Skyrmion-hosting B20-type MnSi films on Si substrates grown by flash lamp annealing

The aim of the current thesis was to investigate the preparation of MnSi film on Si substrates. The preparation process includes room temperature sputtering Mn films with different thicknesses and flash-lamp annealing with different energy density (annealing temperature). Systematic investigations on their structural, electrical, magnetic, and magneto-transport properties were performed. The key findings are summarized below: Thin films with the B20-MnSi phase on Si(100) substrates were fabricated for the first time. They exhibit magnetic skyrmion behaviour. In comparison with Si(111) substrates, Si(100) substrates are more preferred from the practical application point of view. The nucleation of B20-MnSi on Si(100) is believed to be triggered by the fast solid-state phase reaction between Mn and Si via ms-range flash-lamp annealing. Compared with the corresponding bulk material, our films show an increased Curie temperature of around 43 K. The magnetic and transport measurements reveal that skyrmions in B20-MnSi on Si(100) made by sub-seconds solid-state reaction are stable within much broader field and temperature windows than bulk MnSi. The parasitic MnSi1.7 phase can be further minimized or eliminated by optimizing the annealing conditions, the quality of the deposited Mn film, and its interface with the Si substrate. Our work demonstrates a promising route for the fabrication of B20-type transition metal silicides for integrated and/or hybrid spintronic applications on Si(100) wafers, which are more preferable for industry applications. The growth of MnSi films on Si(111) substrates has been widely realized by solid phase epitaxy or molecular beam epitaxy since the lattice mismatch and symmetry fit better. One problem is the parasitic MnSi1.7 phase. By controlling the reaction parameters using strongly non-equilibrium flash lamp annealing, we have achieved full control over the phase formation of Mn-silicides in thin films from single-phase B20-MnSi or MnSi1.7 to mixed phases. The obtained films are highly textured and reveal sharp interfaces to the Si substrate. The obtained B20-MnSi films exhibits a high Curie temperature of 41 K. The skyrmion phase can be stabilized over broad temperature and magnetic field ranges. We propose flash-lamp-annealing-induced transient reaction as a general approach for phase separation in transition-metal silicides and germanides and for growth of B20-type films with enhanced topological stability. By comparing the magnetic properties of MnSi films grown on both Si(111) and Si(100) substrates by ourselves and by others in literature, we found one common feature. It is the increased Curie temperature of around 41-43 K for all MnSi films. It is much higher than 29.5 K for bulk MnSi. We try to understand the puzzling Curie temperature widely reported in MnSi films. We have prepared MnSi films with a large variation regarding their thickness, crystallinity, strain and phase separation. Particularly, polycrystalline MnSi films on Si(100) and textured MnSi films on Si(111), both with different mixture ratio with MnSi1.7 have been grown and systematically characterized. Surprisingly, all obtained MnSi films exhibit a high Curie temperature at around 43 K. The skyrmion phase has also been detected in these films. However, we find no correlation between the increased Curie temperate and the film thickness, strain, lattice volume or the mixture with MnSi1.7. Our work has not provided a conclusive picture for this question, but is rather calling a revisit, especially to the effect by the interface, stoichiometry and point defects. Further studies are essential to understand the B20 transition-metal silicide/germanides films and therefore to utilize them for spintronic applications.:Contents Abstract iii Kurzfassung v 1. Introduction 1 1.1 B20 compounds and magnetic skyrmions 1 1.2 B20 MnSi with magnetic Skyrmions 8 1.2.1 Crystallization process 10 1.2.2 Phase diagram of Mn-Si binary compounds 13 1.2.3 Bulk B20-MnSi 14 1.2.4 B20-MnSi thin film 18 1.2.5 B20-MnSi nanowire 25 1.3 Fast annealing method 27 1.4 Objectives and the structure of the thesis 30 2. Experiment 32 2.1 Sample preparation 32 2.1.1 DC magnetron sputtering 32 2.1.2 Sub-second annealing 35 2.2 Structure characterization: X-ray diffraction 40 2.3 Property characterization 41 2.3.1 Magnetic properties 41 2.3.2 Magneto-transport properties 44 3. B20-MnSi films grown on Si(100) substrates with magnetic skyrmion signature 46 3.1 Introduction 46 3.2 Experiment 47 3.3 Results and Discussions 48 3.4 Conclusions 56 4. Phase selection in Mn-Si alloys by fast solid-state reaction with enhanced skyrmion stability 57 4.1 Introduction 57 4.2 Experiment 59 4.3 Results 61 4.3.1 MnSi and MnSi1.7 phase reaction 61 4.3.2 Magnetic Skyrmion 68 4.3.3 Discussion 76 4.4 Conclusion 78 5. On the Curie temperature of MnSi films 80 5.1 Introduction 80 5.2 Experiment 82 5.3 Results 83 5.4 Conclusion 89 6. Summary and outlook 90 6.1 Summary 90 6.2 Outlook 91 6.2.1 Film thickness effect on formation of (111)-textured B20-MnSi 91 6.2.2 MnSi1.7% influence on Skyrmion stability 96 6.2.3 Preparation of other transition-metal monosilicides and germanides 98 Acknowledgement 99 References 101 Publication list 117 Curriculum Vitae 119 Erklärung 12

26th Symposium on Plasma Physics and Technology

List of abstract

Institute of Ion Beam Physics and Materials Research: Annual Report 2001

Summary of the scientific activities of the institute in 2001 including selected highlight reports, short research contributions and an extended statistics overview

Modifications of radiofrequency capacitive discharge for deposition of carbon coatings

This thesis is an investigation of modifications of RF excited discharge for deposition of carbon coatings. Two separate discharge configurations were examined: an RF capacitive discharge with electron injection and removal by means of an emissive filament and a DC electrode, and an RF discharge with a hollow cathode powered electrode. Plasma characterization was conducted by means of an electrostatic probe and an energy and mass analysis probe. Interpretation of the electrostatic probe data in presence of RF harmonics in plasma has been discussed. Electron injection and removal has been found to control strongly plasma potential and maximum of ion energy at the grounded electrode, reducing them to less than 10 V or increasing above 90 V accordingly. Related changes of electron temperature and density have been measured, with plasma density being increased up to an order of magnitude by electron injection. This effect has been linked with a regime, when hot filament instigates discharge inside of an electron source. A model, based on the analysis of electron movement in an RF matrix sheath, has been developed to investigate an effect of the DC electrode on stochastic heating/cooling of electrons in the sheath. It has been demonstrated that an adequate heating of the EEDF tail and cooling of the bulk electrons could be produced by a combined effect of the sheaths of powered and grounded electrodes, assuming the multi-harmonic grounded sheath and non-equal plasma density at the sheath boundaries. Another model, based on balance of conduction currents from RF plasma to the electrodes, has been aimed at investigation of discharge potentials. It has predicted correctly variation of time-averaged plasma potential with electron injection and removal. Deposition experiments were conducted from both discharge configurations. Carbon coatings were analysed by means of Raman spectroscopy and scanning electron microscope (SEM). Raman spectra of coatings from the plasma with injected electrons revealed a polymeric-like nature of the coatings. SEM study of carbon films from the RF hollow cathode configuration demonstrated a wide variety of coating morphology: from porous films, consisting of separate particles, to nucleation of conical, spherical and cauliflower-like carbon phases

Crescimento, caracterização e estudo magnetoelétrico de heteroestruturas ferromagnéticas-ferroelétricas

Orientadores: Julio Criginski Cezar, Eduardo Granado Monteiro da SilvaTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb WataghinResumo: Os resultados dessa tese podem ser separados em duas linhas principais: i) crescimento e caracterização de filmes finos ferroelétricos; ii) caracterização magnetoelétrica de filmes finos de Fe crescidos sobre substratos de BaTiO3. Conseguimos fabricar filmes finos simples de YMnO3 ortorrômbico, BaTiO3 e SrRuO3 por RF-magnetron sputtering e deposição por laser pulsado. Esses filmes foram caracterizados por diversas técnicas estruturais e espectroscópicas, demonstrando que em alguns casos obtivemos filmes epitaxiais de alta qualidade. Além dos resultados em filmes finos, neste trabalho de doutorado também investigamos a interação entre os domínios ferromagnéticos e os domínios ferroelétricos de um filme fino magnético crescido sobre um substrato ferroelétrico. Esse tipo de sistema se apresenta como uma heteroestrutura multiferroica magnetoelétrica. O objetivo desse trabalho é compreender como se dá a interação de um filme fino de ferro crescido sobre um monocristal ferroelétrico de BaTiO3. Para esse fim empregamos principalmente a técnica de microscopia de emissão de fotoelétrons (PEEM). Essa técnica permite obter informação micrométrica sobre o estado de polarização elétrica e magnetização da amostra. Dessa forma demonstramos que sob condições controladas existe uma interação entre os domínios ferroelétricos e ferromagnéticos nessa heteroestrutura. Nossos resultados mostraram que os domínios ferroelétricos do substrato de BaTiO3 determinam a orientação dos domínios magnéticos de camadas ultrafinas de Fe. Esses resultados demonstram o potencial desse tipo de heteroestrutura para aplicações onde um estado magnético pode ser alterado através de um potencial elétricoAbstract: The results of this thesis can be separated in two main lines: i) growth and characterization of ferroelectric thin films; ii) magnetoelectric characterization of Fe thin films grew on BaTiO3 substrates. We succeed to fabricate simple thin films of orthorhombic YMnO3, BaTiO3 and SrRuO3 by RF-magnetron sputtering and by pulsed laser deposition. The films where characterized by several structural and spectroscopic techniques, showing that in some cases we obtained very high quality epitaxial films. Besides the results on thin film growth, we characterized the ferroelectric domains of a thin iron magnetic film grown on top of a ferroelectric BaTiO3 substrate. This kind of system behaves as a magnetoelectric multiferroic heterostructure. We studied the interaction between ferroelectric and ferromagnetic domains mostly by photoelectron emission microscopy (PEEM). This technique allows one to obtain microscopic information about the electric and magnetic domains in the sample. We were able to demonstrate the interaction between the magnetic and ferroelectric degrees of freedom in such heterostructures. We found that the underlying ferroelectric domains on BaTiO3 in some situations determine the orientation of the top Fe magnetic layer. These results show the potential of such artificial magnetoelectric heterostructure to control the magnetic state through the application of electrical voltagesDoutoradoDoutora em Ciência

New technologies and their characterisation for nanostructured SnO2 Gas sensor devices

[spa] La presente tesis se ha centrado en el desarrollo de nuevas tecnologías para el desarrollo de sensores de gas semiconductores. Por tal motivo se han desarrollado dos técnicas para el desarrollo de nanopartículas de SnO2. Paralelamente se ha diseñado y realizado diversos tipos de substratos basados en alúmina y silicio micromecanizados. Con el objetivo de implementar las nanopartículas en estos substratos se han desarrollado dos nuevas técnicas para dicha implementación, llamadas micropinting y recubrimientos por pulverización. Finalmente, con los dispositivos obtenidos se ha concebido un algortimo para la cuantificación de CO y CH4 en mezclas binarias. Desde un punto de vista científico, se ha realizado una caracterización de SnO2 atendiendo a la temperatura de estabilización del material y hallando la evolución nanostructural de dicho óxido de estaño