Magnetische Halbleiter zum Einsatz in der Spinelektronik : Europiumsulfid und magnetisch dotiertes Galliumnitrid

In this work, magnetic semiconductors were investigated. The work on magnetic semiconductors is motivated by the great interest in the field of spinelectronics in recent years. Of particular interest are magnetic semiconductors which are ferromagnetic with Curie-temperatures well above room temperature. As ferromagnetic compound semiconductor, EuS was investigated as thin film system. Particular attention was paid to the influence of the substrate temperature during growth on the sample properties. Changes in the growth temperature lead to a change in stoichiometry of the films with Eu rich compositions for low groth temperatures. Hence, the carrier concentrations increase for decreasing growth temperatures. Therefore, the Curie-temperatures decrease and the resistances increase with increasing growth temperatures. The increase in the Curie-temperature as well as the characteristic transport properties are caused by the strong f-d interaction between conduction band and localized magnetic moments. The samples grown and investigated here show an anomalous Hall effect. This has been observed for the first time in EuS, bulk samples do not show this effect. As diluted magnetic semiconductors (DMS), GaN films magnetically doped with Mn or Gd were investigated. In both material systems, ferromagnetism far above room temperature was demonstrated.While GaGdN shows a homogeneous magnetism, all ferromagnetic GaMnN samples show small clusters (phase separations). In addition, measurements of the optical absorption and the magnetic circular dichroism (MCD) were performed on the GaMnN samples. In the optical measurements, the Mn was identified as deep acceptor. At low temperatures, both the magneto-absorption and the MCD show a Zeeman-shift of an absorption band for all doping levels. The exchange energies between valence band and localized magnetic moments can be extracted from fits of the Brillouin function to the Zeeman shifts. This yields values between 1.4eV and 1.7eV for the lowest doping levels and between 1.7eV and 2.1eV for the highest doped sample. This fits into the general trend observed in other DMS, where the exchange constants show a pronounced dependence on the lattice constant. At low temperatures, no ferromagnetic behavior was found in the magneto-optical experiments. The signal is dominated by the localized Mn spin system. At room temperature, the MCD shows a ferromagnetic signature. At elevated temperatures, the signal from the Mn spin system is small enough that the influence of the clusters on the band structure of the host lattice becomes visible. Contrary to the GaMnN samples, the GaGdN films show a homogeneous magnetism. Because of the strong localization of the magnetic 4f states of the Gd and the high resistance of the films, direct-, double- and superexchange as well as carrier mediated ferromagnetism can be ruled out as exchange mechanisms. A possible exchange path is an intra-atomic f-d exchange between the localized magnetic 4f and the band states analog to the Euchalcogenides. This is consistent with the increase of the Curie-temperature upon increase of free carriers by Si co-doping. The magnetically doped GaN thin film systems investigated here exhibit different behavior. The exchange mechanism proposed for GaGdN leads to a homogeneous ferromagnetism and is similar to the magnetic exchange in the Eu chalcogenides. The interplay between this intra-atomic f-d exchange and the magnetic end electric properties was demonstrated for the EuS samples. The behavior of the GaMnN samples is contrary to the aforementioned samples. In the ferromagnetic samples, the extrinsic and intrinsic magnetic properties were separated and the magnitude of the exchange between localized magnetic states and the valence band was determined. The samples investigated here do not show the homogeneous carrier mediated ferromagnetism theoretically predicted.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2007

Author dc:creator

• Keller, Jan

Contributors dc:contributor

• Güntherodt, Gernot

• info:eu-repo/classification/ddc/530
• Magnetoelektronik
• Magnetischer Halbleiter
• Festkörperphysik
• Dünnschichttechnik
• Physik
• Verdünnte Magnetische Halbleiter
• DMS
• Magnetischer Zirkulardichroismus
• Magnetotransport
• Spinelektronik
• Diluted Magnetic Semiconductors
• Spintronics
• Magnetic Circular Dichroism
• Material Science

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123937%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62366