Onset of magnetic interface exchange interactions in epitaxially grown Mn/Co(001)
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Onset of magnetic interface exchange interactions in epitaxially grown Mn/Co(001) J.T. Kohlhepp,a) H. Wieldraaijer, and W.J.M de Jonge Eindhoven University of Technology, Department of Applied Physics and Center for NanoMaterials (CNM), 5600MB Eindhoven, The Netherlands (Received 13 October 2006; accepted 20 November 2006)
Manganese (Mn) grows in a metastable expanded (c/a > 1) face-centered-tetragonal (fct) phase on thin fct-Co(001) template films. A layer-by-layer growth mode is observed for small Mn thicknesses. 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 temperature, magnetic exchange interactions are detected at the Co/Mn interface for nominal Mn thicknesses as low as 1.4 monolayer (ML), indicating the onset of AFM in 2-ML-thick Mn islands. However, between 2- 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 coexist in the Mn layers.
I. INTRODUCTION
The interfacial exchange coupling that exists between the spins of an antiferromagnet (AFM) and a ferromagnet (FM) has attracted much attention in 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 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 an epitaxial stabilization of expanded (c/a > 1) face-centeredtetragonal (fct) Mn(001)-films (known to be a metastable phase4) 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 monolayers (ML) Co(001) bilayers with Mn thicker than about 2.5 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 examined the thickness and temperature dependences of the onset of magnetic interface exchange interactions in Mn/Co(001) bilayers in more detail. In contrast to the earlier investigations described in Ref. 5, for the present study, we used smaller Co thicknesses around 10 ML. 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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