Influence of disorder and interfaces on the electronic and magnetic properties of Heusler systems

Anhand von Mössbauer-spektroskopischen Untersuchungen von Fe3Si-Filmen auf GaAs(100) und MgO(100) konnte nachgewiesen werden, dass das Wachstum von Fe3Si auf GaAs(100) durch eine Interdiffusion von Ga/As-Atomen gestört wird. Die Verwendung einer 3 nm dicken MgO-Tunnelbarriere auf der GaAs-Oberfläche unterdrückt diese Interdiffusion deutlich und ermöglicht so ein epitaktisches Filmwachstum von Fe3Si. Ein Vergleich der Röntgenabsorptionsuntersuchungen mit DFT-Rechnungen offenbart die Beiträge der verschiedenen Fe-Untergitter zum XAS und XMCD. Eine Berücksichtigung von Unordnung durch Ga/As-Atome in DFT-Rechnungen zeigt nur eine geringe Abnahme der Spinpolarisation von Fe3Si, so dass das System Fe3Si/GaAs(100) weiterhin ein interessanter Kandidat für spintronische Anwendungen ist. Für die Gemische Co2MnSi und Co2FeSi konnte der Einfluss der 3d-Übergangsmetalle Mn/Fe auf die Hybridisierung durch Röntgenabsorption und DFT-Rechnungen bestimmt werden. Eine tiefenselektive Untersuchung der elektronischen Struktur von Mn in Co2MnSi an einer MgO-Grenzschicht lässt einen Anstieg der Anzahl unbesetzter d-Zustände vermuten und weist somit auf eine MnSi-Terminierung hin. An nicht-stöchiometrischen Co2FeSi-Gemischen konnte eine Magnetfeldabhängigkeit der elektronischen Struktur von Co und Fe in Si-reichem Co2FeSi beobachtet werden. Dies könnte ein Hinweis auf magnetostriktive Effekte in diesem Gemisch sein. Des Weiteren wurde das Heusler-Gemisch Ni51.6Mn32.9Sn15.5 in dieser Arbeit untersucht, das ein Formgedächtnismetall darstellt und einen deutlichen inversen magnetokalorischen Effekt zeigt. In dieser Arbeit lag der Schwerpunkt auf den elementspezifischen magnetischen Eigenschaften von Ni und Mn. Für Mn wurde ein starker Anstieg des Verhältnisses von Bahn- zu Spinmoment ml/mS beobachtet. Während dieses Verhältnis in der Austenitphase 5% beträgt, steigt es in der Martensitphase auf 13.5% an. Gleichzeitig weist Ni einen nahezu konstanten Wert von ml/mS von 28% auf. Legt man in der Martensitphase ein Magnetfeld von 3 T an, so lässt sich der Wert des Verhältnisses von ml/mS für beide Elemente reduzieren, was auf einen feldinduzierten inversen Martensitübergang (FIRMT) hinweist.A Mössbauer-spectroscopic investigation of Fe3Si films on GaAs(100) and MgO(100) revealed a disordered growth mode of Fe3Si on GaAs(100), which is caused by an interdiffusion of Ga/As atoms. Implementing a 3 nm thick MgO tunnelbarrier on the GaAs surface inhibits the interdiffusion and enables an epitaxial film growth of Fe3Si. By comparing experimental X-ray absorption measurements with DFT calculations we are able to resolve the contribution of the different Fe sublattices to the XAS and XMCD signal. Taking into account atomic disorder arising from Ga/As atoms within DFT calculation yields a small reduction of the spin polarization of Fe3Si, indicating that the system Fe3Si/GaAs(100) still is an interesting candidate for spintronic applications. For the Heusler compounds Co2MnSi and Co2FeSi the influence of the 3d transition metals Mn/Fe on the hybridization was determined by X-ray absorption and DFT calculations. A depth-selective study of the electronic structure of Mn in Co2MnSi at the vicinity to a MgO tunnelbarrier indicates an increased number of unoccupied d states referring a MnSi terminated interface. For non-stochiometric Co2FeSi compounds a dependence of the electronic structure of Co and Fe from the external magnetic field was observed. This points to magnetostrictive effects in this compound. Furthermore, the Heusler compound Ni51.6Mn32.9Sn15.5 was studied in this work. The compound is a shape memory alloy exhibiting a large inverse magnetocaloric effect. In this work the focus was put on the element-specific magnetic properties of Ni and Mn. For Mn a strong increase of the ratio of orbital to spin magnetic moment ml/mS was observed. In the austenite phase this ratio accounts for 5%, whereas in the martensite this value becomes 13.5%. For Ni ml/mS remains almost constant at 28%. Applying a magnetic field of 3 T in the martensite phase leads to a reduction of ml/mS for both elements, indicating a field-induced reverse martensitic transition (FIRMT)

Розвиток суспільно-політичного процесу на Волині в період незалежності

У статті розглянуто основні етапи розвитку суспільно-політичного процесу на Волині в період незалежності України. Трансформаційні процеси, що проходять в області, попри свою місцеву особливість, виходять із чергового етапу розвитку політичної системи України, а отже, розвиток суспільно-політичного процесу на Волині прямо залежний від всеукраїнського процесу трансформації.In the article the basic stages of the development of social and political process are considered in Volyn region during the period of independence of Ukraine. The transformation processes, which pass in this region, without regard to the local feature go out from the regular stage of the development of the political system of Ukraine, and consequently, the development of social and political process in Volyn region depend directly upon the all-Ukrainian process of transformation

Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films

The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100) substrate, to an in situ deposited submonolayer of TbPc2 molecules. Because of the element specificity of the X-ray absorption spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film. On both substrates the TbPc2 moleculescouple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface

2,000 Families: identifying the research potential of an origins-of-migration study

Despite recent advances, critical areas in the analysis of European migration remain underdeveloped. We have only a limited understanding of the consequences of migration for migrants and their descendants, relative to staying behind; and our insights of intergenerational transmission is limited to two generations of those living in the destination countries. These limitations stem from a paucity of studies that incorporate comparison with non-migrants – and return migrants – in countries of origin and which trace processes of intergenerational transmission over multiple generations. This paper outlines the theoretical and methodological discussions in the field, design and data of the 2,000 Families study. The study comprises almost 50,000 members of migrant and non-migrant Turkish families across three family generations, living in Turkey and eight European countries. We provide indicative findings from the study, framed within a theoretical perspective of “dissimilation” from origins, and reflect on its potential for future migration research

Hysteretic behaviour in a vacuum deposited submonolayer of single ion magnets

With element-specific X-ray absorption spectroscopy and X-ray magnetic circular dichroism we have investigated submonolayer coverages of TbPc2 and DyPc2 molecules sublimated on highly ordered pyrolytic graphite. We have studied the field dependence of the magnetization of the central lanthanide ion at very low temperatures. Even in zero applied magnetic field we still observe a remanence in the magnetization. Since there are neither intermolecular coupling nor magnetic interactions with the substrate, this remanent behaviour results just from single-ion anisotropy. On the very inert surface of graphite at temperatures between 0.5 K and 2 K the spin relaxation is slow enough to observe a memory effect in the timescale of the experimental measurements

Magnetocrystalline anisotropy and Gilbert damping in iron-rich Fe1−xSix thin films

The magnetocrystalline anisotropy of Fe1−xSix (0≤x≤0.4) epitaxial thin films on MgO(001) was studied by ferromagnetic resonance. The experimental results are in good agreement with theoretical predictions of ab initio electronic structure calculations using the fully relativistic Korringa-Kohn-Rostoker Green's function method within spin-density-functional theory. The Gilbert damping α is found to be isotropic by theory and experiment with a minimum at the composition x=0.2

Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N K x-ray absorption (XA) and Fe L2,3 x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S=2) from an intermediate spin state (S=1) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy

Intergenerational consequences of migration: Socio-economic, family and cultural patterns of stability and change in Turkey and Europe

This book analyzes the impact of migration on the lives of multiple generations of 2000 Turkish families. Exploring education, marriage, fertility, friends, attitudes and religiosity, it reveals transformations and continuities in the lives of migrants and their families in Europe when compared to their non-migrant counterparts in Turkey