Exchange bias by implantation of O ions into Co thin films

Abstract

An original approach for the formation of an exchange bias system is presented. Alternative to surface oxidation or deposition for the formation of Co/CoO bilayer exchange bias systems, implantation of oxygen ions into Co films is applied. The implantation results in the formation of CoxOy embedded in a Co matrix. Comparison with noble gas implantation unambiguously demonstrates that the observed exchange bias effect is induced by the implanted oxygen. Opposed to bilayers formed by surface oxidation, the implantation results in a different morphology of the interface between Co and CoxOy and also gives rise to a radically different magnetization reversal mechanism.

Interdependence between training and magnetization reversal in granular Co-CoO exchange bias systems

“…2, in contrast to O-implanted thin films with Gaussian-like O depth profiles that display inhomogeneous hysteresis loops [34,36,37], the loops are rather symmetric and characterized by sharp descending and ascending branches, indicating that magnetization reversal takes place at well-defined switching fields thus confirming the homogeneity of the O-induced profile. Interestingly, as shown in Fig.…”

“…This also clearly evidences the metastable nature of the system after field cooling, which progressively evolves with consecutively measured loops to a local equilibrium state. In contrast to O-implanted Co films with a Gaussian-like O depth profile, which exhibit both unbiased Co and a variation of EB strength along film depth and no reversal asymmetry [34,37], the behavior of the low-fluence implanted samples seems to somewhat recover the asymmetry found in Co/CoO bilayers, where the first reversal mechanism is governed by domain wall nucleation and motion and the rest occur by coherent rotation [21,22,46]. In analogy to Co/CoO bilayers [43,44], the results could be understood in the framework of the level of magnetocrystalline anisotropy achieved with ion implantation.…”

“…Since surface oxidation is a self-limiting process, it results in an oxide thickness of only a few nanometers, which forms a single interface between Co and CoO. Ion implantation has been demonstrated to be a suitable procedure to control the amount of AFM and, ultimately, the EB properties of FM-AFM systems, such as Co-CoO [34][35][36][37] or Ni-NiO [37], by forming multiple FM-AFM interfaces (i.e., granularlike) controllably distributed throughout the FM matrix.…”

Tuning the ferromagnetic-antiferromagnetic interfaces of granular Co-CoO exchange bias systems by annealing

“…3 The majority of EB research has been focused on thin films, with Co-CoO bilayers constituting a valuable model system. 14 Alternatively, ion implantation has been demonstrated to be a suitable approach to control the EB properties of granular-like FM-AFM systems with multiple interfaces, such as Co-CoO [15][16][17][18] or Ni-NiO. 18 Recently, by the interplay between implantation conditions and sample design, it has been shown that the O-depth profile across the FM layer thickness can be controlled using a single-energy ion implantation approach.…”

Probing Exchange Bias Effects in CoO/Co Bilayers with Pillar-Like CoO Structures

“…The exchange bias found for the samples studied in this work is similar to the result found for epitaxial films Co where deliberate oxygen ion-implantation was performed to form Co/CoO nanocomposites (H ex $ 400 Oe), although there higher ion-beam energies were used (60 keV) presumably resulting in a different overall microstructure. 26) The magnetic bias found in such nanocomposites is not altogether surprising, since a disordered oxide-metal interface could play a similar role to the defects in a diluted antiferromagnet 1) or create random interfacial fields in the closely-related Malozemoff model. 27) Additionally, oxidepillars could act as pinning sights for Co ferromagnetic domain wall motion; provide a larger surface area for ferromagnetic/antiferromagnetic spin interactions and change the effective magnetostatic anisotropy.…”