The region between the infrared and microwave region in the electromagnetic spectrum, the Terahertz (THz) gap, provides an exciting opportunity for future wireless communications as this band has been under utilised. This doctoral work takes a two-pronged approach into closing the THz gap with low-dimensional materials. The first attempt addresses the need for a compact THz source that can operate at room temperature. The second approach addresses the need to build optical elements such as filters and modulators in the THz spectrum. Terahertz quantum cascade lasers (THz QCLs) are one of the most compact, powerful sources of coherent radiation that bridge the terahertz gap. However, their cryogenic requirements for operation limit the scope of the applications. This is because of the electron-electron scattering and heating of the 2-dimensional free electron gas which leads to significant optical phonon scattering of the hot electrons. Theoretical studies in laterally confined QCL structures have predicted enhanced lifetime of the upper state through suppression of the non-radiative intersubband relaxation of carriers, which leads to lower threshold, and higher temperature performance. Lithographically defined vertical nanopillar arrays with electrostatic radius less than tens of nm offer a possible route to achieve lateral confinement, which can be integrated into QCL structures. A typical gain medium in a QCL consists of at least 100 repeat periods, with a thickness of 6-14 micron. For practical implementation of the top-down approach, restrictions are imposed by aspect ratios that can be achieved in present dry-etching systems. Typically, for sub-200 nm radius pillars, the thickness ranges from 1-3.5 micron. It is therefore necessary to work with THz QCLs based on 3-4 quantum well active regions, so as to maximise the number of repeat periods (hence gain) within an ultra-thin active region. After an introductory chapter, Chapter 2 presents a theoretical treatise on the realistic electrostatic potential in a lithographically defined nanopillar by scaling from a single quantum well (resonant tunnelling diode) to a THz QCL. Chapter 2 also discusses, the effect of lateral confinement on the intersubband states and the plasmonic mode in a THz QCL. One of the key experimental challenges in scaling down from QCLs to quantum-dot cascade lasers is the electrical injection into the nanopillars. This involves insulation and planarisation of the high aspect-ratio nanopillar arrays. Furthermore, the choice of the planarising layer is critical since it determines the loss of any optical mode. This experimental challenge is solved in Chapter 3. Chapter 4 presents the electro-optic performance of low-repeat period QCLs with an active region thickness that is less than 3.5 micron. Another topic of recent interest in the THz optics community is plasmonics in graphene. This is because the bound electromagnetic modes (plasmons) are tightly confined to the surface and can also be tuned with carrier concentration. Plasmonic resonance at terahertz frequencies can be achieved by gating graphene grown via chemical vapour deposition (CVD) to a high carrier concentration. THz time domain spectroscopy of such gated monolayer graphene shows resonance features around 1.6 THz superimposed on the Drude-like frequency response of graphene which may be related to the inherent poly-crystallinity of CVD graphene. Chapter 5 discusses these results, as an understanding of these features is necessary for the development of future THz optical elements based on CVD graphene. Chapter 5 finally describes how the gate tunability of THz transmission through graphene can be exploited to indirectly modulate a THz QCL. Chapter 6 presents ideas from this doctoral work, which can be developed in future to address the issues of enhanced temperature performance of THz QCLs and to realise realistic THz devices based on graphene.

Στην παρούσα διδακτορική διατριβή μελετάται η ανάπτυξη λεπτών υμενίων πορωδών ανοδικών οξειδίων του αλουμινίου (αναφέρονται και ως πορώδης ανοδική αλουμίνα) σε υπόστρωμα πυριτίου. Επιπλέον, εξετάζεται η ανάπτυξη εφαρμογών που αφορούν τη χρήση της πορώδους αλουμίνας ως μάσκα και ως μήτρα για την δημιουργία νανονημάτων ή κβαντικών τελειών (νανονησίδων) στο Si. Το πρώτο κεφάλαιο πραγματεύεται τη θεωρία και τους μηχανισμούς που διέπουν την ανάπτυξη πορωδών υμενίων, που προέρχονται από ανοδική οξείδωση (ανοδίωση) τόσο φύλλων αλουμινίου, όσο και υμενίων αλουμινίου σε υπόστρωμα πυριτίου. Επιπροσθέτως, παρατίθεται ο ρόλος που διαδραματίζουν οι ηλεκτροχημικές συνθήκες ανοδίωσης, όπως το pH, η θερμοκρασία και η εφαρμοζόμενη τάση, στα τελικά δομικά χαρακτηριστικά των πορωδών υμενίων. Στο δεύτερο κεφάλαιο παρουσιάζονται τα τεχνολογικά βήματα διεργασιών που αφορούν την προετοιμασία των δειγμάτων τα οποία πρόκειται να ανοδιωθούν, και δίνονται λεπτομέρειες για την πειραματική διάταξη η οποία χρησιμοποιείται κατά την ανοδίωση. Στο τρίτο κεφάλαιο μελετώνται εκτενώς, τρεις παράγοντες που έχουν σημαντική επίδραση στα τελικά δομικά χαρακτηριστικά των πορωδών υμενίων. Κατά τους δύο πρώτους, εξετάζεται η επίδραση του πάχους του προς ανοδίωση υμενίου αλουμινίου πάνω στο Si, καθώς και ο περιορισμός του σε επιφάνειες μερικών τετραγωνικών μικρομέτρων πάνω στο Si, στο μέγεθος και την πυκνότητα των πόρων. Ο τρίτος παράγοντας αφορά το ρόλο της ανοδίωσης του υμενίου του αλουμινίου σε δύο και τρία στάδια σε συνδυασμό με τη χημική εγχάραξή του μετά από κάθε στάδιο ανοδίωσης, στην ανάπτυξη εξαγωνικής συμμετρίας στην κατανομή των πόρων. Το τέταρτο κεφάλαιο, πραγματεύεται την ανάπτυξη εφαρμογών που συνδέονται με τη χρήση της πορώδους αλουμίνας ως μάσκα και ως μήτρα για τη δημιουργία νανοδομών επάνω στο πυρίτιο. Ως εκ τούτου παρουσιάζεται η δημιουργία νανονησίδων Cr, Ti, νανοστηλών Si, και νανονημάτων Au, πάνω στο Si, εφαρμογές στις οποίες τα πορώδη ανοδικά υμένια χρησιμοποιήθηκαν ως ενδιάμεσο στάδιο. Στο πέμπτο κεφάλαιο παρατίθεται η ανάπτυξη διαμέσου της πορώδους αλουμίνας, εξαγωνικά διατεταγμένων νανονησίδων SiO2 στο Si. Επίσης, παρουσιάζεται ο ηλεκτρικός χαρακτηρισμός διατάξεων οι οποίες αποτελούνται από την εν λόγω δομή. Σε ένα επιπλέον βήμα, οι νανονησίδες SiO2 χρησιμοποιούνται για την ανάπτυξη νανοκρυσταλλιτών Si στο εσωτερικό τους μέσω της τεχνικής της ιοντικής σύνθεσης. Τα σημαντικότερα αποτελέσματα και συμπεράσματα που προέκυψαν από την εκπόνηση της παρούσας διδακτορικής διατριβής συνοψίζονται στα εξής: • Βελτίωση της εξαγωνικής συμμετρίας στην κατανομή των πόρων, μέσω ανοδίωσης σε δύο ή τρία στάδια σε συνδυασμό με χημική εγχάραξη του προς ανοδίωση αλουμινίου έπειτα από κάθε στάδιο ανοδίωσης. • Αύξηση της πυκνότητας των πόρων των ανοδικών υμενίων κατά μία τάξη μεγέθους, με περιορισμό του προς ανοδίωση αλουμινίου σε προεπιλεγμένες περιοχές στο Si, επιφάνειας μερικών τετραγωνικών μικρομέτρων. • Ανάπτυξη διατεταγμένων νανοδομών Ti και Cr σε υπόστρωμα Si χρησιμοποιώντας λεπτά υμένια πορώδους αλουμίνας πάνω σε Si. Ιδιαίτερα οι δομές Cr, μπορούν να χρησιμοποιηθούν ως μεταλλική νανοδομημένη μάσκα για την εγχάραξη με ενεργά ιόντα του υποστρώματος Si και τη δημιουργία νανοστηλών Si πάνω σε αυτό. Η δημιουργία νανοστηλών Si, βρίσκει πληθώρα εφαρμογών στη Νανοηλεκτρονική, σε αισθητήρες, Nανοφωτονική, μνήμες κ.τ.λ. • Οι πυκνότητες διεπιφανειακών καταστάσεων που προέκυψαν από τον ηλεκτρικό χαρακτηρισμό της διεπιφάνειας υμενίων πορώδους αλουμίνας με το πυρίτιο, και της διεπιφάνειας πορώδους αλουμίνας – νανονησίδων SiO2 με το πυρίτιο. Οι τιμές που υπολογίστηκαν είναι ενθαρρυντικές, αν ληφθεί υπόψη ο ηλεκτροχημικός τρόπος παρασκευής των εν λόγω σύνθετων υμενίων πάνω στο πυρίτιο. • Ανάπτυξη μεμονωμένων νανοκρυσταλλιτών Si ενσωματωμένων σε νανονησίδες SiO2. Για το σκοπό αυτό συνδυάστηκαν δύο διαφορετικές τεχνολογίες, εκείνη της ιοντικής σύνθεσης και εκείνη της ανάπτυξης νανονησίδων SiO2 διαμέσου λεπτών υμενίων πορώδους αλουμίνας απευθείας σε υπόστρωμα Si. Τέτοιες δομές νανοκρυσταλλιτών έχουν εφαρμογές σε διατάξεις μη πτητικών μνημών, όπου η κατανεμημένη αποθήκευση φορτίου στους νανοκρυσταλλίτες ευνοεί τη χρήση λεπτότερων οξειδίων πύλης και τη δυνατότητα σμίκρυνσης του πάχους των οξειδίων αυτών χωρίς να μειώνεται ο χρόνος αποθήκευσης φορτίου. / In the present thesis, the growth of porous anodic alumina films on Si substrate was studied extensively. Potential applications of porous anodic alumina films formed directly on Si, regarding the use of porous membranes as mask or template for various nanostructures growth directly on Si, are discussed. Chapter one deals with the theory and mechanisms governing porous anodic alumina film growth, either on porous anodic films formed by anodization of aluminum foils, or on porous anodic films developed on Si substrates. Additionally, the effect of different factors (pH, temperature, applied voltage) on the final structural characteristics is presented. In chapter two, the preliminary processing steps regarding sample preparation before the anodization procedure are quoted. Moreover, details about the experimental set-up and the electrochemical conditions used during the sample anodization in the current work are given. In chapter three, the influence of three different factors, in the final structural characteristics, is investigated. Primarily, the impact of the initial aluminum thickness deposited on Si substrate, and secondly the confinement of the aluminum film in areas of a few μm2, in the pore size and pore density are studied. Finally, the influence of the third factor is associated with a three-step instead of a two-step anodization, in combination with an in-between step of aluminum chemical etching, on the ordering and the uniformity of the pores. The deposition of Ti and Cr nanodots arrays on Si, using the porous alumina membrane as a masking layer, is investigated in chapter four. Furthermore, the Ti nanodots are used for the electrodeposition of Au nanodots and nanowires inside the porous alumina films. Additionally, the Cr dots are used as metallic nanostructured mask for the Si etching by reactive ion etching process, that leads to the formation of Si nanopillars on Si substrate. In chapter five the growth of hexagonally ordered SiO2 dots on Si through porous anodic alumina membranes, in various acidic electrolytes, is studied. Moreover, the electrical characterization of the interface of porous alumina film/Si and porous alumina film with SiO2 dots in pore bottoms/ Si is presented. Finally, in the present thesis the technology of fabrication of Si nanocrystals embedded in SiO2 dots arrays through porous alumina membranes on Si substrate is developed for the first time. This was achieved by the combination of ion beam synthesis with the already existing technology of porous anodic alumina growth on Si substrates. The nanocrystals are electrically isolated from the substrate. This technique is promising as an application in non-volatile memory devices. The main achievements accomplished through this study are summarized as follows: • The optimization of pores ordering by developing the porous alumina membrane in two or three processing steps in combination with the chemical etching of Al film, lying above the porous membrane, following each anodization cycle. • The increase of porous density by the confinement of porous alumina film in areas of a few μm2 on Si. • The development of Ti, Cr and nanodots arrays, directly on Si, through porous alumina membranes. The use of Cr nanodots as nanostructured masking layer for the formation of Si nanopillars, formed by etching of Si substrate with RIE, on Si. • The density of interface stages results from the electrical characterization of porous alumina with or without SiO2 dots at each pore bottom, with the Si substrate. The results are encouraging, keeping in mind that the pore membranes and SiO2 dots were electrochemically grown directly on Si substrate. • The development of distinct Si nanocrystalls, embedded in SiO2 dots, combining for the first time two different technologies, that is the fabrication of porous anodic alumina films directly on Si substrate, as well as the ion beam synthesis technique. The proposed technique is promising for the fabrication of non-volatile memory devices.

III-V nanostructures have attracted substantial research effort due to their interesting physical properties and their applications in new generation of ultrafast and high efficiency nanoscale electronic and photonic components. The advances in nanofabrication methods including growth/synthesis have opened up new possibilities of realizing one dimensional (1D) nanostructures as building blocks of future nanoscale devices. For processing of semiconductor nanostructure devices, simplicity, cost effectiveness, and device efficiency are key factors. A number of methods are being pursued to fabricate high quality III-V nanopillar/nanowires, quantum dots and nano disks. Further, high optical quality nanostructures in these materials together with precise control of shapes, sizes and array geometries make them attractive for a wide range of optoelectronic/photonic devices. This thesis work is focused on top-down approaches for fabrication of high optical quality nanostructures in III-V materials. Dense and uniform arrays of nanopillars are fabricated by dry etching using self-assembly of colloidal SiO2 particles for masking. The physico-chemistry of etching and the effect of etch-mask parameters are investigated to control the shape, aspect ratios and spatial coverage of the nanopillar arrays. The optimization of etch parameters and the utilization of erosion of etch masks is evaluated to obtain desired pillar shapes from cylindrical to conical. Using this fabrication method, high quality nanopillar arrays were realized in several InP-based and GaAs-based structures, including quantum wells and multilayer heterostructures. Optical properties of these pillars are investigated using different optical spectroscopic techniques. These nanopillars, single and in arrays, show excellent photoluminescence (PL) at room temperature and the measured PL line-widths are comparable to the as-grown wafer, indicating the high quality of the fabricated nanostructures. The substrate-free InP nanopillars have carrier life times similar to reference epitaxial layers, yet an another indicator of high material quality. InGaAs layer, beneath the pillars is shown to provide several useful functions. It effectively blocks the PL from the InP substrate, serves as a sacrificial layer for generation of free pillars, and as a “detector” in cathodoluminescence (CL) measurements. Diffusion lengths independently determined by time resolved photoluminescence (TRPL) and CL measurements are consistent, and carrier feeding to low bandgap InGaAs layer is evidenced by CL data. Total reflectivity measurements show that nanopillar arrays provide broadband antireflection making them good candidates for photovoltaic applications.  A novel post etch, sulfur-oleylamine (S-OA) based chemical process is developed to etch III-V materials with monolayer precision, in an inverse epitaxial manner along with simultaneous surface passivation. The process is applied to push the limits of top-down fabrication and InP-based high optical quality nanowires with aspect ratios more than 50, and nanostructures with new topologies (nanowire meshes and in-plane wires) are demonstrated.  The optimized process technique is used to fabricate nanopillars in InP-based multilayers (InP/InGaAsP/InP and InP/InGaAs/InP). Such multilayer nanopillars are not only attractive for broad-band absorption in solar cells, but are also ideal to generate high optical quality nanodisks of these materials. Finally, the utility of a soft stamping technique to transfer free nanopillars/wires and nanodisks onto Si substrate is demonstrated. These nanostructures transferred onto Si with controlled densities, from low to high, could provide a new route for material integration on Si. / <p>QC 20130205</p>

Ao passar por uma fina camada magnética uma corrente spin polarizada pode produzir um efeito de torque clássico atuando na camada, sendo capaz de gerar precessão e reversão da magnetização. Esse efeito tem sido alvo de inúmeras pesquisas, em especial pela possibilidade de aplicações em memórias magnéticas não voláteis e em nano-osciladores de alta frequência, entretanto outras características podem ser exploradas. Em particular, devido ao seu caráter não-linear, torques de spin aplicados em camadas magnéticas podem fornecer condições para aparecimento de caos determinístico e ressonância estocástica. Caos determinístico pode ocorrer em sistemas dinâmicos contínuos que tenham ao menos três graus de liberdade. Nesse caso, mesmo que apenas termos determinísticos sejam considerados, a combinação de termos não-lineares e alta sensibilidade em relação a condições iniciais ou pequenas perturbações pode gerar irregularidade e imprevisibilidade no sistema. Ressonância estocástica é o nome que se dá para fenômenos em que a adição de ruído a um sistema pode melhorar a resposta do mesmo, existindo um nível ótimo de ruído. Esse fenômeno pode ser usado para detecção e amplificação de sinais de baixa intensidade, por exemplo. Aqui analisamos a dinâmica da magnetização da camada livre de junções magnéticas em geometrias do tipo nanopilar, com o estudo dividido em dinâmicas determinísticas e estocásticas. Dentro da análise apenas com termos determinísticos, buscamos verificar comportamentos regulares, irregulares e caóticos, caracterizando o sistema através da geração de diagramas com as fases dinâmicas para diferentes valores de parâmetros. Foram vistas duas geometrias diferentes, sendo que em uma delas foi possível fazer a caracterização completa das fases dinâmicas do sistema. No caso de dinâmicas estocásticas, buscamos explorar efeitos não-lineares e flutuações térmicas, analisando ressonância estocástica e sincronização facilitada por ruído em uma junção túnel magnética, além de estudar as respostas dinâmicas quando há apenas o torque de Slonczewski e quando também está presente o torque tipo campo. Foi possível observar a influência de diversos parâmetros, como a amplitude da corrente aplicada e a frequência de entrada, na resposta magnética e na sincronização de dispositivos estocásticos. Além disso, vimos que com a inclusão do torque tipo campo aparece um possível novo comportamento, similar à ressonância, em alta frequência, ainda não detectado experimentalmente. Esses resultados são importantes pela possibilidade de uso desses dispositivos spintrônicos em transmissão segura de dados, comunicação em alta frequência e em uma nova geração de dispositivos bio-inspirados e eficientes energeticamente. / When passing through a fine magnetic layer a spin polarized electric current may result in a classical torque acting on the layer, being capable of causing magnetization precession and reversal. This effect has been object of numerous researches, specially because of possible applications in non-volatile magnetic memories and high frequency nanooscillators. However, other characteristics can be exploited. In particular, because of its non-linear features, spin torques acting on magnetic layers can generate the conditions for deterministic chaos and stochastic resonance to arise. Deterministic chaos may happen in continuous nonlinear dynamical systems with at least three degrees of freedom. In this case, even if only deterministic terms are considered, the combination of nonlinearities with high sensitivity on initial conditions or small perturbations can produce irregularity and unpredictability in the dynamical behaviour. Stochastic resonance is the phenomenon in which the addition of noise in a system can produce a better output, or system response, existing an optimal noise level. This effect can be used as a way to detect and amplify low intensity signal, for example. In this PhD Thesis we study the magnetization dynamics on the free layer of magnetic junctions in nanopillar geometries. The work is divided into two parts: deterministic and stochastic dynamics. When analysing the deterministic case we tried to characterize regular, irregular and chaotic behaviours, producing dynamical phases diagrams for different system parameters. Two different geometries were analysed, being possible to generate a complete characterization of the dynamical phases in one of them. For the stochastic case we tried to explore nonlinear effects and thermal fluctuations, analysing stochastic resonance and noise-enhanced synchronization in a magnetic tunnel junction and studying the dynamical response when only one spin torque is considered, the Slonczewski torque, and also when a perpendicular torque, the field-like torque, is present. We were able to see the influence of several system parameters, such as the amplitude of the applied electric current and the input frequency, on the system response and on the synchronization of stochastic systems. Also, we noticed that with the inclusion of the field-like torque a possibly new high frequency resonance-like behaviour appears. These results are important because of the possibility of using new spintronic devices for secure data transmission, high frequency communications and on a new generation of bio-inspired devices.

Ao passar por uma fina camada magnética uma corrente spin polarizada pode produzir um efeito de torque clássico atuando na camada, sendo capaz de gerar precessão e reversão da magnetização. Esse efeito tem sido alvo de inúmeras pesquisas, em especial pela possibilidade de aplicações em memórias magnéticas não voláteis e em nano-osciladores de alta frequência, entretanto outras características podem ser exploradas. Em particular, devido ao seu caráter não-linear, torques de spin aplicados em camadas magnéticas podem fornecer condições para aparecimento de caos determinístico e ressonância estocástica. Caos determinístico pode ocorrer em sistemas dinâmicos contínuos que tenham ao menos três graus de liberdade. Nesse caso, mesmo que apenas termos determinísticos sejam considerados, a combinação de termos não-lineares e alta sensibilidade em relação a condições iniciais ou pequenas perturbações pode gerar irregularidade e imprevisibilidade no sistema. Ressonância estocástica é o nome que se dá para fenômenos em que a adição de ruído a um sistema pode melhorar a resposta do mesmo, existindo um nível ótimo de ruído. Esse fenômeno pode ser usado para detecção e amplificação de sinais de baixa intensidade, por exemplo. Aqui analisamos a dinâmica da magnetização da camada livre de junções magnéticas em geometrias do tipo nanopilar, com o estudo dividido em dinâmicas determinísticas e estocásticas. Dentro da análise apenas com termos determinísticos, buscamos verificar comportamentos regulares, irregulares e caóticos, caracterizando o sistema através da geração de diagramas com as fases dinâmicas para diferentes valores de parâmetros. Foram vistas duas geometrias diferentes, sendo que em uma delas foi possível fazer a caracterização completa das fases dinâmicas do sistema. No caso de dinâmicas estocásticas, buscamos explorar efeitos não-lineares e flutuações térmicas, analisando ressonância estocástica e sincronização facilitada por ruído em uma junção túnel magnética, além de estudar as respostas dinâmicas quando há apenas o torque de Slonczewski e quando também está presente o torque tipo campo. Foi possível observar a influência de diversos parâmetros, como a amplitude da corrente aplicada e a frequência de entrada, na resposta magnética e na sincronização de dispositivos estocásticos. Além disso, vimos que com a inclusão do torque tipo campo aparece um possível novo comportamento, similar à ressonância, em alta frequência, ainda não detectado experimentalmente. Esses resultados são importantes pela possibilidade de uso desses dispositivos spintrônicos em transmissão segura de dados, comunicação em alta frequência e em uma nova geração de dispositivos bio-inspirados e eficientes energeticamente. / When passing through a fine magnetic layer a spin polarized electric current may result in a classical torque acting on the layer, being capable of causing magnetization precession and reversal. This effect has been object of numerous researches, specially because of possible applications in non-volatile magnetic memories and high frequency nanooscillators. However, other characteristics can be exploited. In particular, because of its non-linear features, spin torques acting on magnetic layers can generate the conditions for deterministic chaos and stochastic resonance to arise. Deterministic chaos may happen in continuous nonlinear dynamical systems with at least three degrees of freedom. In this case, even if only deterministic terms are considered, the combination of nonlinearities with high sensitivity on initial conditions or small perturbations can produce irregularity and unpredictability in the dynamical behaviour. Stochastic resonance is the phenomenon in which the addition of noise in a system can produce a better output, or system response, existing an optimal noise level. This effect can be used as a way to detect and amplify low intensity signal, for example. In this PhD Thesis we study the magnetization dynamics on the free layer of magnetic junctions in nanopillar geometries. The work is divided into two parts: deterministic and stochastic dynamics. When analysing the deterministic case we tried to characterize regular, irregular and chaotic behaviours, producing dynamical phases diagrams for different system parameters. Two different geometries were analysed, being possible to generate a complete characterization of the dynamical phases in one of them. For the stochastic case we tried to explore nonlinear effects and thermal fluctuations, analysing stochastic resonance and noise-enhanced synchronization in a magnetic tunnel junction and studying the dynamical response when only one spin torque is considered, the Slonczewski torque, and also when a perpendicular torque, the field-like torque, is present. We were able to see the influence of several system parameters, such as the amplitude of the applied electric current and the input frequency, on the system response and on the synchronization of stochastic systems. Also, we noticed that with the inclusion of the field-like torque a possibly new high frequency resonance-like behaviour appears. These results are important because of the possibility of using new spintronic devices for secure data transmission, high frequency communications and on a new generation of bio-inspired devices.

Ao passar por uma fina camada magnética uma corrente spin polarizada pode produzir um efeito de torque clássico atuando na camada, sendo capaz de gerar precessão e reversão da magnetização. Esse efeito tem sido alvo de inúmeras pesquisas, em especial pela possibilidade de aplicações em memórias magnéticas não voláteis e em nano-osciladores de alta frequência, entretanto outras características podem ser exploradas. Em particular, devido ao seu caráter não-linear, torques de spin aplicados em camadas magnéticas podem fornecer condições para aparecimento de caos determinístico e ressonância estocástica. Caos determinístico pode ocorrer em sistemas dinâmicos contínuos que tenham ao menos três graus de liberdade. Nesse caso, mesmo que apenas termos determinísticos sejam considerados, a combinação de termos não-lineares e alta sensibilidade em relação a condições iniciais ou pequenas perturbações pode gerar irregularidade e imprevisibilidade no sistema. Ressonância estocástica é o nome que se dá para fenômenos em que a adição de ruído a um sistema pode melhorar a resposta do mesmo, existindo um nível ótimo de ruído. Esse fenômeno pode ser usado para detecção e amplificação de sinais de baixa intensidade, por exemplo. Aqui analisamos a dinâmica da magnetização da camada livre de junções magnéticas em geometrias do tipo nanopilar, com o estudo dividido em dinâmicas determinísticas e estocásticas. Dentro da análise apenas com termos determinísticos, buscamos verificar comportamentos regulares, irregulares e caóticos, caracterizando o sistema através da geração de diagramas com as fases dinâmicas para diferentes valores de parâmetros. Foram vistas duas geometrias diferentes, sendo que em uma delas foi possível fazer a caracterização completa das fases dinâmicas do sistema. No caso de dinâmicas estocásticas, buscamos explorar efeitos não-lineares e flutuações térmicas, analisando ressonância estocástica e sincronização facilitada por ruído em uma junção túnel magnética, além de estudar as respostas dinâmicas quando há apenas o torque de Slonczewski e quando também está presente o torque tipo campo. Foi possível observar a influência de diversos parâmetros, como a amplitude da corrente aplicada e a frequência de entrada, na resposta magnética e na sincronização de dispositivos estocásticos. Além disso, vimos que com a inclusão do torque tipo campo aparece um possível novo comportamento, similar à ressonância, em alta frequência, ainda não detectado experimentalmente. Esses resultados são importantes pela possibilidade de uso desses dispositivos spintrônicos em transmissão segura de dados, comunicação em alta frequência e em uma nova geração de dispositivos bio-inspirados e eficientes energeticamente. / When passing through a fine magnetic layer a spin polarized electric current may result in a classical torque acting on the layer, being capable of causing magnetization precession and reversal. This effect has been object of numerous researches, specially because of possible applications in non-volatile magnetic memories and high frequency nanooscillators. However, other characteristics can be exploited. In particular, because of its non-linear features, spin torques acting on magnetic layers can generate the conditions for deterministic chaos and stochastic resonance to arise. Deterministic chaos may happen in continuous nonlinear dynamical systems with at least three degrees of freedom. In this case, even if only deterministic terms are considered, the combination of nonlinearities with high sensitivity on initial conditions or small perturbations can produce irregularity and unpredictability in the dynamical behaviour. Stochastic resonance is the phenomenon in which the addition of noise in a system can produce a better output, or system response, existing an optimal noise level. This effect can be used as a way to detect and amplify low intensity signal, for example. In this PhD Thesis we study the magnetization dynamics on the free layer of magnetic junctions in nanopillar geometries. The work is divided into two parts: deterministic and stochastic dynamics. When analysing the deterministic case we tried to characterize regular, irregular and chaotic behaviours, producing dynamical phases diagrams for different system parameters. Two different geometries were analysed, being possible to generate a complete characterization of the dynamical phases in one of them. For the stochastic case we tried to explore nonlinear effects and thermal fluctuations, analysing stochastic resonance and noise-enhanced synchronization in a magnetic tunnel junction and studying the dynamical response when only one spin torque is considered, the Slonczewski torque, and also when a perpendicular torque, the field-like torque, is present. We were able to see the influence of several system parameters, such as the amplitude of the applied electric current and the input frequency, on the system response and on the synchronization of stochastic systems. Also, we noticed that with the inclusion of the field-like torque a possibly new high frequency resonance-like behaviour appears. These results are important because of the possibility of using new spintronic devices for secure data transmission, high frequency communications and on a new generation of bio-inspired devices.