Global ETD Search

1	Molecule-based magnetic materials of the ReIV ion Pedersen, Anders Hjordt January 2017 (has links) The [ReCl6]2-, [ReBr6]2- and [ReCl4(ox)]2- anions are crystallised with the organic 4,4’- bipyridinium dication (4,4-H2bipy). Magnetometry reveals exotic behaviour of the [4,4’- H2bipy][ReCl6] and [4,4’-H2bipy][ReBr6] salts which demonstrate spin-canting, antiferromagnetic exchange interactions and metamagnetism. Single crystal X-ray structures at T = 3, 14 and 20 K of the [4,4’-H2bipy][ReBr6] salt reveal the behaviour to be purely of magnetic origin as no structural changes are observed. For the [4,4’-H2bipy][ReCl4(ox)] compound an antiferromagnetic exchange interaction of 10.2 cm-1 between the anions is observed (Chapter 2). The complexes (NBu4)2[(ReCl5)2(μ-pyrazine)], (NBu4)2[(ReBr5)2(μ-pyrazine)], (NBu4)2[(ReBr5)2(μ-pyrimidine)] and (NBu4)2[(ReBr5)2(μ-triazine)] are structurally and magnetically characterised in Chapter 3. Magnetic measurements reveal the ReIV ions bridged by a 1,4-heterocyclic amine to exhibit strong antiferromagnetic coupling induced by the linearity of the bridging ligand. The two dimers bridged by a 1,3-heterocyclic amine exhibit intramolecular ferromagnetic exchange and at low temperature an intermolecular antiferromagnetic coupling is observed for the (NBu4)2[(ReBr5)2(μ-triazine)] complex due to the presence of short intermolecular Br···Br distances. Six molecular ReIVCuII chains of formula {[Cu(L)4][ReCl6]}n (L = imidazole, 1- methylimidazole, 1-vinylimidazole, 1-butylimidazole, 1-vinyl-1,2,4-triazole or dimethylformamide) are characterised structurally and magnetically in Chapter 4. SQUID magnetometry and theoretical calculations reveal the chains to exhibit ferromagnetic exchange interactions, which increase as the Re–Cl–Cu bond angle decreases. The {[Cu(vinylimidazole)4][ReCl6]}n chain exhibit magnetic order at TC = 2.4 K, and the {[Cu(imidazole)4][ReCl6]}n network exhibits ferrimagnetic behaviour. Eight complexes of the [ReCl6]2- and [ReBr6]2- anions crystallised with the [MII(L•)2]2+ (M = Fe, Co or Cu) or [Ni(L•)(CH3CN)3]2+ cations (L• = 4-dimethyl-2,2-di(2-pyridyl)oxazolidine N-oxide) are characterised structurally and magnetically in Chapter 5. The [Co(L•)2]2+ cation shows evidence of a gradual, thermally induced spin-crossover transition in variable-temperature magnetic and structural experiments. The [Ni(L•)(CH3CN)3]2+ cation show exchange of the coordinated acetonitrile molecules for atmospheric water upon drying. The nickel-radical magnetic coupling is ferromagnetic in all cases, demonstrating spin-canting behaviour with an ordering temperature of T = 2.7 K for the [ReCl6]2- based compound, and intermolecular antiferromagnetic exchange interactions for the [ReBr6]2- based complex.
2	Nanočástice na bázi oxidů 3d kovů - korelace struktury a magnetismu / Nanoparticles based on 3d metal oxides - correlation of structure and magnetism Kubíčková, Simona January 2015 (has links) Title: Nanoparticles based on 3d metal oxides - correlation of structure and magnetism Author: RNDr. Simona Kubíčková Department: Institute of Physics CAS, v.v.i. Supervisor: doc. RNDr. Jana Kalbáčová Vejpravová, Ph.D., Institute of Physics CAS, v.v.i. Abstract: The thesis is focused on the correlation of the magnetic response of iron oxide nanoparticles (NPs) with their internal structure. Several complementary methods were used and compared that bring insight into the relative crystallinity of the investigated NPs. The main goal was devoted to the elucidation of the origin of the so-called spin canting angle determined by In-field Mössbauer Spectroscopy (IFMS) by examination of samples with different internal structure. It has been observed that the IFMS is not an unambiguous method to study the surface effects in the NPs as the IFMS is sensitive only to the average value of all spins and does not distinguish between the surface and core effects. Moreover, the IFMS was performed on the epsilon phase of the iron(III) oxide NPs in order to inspect the peculiar behavior of this phase in an external magnetic field. Keywords: iron oxide nanoparticles, magnetism, In-field Mössbauer Spectroscopy, spin canting
3	Hollow Magnetic Nanoparticles : experimental and numerical studies / Nanoparticules magnétiques creuses : études expérimentale et numérique Sayed, Fatima 16 December 2016 (has links) Cette thèse concerne l'étude des propriétés structurales et magnétiques de nanoparticules magnétiques creuses (HMNPs), coquille et coquille/coquille. Les effets de surface sont exaltés de par la présence des surfaces interne et externe. L'étude expérimentale de HMNPs basée sur des mesures magnétiques et de spectrométrie Mössbauer du 57Fe a montré une structure magnétique complexe. Les HMNPs ayant une épaisseur ultrafine présentent une structure magnétique décrite par 2 sous-réseaux spero-magnétiques opposés, en plus de la présence d’un champ d'échange bias significatif. L'effet de la taille et de l'épaisseur des HMNPs a été également étudié. Les spectres Mössbauer obtenus sous champ magnétique montrent que la structure magnétique est fortement corrélée au rapport surface/volume. Ces résultats expérimentaux ont été confirmés par simulation Monte Carlo. Après optimisation du modèle, l’approche numérique montre d’abord que l'anisotropie de surface Ks gouverne le comportement magnétique des HMNPs et ensuite que la valeur critique de Ks nécessaire pour obtenir une configuration radiale (spike) diminue lorsque la taille des HMNPs augmente. L'étude numérique menée pour différentes tailles et épaisseurs de coquille, a permis de suivre leurs effets sur la structure magnétique des HMNPs. Par ailleurs, l'étude expéri-mentale menée sur des HMNPs shell/shell, montre que le désordre des spins et le champ d'échange bias deviennent plus importants lorsque les HMNPs sont recouvertes d’une coquille antiferromagnétique (NiO). De ces résultats, on peut déduire l'effet du désordre des spins sur les phénomènes d'échange bias dans un tel système. / This thesis concerns the study of structural and magnetic properties of hollow magnetic nanoparticles (HMNPs), shell and shell/shell. These HMNPs present enhanced surface effects resulting from the presence of both inner and outer surface layers. The experimental investigation combining magne-tic measurements and 57Fe Mössbauer spectrometry of such HMNPs has revealed a complex spin magnetic structure. Small HMNPs with ultrathin thickness show highly disordered magnetic structure and the corresponding in-field hyperfine structure can be described by means of 2 speromagnetic antiferromagnetically coupled, in addition to the significant exchange bias phenomenon. The in-field Mössbauer study of the effect of size and thickness of HMNPs shows that the spin disorder is strongly correlated to the surface to volume ratio. Those experimental magnetic behaviors were confirmed using Monte Carlo simulation. Indeed, after improving the numeric model, it is concluded that surface anisotropy Ks has a dominant role in the magnetic behavior of HMNPs and the value of critical Ks necessary to obtain radial (spike) configuration decreases as the size of HMNPs increases, keeping the same thickness. The numeric study for different sizes and shell thicknesses allows the effect of these parameters on the spin structure of HMNPs to be followed. Then, the experi-mental study extended to shell/shell HMNPs indicates that the spin disorder is enhanced in HMNPs with antiferromagnetic shell (NiO) in addition to larger exchange bias field. From those results, one can try to deduce the effect of spin disorder on the exchange bias phenomena in such system. Nanoparticules magnétiques creuses Structure coquille/coquille Oxyde de fer, NiO Anisotropie et effet de surface Couplage d'échange bias Spectrométrie Mössbauer du 57Fe Mesures magnétiques Simulation Monte Carlo Hollow magnetic nanoparticles (HMNPs) Shell/shell structure Iron oxide, NiO Surface anisotropy and surface effects Spin canting and spin disorder Exchange bias effect 57Fe Mössbauer spectrometry Magnetic measurements 538.4
4	Theoretical investigation of size effects in multiferroic nanoparticles Allen, Marc Alexander 05 August 2020 (has links) Over the last two decades, great progress has been made in the understanding of multiferroic materials, ones where multiple long-range orders simultaneously exist. However, much of the research has focused on bulk systems. If these materials are to be incorporated into devices, they would not be in bulk form, but would be miniaturized, such as in nanoparticle form. Accordingly, a better understanding of multiferroic nanoparticles is necessary. This manuscript examines the multiferroic phase diagram of multiferroic nanoparticles related to system size and surface-induced magnetic anisotropy. There is a particular focus on bismuth ferrite, the room-temperature antiferromagnetic-ferroelectric multiferroic. Theoretical results will be presented which show that at certain sizes, a bistability develops in the cycloidal wavevector. This implies bistability in the ferroelectric and magnetic moments of the nanoparticles. This novel magnetoelectric bistability may be of use in the creation of an electrically-written, magnetically-read memory element. / Graduate bismuth ferrite bfo nanoparticle nanomagnetism multiferroics optimization magnetism ferroelectric bistability anisotropy surface anisotropy weak ferromagnetism antiferromagnetism Dzyaloshinskii-Moriya interaction cycloid numerical methods condensed matter solid state physics magnetoelectric effect room-temperature multiferroic superexchange magnetic memory spin canting g-type antiferromagnetism Heisenberg Hamiltonian Nelder-Mead L-BFGS-B mathematica python ferroelectricity perovskite antiferromagnet bifeo3 rhombohedral pseudocubic r3c trigonal physics magnetic memory electrically-written memory curie temperature néel temperature materials science

1

Page generated in 0.0873 seconds