Local patterning of damping by ion-irradiation In ferromagnetic-antiferromagnetic thin films

The control of the effective magnetic anisotropy, saturation magnetization as well as the dynamic magnetic properties in ferromagnetic thin films is of significant importance for most applications in spin electronics. The two materials’ parameters which control the dynamic response of soft magnetic thin films are the precessional frequency f_res and the effective damping parameter alpha. Whereas anisotropy and saturation magnetization together determine the precessional frequency of the films, the magnetic damping parameter cannot be varied easily. However it was shown that exchange coupled ferromagnetic/antiferromagnetic (F/AF) systems exhibit strong changes in f_res and alpha independent of the occurrence of exchange bias [1, 2]. On the other hand, ion irradiation of magnetic thin films [3] has been shown to provide an alternative method to alter soft magnetic properties of magnetic multilayer films due to ion induced changes in the interfacial structure. Here we use Ni-ion irradiation to control the dynamic magnetic properties in Ni81Fe19/IrMn/Ni81Fe19 structures in a controlled way. We show that for very thin AF layers alpha can be increased significantly [4]. Selecting an IrMn thickness below the onset of exchange bias, it is possible to solely alter alpha. This effect is reversible by low fluence Ni ion irradiation without changing the composition of the ferromagnetic alloy and thus can be used to adjust alpha over a wide range. As a consequence, the method can be used to laterally pattern samples only by the damping parameter. As shown in Fig. 1 the overall integral damping parameter then is determined by the relative fraction of materials with high and low damping parameter. alpha is thereby varied linearly without any significant change in f_res. The demonstrated paths of film alteration by ion irradiation provide an additional degree of freedom for the tailoring of dynamic magnetic properties of softmagnetic thin films. The method not only allows for general adjustment of magnetic properties, but also for a local setting of dynamic magnetic properties.
This work was supported by the German Science Foundation DFG under grant no. MC9-8/2.

[1] J. McCord et al., Phys. Rev. B 70, 094420 (2004)
[2] J. McCord et al., Phys. Rev. B 75, 134418 (2007).
[3] J. Fassbender and J. McCord, J. Magn. Magn. Mater. 320, 579 (2008).
[4] J. McCord, et al., Appl. Phys. Lett. 92 162506 (2008).