Tesis doctoral de Miroslavna Kovylina
The present work is devoted to the study of the exchange bias (eb) effect in ni/fef2 heterostructures, both unpatterned and nanostructured by focused lon beam (fib) lithography. the eb effect, discovered more than 50 years ago, still attracts considerable attention due to the incomplete understanding of its microscopic origin and its tremendous utility in magnetic recording and magnetic sensors in general. This ph.D. Thesis describes how the exciting ability of fib to structure a given material at the nanoscale level, combined with the high sensitivity of magnetoresistance measurements (mr) to local configurations of the magnetic moments, open new possibilities forgaining deeper insights into the open questions in eb, for example, about the size and distribution of the uncompensated pinned af spins (eb domains) or the magnetization reversal mechanisms. The present work also describes the results of extensive micromagnetic simulations proving our hypothesis regarding certain experimentally observed eb phenomenology. chapter l reviews the most relevant studies about systems with so-called positive and double hysteresis loops, in addition to providing a general introduction to the eb phenomenon and motivation to use the nanostructuring tools. Chapter 2 is devoted to the detailed explanation of the experimental set-up developed during this ph.D. Thesis and used for the mr measurements: an electrical characterization system (lcr/dc) equipped with a closed cycle he cryostat and electromagnet. Chapter 3 describes the samples preparation and mr results for ni/fef2 thin films, including the striking mirror symmetry between negative and positive eb observed in the magnetization reversal mechanisms and coexistence of eb domains of opposite sign, which allows explaining the experimental mr curves. The micromagnetic simulations of the magnetization reversal in ni thin films coupled to a frozen af layer, either with one or two signs for the eb domains, are presented in chapter 4. The fib fabrication of ordered arrays of antidots in exchange biased ni/fef2 bilayers are described in chapter 5. Finally, chapter 6 explains how the introduction of increasing densities of antidot arrays into ni/fef2 bilayers leads to the systematic tuning of negative and positive eb, favoring the onset of positive exchange bias at lower cooling fields due to non-interfacial magnetic moments created at the antidot faces.
Datos académicos de la tesis doctoral «Exchange bias in ni/fef2 thin films and nanoestructures«
- Título de la tesis: Exchange bias in ni/fef2 thin films and nanoestructures
- Autor: Miroslavna Kovylina
- Universidad: Barcelona
- Fecha de lectura de la tesis: 31/03/2011
Dirección y tribunal
- Director de la tesis
- Xavier Batlle Gelabert
- Tribunal
- Presidente del tribunal: amílcar Labarta rodríguez
- Fernando Bartolomé usieto (vocal)
- (vocal)
- (vocal)