Detailed magnetometry and polarized neutron reflectometry studies were conducted on MnSi thin films grown epitaxially on Si(111) substrates. It is demonstrated that with an in-plane applied field H || [110], a broadly stable skyrmion phase exists at elevated temperatures and fields. Magnetometry and transport measurements with an out-of-plane applied field H || [111] prove that no skyrmion phase exists in this geometry. However, Hall effect measurements in this geometry show unexpected evidence of a topological Hall effect. This can be explained with a multi-dimensionally modulated cone phase, which proves that contrary to recent literature, a topological Hall effect is not sufficient proof of skyrmions. The results of this thesis represent a significant step towards a technologically relevant material in which skyrmions are broadly stable. A material of this type could be used in novel magnetic storage devices and signi ficantly impact our future computing capabilities.

No description available.

Spin-dependent electron transport in submicron/nano sized magnetic thin film devices fabricated using the optical lithography, e-beam lithography and focused ion beam (FIB) was investigated with the primary aim to find the ballistic magnetoresistance (BMR) in thin film nanoconstrictions. All experimental results were analysed in combination with micromagnetic simulations. The magnetisation reversal processes were investigated in a submicron half-pinned NiFe stripe with a microconstriction. An asymmetric MR curve was observed, and micromagnetic simulations verified it was due to the exchange-bias on the left side, which changed the magnetic switching mechanism. The effects of different pinning sites on the magnetisation switching and domain wall displacement were studied in NiFe film and spin-valve based nanodevices. A sign of domain wall MR was seen on the transversal MR curve of the NiFe nanodevice due to the domain wall induced electron scattering. The size effect on the magnetisation switching and interlayer magnetostatic coupling was demonstrated and characterised in synthetic antiferromagnet (SAF)-pinned spin-valve nanorings. It has been clarified by micromagnetic simulations that these nanorings exhibit a double or single magnetisation switching process, which is determined by the magnetostatic coupling as a function of the ring diameter. The interlayer magnetostatic coupling was efficiently reduced in large SAF-pinned nanorings, resulting in a small shift of the minor MR curve, which is beneficial to the magnetic memory applications. In-situ MR measurements and the investigation of domain wall properties have been carried out in FIB patterned NiFe film nanoconstrictions. Spin-valve like sharp transitions were observed on the MR curves in the 80 nm/130 nm wide nanoconstriction devices. However, our analysis of the results by micromagnetic simulations and domain observations with scanning electron microscopy with polarisation analysis (SEMPA) concluded that these sharp MR transitions originated from the anisotropic magnetoresistance (AMR) effect, due to the fast magnetisation rotation in the nanoconstriction, and not from BMR. The numerical investigation has proved that a further reduction of the constriction width/length is necessary for large MR values.

Magnetic nanoscale devices have shown great promise in both research and industry. Magnetic nanostructures have potential for non-volatile data storage applications, reconfigurable logic devices, biomedical devices and many more. The S-state magnetic element is one of the promising structures for non-volatile data storage applications and reconfigurable logic devices. It is a single-layer logic element that can be integrated in magnetoresistive structures. We present a detailed micromagnetic analysis of the geometrical parameter space in which the logic operation is carried out. The influence of imperfections, such as sidewall roughness and roundness of the edge is investigated. Magnetic nanowires are highly attractive materials that has potential for applications in ultrahigh magnetic recording, logic operation devices, and micromagnetic and spintronic sensors. To utilize applications, manipulation and assembly of nanowires into ordered structures is needed. Magnetic self-alignment is a facile technique for assembling nanowires into hierarchical structures. In my thesis, I focus on synthesizing and assembling nickel nanowires. The magnetic behaviour of a single nickel nanowire with 200~nm diameter is investigated in micromagnetic simulations. Nickel nanowires with Au caps at the ends were synthesized by electrochemical deposition into nanopores in alumina templates. One-dimensional alignment, which forms chains and two-dimensional alignment, which forms T-junctions as well as cross-junctions are demonstrated. Attempts to achieve three-dimensional alignment were not successful yet. I will discuss strategies to improve the alignment process.

Magnetic nanoscale devices have shown great promise in both research and industry. Magnetic nanostructures have potential for non-volatile data storage applications, reconfigurable logic devices, biomedical devices and many more. The S-state magnetic element is one of the promising structures for non-volatile data storage applications and reconfigurable logic devices. It is a single-layer logic element that can be integrated in magnetoresistive structures. We present a detailed micromagnetic analysis of the geometrical parameter space in which the logic operation is carried out. The influence of imperfections, such as sidewall roughness and roundness of the edge is investigated. Magnetic nanowires are highly attractive materials that has potential for applications in ultrahigh magnetic recording, logic operation devices, and micromagnetic and spintronic sensors. To utilize applications, manipulation and assembly of nanowires into ordered structures is needed. Magnetic self-alignment is a facile technique for assembling nanowires into hierarchical structures. In my thesis, I focus on synthesizing and assembling nickel nanowires. The magnetic behaviour of a single nickel nanowire with 200~nm diameter is investigated in micromagnetic simulations. Nickel nanowires with Au caps at the ends were synthesized by electrochemical deposition into nanopores in alumina templates. One-dimensional alignment, which forms chains and two-dimensional alignment, which forms T-junctions as well as cross-junctions are demonstrated. Attempts to achieve three-dimensional alignment were not successful yet. I will discuss strategies to improve the alignment process.

No description available.

À l'approche d'un point critique, la divergence de la longueur de corrélation des fluctuations peut être tronquée par le confinement du système. Cette troncature engendre des effets de taille finie présentant des caractères universels au sein d'un classe de transitions de phases.Nous nous sommes intéressés particulièrement à la classe d'universalité du modèle d'Ising, regroupant notamment les transitions de phase ferro/paramagnétique pour les systèmes magnétiques uniaxiaux, la transition liquide/gaz et encore la démixtion de mélanges binaires. Nous présentons tout d'abord une introduction aux phénomènes critiques, à l'universalité, au « finite-size scaling » et aux simulations Monte Carlo du modèle d'Ising, sur lesquelles se fondent la majeur partie de ce travail.Un effet de taille finie ayant attiré une grande attention durant les dernières dizaines d'années est la force de Casimir critique. Les travaux théoriques et numériques concernant cette force ont, dans leur quasi totalité, été menés dans des systèmes magnétiques modèles, tel que les modèles d'Ising ou XY. Par contre, les approches expérimentales ont toutes été réalisées dans des systèmes fluides, tels que des mélanges binaires ou de l'hélium IV proche de la transition superfluide.Une motivation de ce travail a été de chercher a résoudre cette situation paradoxale en proposant, d'une part, un protocole expérimental pour la mesure de la force de Casimir dans une couche mince magnétique et, d'autre part, une approche numérique dans un mélange binaire de type Lennard-Jones. Cette dernière approche présente l'avantage d'ouvrir la porte à des études des fluctuations de la force de Casimir ou encore hors-équilibre. / Approaching a critical point, the divergence of the correlation length of fluctuations can be cut-off by a confinement of the system. This truncation fosters finite size effects with universal features in a class of continuous phase transitions. We are particularly interested in the Ising universality class, regrouping transitions such as the ferromagnetic/paramagnetic transition for uniaxial magnetic systems, the liquid/gas tran- sition and the demixing of binary mixtures. We will first present an introduction to critical phenomena, universality, finite-size scaling and Monte Carlo simulations of the Ising model, on which a major part of this work relies.A finite size effect that has particularly drawn attention in the past decades is the critical Casimir force. On the one hand, theoretical and numerical works on the subject have almost systematically been performed in magnetic model systems, such as the Ising or XY models. On the other hand, experimental approaches were all realized in fluid systems, such as binary mixtures or helium IV close to the superfluid transition.A motivation of this work was to bridge this gap by proposing, firstly, an experimental protocol for measuring the critical Casimir force in a magnetic layer and, secondly, a numerical approach in a Lennard-Jones binary mixture. The latter is of particular interest as it could lead the way to studying fluctuations of the Casimir force or out-of-equilibrium phenomena.