The dilute magnetic semiconductor (Ga,Mn)As , which is the most interesting and promising material for spintronics applications, has been theoretically studied by using Density Functional Theory. First of all, calculations on GaAs were done and it was found that GaAs is a semiconductor with a direct band gap. The calculated value of the band gap is ~ 0.5eV. Secondly, the material iron was considered and it was confirmed that iron is a ferromagnetic metal with 2.2µB net magnetic moment. Then a magnetic impurity of manganese, Mn was introduced in the nonmagnetic GaAs and it became ferromagnetic with a net magnetic moment of 4µB. The origin of the ferromagnetic behaviour is discussed and also the Curie temperature TC of the material. It appeared that (Ga,Mn)As is a suitable material for DMS but TC has to be increased before (Ga,Mn)As could be used for spintronics applications and on that account some methods of increasing TC are considered at the end. / Den magnetiska halvledaren (Ga,Mn)As som är det mest intressanta och lovande materialet för spinelektroniska tillämpningar har teoretiskt undersökts med hjälp av Täthetsfunktionalteorin. Först gjordes beräkningar på GaAs och det visade sig att GaAs är en halvledare med direkt bandgap. Det beräknade värdet på bandgapet är ca 0.5eV. Sedan var det järn som undersöktes och det blev bekräftat att järn är en ferromagnetisk metall med netto magnetisk moment lika med 2.2μB. Då magnetiska störningar i form av mangan atomer, Mn, infördes i det omagnetiska GaAs blev halvledaren ferromagnetisk med netto magnetisk moment lika med 4μB. Orsakerna till den ferromagnetiska ordningen diskuteras och även Curie temperaturen TC för materialet. Det visade sig att (Ga,Mn)As är ett lämpligt material för tillverkning av magnetiska halvledare men TC måste ökas innan (Ga,Mn)As skulle kunna användas i spinntroniska tillämpningar och av det skälet anges i slutet vissa metoder för att öka TC.

The cyanide ligand has frequently been used to prepare clusters with novel magnetic properties due to its ability to provide an efficient pathway for superexchange between metal centers that are bound in an end-to-end fashion. One of the common synthetic approaches in this chemistry is to design suitable cyanide containing precursors and then to react such building blocks with metal complexes consisting of accessible sites. The triphos ligand (triphos: 1,1,1-tris(diphenylphosphinomethyl)ethane) has been employed in this vein to prepare metal complexes, one of which is a five coordinate paramagnetic complex (S = 1/2) with a square pyramidal metal center, [CoII(triphos)(CN)2]. A family of molecular squares, [{MIICl2}2{CoII(triphos)(CN)2}2] (M= Mn (2), Fe (3), Co (4), Ni (5), and Zn (6)), has been synthesized by the reaction of CoII(triphos)(CN)2 and MCl2 (M= Mn, Co, Ni, Zn) or Fe4Cl8(THF)6 in CH2Cl2/EtOH mixture. A series of cyanide-bridged trinuclear complexes, {[Co(triphos)(CN)2]2 [M(MeOH)4]}(ClO4)2 ( M = Mn (7), Fe (8), Co (9), and Ni (10)) and tetranuclear complexes, {[Co(triphos)(CN)2]2[M(MeOH)4]2}(ClO4)4 ([Co2M2] M = Mn (11) and Ni (12)) have been synthesized in a similar fashion by the reaction of CoII(triphos)(CN)2 and M(ClO4)2.6H2O (M= Mn, Fe, Co, Ni) in methanol. The trinuclear compounds (7-9), and tetranuclear complexes (2-6, 11, 12), are characterized by antiferromagnetic coupling between metal centers while magnetic behavior of 10 indicates the presence of ferromagnetic interactions between the paramagnetic metal centers. Interactions between magnetic orbitals of Co(II) and M(II) ions were also investigated by means of the density functional theoretical (DFT) calculations. Another triphos containing building block, [(triphos)Re(CN)3] anion (13), has been employed to prepare derivatives of a cubic SMM cluster with four octahedral Re(II) ions and four tetrahedral Mn(II) sites bridging through cyanide ligand. The reactions of Re(II) precursor with MnI2 and solvated Mn(II) ions resulting in derivatives of Re4Mn4 cube with different ligands attached to the Mn center other than the chloride atom were reported. Our efforts on linking these cubes using organo cyanide ligands such as dicyanamide (dca) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) to form extended networks were also discussed.

The magnetic properties of transition metal compounds have been studied using SQUID-magnetometry, magnetic force microscopy and Lorentz transmission electron microscopy. New magnetic materials have been found and their magnetic properties have been determined. How the magnetic properties of a material can be changed through e.g. chemical substitution of magnetic and nonmagnetic atoms and shape and size effects have also been studied. Three different sets of samples have been investigated: three new Mn-compounds, two substitution series of layered magnetic structures and ferromagnetic micronsized thin film elements. The three Mn-compounds, Mn3IrSi, IrMnSi and Mn8Pd15Si7, show different magnetic ordering. Mn3IrSi orders 'antiferromagnetically' at 210 K. IrMnSi forms a double cycloidal spin spiral below 460 K. Mn8Pd15Si7 only shows short-range magnetic ordering. Substituting Se with S in TlCo2Se2-xSx changes the magnetic order from a spin spiral to a colinear ferromagnet for a composition of x=1.75. An intermediate region exists where the compound is neither a pure ferromagnet, nor purely a spin spiral, as evidenced by the magnetization versus field measurements for the x=1.3 and 1.5 samples. This is also seen in the temperature dependent susceptibility measurements. For the TlCu2-xFexSe2 compounds it was found that the ordering temperature and saturation magnetic moment per Fe-atom changed with composition x. Ferromagnetic micronsized thin film elements in permalloy, Fe20Ni80, and epitaxial Fe/Co multilayers were studied. For the Fe/Co multilayer thin film elements it was found that it is possible to change the magnetization reversal process, by aligning the easy shape anisotropy axis with either the easy or the hard magnetocrystalline anisotropy axis. In the permalloy elements the effect of inter-elemental distance was found to determine the interval of fields where multidomain states were stable, so that for shorter inter-elemental distances multidomain states were stable for a shorter interval of fields. The domain structure of permalloy elements in rotating magnetic fields was also studied. Higher applied fields led to a broader interval of angles in which saturated states were stable.

Orientadores: Pedro Luiz Onofrio Volpe, Maria Regina Alcantara / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-05T01:16:47Z (GMT). No. of bitstreams: 1 Bombard_AntonioJoseFaria_D.pdf: 17369274 bytes, checksum: 8d592533d59e6cf4325ee9574b58724c (MD5) Previous issue date: 2005 / Doutorado / Físico-Química / Doutor em Ciências

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Structural characterization is essential for the understanding of the magnetic properties of thin films and multilayers. In this thesis, both crystalline and amorphous thin films and multilayers were analyzed utilizing transmission electron microscopy (TEM). High resolution TEM and electron diffraction studies emphasize on the growth of amorphous Fe91Zr9 and Co68Fe24Zr8 on both Al2O3 and Al70Zr30 in multilayer structures by magnetron sputtering. The properties of the growth surfaces were found to strongly influence the formation of nano-crystallites of the magnetic material at interfaces. Field induced uniaxial magnetic anisotropy was found to be possible to imprint into both fully amorphous and partially crystallized Co68Fe24Zr8 layers, yielding similar magnetic characteristics regardless of the structure. These findings are important for the understanding of both growth and magnetic properties of these amorphous thin films. As magnetic systems become smaller, new analysis techniques need to be developed. One such important step was the realization of electron energy-loss magnetic circular dichroism (EMCD) in the TEM, where information about the ratio of the orbital to spin magnetic moment (mL/mS) of a sample can be obtained. EMCD makes use of angular dependent inelastic scattering, which is characterized using electron energy-loss spectroscopy. The work of this thesis contributes to the development of EMCD by performing quantitative measurements of the mL/mS ratio. Especially, methods for obtaining energy filtered diffraction patterns in the TEM together with analysis tools of the data were developed. It was found that plural inelastic scattering events modify the determination of the mL/mS ratio, wherefore a procedure to compensate for it was derived. Additionally, utilizing special settings of the electron gun it was shown that EMCD measurements becomes feasible on the nanometer level through real space maps of the EMCD signal.

Tetramethyl ammonium manganese trichloride ([CH3)4N][Mn(II)Cl3]) known as TMMC, has been one of the most interesting systems in experimental magnetism due to its highly one-dimensional magnetic Heisenberg behaviour. The focus of this research programme was to study its magnetic and structural properties as a function of pressure. TMMC crystals were prepared by slow evaporation technique at room temperature; however it quickly became apparent that the material is only weakly magnetic and requires a pressure cell with a very low background. This discovery lead to the programme of instrumentation development for studies of weakly magnetic materials and gave a dualistic nature to the project. The first pressure cell developed was a piston-cylinder type cell for magnetic susceptibility measurements in a Magnetic Properties Measurement System (MPMS®) based on Superconducting Quantum Interference Device (SQUID) technology from Quantum Design, USA. It has been carefully designed in order to reduce the magnetic background. One way in which this has been achieved was through making the pressure cell symmetric with respect to the sample in order to provide an integrable response in the SQUID magnetometer. The cell was made of beryllium copper alloy which has a low background even at low temperature. The use of a multi-layered cylinder with the interference fit method has resulted in the increased strength of the cell and allowed larger sample volume. The use of Lamé equation and finite element method to calculate the change of the cell diameter or cell length as a function of internal pressure enables us to eradicate the need of superconductive manometer. The cell has been successfully tested up to a maximum pressure of 10 kbar. Further development of the cell has resulted in development of an electrical plug for in situ pressure measurement inside the pressure cell. This has been achieved by means of a manganin pressure sensor calibrated to provide pressure reading at any given temperature. For structural studies, a diamond anvil cell (DAC) was designed to conduct singlecrystal X-ray diffraction measurements at low temperature. The design was based on the well-known Merrill-Bassett DAC and on the design of the miniature DAC which has been developed for use within He-3 system in the Physical Properties Measurement System (PPMS®), Quantum Design. The cell has been tested down to liquid nitrogen temperatures with a cryostream cooling system and has shown a significant improvement compared to the standard pressure cells. The frost formation on the surface of the cell has slowed down significantly compared to the tests on the Merrill-Bassett cell, which led to a better quality diffraction pattern from the sample inside the cell. This result has been achieved due to the high thermal conductivity of the materials used in the construction and the minimisation of the DAC, which was effectively built around the Boehler-Almax diamond anvils. With the help of some of the high-pressure instruments mentioned above, highpressure properties of TMMC have been studied in this project. The structuremagnetism relationship was established from the results of magnetic and structural measurements under pressure. The magnetic susceptibility data helped to establish the change of the intrachain antiferromagnetic coupling constant as a function of pressure, while X-ray structures of TMMC were refined from ambient pressure to 17 kbar using a synchrotron X-ray diffraction technique. The structure of TMMC at room temperature was confirmed to be hexagonal. However, indirect evidence of the hexagonal-monoclinic structural phase transition was observed at above 17 kbar and room temperature. The combination of the magnetic and structural data has helped to establish that the interaction between high spin d5 metal orbitals (Mn(II)) in facesharing octahedral has a contribution from both direct exchange and superexchange interactions. The power-law relationship developed by Bloch was also observed in this system.

Road Deposited Sediment (RDS) is an important pathway of pollution material in the urban environment. Traditional particulate matter (PM) monitoring methods are typically expensive and time consuming. To date, urban sediment studies have not fully explored the application of mineral magnetic technologies as an alternative to characterise RDS or, perhaps more importantly, their use as particle size proxy. Therefore, this study addresses these issues by determining the extent of any linkages between magnetic properties and the physio-chemical concentrations of RDS. Investigations have focussed on a spatial temporal study (2008-10) of RDS from the City of Wolverhampton (n = 546) and a similar ‘snap-shot’ study of eight selected town and cities across the UK (n = 306), plus a comparison investigation linked to regional monitoring of air sampling units (ASU) (n = 208). A suite of analytical approaches, namely mineral magnetism, laser diffraction, X-Ray Fluorescence spectroscopy (XRF), Scanning Electron Microscopy (SEM) and Loss on Ignition (LOI), were employed to characterize sample properties. Data interrogation identified mainly weak correlations exist between most mineral magnetic parameters and particle size classes (i.e. sand, silt and clay) and respiratory health-related size classes (i.e. PM10, PM2.5 and PM1.0). The few strongest correlations (p <0.001) were found between mineral magnetic concentration and <PM10 parameters. In Wolverhampton this occurred for samples collected during the spring months (March and May), indicating possible seasonal influences on RDS dynamics and sources. Elsewhere in the UK, and at ASU stations, results revealed mainly limited or insignificant (p >0.05) correlations exist between mineral magnetic parameters and particle size. However, for some locations (most notably, London and Scunthorpe), results exhibit signatures perceived to be associated with environmental factors. Detailed multivariate Factor Analysis plots and Geographical Information System (GIS) images have been used to explore these findings further. These illustrate RDS properties of road types (arterial and residential) display significantly different characteristics, with raised mineral magnetic concentrations for arterial roads, compared to lesser concentrations for residential roads, which corresponds to traffic flow data. This is supported by SEM analyses that reveal elevated concentrations of iron oxide spheres in samples collected from arterial roads, which are indicative of inputs from anthropogenic combustion sources. Contextualising these findings within the framework of existing knowledge, a conceptual approach has been presented that explores factors (i.e. sampling area, topography, land use, sediment source and potential mixing), which influence the reliability of using mineral magnetic techniques a particle size proxy. This demonstrates that any increase in the complexity of these factors (sampling area dynamics) can be used to predict the likelihood of being able to employ mineral magnetic measurements as a proxy. To surmise the work overall, despite mineral magnetic technologies offering an inexpensive and rapid means of analysing RDS, its use as a proxy measure for particulate matter appears to be limited by a series of site-related factors but the technique seems to offer valuable insights for pollution source studies.

A Fourier transform far infrared spectrometer bas been constructed to perform reflectivity and transmission measurements on small semiconductor samples in the temperature range 10-300 grad. K. Far infrared and Raman spectra selection rules have been obtained by the Correlation method from the factor group analysis of the 5nS subset. Crystals of n- and p- type SnS bave been prepared, and the preparation methods are critically reviewed with reference to the T-p-x diagram. The electrical properties (Hall effect, carrier concentration, Hall mobility and barrier height) are measured for these specimens in the temperature range 40-3000 K. Methods of obtaining electrical contacts to SnS are exhaustively studied. Broadband photoconductivity and photovoltaic measurements are reported on n- and p-type specimens in the spectral range 0.5-2 .1 eV at temperatures from 300 grad. K to 10 grad. K. Band structure and impurity energies are suggested to account for the observed results. The band edge shift with temperature is also measured. Far infrared reflectivity and transmission measurements are made between 300 grad. K and 10 grad. K. in order to determine the lattice vibration frequencies. Kramers-Kroenig analysis and computer fitting routines are used to obtain this information from the reflectivity data. The interlayer forces are compared with those obtained in the isomorphic materials GeS and GeSe. Similar spectroscopic and analytic techniques have also been used to examine the room temperature optical properties of ZnS+Fe (Marmatite).

High resolution rotational spectra of ¹⁰BH₃CO, ¹¹BH₃CO, ¹¹BD₃CO, PH₂D and CD₃CN were studied to obtain information concerning the electronic and magnetic properties of these molecules. From the analysis of the hyperfine structure of the rotational spectra of the molecules the following constants were determined: rotational constants (B₀), nuclear quadrupole coupling constants (eOq [x], spin-spin constants D (X-Y) and spin rotation constants (C[x]). The two instruments used in these experiments were a Stark modulated microwave spectrometer and a molecular beam maser spectrometer. The J = 0 to 1, F₁ = 3 to 3 and F₁ = 3 to 4 transitions of ¹⁰BH₃CO were studied. The molecular constants in KHz which were derived from the spectral analysis were: B(,0) = 8980060.3 (+/-) 1.0, eQqₐₐ (¹⁰B) = 3463.- (+/-) 8.0, C(¹⁰B) = .2 (+/-) .2, D(¹⁰B-H) = -2.8 (+/-) .2, D(H-H) = -7.2 (+/-) .5 and C(H) = 0.0 (+/-) .7. All three of the F₁ components of the J = 0 to 1 rotational transition of ¹¹BH₃CO were observed. The molecular constants in KHz which were obtained from the data analysis were: B₀ = 8657333.8 (+/-) .6, eQqₐₐ (¹¹B) = 1661.9 (+/-) 2.3, C(¹¹B) = .7 (+/-) .3, D(¹¹B-H) = 8.3 (+/-) .4, D(H-H) = -6.8 (+/-) .2 and C(H) = .4 (+/-) .4. Only spectra of the J = 0 to 1, F₁ = 3/2 to 3/2 transition of ¹¹BD₃CO were obtained. From the analysis of the spectra the following molecular constants were derived: eQqₐₐ (D) = -48.5 (+/-) 2.3, eQq(,zz)(D) = 116.9 (+/-) 5.4, C(D) = 0.0 (+/-) .8 and D(¹¹B-H) = -1.2 (+OR-) .3. The investigation of the 4₀₄ to 4₁₄ and 1₁₁ to 1₀₁ rotational transitions of PH₂D was unsuccessful in determining any molecular parameters. The spectra of the 4₀₄ to 4₁₄ rotational transition were not amenable to analysis and no spectra were obtained for the 1₁₁ to 1₀₁ rotational transitions. The three components of the J = 0 to 1 rotational transitions of CD₃CN were studied with the maser in the two cavity configuration. The constants in KHz which were determined were: B₀ = 7857978.7 (+/-) .1, eQqₐₐ(N) = -4229.2 (+/-) .6, eQqₐₐ(D) = -55.1 (+/-) .4, eQq(,zz)(D) = 165.5 (+/-) 5.0, C(,N) = 1.7 (+/-) .1 and C(,D) = 0.0 (+/-) .03.