| 1. |
- Wang, Duo
(author)
-
Ab initio studies of advanced functional materials with complex magnetism
- 2022
-
Doctoral thesis (other academic/artistic)abstract
- For centuries, magnetism of materials has been an inevitable part of human civilization. Only in the last century, the mysteries of magnetism started to unfold thanks to the development of quantum theory of solids. Nevertheless, even today, new exotic phenomena related to magnetism keep on surprising us and provide an enormous playground for theoreticians and experimentalists to unravel the complexities. In this thesis, the magnetic properties of materials are studied from different aspects by using first-principle density functional theory. Specifically, we investigated the substituted quadruple perovskite compounds ACu2Fe2Re2O12 (A=Ca, Sr, Ba, Pb, Sc, Y, La). Seven different A-site doped structures are studied, including divalent and trivalent charge substitutions. We found that all these compounds are half-metallic ferrimagnets with large magnetization and high transition temperatures (above 405K). Interestingly, the trivalent atom doping at the A-site can significantly increase the transition temperature. The exchange mechanism is explained by the super-exchange in the Re-Cu and Re-Fe pairs. Moreover, we investigated three different two-dimensional magnets, CrI3, FeS2, and CrO. For the first project, we studied stacking dependent magnetic properties of CrI3. It was found that the magnetic ground state can be tuned by the stacking sequences. In the second project, we studied the monolayer FeS2. The results show that the structures with FM and AFM configuration are close in energy. By performing further spin-spiral calculations, we found that the ground state magnetic configurations are different with different crystal structures. This structure dependent magnetic property indicates the existence of spin-lattice coupling in this material. In the third project, we predicted a monolayer CrO, which is a Weyl semimetal with antiferromagnetism up to room temperature. Finally, a heterostructure structure with G-type SrMnO3 supported on SrTiO3 substrate is investigated. We found that with a 2.9% tensile strain introduced by the substrate, the SrMnO3 keeps as G-type AFM. Moreover, oxygen vacancy intends to stay at the surface. Interestingly, this vacancy induces the AFM-FM transition on the specific layer due to the double exchange mechanism.
|
|
| 2. |
- Maltoni, Pierfrancesco, 1994-
(author)
-
Design, Synthesis And Characterization Of Magnetic Ferrite Nanostructures : Toward Novel Permanent Magnets
- 2023
-
Doctoral thesis (other academic/artistic)abstract
- Magnetic oxide nanoparticles (NPs) may interact with each-other for example via dipolar or superexchange interactions, depending whether they are in direct contact. These interparticle interactions yield both ferro- and/or antiferromagnetic coupling and modify the energy barrier of the magnetic particle, depending upon the strength of the coupling and orientation of the particles. The corresponding perturbation of the magnetic order can readily be investigated by measuring the dc-magnetization or/and the ac-susceptibility of the samples as a function of the temperature or magnetic field. Furthermore, remanence plots, First Order Reversal Curves (FORCs) analysis and magnetic relaxation measurements are ideal methods to investigate reversal mechanisms and magnetic interactions. As we show in this thesis for a set of reference nanoparticle systems comprising magnetically hard/soft ferrites, the strong interaction regime leads to interesting phenomena, including collective dynamics, exchange bias-like hysteresis loop shifts, and interface-mediated exchange-coupling of hard and soft phases. The strength of the interparticle interactions has been investigated for a set of dense assemblies of equally sized magnetically soft maghemite NPs coated with different fractions of oleic acid layer, and compared to the dilute case of silica-coated NPs. When hard exchange-biased Co-doped maghemite NPs are mixed with unbiased soft particles with equal size, we observe that dipolar interactions yield a horizontal magnetic hysteresis loop shift. We observe that the measured hysteresis bias of this system is larger than that of the exchange bias of the unmixed Co-doped particles, and assign the extra contribution to have dipolar origin ("dipolar bias"). A more complex scenario is reported for hard/soft nanostructured powder systems of Sr and Co ferrite whose morphology (epitaxial texture or lacking coherence) strongly alter the existing interparticle magnetic interactions, and in turn the reversal process of magnetization: the magnetic coherence length scales have been estimated and thus limit for rigid coupling uncovered. Doping strategies by chemical substitution with diamagnetic cations have been also investigated, to tailor the hard/soft properties: eventually, the plasma sintered compacted ferrite composites exhibit a larger energy product compared to the single phased components, establishing a strategy to produce permanent magnets with large coercivity. We believe that our studies provide new and useful knowledge into the role of magnetic interactions at the nanoscale.
|
|
