Exchange bias by ion implantation: From the classical bilayer system to a new induction method.

Exchange bias, referring to the unidirectional anisotropy in a ferromagnet when in contact with an antiferromagnet, has been an extremely intriguing phenomenon in the more than fifty years since its discovery. Due to its complexity, it has been mainly studied in bilayers, consisting of a thin ferromagnetic film having a single planar interface with a thin antiferromagnetic film. In this simple model system, the mechanism for exchange bias is fairly well understood, namely, the magnetic coupling between the ferromagnet and the uncompensated antiferromagnetic spins at their common interface. Nevertheless, this single interface puts a strong restriction on the thickness of the ferromagnet for exchange bias to occur in bilayers. More in particular, above a certain thickness of the ferromagnet, the coupling with the antiferromagnet is not strong enough to maintain the unidirectional anisotropy.In this thesis, the use of ion implantation to form an antiferromagnet embedded in a ferromagnetic thin film is studied. This is a hitherto unexplored method to form an exchange bias system. The underlying idea is to form a structure where there is not a single planar interface between the ferromagnet and the antiferromagnet, but rather an interface distributed throughout the structure. In a sense, the unidirectional anisotropy does not originate at the boundaries of the ferromagnet, but inside the ferromagnet. It is hoped that in this way the limitation on the thickness of the ferromagnetic film for exchange bias to occur, as observed in bilayers, can be overcome. Besides, this new method to form an exchange bias system also challenges the existing knowledge on exchange bias, which is mainly based on the study of bilayers.First of all, the feasibility of this approach is tested. It is shown that by the implantation of O ions into thin Co films exchange bias can be induced. By comparing to well-selected reference systems, it is unambiguously proven that the observed exchange bias effect is originating from the formation of antiferromagnetic oxides inside the ferromagnetic film. Surface oxidation and effects related to the ion implantation process are ruled out as a cause of the observed exchange bias effect. Moreover, this approach is successful in a film thickness regime where exchange bias in bilayers does not occur. To further elucidate the mechanisms underlying exchange bias by ion implantation, a number of aspects related to the film growth and implantation process are studied. Specifically, the influence of the implantation matrix, the implantation fluence, and the implantation profile on the exchange bias properties is scrutinized. First of all, these experiments reveal how the strength of exchange bias by ion implantation can be controlled. Furthermore, the experimental findings support the hypothesis of formation of grain boundary oxides as the mechanism for exchange bias by ion implantation. Finally, a detailed modeling of the implantation profile discloses the particular distribution of coupling strengths within the implanted films. This distribution has a large impact on the global magnetic behavior of the system, more in particular, on the shape of the hysteresis loop. Finally, exchange bias by ion implantation is compared to exchange bias in the bilayer system, the starting point of this research. This comparison is focused on the magnetocrystalline anisotropy and the magnetization reversal mechanism and the relation between both. It is shown that in bilayers the magnetization reversal mechanism is determined to a large extent by the magnetocrystalline anisotropy. However, in the implanted system, magnetocrystalline anisotropy is largely suppressed by the implantation process, minimizing its influence on any of the exchange bias properties. In addition, the asymmetry in the reversal mechanism, observed in a number of bilayer systems, is absent in the implanted system. These differences between both systems demonstrate the large influence of this new preparation method on the exchange bias properties.nrpages: 199status: publishe