Another spin in the wall : domain wall dynamics in perpendicularly magnetized devices

The world as we know it today would be completely different without spintronics. It has revolutionized the way we carry, store and exchange information in our daily lives. What is it? It is a research realm that combines the fundamental property of the electron, spin, and the charge property driving conventional electronics, hence: spin-tronics. Spintronics has until recently concentrated mainly on the manipulation of chargecurrent through the control of the magnetic state. Currently, a new paradigm has emerged that reverses this idea, basically by applying Newton’s law of action implies reaction. Instead of manipulating the charge-current by the magnetic state, it manipulates the magnetic state by a spin-polarized current, and it is gradually gaining interest. Spin-polarized current induced motion of a magnetic domain wall is the subject of this thesis.We try to use a spin polarized current to push a magnetic domain wall. The interaction between spin-current and the magnetic domain wall is still an open area for exploration; there are still many unanswered questions on the fundamental physics that brings it about. The prospect of new data storage, memory and even bio-related devices makes it a very lively and competitive research topic possibly relevant in shaping our future world. We chose perpendicularly magnetized devices as our material, and for a reason. In this class of materials the magnetic domain walls are very narrow; and this, was our assumption, should increase the interaction between the spin polarized current and the magnetization. Our research has made use of a great variety of experimental and nano-fabrication techniques. Since the nano-fabrication techniques were particularly new to our research group, they have been given ample attention in chapter 2. The material used in our research are perpendicularly magnetized ultrathin Co(FeB) layers

Unidirectional magnetic coupling

We show that interlayer Dzyaloshinskii-Moriya interaction in combination with non-local Gilbert damping gives rise to unidirectional magnetic coupling. That is, the coupling between two magnetic layers -- say the left and right layer -- is such that dynamics of the left layer leads to dynamics of the right layer, but not vice versa. We discuss the implications of this result for the magnetic susceptibility of a magnetic bilayer, electrically-actuated spin-current transmission, and unidirectional spin-wave packet generation and propagation. Our results may enable a route towards spin-current and spin-wave diodes and further pave the way to design spintronic devices via reservoir engineering.Comment: 6 pages, 3 figure

Spin motive forces due to magnetic vortices and domain walls

We study spin motive forces, i.e, spin-dependent forces, and voltages induced by time-dependent magnetization textures, for moving magnetic vortices and domain walls. First, we consider the voltage generated by a one-dimensional field-driven domain wall. Next, we perform detailed calculations on field-driven vortex domain walls. We find that the results for the voltage as a function of magnetic field differ between the one-dimensional and vortex domain wall. For the experimentally relevant case of a vortex domain wall, the dependence of voltage on field around Walker breakdown depends qualitatively on the ratio of the so-called $\beta$-parameter to the Gilbert damping constant, and thus provides a way to determine this ratio experimentally. We also consider vortices on a magnetic disk in the presence of an AC magnetic field. In this case, the phase difference between field and voltage on the edge is determined by the $\beta$ parameter, providing another experimental method to determine this quantity.Comment: 8 pages, 9 figures, submitted to PR

Correlation between magnetism and spin-dependent transport in CoFeB alloys

We report a correlation between the spin polarization of the tunneling electrons (TSP) and the magnetic moment of amorphous CoFeB alloys. Such a correlation is surprising since the TSP involves s-like electrons close to the Fermi level (EF), while the magnetic moment mainly arises due to all d-electrons below EF. We show that probing the s and d-bands individually provides clear and crucial evidence for such a correlation to exist through s-d hybridization, and demonstrate the tuneability of the electronic and magnetic properties of CoFeB alloys.Comment: Accepted for publication in Physical Review Letters. Letter (4 pages) and Supplementary material (4 pages

Magnetization dynamics and Gilbert damping in ultrathin Co48Fe32B20 films with out-of-plane anisotropy

Time resolved magneto-optical Kerr measurements are carried out to study the precessional dynamics of ferromagnetic thin films with out-of-plane anisotropy. A combined analysis of parameters, such as coercive fields, magnetic anisotropy, and Gilbert damping a, is reported. Using a macrospin approximation and the Landau-Lifshitz-Gilbert equation, the effective anisotropy and a are obtained. A large damping varying with the applied field as well as with the thickness of the ferromagnetic layer is reported. Simulations using a distribution in the effective anisotropy allow us to reproduce the field evolution of a. Moreover, its thickness dependence correlates with the spin pumping effect. © 2009 American Institute of Physics

Efficient Gold(I) Acyclic Diaminocarbenes for the Synthesis of Propargylamines and Indolizines

Mononuclear gold(I) acyclic diaminocarbenes (ADCs) were prepared by the reaction of 1, 2-cyclohexanediamine with the corresponding isocyanide complexes [AuCl(CNR)] (R = Cy, tBu). The three-component coupling of aldehydes, amines, and alkynes was investigated by using these gold(I) ADC complexes. The new gold(I) metal complexes are highly efficient catalysts for the synthesis of propargylamines and indolizines in the absence of solvent and in mild conditions. This method affords the corresponding final products with excellent yields in short reaction times. Additionally, chiral gold(I) complexes with ADCs have been prepared and tried in the enantioselective synthesis of propargylamines

Sputter grown Fe and Fe/Cr multilayers with fourfold magnetic anisotropy on GaAs

Thin films of Fe have been epitaxially sputtered on GaAs substrates with native oxide removal prior to the deposition carried out by an Ar ion milling. Films grown at substrate temperatures above 100 °C show well-defined fourfold anisotropies. The onset of epitaxial growth is accompanied by an increase in the surface roughness with growth occurring in a distinct island-like pattern. The Fe layers show significantly reduced moments, which decrease with increasing temperature. Antiferromagnetic coupling between Fe layers with Cr spacers was measured in a multilayer with a Cr thickness of 2.7 nm, around the second antiferromagnetic peak. The magnetic properties of the films are discussed in the context of multilayer storage applications

Substrate conformal imprint fabrication process of synthetic antiferromagnetic nanoplatelets

Methods to fabricate and characterize monodisperse magnetic nanoplatelets for fluid/bio-based applications based on spintronic thin-film principles are a challenge. This is due to the required top-down approach where the transfer of optimized blanket films to free particles in a fluid while preserving the magnetic properties is an uncharted field. Here, we explore the use of substrate conformal imprint lithography (SCIL) as a fast and cost-effective fabrication route. We analyze the size distribution of nominal 1.8 um and 120 nm diameter platelets and show the effect of the fabrication steps on the magnetic properties which we explain through changes in the dominant magnetization reversal mechanism as the size decreases. We show that SCIL allows for efficient large-scale platelet fabrication and discuss how application-specific requirements can be solved via process and material engineering

Asymmetric magnetic bubble expansion under in-plane field in Pt/Co/Pt: effect of interface engineering

We analyse the impact of growth conditions on asymmetric magnetic bubble expansion under in-plane field in ultrathin Pt / Co / Pt films. Specifically, using sputter deposition we vary the Ar pressure during the growth of the top Pt layer. This induces a large change in the interfacial structure as evidenced by a factor three change in the effective perpendicular magnetic anisotropy. Strikingly, a discrepancy between the current theory for domain-wall propagation based on a simple domain-wall energy density and our experimental results is found. This calls for further theoretical development of domain-wall creep under in-plane fields and varying structural asymmetry.Comment: 16 pages, 3 figure