Onset of Exchange Anisotropy in Epitaxial fct-Cobalt(001)/fct-Manganese(001) Bilayers
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0941-Q04-06
Onset of Exchange Anisotropy in Epitaxial fct-Cobalt(001)/fct-Manganese(001) Bilayers Jürgen T. Kohlhepp, Harm Wieldraaijer, and Wim J. M. de Jonge Department of Applied Physics and center for NanoMaterials (cNM), Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands
ABSTRACT Manganese (Mn) grows in a meta-stable expanded (c/a > 1) face-centered-tetragonal (fct) phase on thin fct-Co(001) template films. For small Mn thicknesses a close to a layer-by-layer growth is observed. Antiferromagnetism (AFM) of fct-Mn is evidenced by the observation of shifted magnetization loops antiparallel to the cooling field direction (negative bias) and enhanced coercive fields for Co/Mn bilayers. At low temperatures magnetic Co/Mn interface exchange interactions are detected for nominal Mn thicknesses as low as 1.3 monolayer (ML), indicating the onset of AFM in 2 ML thick Mn islands. However, between 2 ML and 5 ML Mn a positive bias of the magnetization loops is observed in a certain temperature range below the blocking temperature (TB) of the films. This peculiar behavior is explained by the dependence of TB on both the local Mn ML thickness and the Mn island size, leading to a unidirectional coercivity enhancement in a temperature range where both blocked and unblocked regions are coexisting in the Mn layers. INTRODUCTION The interfacial exchange coupling that exists between the spins of an antiferromagnet (AFM) and a ferromagnet (FM) has attracted much attention in the past years [1-3]. Mainly triggered by the direct application perspectives in modern spintronic devices, the majority of the reported studies dealt with polycrystalline samples. These structures, however, suffer from different sorts of imperfections, such as i.e. interdiffusion, grain size distributions and coexisting crystal phases, which makes the interpretation of experimental results difficult and sometimes ambiguous. Recently we have introduced a new single-crystalline AFM/FM model system, which is based on the epitaxial stabilization of expanded (c/a > 1) face-centered-tetragonal (fct) Mn(001)films (known to be a meta-stable phase [4]) on thin fct-Co(001) template layers by molecular beam epitaxy (MBE)[5]. We have demonstrated that at room temperature (RT) distinctly enhanced coercive fields HC (i.e. the half-width of the magnetic hysteresis loop) are observed for Mn(001)/20 ML Co(001) bilayers with Mn thicker than about 2.5 monolayer (ML) [5]. Additionally, the onset of exchange bias (shift of the hysteresis loop along the field axis) was found around 5-7 ML Mn(001). In the present study we use this model system to examine in detail the thickness and temperature dependence of the onset of magnetic interface exchange interactions in Mn/Co(001) bilayers using now Co thicknesses of about 10 ML. The difference to the earlier investigation lies in the fact that below about 15 ML the Co growth on Cu(001) is almost ideal layer-by-layer and with a structural quality corresponding to virtually perfect single-crystalline fct-Co, a
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