Interlayer exchange coupling

The interlayer exchange coupling between two ferromagnetic layers, coupled via a metallic, non-magnetic interlayer, is very sensitive to the interface morphology. By means of ion irradiation this morphology can be modified easily. A transition from antiferromagnetic to ferromagnetic coupling can be induced [1]. This effect has been used for the pure magnetic patterning of such layer systems by means of focused ion irradiation. The magnetic domain are observed by means of transmission x-ray microscopy  at the Advanced Light Source in Berkeley. In order to use the element specific imaging capabilities of this technique, which rely on the magnetic circular dichroism effect, a layer system with different ferromagnetic materials have been chosen. The samples have been sputter deposited onto x-ray transparent Si3N4 membranes. The antiferromagnetically coupled sandwich structure consists of a 16 nm Ni81Fe19-, a 0.8 nm Ru- and a 12 nm Co90Fe10 layer. The Ru film thickness was chosen to achieve the strongest antiferromagnetic coupling strength between both ferromagnetic layers (1st maximum of the antiferromagnetic coupling). In addition a 3 nm Ru cap layer has been deposited in order to circumvent the potential sputter erosion of the magnetic films during ion irradiation. The magnetic patterning has been performed wit a 60 keV Co2+ focused ion beam (IMSA-Orsay Physics) with a spot diameter of 50 nm. The ions have a mean projected range of 18 nm and thus are implanted into the magnetic layer stack. In order to achieve a pure magnetic patterning and to avoid chemical effects due to ion implantation Co ions are used instead of the common Ga. In Fig. 1 the magnetization reversal behavior is shown.  

Figure 1:

Transmission x-ray microscopy magnetic domain images aquired at the Co L3-edge as a function of applied magnetic field. Shown are difference images between the saturated state and the image taken at the specified magnetic field strength. In the dark areas the layer sys-tem has been ion irradiated and is thus ferromagnetically coupled. The magnetization reversal behavior with the creation of about 200 nm large domain wall areas is clearly observed. For further details see Ref. [2].



1. Control of interlayer exchange coupling Fe/Cr/Fe trilayers by ion beam irradiation
S. O. Demokritov, C. Bayer, S. Poppe, M. Rickart, J. Fassbender, B. Hillebrands, D. I. Kohlin, N. M. Kreines, M. O. Liedke
Phys. Rev. Lett. 90, 097201 (2003).

2. Magnetic domains and magnetization reversal of ion-induced magnetically patterned RKKY-coupled Ni81Fe19/Ru/Co90Fe10 films
J. Fassbender, L. Bischoff, R. Mattheis, P. Fischer
J. Appl. Phys. in press (2005).