Tese
Exchange bias em bicamadas de NiFe/FeMn: um estudo em função da espessura da camada antiferromagnética
Fecha
2018-12-04Autor
Kern, Paula Roberta
Institución
Resumen
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.