Magnetization reversal of ferromagnetic elements surrounded by a synthetic antiferromagnet

We investigated patterning of magnetic thin films by ion implantation. As shown in Ref. 1 by using ion implantation of an exchange coupled Fe/Cr/Fe system one can control the strength and sign of the antiferromagnetic (AF) coupling. Starting with a synthetic AF (SAF) trilayer (Co90Fe10/Ru/Co90Fe10) we used lithographic masks to irradiate spatially restricted areas. By doing this the trilayer structure was intermixed and resulted in ferromagnetic (FM) patterned elements surrounded by a SAF trilayer. At low fields the magnetization in the two Co90Fe10 layers of the SAF is antiparallel to each other and therefore behaves like an environment with a much lower susceptibility than the soft-magnetic Co90Fe10 film. The advantage of patterning by implantation compared to etching is that the magnetic elements don’t have a structural edge which usually is not free from roughness. The boundary of the patterned structures is only a magnetic boundary. Comparing to a previously used method, where we reduced the saturation magnetization locally by implanting Cr ions into a Ni81Fe19 layer [2], the method presented here needs a much lower ion fluence and therefore results in less irradiation damage and sputter losses.

The patterning was done by implanting Co ions with an energy of 80 keV into the following stack structure (deposited by dc-sputtering in an UHV vacuum system onto an oxidized Si wafer): Ta(4nm)/Co90Fe10(10nm)/Ru(1.15nm)/Co90Fe10(10nm)/Ru(3nm). Using optical lithography a mask was formed in photoresist and partly removed. At the used fluence of 5×1015 cm-2 the Co-ions intermixed the two Co90Fe10 layers with the Ru interlayer in the open areas of the mask (shown in Fig. 1 is a simulation of the intermixing during implantation using the Tridyn software package [3]). By this method FM elements were created that are surrounded by an AF-coupled Co90Fe10 bilayer. The magnetization reversal and domain structures were compared to patterned FM Ta(4nm)/Co90Fe10(20nm)/Ru(3nm) where the structures are etched.

Magnetic domain imaging was done using wide-field Kerr microscopy. By analyzing the gray scale intensity of individual stripes the magnetization loops of the FM stripes could be extracted. In the etched 20 µm wide stripes domains with anti-parallel magnetization and 180° domain walls are formed throughout the stripe during hard axis magnetization reversal. In the stripes surrounded by the SAF the magnetization is evolving into a different pattern. Along the FM-SAF interface edge domains evolve that depend on the magnetic field history. Fig. 2 shows the domain states during hard axis reversal for the implanted and etched sample as well as the hysteresis loops for the two different samples. The implanted sample has a smaller coercivity. Therefore even a small misalignment of the magnetic field with the hard axis has a stronger influence than in the etched sample. So in the implanted sample the magnetization in the center of the stripe is turning towards one direction due to the external field but the magnetization at the edge does not flip and is forming an edge domain. This can also explain the different domain size in the two images shown in Fig. 2. The smaller coercivity indicates that there are less pinning sites in the implanted sample compared to the etched stripes.

In narrower stripes with a nominal width of 2 µm Hc and Hk are the same for the etched and implanted stripes. Also here the coercivity is smaller in the implanted sample. The different anisotropy field for the implanted and etched stripes is caused from a different resulting width of the FM stripes for the two different preparation methods (stripe width obtained from AFM/MFM microscopy, not shown here).

To summarize, we used ion implantation to pattern extended magnetic thin films into elements of different sizes without introducing additional structural edges. Only the magnetic behavior is patterned (FM and SAF) and no structural edges are created. By using asymmetric Co90Fe10 thicknesses also patterning into areas of high and low effective saturation magnetization is possible. More examples of the resulting magnetic behavior will be shown during the presentation.

Support by DFG (FA 314/3 and MC 9/7) is gratefully acknowledged