The structure and magnetism of ultrathin Mn layers grown by molecular beam epitaxy (MBE) on Si(001) are investigated. X-ray absorption fine structure (XAFS) shows that a metastable MnSi phase with a B2-like (CsCl) crystal structure forms and superconducting quantum interference device (SQUID) magnetometry measurements reveal that the B2 structure is ferromagnetic with a Mn moment of 0.33 µB and a Curie temperature TC > 400 K. A change in the Si capping layer growth temperature, TSi, produces a MnSi phase with a B20-like structure, which exhibits a small moment and a TC below 20 K. Surfactant mediated growth of Mn ?-doped Si films and co-deposited Si1-xMnx alloys shows that a Pb surfactant strongly alters the structure and magnetism of these systems. For the ?-doped films, analysis of the crystal structure and magnetic properties over a range of growth parameters identified three distinct Mn-Si phases. With Pb, a sample with a coverage ?Mn = 0.26 monolayer and TSi = 200 ºC develops a ferromagnetic phase with a Mn moment of 1.56 µB and a TC > 400 K, whereas TC drops to zero for a sample grown without Pb. For TSi > 200 ºC, nano-disks with MnSi-B20 type structure form with a TC ? 170 K. A possible correlation exists between the remanent moment and the fraction of Mn occupying Si substitutional sites, which suggests that a dilute Si1-xMnx may be forming in the matrix. Density functional theory (DFT) shows that Pb lowers the formation energy of Si vacancies by 0.92 eV, which enhances the substitutional incorporation of Mn. In the absence of Pb, the co-deposited Si1-xMnx films undergo a 2D spinodal decomposition and form nanocolumns. The nanocolumns are amorphous and paramagnetic. In contrast, in the samples grown with Pb, MnSi1.7 nanorods form in the plane of the films and exhibit two ferromagnetic transitions at TC1 ? 40 K and TC2 > 400 K when x = 0.5%. While TC1 is consistent with TC of bulk MnSi1.7 crystals, TC2 is believed to originate from Mn diluted in the Si matrix.

This work is devoted to the investigation of a spin polarization in ferromagnetic semiconductor Ga1-xMnxAs with a broad nominal concentration of manganese ions using one method of the ultrafast laser spectroscopy - the time-resolved Kerr rotation. At first, the experimental setup was optimized for the investigation of the dynamics of spin polarized charge carriers in semiconductors which were photo-generated by circularly polarized laser pulses. It was observed that the measured signal is induced by spin-polarized electrons. Due to a small thickness of the investigated ferromagnetic films the measured signal probably monitored the dynamics of fotogenerated electrons in GaAs substrate. Nevertheless, the measured data show that the electron spin dynamics in the substrate is significantly influenced by proximity effect due to the deposited ferromagnetic layer.

As the switching rates in electronic and optoelectronic devices are pushed to even higher frequencies, it is crucial to probe carrier dynamics in semiconductors on femtosecond timescales. Time resolved spectroscopy is an excellent tool to probe the relaxation dynamics of photoexcited carriers; where after the initial photoexcitation, the nonequilibrium population of electrons and holes relax by a series of scattering processes including carrier-carrier and carrier-phonon scattering. Probing carrier and spin relaxation dynamics in InAs and InSb based narrow-gap semiconductors is crucial to understand the different scattering mechanisms related to the systems. Similar studies in InSb quantum wells are also intriguing, especially for their scientifically unique features (such as small effective mass, large g-factor etc). Our time resolved techniques demonstrated tunability of carrier and spin dynamics which might be important for charge and spin based devices. The samples studied in this work were provided by the groups of Prof. Wessels (Northwestern University) and Prof. Santos (University of Oklahoma). Theoretical calculations were performed by the group of Prof. Stanton (University of Florida). The THz measurements were performed at Wright State University in collaboration with Prof. Jason Deibel. This work has been supported by the National Science Foundation through grants Career Award DMR-0846834, AFOSR Young Investigator Program 06NE231. A portion of this work was performed at the National High Magnetic Field Laboratory (in collaboration with Dr. Stephen McGill), which is supported by National Science Foundation Cooperative Agreement No. DMR-0654118, the State of Florida, and the U.S. Department of Energy. / Ph. D.

Dans ce travail, différent systèmes ont été étudiés par des méthodes de microscopie électronique en transmission (TEM) : nanoparticules (NPs) de Pt sur du carbone amorphe, boîtes quantiques (QDs) de Ge, l'incorporation du Mn dans les QDs de Ge and des nanocolonnes (NCs) GeMn dans une matrice de Ge pure ou de GeSn. Le mûrissement de NPs de Pt sur un film de carbone amorphe a été étudié par TEM haute résolution (HRTEM) après des recuits à des températures comprises entre 200 °C et 300 °C pour des durées allant jusqu'à 160 h. Une augmentation significative de la taille moyenne des particules est observé en augmentation la durée du recuit pour toutes les températures étudiées. Une expérience de recuit in-situ a révélée deux étapes de mûrissement. La première est dominée par le mûrissement de Smoluchowski tandis que la seconde est dominée par le mûrissement d'Oswald de surface. La dépendance de type Arrhenius du coefficient de transport de masse de surface donne une énergie d'activation de Ed = 0.84 ± 0.08 eV/atome pour la diffusion des atomes de Pt sur un substrat de carbone amorphe. Des méthodes de TEM avancée ont été utilisé pour déterminer directement des profiles de concentration à l'échelle atomique et grand champ de vue par corrélation de signaux de champ sombre annulaire à grand angle (HAADF) et de spectroscopie de perte d'énergie d'électron (EELS). Cette méthode a été appliquée à l'étude de la concentration de Ge à l'échelle atomique dans le system SiGe. Le profile de concentration le long de la direction de croissance est expliqué par la ségrégation de surface des atomes de Ge pendant la croissance avec un modèle d'échange à deux états. L'incorporation de Mn dans les boîtes de Ge a été effectuée par croissance par jets moléculaire (MBE) de GeMn. Des précipités de SiMn sont formés pour des températures de croissance de 380 °C. La diminution de la température de croissance à 220 °C permet de limiter la ségrégation latérale de Mn et d'incorporer le Mn dans les QDs de Ge. Les compositions chimiques absolues obtenues par STEM-EELS prouvent que la densité atomique totale dans les NCs de GeMn est presque deux fois supérieure par rapport à la matrice de Ge. Des études structurales par HRTEM montrent les NCs cristallines sont très désordonnées. Les observations expérimentales peuvent être modélisées par une structure de phase α modifié, si des variants sont introduits pour annuler des réflexions de Bragg et des atomes de Ge sont substitués par des atomes de Mn. Les propriétés structurales et magnétiques de films GeSnMn croît par MBE à basse température (LTMBE) ont été étudiées. De manière similaire aux films GeMn, les atomes de Mn diffusent pendant la croissance et s'agrègent pour former des NCs de quelques nanomètres de diamètre, alignées verticalement et riche en Mn. Les observations TEM en vue plane montrent clairement que l'incorporation de Sn n'est pas homogène avec des concentrations en Sn dans les NCs inférieures à la limite de détection de l'EELS. La matrice présente une solution solide tandis qu'une coquille riche en Sn est formée autour des NCs de GeMn. La magnétisation dans les couches de GeSnMn est plus élevée que dans celles de GeMn. L'augmentation du moment magnétique dans les couches de GeSnMn est probablement due à la modification de la structure électronique des atomes de Mn in the NCs par la coquille de Sn. / In this work, different system have been studied using transmission electron microscopy (TEM) methods: Pt nanoparticles (NPs) on amorphous carbon, Ge quantum dots (QDs), Mn incorporation in Ge QDs and GeMn nanocolumns (NCs) embedded in Ge or GeSn matrix. The coarsening of Pt NPs on amorphous carbon film was studied by high resolution TEM (HRTEM) after annealing at temperatures between 200°C and 300°C for periods of up to 160 hours. A significant increase of the average particle size is observed with increasing annealing time for all investigated temperatures. An in-situ annealing experiment reveals two coarsening stages. The first coarsening stage is dominated by Smoluchowski ripening whereas the second coarsening stage is dominated by surface Ostwald ripening. The Arrhenius-type dependence of the derived surface mass-transport coefficients yields an activation energy Ed = 0.84 ± 0.08 eV/atom for the surface diffusion of Pt atoms on an amorphous carbon substrate. Advanced TEM methods have be used to obtain direct determination of composition profiles with atomic resolution and large field of view by correlation of high angle annular dark field (HAADF) and electron energy loss spectroscopy (EELS) signals. This method was used to obtain a direct and precise quantification of Ge concentration at the atomic level for the SiGe system. The Ge concentration profile along the growth direction was explained by Ge surface segregation during the growth with a two-state exchange model. The incorporation of Mn in Ge QDs have been performed by molecular beam epitaxy (MBE) growth of GeMn. At growth temperature of 380°C, SiMn precipitates are formed. Lowering the growth temperature at 220°C allows limiting the lateral segregation of Mn in Ge and incorporating Mn in Ge QDs. Absolute chemical composition by STEM-EELS evidenced that the total atomic density in Ge(Mn) NCs is almost two times higher than in the Ge matrix. Structural analysis by HRTEM shows that the crystalline NCs exhibit a high degree of disorder. Experimental observation can be model with a modified α-phase structure if variants are introduced to cancel reflexions and Ge atoms are substituted by Mn atoms. The structural and magnetic properties of GeSnMn films grown on Ge(001) by low temperature MBE (LTMBE) have been studied. Like in Ge(Mn) films, Mn atoms diffuse during the growth and aggregate into vertically aligned Mn-rich NCs of a few nanometers in diameter. TEM observations in plane view clearly indicate that the Sn incorporation is not uniform with concentration in Mn rich vertical NCs lower than the EELS detection limit. The matrix exhibits a GeSn solid solution while there is a Sn-rich GeSn shell around GeMn NCs. The magnetization in GeSnMn layers is higher than in GeMn films. This magnetic moment enhancement in GeSnMn is probably related to the modification of the electronic structure of Mn atoms in the NCs by the Sn-rich shell, which is formed around the NCs.

Manganese (Mn) doped III-V dilute ferromagnetic semiconductors (DFSs) are a candidate materials for semiconductor spintronics due to their intrinsic ferromagnetism mediated by holes. In this thesis, Mn doped III-V dilute ferromagnetic semiconductors (DFSs), including (Ga,Mn)As, (In,Mn)As, (Ga,Mn)P, and (In,Ga,Mn)As have been successfully prepared by ion implantation and pulsed laser melting. All (In,Ga,Mn)As films are confirmed to be well recrystallized and ferromagnetic while their Curie temperatures depend on the Ga concentration. (Ga,Mn)As and (Ga,Mn)P have an inplane easy axis, while an out-of-plane easy axis for (In,Mn)As is observed. However, all of them do not present strong in-plane uniaxial anisotropy between [110] and [110] directions, which always occurs in low temperature molecular beam epitaxy (LT-MBE) grown (Ga,Mn)As samples. The reason is ascribed to the fact that the ultra-fastrecrystallization induced by pulsed laser melting weakens the formation of Mn-Mn dimers along the [100] direction which occurs in LT-MBE grown (Ga,Mn)As. Then selected samples were co-doped with Zn or irradiated with He ions. The Zn co-doping leads to the increase of conductivity of (Ga,Mn)P, however both the Curie temperature and magnetization decrease, which is probably due to the suppression of active Mn substitution by Zn co-doping. By using Rutherford Backscattering Spectroscopy and Particle-Induced X-ray Emission, the substitutional Mn atoms in (Ga,Mn)As are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites. This is consistent with first-principles calculations, showing that the formation energy of substitutional Zn and interstitial Mn is 0.7 eV lower than that of interstitial Zn and substitutional Mn. For ion irradiated (Ga,Mn)As, (In,Mn)As and (Ga,Mn)P, both Curie temperature and magnetization decrease due to the hole compensation. However, the compensation effect is the strongest in (In,Mn)As and the least in (Ga,Mn)P. This is due to the different energy level of the produced defect relative to the band edges in different semiconductors. The results in the thesis point to an important issue: the difference in the band alignment and the hole binding energy of Mn dopants in different III-Mn-V dilute ferromagnetic semiconductors have strong influence on their magnetic properties and should be taken into account in the material design.

À la fois semi-conducteur et ferromagnétique, le Ga1-xMnxAs offre des potentialités intéressantes pour l'électronique de spin. Cette double propriété est due à l'interaction d'échange entre les spins localisés des atomes de manganèse Mn et les spins des porteurs de charge. Le travail présenté dans cette thèse est centré sur le contrôle de l'aimantation de ces structures magnétiques. Une étude expérimentale, comparative et détaillée, de l'anisotropie magnétique a été menée sur deux séries d'échantillons. Par ailleurs, une méthode d'analyse basée sur l'étude de la densité d'énergie libre magnétocristalline des systèmes observés a été développée en vue de confronter les résultats aux prédictions théoriques. Les mesures d'effet Hall et d'aimantation par SQUID sur des monocouches à aimantation planaire ont permis de mettre en évidence deux types d'anisotropie : une anisotropie cubique pour T < TC/2 avec un retournement d'aimantation par sauts de 90°, et une anisotropie uniaxiale pour des températures TC/2 < T < TC avec un renversement d'aimantation à 180°. La technique du recuit post-croissance réduit cependant l'anisotropie cubique au profit de l'anisotropie uniaxiale. Les structures à aimantation perpendiculaire présentent, quant à elles, un retournement d'aimantation à 180° pour toutes les températures T < TC. Par conséquent, dans ces dispositifs, l'anisotropie magnétique est fortement uniaxiale. En dernier lieu, cette étude porte sur la dynamique des domaines magnétiques et la détermination des vitesses de propagation de parois de domaines, induites par un champ magnétique. Les résultats révèlent une anisotropie de propagation de parois suivant les axes cristallographiques <110> avec deux régimes de vitesses distincts, dont l'un est fortement contrôlé par des défauts de structure. / At the same time semiconductor and ferromagnetic, the Ga1-xMnxAs offers interesting potentialities for spintronic. This double property is due to the exchange interaction between localized spin of Mn atoms and the spin charge carrier. The work presented in this thesis is centred on the magnetization control of these magnetic structures. A comparative and detailed experimental study of the magnetic anisotropy is carried out on two series of samples. An analysis method based on the free energy density study of the observed systems was also been developed to confront the results with theoretical predictions. The Hall effect and SQUID measurements on the structures with planar magnetization allowed us to bring to light two types of anisotropy: a cubic anisotropy for T < TC / 2 with a magnetization reversal by jumps of 90 ° and an uniaxial anisotropy for temperatures TC / 2 < T < TC with a reversal of magnetization in 180°. The structure with perpendicular magnetization present a magnetization switch at 180° for all temperatures T < TC. Consequently in these compounds, the magnetic anisotropy is strongly uniaxiale. Lastly this study deals the magnetic domains structures and the determination of domain wall propagation velocity. The results reveal an anisotropic domain wall propagation along crystallographic axes <110> with two distinct velocity regimes, including one strongly controlled by structural defects.

Spintronics is a new branch of electronics, which uses not only the charge of electron but also its spin for transfer and processing of information. For re- al applications it is necessary to understand, how the magnetic state of matter changes not only in time but also in space. This diploma thesis is therefore de- voted to the imaging of magnetic domains in ferromagnetic semiconductors by purely optical methods, which are based on the usage of the magneto-optical ef- fects. In first part of the thesis we concentrated on the optimalization of existing polarizing microscope, whose function is based solely on the polar Kerr effect. After the optimalization we try to use this setup to observe magnetic domains in ferromagnetic semiconductor GaMnAs by the means of the magnetic linear dichroism. 1

In this work we study ultrafast laser-induced magnetization dynamics in samples of ferromagnetic semiconductor Ga1−xMnxAs with a nominal concentration of Mn within the range of x = 0,015-0,14. To get information about magnetization movement we use magneto-optic phenomena PKE and MLD in a time-resolved pump & probe experiment. Thorough analysis of the measured magneto-optical signal allows us to disentangle contributions due to angular movement of magnetization and due to demagnetization and to reconstruct 3D motion of magnetization vector without any numerical modeling. First we explain the basis of this experimental method and we demonstrate its utilization on the measured data. After that we study angular movement of magnetization vector and its dependence on the external magnetic field, excitation intensity and Mn concentration. The pump pulse helicity dependent and independent dynamics were treated separately. In the case of demagnetization we have been able to observe not only its intensity and Mn doping dependence but also the magnetic field dependence, which has not been reported so far in the literature.

No description available.