Τα τελευταία χρόνια υπάρχει εκτεταμένη πειραματική και θεωρητική μελέτη στην περιοχή των νανοσωματιδίων διότι βρίσκουν εφαρμογές σαν μέσα μαγνητικής εγγραφής και αποθήκευσης πληροφοριών και πιο πρόσφατα στην ιατρική. Στόχος των ερευνών είναι να κατασκευαστούν όσο το δυνατό μικρότερα σε μέγεθος νανοσωματίδια, στα οποία όμως οι μαγνητικές ιδιότητες να είναι σταθερές σε θερμοκρασία δωματίου, δηλαδή με μεγάλη ανισοτροπία. Τα σύνθετα νανοσωματίδια με μορφολογία σιδηρομαγνητικού πυρήνα/αντισιδηρομαγνητικού φλοιού εμφανίζουν μία επιπλέον ανισοτροπία την ανισοτροπία ανταλλαγής η οποία τα καθιστά θερμικά σταθερά σε μικρό μέγεθος. Στην παρούσα εργασία χρησιμοποιήθηκε η τεχνική Μetropolis Monte Carlo για την μελέτη της μαγνητικής συμπεριφοράς νανοσωματιδίων με μορφολογία πυρήνα/φλοιού τα οποία εμφανίζουν την ανισοτροπία ανταλλαγής. Η τεχνική αυτή έχει το πλεονέκτημα ότι μπορεί να περιλάβει στους υπολογισμούς τις λεπτομέρειες της μικροδομής του συστήματος και την θερμοκρασία. Στόχος της εργασίας μας ήταν με την ανάπτυξη κατάλληλου θεωρητικού μοντέλου να μελετηθεί ο μηχανισμός που προκαλεί την εμφάνιση της ανισοτροπίας ανταλλαγής και οι παράγοντες που επηρεάζουν τόσο την εμφάνιση της όσο και την ισχύ της. Βρήκαμε ότι η ανισοτροπία ανταλλαγής η οποία επάγεται από την αλληλεπίδραση ανταλλαγής κατά μήκος της σιδηρομαγνητική/αντισιδηρομαγνητική διεπιφάνειας οφείλεται στην ύπαρξη μη αντισταθμισμένων μαγνητικών ροπών στην διεπιφάνεια μεταξύ σιδηρομαγνητικού πυρήνα του νανοσωματιδίου και αντισιδηρομαγνητικού φλοιού και οι παράγοντες που επηρεάζουν το μέγεθος της είναι α)το πάχος του φλοιού, έχουμε εμφάνιση της μετά το δεύτερο στρώμα αντισιδηρομαγνητικού φλοιού β)το μέγεθος της σταθεράς ανταλλαγής στην διεπιφάνεια και λιγότερο στον φλοιό που ενισχύουν το φαινόμενο, γ)το και το είδος της ανισοτροπίας στον φλοιό, είναι πιο έντονη για ανισοτροπία z-άξονα στο φλοιό και δ)από το μέγεθος της ανισοτροπίας της διεπιφάνειας. Ένα άλλο φαινόμενο που συνδέεται με την εμφάνιση της ανισοτροπίας ανταλλαγής είναι η κάθετη μετατόπιση. Δηλαδή η ασσυμετρία του βρόχου υστέρησης των σύνθετων νανοσωματιδίων με μορφολογία πυρήνα φλοιού στον κάθετο άξονα της μαγνήτισης. Οι υπολογισμοί μας έδειξαν ότι σε αντίθεση με την ανισοτροπία ανταλλαγής η κάθετη μετατόπιση οφείλεται στην ύπαρξη του συνολικού αριθμού των μη αντισταθμισμένων μαγνητικών ροπών του φλοιού και όχι μόνο της διεπιφάνειας. Οι υπολογισμοί μας συγκρίθηκαν με πρόσφατα πειραματικά αποτελέσματα, και βρέθηκαν σε πολύ καλή συμφωνία. Επιβεβαιώνοντας ότι οι ανταγωνιστικές αλληλεπιδράσεις που οφείλονται στην ύπαρξη ανταγωνιστικών αλληλεπιδράσεων ανταλλαγής σε σύνθετα νανοσωματίδια με μορφολογία σιδηρομαγνητικού πυρήνα/αντισιδηρομαγνητικού φλοιού είναι υπεύθυνες για τη μεταβολή των μαγνητικών τους ιδιοτήτων σε σχέση με αυτές των σιδηρομαγνητικών νανοσωματιδίων. / The magnetic properties of nanoparticles have been subject of continuously growing interest, driven by fundamental research and technological interest especially in the magnetic recording industry and more recently in medicine. The small size of the nanoparticles and the reduced symmetry at their surface result in high coercivity. More recently, the requirement for stable magnetic behavior of the nanoparticles at room temperature led to the development of complex spin nanostructures with core/shell morphology that results to enhanced magnetic anisotropy. Composite nanoparticles with ferromagnetic core/ antiferromagnetic shell morphology have an extra anisotropy, the exchange anisotropy which makes them thermally stable in room temperature even in the case of very small size. In the current work we use the Metropolis Monte Carlo method to study the magnetic behaviour of nanoparticles with core/shell morphology which exhibit exchange anisotropy. The advantages of this method is that the microstructure of the nanoparticles is explicitly included and the temperature. The main goal of this thesis was to develop an appropriate theoretical model for the study of the origin of the exchange anisotropy and the parameters which affect its appearance and its magnitude. We find that the exchange anisotropy which is induced by the exchange interaction between ferromagnetic core and antiferromagnetic shell in the interface can is due to the existence of uncompensated bonds along the interface. The size of the exchange anisotropy depends on a) the shell thickness, at least two antiferromagnetic shell layers are necessary for the appearance of the effect, b) the magnitude of exchange interaction at the interface and at the shell influence the strength of the exchange anisotropy also c) the type of anisotropy in the antiferromagnetic shell, it is bigger for z-axis shell anisotropy and d) the magnitude of anisotropy at the interface. Another phenomenon which is related with exchange anisotropy is the vertical shift i.e. the asymmetry of the hysterisis loop on the vertical axis. Our calculations show that this shift depends on the total number of the uncompensated spins in the antiferromagnetic shell and not only from the interface. Our calculations are in good agreement with recent experimental results. This confirms that exchange interaction between ferromagnetic core and antiferromagnetic shell is responsible for the exchange anisotropy effect in these composite nanoparticles with ferromagnetic core/antiferromagnetic shell morphology that results to magnetic behaviour different from that of pure ferromagnetic nanoparticles.

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Made available in DSpace on 2014-12-17T15:15:01Z (GMT). No. of bitstreams: 1 MariaGDS_TESE.pdf: 10253325 bytes, checksum: a11a4b9893c49f999607f55737b5aded (MD5) Previous issue date: 2014-07-28 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The effect of confinement on the magnetic structure of vortices of dipolar coupled ferromagnetic nanoelements is an issue of current interest, not only for academic reasons, but also for the potential impact in a number of promising applications. Most applications, such as nano-oscillators for wireless data transmission, benefit from the possibility of tailoring the vortex core magnetic pattern. We report a theoretical study of vortex nucleation in pairs of coaxial iron and Permalloy cylinders, with diameters ranging from 21nm to 150nm, and 12nm and 21nm thicknesses, separated by a non-magnetic layer. 12nm thick iron and Permalloy isolated (single) cylinders do not hold a vortex, and 21nm isolated cylinders hold a vortex. Our results indicate that one may tailor the magnetic structure of the vortices, and the relative chirality, by selecting the thickness of the non-magnetic spacer and the values of the cylinders diameters and thicknesses. Also, the dipolar interaction may induce vortex formation in pairs of 12nm thick nanocylinders and inhibit the formation of vortices in pairs of 21nm thick nanocylinders. These new phases are formed according to the value of the distance between the cylinderes. Furthermore, we show that the preparation route may control relative chirality and polarity of the vortex pair. For instance: by saturating a pair of Fe 81nm diameter, 21nm thickness cylinders, along the crystalline anisotropy direction, a pair of 36nm core diameter vortices, with same chirality and polarity is prepared. By saturating along the perpendicular direction, one prepares a 30nm diameter core vortex pair, with opposite chirality and opposite polarity. We also present a theoretical discussion of the impact of vortices on the thermal hysteresis of a pair of interface biased elliptical iron nanoelements, separated by an ultrathin nonmagnetic insulating layer. We have found that iron nanoelements exchange coupled to a noncompensated NiO substrate, display thermal hysteresis at room temperature, well below the iron Curie temperature. The thermal hysteresis consists in different sequences of magnetic states in the heating and cooling branches of a thermal loop, and originates in the thermal reduction of the interface field, and on the rearrangements of the magnetic structure at high temperatures, 5 produce by the strong dipolar coupling. The width of the thermal hysteresis varies from 500 K to 100 K for lateral dimensions of 125 nm x 65 nm and 145 nm x 65 nm. We focus on the thermal effects on two particular states: the antiparallel state, which has, at low temperatures, the interface biased nanoelement with the magnetization aligned with the interface field and the second nanoelement aligned opposite to the interface field; and in the parallel state, which has both nanoelements with the magnetization aligned with the interface field at low temperatures. We show that the dipolar interaction leads to enhanced thermal stability of the antiparallel state, and reduces the thermal stability of the parallel state. These states are the key phases in the application of pairs of ferromagnetic nanoelements, separated by a thin insulating layer, for tunneling magnetic memory cells. We have found that for a pair of 125nm x 65nm nanoelements, separated by 1.1nm, and low temperature interface field strength of 5.88kOe, the low temperature state (T = 100K) consists of a pair of nearly parallel buckle-states. This low temperature phase is kept with minor changes up to T= 249 K when the magnetization is reduced to 50% of the low temperature value due to nucleation of a vortex centered around the middle of the free surface nanoelement. By further increasing the temperature, there is another small change in the magnetization due to vortex motion. Apart from minor changes in the vortex position, the high temperature vortex state remains stable, in the cooling branch, down to low temperatures. We note that wide loop thermal hysteresis may pose limits on the design of tunneling magnetic memory cells / Os efeitos de confinamento e o forte acoplamento dipolar na estrutura de v?rtices de nano-elementos ferromagn?ticos ? um tema de interesse atual, n?o apenas pelo valor puramente acad?mico, mas tamb?m pelo impacto em grande n?mero de dispositivos da ?rea de spintr?nica. Muitos dispositivos, como nano-osciladores para transmiss?o de dados sem fio, podem tirar grande proveito da possibilidade de controlar o padr?o magn?tico do n?cleo do v?rtice magn?tico. Relatamos um estudo te?rico da nuclea??o de v?rtices em um par de cilindros coaxiais de ferro e de Permalloy, com di?metros desde 21nm at? 150nm e espessuras de 12nm e de 21nm, separados por uma fina camada n?o-magn?tica. Cilindros isolados de ferro e Permalloy com espessura de 12nm n?o permitem a forma??o de v?rtices, enquanto que cilindros de espessura de 21nm possuem v?rtices quando isolados em reman?ncia. Nossos resultados indicam que ? poss?vel controlar a estrutura magn?tica dos v?rtices, bem como a chiralidade e polaridade relativa dos dois v?rtices, pela escolha apropriada dos valores dos di?metros e da separa??o dos dois cilindros ferromagn?ticos. Dependendo do valor da separa??o entre os cilindros, a intera??o dipolar pode induzir a forma??o de v?rtices em pares de cilindros de espessura de 12nm e inibir a forma??o de v?rtices em pares de cilindros de 21nm de espessura. Al?m disso, mostramos que a rota de prepara??o do estado magn?tico em campo nulo, pode ser usada para determinar a chiralidade e polaridade relativa dos dois v?rtices. Por exemplo: partindo da satura??o da magnetiza??o de um par de cilindros de ferro com di?metro de 81nm e espessura de 21nm, na dire??o do eixo f?cil da anisotropia uniaxial do ferro, resulta um par de v?rtices com n?cleo de 36nm, mesma chiralidade e mesma polaridade. Partindo do estado saturado em uma dire??o no plano e perpendicular ao eixo de anisotropia uniaxial, resulta um par de v?rtices com n?cleo de 30nm de di?metro, com chiralidade e polaridade opostas. Relatamos tamb?m um estudo te?rico do impacto de v?rtices magn?ticos na histerese t?rmica de um par de nanoelementos el?pticos de ferro, de 10nm de espessura, separados por um espa?ador n?o-magn?tico e acoplados com um substrato antiferromagn?tico por energia de 3 troca. Nossos resultados indicam que h? histerese t?rmica em temperatura ambiente (muito menor do que a temperatura de Curie do ferro), se o substrato for uma superf?cie n?o compensada de NiO. A histerese t?rmica consiste na diferen?a da sequ?ncia de estados magn?ticos nos ramos de aquecimento e resfriamento de um ciclo t?rmico, e se origina na redu??o do valor do campo de interface em altas temperaturas, e na reestrutura??o das fases magn?ticas impostas pela intera??o dipolar forte entre os dois nanoelementos de ferro. A largura da histerese t?rmica varia entre 500K ? 100K para dimens?es laterais de 125nm x 65nm e 145nm x 65nm. Focamos nos ciclos t?rmicos de dois estados especiais: o estado antiparalelo, com o nanoelmento em contato com o substrato alinhado na dire??o do campo de interface e o outro nanoelemento alinhado em dire??o oposta; e o estado paralelo em que os dois nanoelementos est?o alinhados com o campo de interface em temperaturas baixas. Esses s?o os dois estados magn?ticos b?sicos de c?lulas de mem?rias magn?ticas de tunelamento. Mostramos que a intera??o dipolar confere estabilidade t?rmica ao estado antiparalelo e reduz a estabilidade t?rmica do estado paralelo. Al?m disso, nossos resultados indicam que um par de cilindros com dimens?es de 125nm x 65nm, separados por 1.1nm, com campo de interface de 5.88kOe em temperatura de 100K, est? no estado paralelo. Essa fase se mant?m at? 249K, quando h? uma redu??o de 50% da magnetiza??o devido ? nuclea??o de um v?rtice no nanoelemento com superf?cie livre. Pequenas varia??es da magnetiza??o, devidas ao movimento do v?rtice, s?o encontradas no ramo de aquecimento, at? 600K. O estado encontrado em 600K se mant?m ao longo do ramo de resfriamento, com pequenas mudan?as na posi??o do v?rtice. A exist?ncia de histerese t?rmica pode ser um s?rio limite de viabilidade de mem?rias magn?ticas de tunelamento