@phdthesis{Keller2004,
author = {Keller, Dirk},
title = {Optische Eigenschaften ZnSe-basierter zweidimensionaler Elektronengase und ihre Wechselwirkung mit magnetischen Ionen},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-14774},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2004},
abstract = {In dieser Arbeit wurden nichtmagnetische und semimagnetische ZnSe-basierte Quantentr{\"o}ge untersucht. Im Mittelpunkt des Interesses standen hierbei vor allem die Modifikation der optischen Spektren mit einer zunehmenden Modulationsdotierung der Strukturen und der Einfluss von Spinflip-Streuungen der freien Band-Elektronen an den Mn-Ionen auf die Magnetisierung und somit die Zeeman-Aufspaltung der Strukturen. Als experimentelle Methoden wurden Photolumineszenz (PL), Photolumineszenzanregung (PLE) und Reflexionsmessungen verwendet, die in Magnetfeldern von bis zu B=48 T und bei Temperaturen im Bereich von 1.6 K bis 70 K durchgef{\"u}hrt wurden. Dar{\"u}ber hinaus wurde die Abh{\"a}ngigkeit der Spin-Gitter-Relaxationszeit der Mn-Ionen von der Mn-Konzentration und der Elektronengasdichte in den Quantentr{\"o}gen durch zeitaufgel{\"o}ste Lumineszenzmessungen untersucht. Der Einfluss eines Gradienten in der s/p-d-Austauschwechselwirkung auf die Diffusion der Ladungstr{\"a}ger bildet einen weiteren Schwerpunkt dieser Arbeit. Als experimentelle Methode wurde hierbei ortsaufgel{\"o}ste Lumineszenz verwendet.},
subject = {Zinkselenid},
language = {de}
}
@phdthesis{Pappert2007,
author = {Pappert, Katrin},
title = {Anisotropies in (Ga,Mn)As - Measurement, Control and Application in Novel Devices},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-23370},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {Ferromagnetic semiconductors (FS) promise the integration of magnetic memory functionalities and semiconductor information processing into the same material system. The prototypical FS (Ga,Mn)As has become the focus of semiconductor spintronics research over the past years. The spin-orbit mediated coupling of magnetic and semiconductor properties in this material gives rise to many novel transport-related phenomena which can be harnessed for device applications. In this thesis we address challenges faced in the development of an all-semiconductor memory architecture. A starting point for information storage in FS is the knowledge of their detailed magnetic anisotropy. The first part of this thesis concentrates on the investigation of the magnetization behaviour in compressively strained (Ga,Mn)As by electrical means. The angle between current and magnetization is monitored in magnetoresistance(MR) measurements along many in-plane directions using the Anisotropic MR(AMR) or Planar Hall effect(PHE). It is shown, that a full angular set of such measurements displayed in a color coded resistance polar plot can be used to identify and quantitatively determine the symmetry components of the magnetic anisotropy of (Ga,Mn)As at 4 K. We compile such "anisotropy fingerprints" for many (Ga,Mn)As layers from Wuerzburg and other laboratories and find the presence of three symmetry terms in all layers. The biaxial anisotropy term with easy axes along the [100] and [010] crystal direction dominates the magnetic behaviour. An additional uniaxial term with an anisotropy constant of ~10\% of the biaxial one has its easy axis along either of the two <110> directions. A second contribution of uniaxial symmetry with easy axis along one of the biaxial easy axes has a strength of only ~1\% of the biaxial anisotropy and is therefore barely visible in standard SQUID measurements. An all-electrical writing scheme would be desirable for commercialization. We report on a current assisted magnetization manipulation experiment in a lateral (Ga,Mn)As nanodevice at 4 K (far below Tc). Reading out the large resistance signal from DW that are confined in nanoconstrictions, we demonstrate the current assisted magnetization switching of a small central island through a hole mediated spin transfer from the adjacent leads. One possible non-perturbative read-out scheme for FS memory devices could be the recently discovered Tunneling Anisotropic MagnetoResistance (TAMR) effect. Here we clarify the origin of the large amplification of the TAMR amplitude in a device with an epitaxial GaAs tunnel barrier at low temperatures. We prove with the help of density of states spectroscopy that a thin (Ga,Mn)As injector layer undergoes a metal insulator transition upon a change of the magnetization direction in the layer plane. The two states can be distinguished by their typical power law behaviour in the measured conductance vs voltage tunneling spectra. While all hereto demonstrated (Ga,Mn)As devices inherited their anisotropic magnetic properties from their parent FS layer, more sophisticated FS architectures will require locally defined FS elements of different magnetic anisotropy on the same wafer. We show that shape anisotropy is not applicable in FS because of their low volume magnetization. We present a method to lithographically engineer the magnetic anisotropy of (Ga,Mn)As by submicron patterning. Anisotropic strain relaxation in submicron bar structures (nanobars) and the related deformation of the crystal lattice introduce a new uniaxial anisotropy term in the energy equation. We demonstrate by both SQUID and transport investigations that this lithographically induced uniaxial anisotropy overwrites the intrinsic biaxial anisotropy at all temperatures up to Tc. The final section of the thesis combines all the above into a novel device scheme. We use anisotropy engineering to fabricate two orthogonal, magnetically uniaxial, nanobars which are electrically connected through a constriction. We find that the constriction resistance depends on the relative orientation of the nanobar magnetizations, which can be written by an in-plane magnetic field. This effect can be explained with the AMR effect in connection with the field line patterns in the respective states. The device offers a novel non-volatile information storage scheme and a corresponding non-perturbative read-out method. The read out signal is shown to increase drastically in samples with partly depleted constriction region. This could be shown to originate in a magnetization direction driven metal insulator transition of the material in the constriction region.},
subject = {Anisotropie},
language = {en}
}
@phdthesis{Wenisch2008,
author = {Wenisch, Jan},
title = {Ferromagnetic (Ga,Mn)As Layers and Nanostructures: Control of Magnetic Anisotropy by Strain Engineering},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-34552},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2008},
abstract = {This work studies the fundamental connection between lattice strain and magnetic anisotropy in the ferromagnetic semiconductor (Ga,Mn)As. The first chapters provide a general introduction into the material system and a detailed description of the growth process by molecular beam epitaxy. A finite element simulation formalism is developed to model the strain distribution in (Ga,Mn)As nanostructures is introduced and its predictions verified by high-resolution x-ray diffraction methods. The influence of lattice strain on the magnetic anisotropy is explained by an magnetostatic model. A possible device application is described in the closing chapter.},
subject = {Magnetischer Halbleiter},
language = {en}
}
@phdthesis{Kehl2010,
author = {Kehl, Christian},
title = {Magnetic soft mode behaviour investigated via Multi-Spin Flip Raman Spectroscopy on near surface Cd1-xMnxTe/Cd1-yMgyTe Quantum wells},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56088},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2010},
abstract = {In the context of the ongoing discussion about a carrier-induced ferromagnetic phase transition in diluted-magnetic II-VI semiconductors (DMS), theoretical studies on coherent dynamics of localized spins coupled with a two-dimensional hole gas (2DHG) in DMS quantum wells (QWs) were done by K.V. KAVOKIN. His key for studying the exchange interaction of the localized spin ensemble (e.g. Mn2+) with the 2DHG is the Larmor frequency of the localized Mn-ion spins and thus their Mn-g-factor. It was shown that the 2DHG affects a time evolution of the (Mn-) spin system in an in-plane magnetic field resulting in the reduction of its Larmor frequency (Mn-g-factor) under the influence of an oscillating effective field of holes. This is called magnetic soft mode (behaviour). The experimental access for demonstrating this Mn-g-factor reduction with increasing hole concentration is the method of Multi-Spin-Flip (SF) Raman scattering combined with the variation of the carrier concentration by photo-excitation with an additional light source (two-colour experiment). The main motivation for this thesis was the experimental confirmation of the theoretically predicted magnetic soft mode and the analysis of its dependence on the hole-concentration and external B-field, as well as its disappearance with increasing sample temperature. For that purpose, CdMnTe/CdMgTe QWs (Mn: 0.6\%, 1.0\%) positioned close to the sample surface (13-19nm) were investigated in an in-plane applied external magnetic field (up to 4.5T in Voigt-geometry) via a two-colour experiment i.e. using two light sources. This allows the spin excitation of Mn-ions by simultaneously tuning the hole-concentration towards the ferromagnetic phase transition by photo-generated carriers. Thus, one tuneable laser is responsible for resonant below-barrier excitation as a probe for Multi-SF Raman scattering. The other laser excites photo-generated carriers from above barrier (2.41eV) for tuning the hole concentration in the QW. Positioning the QW close to the sample surface causes a surface-induced p-doping of the QW (intrinsic hole concentration in the QW) and enables the active tuning of the hole concentration by photo-generated carriers due to different tunnelling behaviour of electrons and holes from the QW to the surface. The Mn-g-factor was decreased by quasi-continuously increasing the above-barrier illumination (and thus the hole concentration), while the below-barrier excitation (Multi-PR probe) was kept at a constant low power. This results in a Mn-g-factor reduction starting from its atomic value g=2.01 to lowest evaluated Mn-g-factor in this thesis g=1.77. This is a magnetic softening of 12\%. Apart from the general magnetic soft mode behaviour at low temperatures, one of the main experimental results in this thesis is the confirmation of the theoretical prediction that the magnetic soft mode behaviour in the external B-field does not only depend on the carrier concentration but also on the B-field strength itself. An additional aspect is the temperature dependence of the magnetic soft mode. The Mn-g-factor decrease is suppressed with increasing temperature almost reaching the atomic Mn-g-factor at 4.2K (g=1.99). This behaviour is due to the T-induced weakening of the transverse 2DHG spin susceptibility. The results of the investigations concerning the cap layer thickness impact on the QW carrier characteristics were investigated in the cap thickness range of 13nm to 19nm. The cap thickness configures on the one hand the intrinsic hole concentration of the QW ("2DHG offset") due to the surface-induced p-doping and sets the "starting point" for the Mn-g-factor reduction. On the other hand the cap thickness determines the probability of electron tunnelling to the surface and thus the efficiency of the hole tuning by light. The latter is the criterion for the range of Mn-g-factor reduction by light. This two dependences were pointed out by the photo-generated hole influence on the QW PL-spectra which results in tuning the exciton-trion ratio. In summary both mechanisms are of relevance for the hole tuning and thus for the magnetic soft-mode behaviour. The mechanism of tunnelling time prevails at small cap layer thicknesses while the surface-induced p-doping plays the major role for larger cap thicknesses (> 25nm). In conclusion, the presented method in this thesis is a sensitive tool to study the dynamics of the spin excitations and the paramagnetic susceptibility in the vicinity of the hole-induced ferromagnetic phase transition.},
subject = {Raman-Spektroskopie},
language = {en}
}
@phdthesis{Ebel2013,
author = {Ebel, Lars Frederik},
title = {Molecular Beam Epitaxy and Characterization of Ferromagnetic Bulk and Thin (Ga,Mn)As Layers/Heterostructures},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-83942},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2013},
abstract = {Die vorgelegte Arbeit untersucht den ferromagnetischen Halbleiter (Ga,Mn)As mit seinen komplexen Eigenschaften im Hinblick auf die Optimierung der Materialeigenschaften sehr d{\"u}nner (4 nm) (Ga,Mn)As Schichten, welche mit der Technologie der Molekularstrahlepitaxie (MBE) hergestellt wurden. Zuerst werden die strukturellen, ferromagnetischen und elektrischen Eigenschaften von (Ga,Mn)As vorgestellt. Die Einfl{\"u}sse von Punktdefekten, Grenzfl{\"a}chen- und Oberfl{\"a}chen-Effekten auf dicke und d{\"u}nne (Ga,Mn)As Schichten werden mit Hilfe von vereinfachten, selbstkonsistenten Berechnungen der Bandkantenverl{\"a}ufe diskutiert. Der Experimental-Teil ist in drei Teile unterteilt: Der erste Teil untersucht den Einfluss der epitaktischen Wachstumsbedingungen auf die elektrischen und magnetischen Eigenschaften von dicken (70 nm) (Ga,Mn)As Schichten. Der zweite Teil f{\"u}hrt ein alternatives, parabolisches Mn-Dotierprofil mit effektiver Schichtdicke von 4 nm ein im Vergleich zu einer gleich d{\"u}nnen Schicht mit homogenem Mn-Dotierprofil. Es konnten einerseits verbesserte Eigenschaften dieser parabolischen Schicht erreicht werden, anderseits sind die magnetischen und elektrischen Eigenschaften vergleichbar zu dicken (Ga,Mn)As Schichten mit gleichem Mn-Gehalt von 4\%. MBE Wachstumsbedingungen f{\"u}r (Ga,Mn)As Schichten mit parabolischem Mn-Dotierprofil und verringertem nominellem Mn-Gehalt von 2.5\% wurden ebenfalls untersucht. Ein schmales Wachstumsfenster wurde hierbei ermittelt, in dem die Tieftemperatur-Eigenschaften verbessert sind. Der letzte Teil der Arbeit pr{\"a}sentiert eine Anwendung der magnetischen Anisotropiekontrolle einer dicken (Ga,Mn)As Schicht.},
subject = {Molekularstrahlepitaxie},
language = {en}
}
@phdthesis{Constantino2013,
author = {Constantino, Jennifer Anne},
title = {Characterization of Novel Magnetic Materials: Ultra-Thin (Ga,Mn)As and Epitaxial-Growth MnSi Thin Films},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-90578},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2013},
abstract = {The study of magnetic phases in spintronic materials is crucial to both our fundamental understanding of magnetic interactions and for finding new effects for future applications. In this thesis, we study the basic electrical and magnetic transport properties of both epitaxially-grown MnSi thin films, a helimagnetic metal only starting to be developed within our group, and parabolic-doped ultra-thin (Ga,Mn)As layers for future studies and applications.},
subject = {Galliumarsenid},
language = {en}
}