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Molecular dynamics simulation combined magnetic theory on investigation of the magnetic properties and nano-structural variation of Co-Cu nanoparticle.Lo, Yu-Chieh 25 July 2005 (has links)
The Co-Cu nanoparticles is one of the magnetic materials that have considerable potential for a variety of industrial applications, including giant magnetoresistance (GMR) digital storage devices and have therefore attracted a great deal of attention in recent years. For this reason, it will be an important reference to the development of the magnetic digital storage devices if we can go deep into study the material properties of the Co-Cu nanoparticles.
This study uses molecular dynamics simulations to investigate the crystalline process of Co-Cu nanoparticles of high and low Co concentrations (5~25 %) during the annealing process. The modified many-body tight binding potential is adopted to accurately model the Cu-Cu, Co-Co, and Co-Cu pair inter-atomic interactions. The structural transformations at the upper and lower melting points are observed by the radial distribution function (RDF), the angle correction function (ACF) and the average bond lengths. finally, we employs molecular dynamics simulations to predict the distribution function of diluted magnetic Co atoms in a Cu host and then uses the the Ruderman-Kittel-Kasuya-Yosida (RKKY) theory and quantum magnetism theory to calculate the magnetic properties of the Co-Cu alloys at different temperature, including their Curie temperature.
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Observation of Magnetic Multilayers by Electron HolographyTanji, T., Hasebe, S., Nakagami, Y., Yamamoto, K., Ichihashi, M. 02 1900 (has links)
No description available.
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Nanostruktur ionenbestrahlter Fe/Al- und Co/Cu-GrenzschichtenNoetzel, Joachim 16 August 2000 (has links) (PDF)
In dieser Arbeit wird die nanoskalige Struktur von Grenzschichten in binären metallischen Multischichten untersucht. Ausgangspunkt sind laserdeponierte Multischichten des mischbaren Systems Fe/Al und des nichtmischbaren Systems Co/Cu. Die Struktur der durch die hochenergetischen Teilchen bei der Deposition entstandenen Grenzschichten wird mit Hilfe von zahlreichen Analyseverfahren (RBS, CEMS, EXAFS, Röntgenverfahren, TEM, AES und magnetische Messungen), sowie Simulationsrechnungen auf Basis des ballistischen Mischens (TRIDYN) untersucht. Anschließend wird mit Hilfe von Ionenstrahlmischen und thermischem Anlassen die Grenzschichtstruktur weiter modifiziert.
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Nanostruktur ionenbestrahlter Fe/Al- und Co/Cu-GrenzschichtenNoetzel, Joachim 17 July 2000 (has links)
In dieser Arbeit wird die nanoskalige Struktur von Grenzschichten in binären metallischen Multischichten untersucht. Ausgangspunkt sind laserdeponierte Multischichten des mischbaren Systems Fe/Al und des nichtmischbaren Systems Co/Cu. Die Struktur der durch die hochenergetischen Teilchen bei der Deposition entstandenen Grenzschichten wird mit Hilfe von zahlreichen Analyseverfahren (RBS, CEMS, EXAFS, Röntgenverfahren, TEM, AES und magnetische Messungen), sowie Simulationsrechnungen auf Basis des ballistischen Mischens (TRIDYN) untersucht. Anschließend wird mit Hilfe von Ionenstrahlmischen und thermischem Anlassen die Grenzschichtstruktur weiter modifiziert.
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Screened Korringa-Kohn-Rostoker-Methode für Vielfachschichten / Screened Korringa-Kohn-Rostoker-method for multilayered systemsZahn, Peter 24 July 2005 (has links) (PDF)
Im Rahmen der vorliegenden Arbeit wird eine Tight-Binding-Formulierung der Korringa-Kohn-Rostoker-Greenschen-Funktionsmethode vorgestellt. Dabei werden mittels eines geeignet gewählten Referenzsystems abgeschirmte Strukturkonstanten konstruiert. Es werden die Vorteile und Grenzen dieser Transformation des Formalismus diskutiert. Es wird gezeigt, daß der numerische Aufwand zur erechnung der Elektronenstruktur von Systemen mit langgestreckter Elementarzelle linear mit der Systemgröße wächst. Damit ist eine Behandlung von Systemen mit 500 und mehr Atomen pro Elementarzelle möglich. Anhand von umfangreichen Testrechnungen kann demonstriert werden, daß das neue Verfahren bezüglich seiner Genauigkeit mit dem traditionellen KKR-Verfahren vergleichbar ist. Es werden Anwendungen zur Berechnung der Elektronenstruktur sowie zur Zwischenlagenaustauschkopplung von Co/Cu(100)-Vielfachschichten vorgestellt. / A newly developed ab initio tight-binding-formulation of the Korringa-Kohn-Rostoker-Green's function method for layered systems is presented. Screened structure constants are calculated by means of a repulsive reference system. Advantages and limits of this transformation of the formalism are discussed in detail. The numerical effort for self consistent electronic structure calculations of systems with a large prolonged supercell scales linearly with the system size. Systems with up to 500 atoms per unit cell can be treated easily. The accuracy of the new method is of the same order as the traditional KKR method. Applications to electronic structure calculations and magnetic interlayer exchange coupling in Co/Cu(100) multilayers are presented.
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ESTUDO DE COMPOSTOS LAMELARES CONTENDO Fe USANDO ESPECTROSCOPIA MÃSSBAUER DE 57Fe E TÃCNICAS COMPLEMENTARES.Daniel Xavier Gouveia 20 April 2006 (has links)
Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / The structural and thermal decomposition properties of Mg-Fe and Co-Cu-Fe hy-
drotalcites (HT) have been studied through thermogravimetric analysis, X ray powder
difraction (XRD), Fourier transform infrared spectroscopy and 57Fe MÃssbauer spec-
troscopy. In the Mg-Fe system, the destruction of the layered structure took place at
about 300 oC. The broad peaks observed in the X ray difractograms suggests that the
resultant oxides constitute a solid solution. For samples treated at temperatures higher
than 500 oC the formation of the MgO and MgFe2O4 formation of the MgO and MgFe2O4
spinel phases is observed. 57Fe MÃssbauer spectroscopy was employed to monitor the Fe
chemical environment for the samples annealed at diferent temperatures (100-900 oC).
In situ XRD experiments revealed that the HTs start an interlayer contraction at about
180 oC. This phenomenon is identified as being due to a grafting process for which the
interlamellar anions attach to the layers through a covalent bond. The reconstruction of
the Mg-Fe HTs was also investigated and its eficiency depends on the thermal anneal-
ing temperature and on the Mg/Fe ratio. The structure of the reconstructed samples
was found to be exactly the same as the parent structure. The in situ 57Fe MÃssbauer
experiments were performed in the 100-500 oC temperature range confirm an increasing
structural disorder in this temperature range. The quadrupolar splitting indicates that
the maximum disorder occurs at 300 oC. Regarding the Co-Cu-Fe ternary system we have
observed that due to the strong Jahn-Teller effect the Cu-Fe layered system is stabilized
only in the presence of Co2+. At low Co2+ contents, additional phases are segregated
in the solids. X ray patterns diffraction show the presence of Cu(OH)2 and CuO. The
decomposition process was investigated by in situ X ray, in situ MÃssbauer and FTIR
experiments. By increasing the temperature from 25 oC up to 180 oC we observed that
the structural disorder increases. This effect has been likely attributed to the Co2+ to
Co3+ oxidation since thermal decomposition was carried out under static air atmosphere.
Part of the Co3+ cations could migrate to the interlayer region, thus forming a metastable
compound that still has a layered structure. Collapse of the layered structure was ob-
served at about 200 oC. By further increasing the temperature the system becomes more
crystalline and the formation of Co3O4 is observed in the X ray patterns. In Cu-rich HT,
some of the carbonate anions are released at temperatures higher than 550 oC and this
phenomenon is attributed to the formation of a carbonate-rich phase. The specific surface
area data present its highest values in the temperature range where the collapse of the
layered structure takes place. / As propriedades estruturais e de decomposiÃÃo tÃrmica das hidrotalcitas Mg-Fe e Co-Cu-Fe foram estudadas atravÃs de espectroscopia MÃssbauer de 57Fe, anÃlise termo-
gravimÃtrica, difraÃÃo de raios X, e espectroscopia de absorÃÃo no infravermelho (FTIR).
No sistema Mg-Fe a destruiÃÃo da estrutura lamelar ocorre em torno de 300 oC. O alargamento dos picos de difraÃÃo de raios X observados nos difratogramas sugerem que os Ãxidos resultantes constituem uma soluÃÃo sÃlida. Para as amostras tratadas em temperaturas maiores do que 500 oC a formaÃÃo de fases do tipo MgO e MgFe2O4 à observada. A espectroscopia MÃssbauer de 57Fe foi empregada para monitorar o ambiente quÃmico do Fe na faixa 100-900 oC de temperatura. As medidas in situ de difraÃÃo de raios X revelam que em 180 oC inicia-se uma contraÃÃo interlamelar. Este fenÃmeno à atribuÃdo ao processo de âgrafting" no qual os Ãnions interlamelares ligam-se nas camadas atravÃs de uma ligaÃÃo covalente. A reconstruÃÃo estrutural da hidrotalcita Mg-Fe tambÃm foi investigada. A eficiÃncia da reconstruÃÃo estrutural depende da temperatura de tratamento e da razÃo molar Mg/Fe. A estrutura das amostras reconstruÃdas sÃo as mesmas da amostra inicial. As medidas in situ de espectroscopia MÃssbauer de 57Fe foram realizadas na faixa 100-500 oC confirmaram uma desordem estrutural crescente nesta regiÃo de temperaturas.
Os valores do desdobramento quadrupolar indicam que o mÃximo de desordem ocorre em 300 oC. Com relaÃÃo ao sistema ternÃrio Co-Cu-Fe observamos que devido ao efeito Jahn-Teller o sistema Cu-Fe somente à estabilizado na presenÃa de Co2+. Para baixas concentraÃÃes Co2+ fases adicionais segregadas sÃo observadas nos sÃlidos. Os padrÃes de
difraÃÃo de raios X indicam a presenÃa de Cu(OH)2 e CuO. O processo de decomposiÃÃo
tÃrmica foi investigado atravÃs de difraÃÃo de raios X, espectroscopia MÃssbauer de 57Fe
in situ e de espectroscopia de absorÃÃo no infravermelho (FTIR). Aumentando a temperatura de tratamento tÃrmico das amostras de 25 oC a 180 oC observamos um aumento
da desordem estrutural. Este efeito tem sido atribuÃdo a oxidaÃÃo Co2+ para Co3+ uma vez que a decomposiÃÃo foi realizada ao ar. Parte dos cÃtions Co3+ migram para a regiÃo interlamelar formando um composto metastÃvel que ainda possui uma estrutura lamelar. O colapso da estrutura lamelar à observado a 300 oC. Com o aumento posterior da temperatura o sistema torna-se mais cristalino e a formaÃÃo de Co3O4 à observada atravÃs do ensaio de raios X. Nas hidrotalcitas com maior teor de Cu, alguns dos Ãnions carbonato sÃo liberados somente acima de 550 oC sendo este fenÃmeno atribuÃdo a formaÃÃo de uma
fase rica em carbonato. Os valores de Ãrea superficial especÃfica apresentam um mÃximo
na faixa de temperatura onde ocorre o colapso da estrutura lamelar.
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Screened Korringa-Kohn-Rostoker-Methode für VielfachschichtenZahn, Peter 24 July 2005 (has links)
Im Rahmen der vorliegenden Arbeit wird eine Tight-Binding-Formulierung der Korringa-Kohn-Rostoker-Greenschen-Funktionsmethode vorgestellt. Dabei werden mittels eines geeignet gewählten Referenzsystems abgeschirmte Strukturkonstanten konstruiert. Es werden die Vorteile und Grenzen dieser Transformation des Formalismus diskutiert. Es wird gezeigt, daß der numerische Aufwand zur erechnung der Elektronenstruktur von Systemen mit langgestreckter Elementarzelle linear mit der Systemgröße wächst. Damit ist eine Behandlung von Systemen mit 500 und mehr Atomen pro Elementarzelle möglich. Anhand von umfangreichen Testrechnungen kann demonstriert werden, daß das neue Verfahren bezüglich seiner Genauigkeit mit dem traditionellen KKR-Verfahren vergleichbar ist. Es werden Anwendungen zur Berechnung der Elektronenstruktur sowie zur Zwischenlagenaustauschkopplung von Co/Cu(100)-Vielfachschichten vorgestellt. / A newly developed ab initio tight-binding-formulation of the Korringa-Kohn-Rostoker-Green's function method for layered systems is presented. Screened structure constants are calculated by means of a repulsive reference system. Advantages and limits of this transformation of the formalism are discussed in detail. The numerical effort for self consistent electronic structure calculations of systems with a large prolonged supercell scales linearly with the system size. Systems with up to 500 atoms per unit cell can be treated easily. The accuracy of the new method is of the same order as the traditional KKR method. Applications to electronic structure calculations and magnetic interlayer exchange coupling in Co/Cu(100) multilayers are presented.
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