Spin transfer torque in ferro/non-ferromagnetic pillar structures

In der vorliegenden Arbeit wird der Spin-Transfer-Torque-Effekt in säulenartigen ferro-/nichtferromagnetischen Schichtstrukturen untersucht. Durch ihn kann die Richtung der Magnetisierung allein mit Hilfe eines Stromes beeinflusst werden, was üblicherweise über den Riesenmagnetowiderstands-Effekt nachgewiesen wird. In dieser Arbeit wird zum ersten Mal die ferromagnetische Resonanz für dessen Nachweis benutzt, indem die Stromabhängigkeit der Dämpfung der Magnetisierung gemessen wird. Der entscheidende Vorteil gegenüber der Widerstandsmessmethode besteht darin, dass die Effekte bei erheblich geringeren Stromdichten untersucht werden können. Die Untersuchungen werden zum einen an Co/Cu/Py-Pillarstrukturen durchgeführt, wobei Co als Spinpolarisator dient und die Auswirkung des Spin-Transfer-Torque anhand der Py-Schicht detektiert wird. Des Weiteren werden Messungen an Schichtsystemen durchgeführt, in denen als Polarisatoren [Co/Pt]- und [Co/Ni]-Multilagen mit senkrechter magnetischer Anisotropie verwendet werden. Damit kann der Winkel zwischen Polarisator- und Analysatormagnetisierung und somit der Einfluss des Spin-Transfer-Torques auf die Dämpfung der Py-Magnetisierung vergrößert werden. Die Strukturen werden mit Hilfe von Elektronenstrahllithographie hergestellt und kontaktiert, wobei eine große Anzahl identischer Pillarstrukturen präpariert werden, die alle in Reihe miteinander elektrisch verbunden sind. Damit kann die Stromdichte in den Strukturen maximiert werden. Messungen am Co/Cu/Py-System zeigen einen Einfluss des Spin-Transfer-Torques auf die Dämpfung der Py-Magnetisierung. Abhängig von der Stromrichtung kann diese gedämpft oder entdämpft werden, was anhand der veränderten FMR-Linienbreite detektiert werden kann. Es zeigt sich zudem der von der Theorie vorhergesagte lineare Zusammenhang zwischen Linienbreite und Stromdichte. Analytische Berechnungen der Größe des Spin-Transfer-Torques zeigen außerdem auch eine quantitative Übereinstimmung mit dem Experiment. Messungen an [Co/Pt]/Cu/Py-Proben zeigen überraschenderweise keinen Einfluss des Spin-Transfer-Torques auf die Linienbreite. Vermutlich führen die Pt-Schichten der Multilage zu einer starken Verringerung der Spinpolarisation, obwohl sie andererseits das Auftreten einer vertikalen Magnetisierung bewirken. Die Messungen mit [Co/Ni]-Multilagen als Polarisator zeigen, dass aufgrund der ursprünglich als vorteilhaft angesehenen Vermeidung der Pt-Schichten und der damit verbundenen niedrigen Anisotropie der Schaltstrom geringer als der der Py-Schicht ist.In the present thesis, the spin transfer torque effect is investigated in ferro/non-ferromagnetic pillar structures. The effect describes the manipulation of the magnetization direction only by a spin polarized current. Usually giant magnetoresistance measurements are used to detect the spin transfer torque effect. In the present thesis, for the first time, conventional ferromagnetic resonance is used to detect the influence of spin transfer torque on the intrinsic damping of the magnetization. The decisive benefit of the ferromagnetic resonance is that it gives access to effects in regions of low current densities which can not be investigated by magnetoresistance measurements. The measurements are performed on Co/Cu/Py pillar structures, in which Co serves as the hard-magnetic spin polarizing layer, and the influence of the spin transfer torque effect is measured by the Py layer. In addition, [Co/Pt] and [Co/Ni] multilayers are also used as spin polarizing layers instead of Co. The magnetization of the multilayers points out-of-plane in remanence. Therefore, the magnetization of the polarizer and the magnetization of the analyzer are oriented perpendicular to each other. In this manner, the effect of the spin tansfer torque can be increased with respect to the case when the magnetizations are parallel. The pillar structures are prepared and electrically connected by means of electron beam lithography. A large number of identical structures are prepared in an array, in which all structures are connected in series so that the current density reaches values high enough to observe the spin transfer torque effect on the FMR signal. Measurements on Co/Cu/Py samples show an influence of the spin transfer torque effect on the damping of the magnetization of the Py layer. Depending on the current direction, the damping can be increased or decreased and can be measured by analyzing the FMR line-width. A linear increase or decrease of the line-width as the function of current density can be observed that is consistent with theory. Analytical calculations of the magnidude of the spin transfer torque also agree very well with the values measured in the experiments. When [Co/Pt] multilayers function as the polarizing layer, the data show no influence of the spin transfer torque on the line-width of the Py layer. The Pt layers cause the perpendicular magnetization. The results on [Co/Ni] multilayers acting as the polarizer show that due to the elimination of the Pt layers, the perpendicular anisotropy is decreased, so that the critical current density for switching is lower than the value for switching the Py layer

Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

Here, we investigate the nature of the moment coupling between 10-nm DMSA-coated magnetic nanoparticles, in both colloidal dispersion and in powder form. The individual iron oxide cores were composed of > 95% maghemite and agglomerated to clusters. At room temperature the ensemble behaved as a superparamagnet according to M\"ossbauer and magnetization measurements, however, with clear signs of dipolar interactions at low temperatures. Analysis of temperature-dependent AC susceptibility data in the superparamagnetic regime indicates a tendency for dipolar coupled anticorrelations of the core moments within the clusters. To resolve the directional correlations between the particle moments we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross-section of the powder in low magnetic field at 300 K. We extract the underlying pair distance distribution function of the magnetization vector field by an indirect Fourier transform of the cross-section, and which suggests positive as well as negative correlations between nearest neighbor moments, with anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo simulations of such core-clusters.Comment: 11 pages, 6 figure

Standardisation of magnetic nanoparticles in liquid suspension

Suspensions of magnetic nanoparticles offer diverse opportunities for technology innovation, spanning a large number of industry sectors from imaging and actuation based applications in biomedicine and biotechnology, through large-scale environmental remediation uses such as water purification, to engineering-based applications such as position-controlled lubricants and soaps. Continuous advances in their manufacture have produced an ever-growing range of products, each with their own unique properties. At the same time, the characterisation of magnetic nanoparticles is often complex, and expert knowledge is needed to correctly interpret the measurement data. In many cases, the stringent requirements of the end-user technologies dictate that magnetic nanoparticle products should be clearly defined, well characterised, consistent and safe; or to put it another way—standardised. The aims of this document are to outline the concepts and terminology necessary for discussion of magnetic nanoparticles, to examine the current state-of-the-art in characterisation methods necessary for the most prominent applications of magnetic nanoparticle suspensions, to suggest a possible structure for the future development of standardisation within the field, and to identify areas and topics which deserve to be the focus of future work items. We discuss potential roadmaps for the future standardisation of this developing industry, and the likely challenges to be encountered along the way

Colloidal Flower-Shaped Iron Oxide Nanoparticles: Synthesis Strategies and Coatings

The assembly of magnetic cores into regular structures may notably influence the properties displayed by a magnetic colloid. Here, key synthesis parameters driving the self-assembly process capable of organizing colloidal magnetic cores into highly regular and reproducible multi-core nanoparticles are determined. In addition, a self-consistent picture that explains the collective magnetic properties exhibited by these complex assemblies is achieved through structural, colloidal, and magnetic means. For this purpose, different strategies to obtain flower-shaped iron oxide assemblies in the size range 25-100 nm are examined. The routes are based on the partial oxidation of Fe(OH)(2), polyol-mediated synthesis or the reduction of iron acetylacetonate. The nanoparticles are functionalized either with dextran, citric acid, or alternatively embedded in polystyrene and their long-term stability is assessed. The core size is measured, calculated, and modeled using both structural and magnetic means, while the Debye model and multi-core extended model are used to study interparticle interactions. This is the first step toward standardized protocols of synthesis and characterization of flower-shaped nanoparticles

How shape and internal structure affect the magnetic properties of anisometric magnetite nanoparticles

A three-step aqueous approach to obtain large (>50 nm) magnetite single-core particles has been developed. The steps are a) synthesis of antiferromagnetic nanoparticles, b) particle coating and c) subsequent reduction of the core material to magnetite. By variation of precursor material and process conditions, the synthesis yielded rhombohedra, discs or needles below 200 nm. A combination of X-ray diffraction, 57Fe Mössbauer spectroscopy and infrared spectroscopy confirmed magnetite to be the dominant final core material. From transmission electron microscopy, we identified porous structures after the reduction. Magnetic characterization of the different magnetic nanopaticles revealed strikingly different magnetic behaviour depending on their shape, internal structure and reduction process. We conclude that each of these parameters have to be considered in further characterization of large magnetite nanoparticles.This work was supported by the EC FP-7 grant “NanoMag” (grant agreement no 604448) and the Spanish Government by MAGO project (MAT2014-52069-R). The magnetic characterization was supported by COST Action TD1402. LG acknowledges financial support from the Ramón y Cajal subprogram (RYC-2014-15512).Peer reviewe

Interpreting the magnetorelaxometry signal of suspended magnetic nanoparticles with Kaczmarz’ algorithm

Magnetic nanoparticles in colloidal dispersions are important for biomedical applications like magnetic drug targeting, magnetic particle hyperthermia, and several imaging applications. For a physical understanding of these applications, the particles' hydrodynamic size distribution should be well characterized. Magnetorelaxometry is a fast method to determine this property, but until now had the drawback that a priori information, like a functional form of the expected size distribution, was necessary. Following recent advances, where Kaczmarz' algorithm was used to determine the core size distribution from static magnetization curves without any such assumptions, we present a similar study for the determination of the hydrodynamic size distribution. Here, the performance of several implementations of Kaczmarz' algorithm are investigated for both simulated and measured magnetorelaxometry data. Our results show that this method is able to determine the hydrodynamic size distribution in agreement with either the known input distribution, in the case of simulated data, or other size estimates determined with different methods such as thermal magnetic noise spectroscopy and dynamic light scattering in the case of measured data

Учебная программа по учебной дисциплине "Теоретическая механика"

Учебная программа "Теоретическая механика" кафедры "Механика" для дневной формы получения образования: общее количество часов-350, трудоемкость учебной дисциплины — 10 з.е., форма контроля знаний — зачет, экзамен, РГР

Polymer/Iron Oxide Nanoparticle Composites-A Straight Forward and Scalable Synthesis Approach

Magnetic nanoparticle systems can be divided into single-core nanoparticles (with only one magnetic core per particle) and magnetic multi-core nanoparticles (with several magnetic cores per particle). Here, we report multi-core nanoparticle synthesis based on a controlled precipitation process within a well-defined oil in water emulsion to trap the superparamagnetic iron oxide nanoparticles (SPION) in a range of polymer matrices of choice, such as poly(styrene), poly(lactid acid), poly(methyl methacrylate), and poly(caprolactone). Multi-core particles were obtained within the Z-average size range of 130 to 340 nm. With the aim to combine the fast room temperature magnetic relaxation of small individual cores with high magnetization of the ensemble of SPIONs, we used small (<10 nm) core nanoparticles. The performed synthesis is highly flexible with respect to the choice of polymer and SPION loading and gives rise to multi-core particles with interesting magnetic properties and magnetic resonance imaging (MRI) contrast efficacy

Size analysis of single-core magnetic nanoparticles

Single-core iron-oxide nanoparticles with nominal core diameters of 14\ua0nm and 19\ua0nm were analyzed with a variety of non-magnetic and magnetic analysis techniques, including transmission electron microscopy (TEM), dynamic light scattering (DLS), static magnetization vs. magnetic field (M-H) measurements, ac susceptibility (ACS) and magnetorelaxometry (MRX). From the experimental data, distributions of core and hydrodynamic sizes are derived. Except for TEM where a number-weighted distribution is directly obtained, models have to be applied in order to determine size distributions from the measurand. It was found that the mean core diameters determined from TEM, M-H, ACS and MRX measurements agree well although they are based on different models (Langevin function, Brownian and N\ue9el relaxation times). Especially for the sample with large cores, particle interaction effects come into play, causing agglomerates which were detected in DLS, ACS and MRX measurements. We observed that the number and size of agglomerates can be minimized by sufficiently strong diluting the suspension