Browsing by Author "Brück, Sebastian"

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    Magnetic resonant reflectometry on exchange bias systems
    (2009) Brück, Sebastian; Schütz, Gisela (Prof. Dr.)
    Modern magnetic thin film devices owe their success in large part to effects related to interlayer coupling and exchange interaction at interfaces. One prominent example of such an interlayer coupling is exchange bias. Exchange bias is an interfacial exchange coupling which can occur if a ferromagnet is in direct contact to an antiferromagnet. The presence of the antiferromagnet induces an additional unidirectional anisotropy to the ferromagnet which results in a horizontal shift of the hysteresis loop. The aim of this thesis was to investigate several open questions and problems related to the basic physics of the exchange bias effect: First of all to find direct evidence for the existence and location of pinned uncompensated moments in the antiferromagnet. These moments are discussed as the origin of the exchange bias loop shift already since the time of its discovery. But direct evidence for their existence, amount and correlation with the effect were mostly missing. Another very important point is the magnetic configuration at the interface between ferro- and antiferromagnet. Finally experimental results indicating the microscopic mechanism responsible for the unidirectional character of the loop shift were completely missing. In this thesis, X-ray magnetic reflectivity (XRMR) and X-ray magnetic circular dichroism (XMCD) were used to carry out a detailed investigation on three prominent exchange bias systems in order to answer the questions just framed. XRMR uses standard X-ray reflectivity and extends it by the additional contrast from the XMCD. As a result, XRMR provides structural, i.e., thickness and roughness information of a sample and also yields information on the magnetic profile along the z-direction. The magnetic configuration at the interface between ferro- and antiferromagnet was investigated in the Fe/MnPd exchange bias system. Energy dependent scattering was performed on the Co/FeMn system to reveal the spectroscopic character of the pinned uncompensated moments in this system. Finally, the exchange bias system Co/NiCoO, a system with an oxide antiferromagnet, was investigated to reconfirm the former findings for this class of systems as well. A precise map of the magnetic configuration at the interface with the ferromagnetic Fe is obtained from the XRMR investigation of Mn in the antiferromagnetic alloy MnPd. A roughly 1 to 2 monolayer, or (4 ± 1)Å thick region of rotatable Mn is found directly at the interface with the Fe. This result clearly proves the existence of rotatable Mn in the antiferromagnet. The magnetic depth profile of pinned moments in the Mn was also obtained from the same XRMR investigation of the Fe/MnPD exchange bias system. It is found that pinned uncompensated Mn moments exist deeper in the antiferromagnet in a (13 ± 1)Å broad region close to the interface. A direct correlation of these moments with the exchange bias loop shift Heb is found from the azimuthal dependence. The second investigated system is a bilayer structure consisting of ferromagnetic cobalt grown on antiferromagnetic FeMn. By measuring the energy dependent scattering at the Fe L2,3 edge, the spectral character of the pinned uncompensated Fe moments in this system is investigated. To prove the direct correlation of the observed spectrum with the exchange bias effect in the system, two different samples with a unique on-off character of the exchange bias effect are compared. The energy dependent scattering at a fixed qz value was investigated in both samples at the Fe L edge to identify the signal from the pinned uncompensated Fe moments in the antiferromagnet. A direct correlation of the signal with the loop shift is found from comparing the signal for the situation with exchange bias to the one without. A relation between the observed difference signal attributed to pinned mo221 ments in the antiferromagnet and the exchange bias loop shift observed by magnetometric means is proven. The shape of the difference signal from the pinned uncompensated Fe moments in the Co/FeMn exchange bias system clearly resembles a spectrum which is only dominated by an orbital momentum. This behaviour is found in both samples and comes/goes in both cases with the loop shift. This implies that the magnetic moments responsible for the exchange bias loop shift are mostly orbital moments. The shape of the signal and also the relation of this signal with the exchange bias loop shift is ascertained beyond doubt by the comparative character of the investigation and the tests performed.