Exchange bias em bicamadas de NiFe/FeMn: um estudo em função da espessura da camada antiferromagnética

Abstract

The exchange bias (EB) arises, generally, from the interfacial coupling between a ferromagnetic layer (FM) and an antiferromagnetic layer (AFM). The most well-known manifestations of this phenomenon, in samples with a FM/AFM interface, are the field shift in the magnetization curves and the increase in the coercive field when compared to the uncoupled FM material. Many experimental results have shown that the exchange bias field (Heb) and coercivity (HC) in FM/AFM bilayers are inversely proportional to the FM layer thickness. On the other hand, the relationship between these parameters and the AFM layer is more complicated and the particularities of each system will define how the evolution of Heb and HC with the AFM thickness will be. The general rule is that there exists a critical thickness of the AFM below which the EB vanishes. In this thesis, in order to investigate the emergence and evolution of EB as a function of the AFM layer thickness, NiFe(50 nm)/FeMn(tAFM) bilayers were produced with 3 nm tAFM 30 nm. The work was carried out by means of structural and magnetic characterization techniques, such as X-ray reflectivity and diffraction, magnetometry and ferromagnetic resonance (at a fixed frequency and broadband). The results obtained by static and dynamic magnetic characterization have shown that, for this FM/AFM system, the EB arises when tAFM 6 nm. A description of the angular evolution of the ferromagnetic resonance field, considering the Zeeman, demagnetization, uniaxial, unidirectional and rotatable energies, confirmed the existence of a misalignment between the uniaxial and unidirectional anisotropies axes, which was found to be dependent on the AFM layer thickness. In addition, the evaluation of the ferromagnetic resonance linewidth as a function of temperature, performed for two samples, presented a peak at approximately 75 K, which was interpreted in terms of inomogeneities in the samples.