Fluence dependence of ion implantation-induced exchange bias in face centered cubic Co thin films

Abstract

The fluence dependence of exchange bias induced by oxygen ion implantation has been studied in highly textured face centered cubic Co films. These films exhibit a strong magnetocrystalline anisotropy prior to implantation. Upon implantation, the crystalline order is strongly reduced, even for the lowest implantation fluence, as shown by an isotropic magnetic behavior. Detailed analysis of the structural changes shows that the crystallite size remains basically unaltered upon implantation, suggesting that CoxOy is formed at the Co grain boundaries. A large suppression of the magnetocrystalline anisotropy is observed after implantation. This anisotropy has no influence on the unidirectional anisotropy associated to the exchange bias effect. Our study identifies a narrow implantation fluence window in which exchange bias by oxygen ion implantation is established. With increasing oxygen fluence, an increase in the magnitude of the exchange bias effect for higher fluences and, finally, a saturation of the exchange bias effect is observed in the studied fluence window. Moreover, the particular shape of the measured hysteresis loop is ascribed to a distribution of switching fields, which results from the implantation depth profile of oxygen throughout the Co film.

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

“…This could be correlated not only to an increased density of defects for samples implanted at higher fluences (i.e., slightly less efficient structural recovery by annealing) but also to a diminution of the exchange interactions between Co grains, which usually has a detrimental effect on coercivity. 16 This is consistent with the phase separation in the O-implanted layer which yields Co rich pure areas embedded in a structurally well-defined CoO.…”

“…17,19 This decrease in H E and H C might be mainly ascribed to the decreased number of FM/AFM interfaces since annealing promotes phase separation. 16,17 Moreover, in the framework of the domain state model, 26 annealing may result in an AFM CoO with decreased volume domains which negatively influence EB. This decrease could also be related to the size increase of the Co regions, as evidenced in the structural analysis.…”

“…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.…”

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

“…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.…”

Exchange bias in polycrystalline magnetite films made by ion-beam assisted deposition

“…7 Some iron oxide thin films prepared by radio frequency magnetron sputtering show more than one phase, the details of which are controlled by the bias voltage. 1 In other systems, oxygen ion implantation in single phase ferromagnetic thin films 8,9 and oxidation of grain boundaries can lead to unexpected exchange bias. 10 Here, we report an unusual exchange bias effect in magnetite thin films on silicon substrates which nominally show absence of a second crystalline phase.…”