchange bias effect of a ferromagnetic/antiferromagnetic bilayer in the presence of a second ferromagnetic layer at the other interface of the antiferromagnetic layer can yield important information about the nature of the pinning of spins in the antiferromagnetic layer. We have studied (Co/)Ni/Ni25Mn75/Ni(/Co) trilayers, epitaxially grown on a Cu3Au(001) substrate. The optional Co layers were used to change the magnetization direction of the Ni films from perpendicular to the film plane into the film plane. The exchange bias (bottom part of the graph) of the bottom Ni/Co ferromagnetic film with in-plane magnetization (green curve) nearly does not change upon the deposition of a Ni layer with out-of-plane magnetization on top of the antiferromagnetic Ni25Mn75 layer (red curve), but shrinks drastically once the magnetization direction of the top Ni layer is turned into the plane by a Co overlayer (blue curve). We interpret this in a model of competing non-collinear pinning centers throughout the entire thickness of the antiferromagnetic Ni25Mn75 layer.
The antiferromagnetic ordering temperature, that is the temperature below which an antiferromagnetic material displays antiferromagnegic spin order, of an ultrathin film may be influenced by proximity effects from adjacent ferromagnetic layers. We could demonstrate the directional dependence of such proximity effects in Ni/FeMn bilayers, epitaxially grown on a Cu(001) single crystal. The magnetization direction of the ferromagnetic Ni layer can be turned from perpendicular to the film plane into the film plane by the deposition of a Co overlayer. The antiferromagnetic ordering temperature of the antiferromagnetic FeMn film differs by up to 60 K for the two magnetization directions of the ferromagnetic layer! This means that it is possible to switch on and off the antiferromagnetic order in the FeMn layer at certain constant temperatures by just turning the magnetization direction of the Ni adlayer by 90°.
A more complete view about the thickness dependence of the interlayer coupling on both the Co bottom layer thickness and the FeMn layer thickness is obtained from magnetic imaging experiments using PEEM, performed on crossed double-wedge samples. The figure shows layer-resolved magnetic domain images of a NiFe/FeMn/Co trilayer on Cu(001), in which the Co bottom FM layer and the FeMn AF layer have been deposited as crossed micro-wedges. The upper panel (a) shows the as-grown domain image of the top FeNi layer, panel b the domain image of the bottom Co layer, as seen through the FeNi and FeMn overlayers. The Co layer thickness increases up to 8 ML from left to right as indicated at the bottom axis, and then stays constant at 8 ML. The FeMn thickness is indicated at the left axis.
Regions with parallel and antiparallel coupling between the two FM layers can be immediately recognized as horizontal stripes in the magnetization of the NiFe layer (panel a). They appear periodically as a function of FeMn thickness, with a period of 2 ML. In addition the stripes show a sawtooth-like corrugation in the left part of the image. This is the region where the Co thickness increases, demonstrating the influence of the Co bottom layer thickness.
Changing the thickness of the bottom Co layer by less than a monolayer modifies the atomic-scale morphology of the interface to the antiferromagnetic FeMn layer, and thus the magnetic coupling. The results highlight the role of atomic steps, islands, and vacancy islands at the interface for the magnetic interaction between AF and FM layers. We find a pronounced systematic dependence of the AF layer thickness for maximum interlayer coupling strength on the atomic-scale interface morphology, which shows that islands and vacancy islands at the interface lead to a quite distinct coupling behavior. Our experimental results shed light on the detailed coupling mechanism between an AF layer and adjacent FM layers, and demonstrate the importance not only of the presence of atomic steps at the interface, but also their detailed arrangement.
We suggest that in general the interface coupling of systems with compensated AF interface spin structure can be enhanced by the controlled incorporation of atomic level roughness features with small lateral size. With the advent of atomic scale manipulation in nanotechnology coming up, this may be a feasible way to controllably modify the coupling strength in FM/AF systems.
These experiments have been conducted in collaboration with L. I. Chelaru, F. Offi, J. Wang, M. Kotsugi, and J. Kirschner, Max-Planck-Institut für Mikrostrukturphysik Halle (Germany).
M. Y. Khan, C.-B. Wu, and W. Kuch
Pinned magnetic moments in exchange bias: Role of the antiferromagnetic bulk spin structure
Phys. Rev. B 89, 094427 (2014).
M. Stampe, P. Stoll, T. Homberg, K. Lenz, and W. Kuch
Influence of ferromagnetic–antiferromagnetic coupling on the antiferromagnetic ordering temperature in Ni/FexMn1–x bilayers
Phys. Rev. B 81, 104420 (2010).
R. Abrudan, J. Miguel, M. Bernien, C. Tieg, M. Piantek, J. Kirschner, and W. Kuch
Structural and magnetic properties of epitaxial Fe/CoO bilayers on Ag(001)
Phys. Rev. B 77, 014411 (2008)
K. Lenz, S. Zander, and W. Kuch
Magnetic Proximity Effects in Antiferromagnet/Ferromagnet Bilayers: The Impact on the Néel Temperature
Phys. Rev. Lett. 98, 237201 (2007).
W. Kuch, L. I. Chelaru, F. Offi, J. Wang, M. Kotsugi, and J. Kirschner
Tuning the magnetic coupling across ultrathin antiferromagnetic films by controlling atomic-scale roughness
Nature Materials 5, 128 (2006).