Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / NanopartÃculas de FeCo recobertas com (Fe,Co)3O4 (Magnetita dopadas com cobalto) na estrutura casca caroÃo (Core-Shell), foram sintetizadas pela rota quÃmica conhecida como Sol-Gel Proteica. Os materiais sintetizados foram caracterizados por Termogravimetria (TG), DifraÃÃo de Raios-X (DRX), Magnetometria de Amostra Vibrante (VSM), Espectroscopia MÃssbauer, Microscopia EletrÃnica de Varredura (MEV) e Microscopia EletrÃnica de TransmissÃo (MET). Os resultados mostram que o aumento da temperatura de re-oxidaÃÃo influi diretamente no tamanho da casca (Shell) bem como o aumento da razÃo entre a magnetizaÃÃo remanente pela magnetizaÃÃo de saturaÃÃo. O efeito de Exchange Spring e Exchange Bias foram observados nas curvas de histerese. A estrutura Core-Shell foi formada de maneira nÃo homogÃnea, ou seja, nem todas as partÃculas foram recobertas pela magnetita. / FeCo Nanoparticles coated with (Fe,Co)3O4 (Magnetite doped with cobalt) were synthesized by the chemical route known as Sol-Gel Protein. The synthesized materials were characterized by thermogravimetry (TG), X-Ray Diffraction (XRD), Vibrating Sample Magnetometry (VSM), MÃssbauer spectroscopy, Scanning Electronic Miscroscopy (SEM) and Transmission Electronic Microscopy (TEM). The results show that the increase in temperature directly influences the size of the shell (Shell) and increased the ratio of remanent magnetization to the saturation magnetization. The effect of Exchange Spring and Exchange Bias were observed in the hysteresis curves. The Core-Shell structure was formed a non-homogeneous manner, ie not all the magnetite particles were coated.

Cette thèse contribue à l'étude des aimants à effet ressort, connu sous le nom de "Exchange Spring Magnets" (ESM). Nous avons divisé nos travaux en deux parties : une première dite expérimentale et une seconde dite théorique. Dans la première partie, nous avons étudié la synthèse de ESM. Le processus de fabrication débute par le mélange des nano-poudres magnétiques dures et douces. Cette étape a été réalisé par broyage mécanique ainsi que par ultrasons. Les mélanges sont ensuite frittés via la technique de frittage par plasma d'arc (Spark Plasma Sintering (SPS)) afin d'obtenir des aimants massifs. La structure ainsi que les propriétés magnétiques de ces aimants sont étudiées. Les structures obtenues présentent des phases intermédiaires ainsi qu'une réduction de la phase magnétique dure. Nous avons trouvé que ces aimants ont des cycles magnétiques à deux phases indiquant un couplage faible entre les phases précurseurs. De plus, les aimants obtenus à partir du mélange à ultrasons possèdent des meilleurs propriétés magnétiques que ceux obtenus par broyages mécanique ainsi qu'une plus grande capacité à obtenir un meilleur couplage. Dans la seconde partie, nous avons étudié l'utilité des ESM pour l'enregistrement magnétique perpendiculaire. Il a été montré qu'en y appliquant un faible champ micro-onde , l'ESM présente un basculement facilité ainsi qu'une meilleur stabilité thermique, ce qui est désirable pour l'enregistrement magnétique. Nous avons étudié les dynamiques non-linéaires de basculement magnétique dans un ESM perpendiculaire en utilisant l'équation de Landau-Lifshitz. A partir d'un nouveau code FORTRAN basé sur la méthode des différences finies, nous avons étudié les solutions numériques de cette équation. Le système bi-couche présente des solutions stables nommés modes-P globaux et une diminution du champ nécessaire au basculement pour une grande gamme de fréquence. Le profil de la magnétisation dans le système étendu à été étudié numériquement. L'étude analytique des dynamiques est commentée en faisant l'approximation que deux modes-P sont couplés par un champ d'interaction. L'"ansatz" utilisé pour décrire le champ d'interaction est validé par comparaison avec les résultats numériques. Cette approche s'est révélée être équivalente à l'étude de deux macro-spins couplés. / This thesis is divided into two parts, experimental part presenting the synthesis of exchange spring magnets and theoretical part showing the magnetization dynamics of exchange spring magnets. For the synthesis, ferrite and metal alloy based magnets using mechanical milling and ultrasonic mixing are studied. This part discusses the difficulty in controlling the microstructure during synthesis. Several samples with varying volume fractions of the soft phase are synthesized. It is found that mechanical milling initiates a reaction and this leads to disintegration of the ferrite phase into its intermediate phases. Samples prepared using ultrasonic mixing however show presence of the ferrite phase up to very large volume fraction of the soft alloy phase which provides better perspective for the synthesis. Dynamics of the exchange spring system is studied theoretically using micromagnetic theory. Microwave assisted magnetization reversal are studied in the bulk bilayer exchange coupled system. We investigate the nonlinear magnetization reversal dynamics in a perpendicular exchange spring media using the Landau-Lifshitz equation. In the limit of the infinite thickness of the system, the propagation field leads the reversal of the system. The reduction of the switching field and the magnetization profile in the extended system are studied numerically. The possibility to study the dynamics analytically is discussed and an approximation where two P-modes are coupled by an interaction field is presented. The ansatz used for the interaction field is validated by comparison with the numerical results. This approach is shown to be equivalent to two exchange coupled macrospins.

When a permanent magnet is considered for an application, the quantity that quantifies the usability of that material is the magnetic energy product (BH)max. In today’s world, rare earth transition metal permanent magnets like Nd-Fe-B, Sm-Co possesses the maximum magnetic energy product. But still for the industrial application, the ferrite permanent magnets are the primary choice over these rare transition metal magnets. Thus, in the present context, the magnetic energy product of the low cost ferrite system makes it unsuitable for the high magnetic energy application. In this regard, exchange spring magnets which combine the magnetization of the soft phase and coercivity of the hard magnetic phases become important in enhancing the magnetic energy product of the system. In this thesis, the exchange spring behaviour is reported for the first time in hard/soft oxide nanocomposites by microstructural tailoring of hard Barium Ferrite and soft Nickel Zinc Ferrite particles. We have analyzed the magnetization reversal and its correlation with the coercivity mechanism in the Ni0.8Zn0.2Fe2O4/BaFe12O19 exchange spring systems. Using this exchange spring concept, we could enhance the magnetic energy product in Iron Oxide/ Barium Calcium Ferrite nanocomposites compared to the bare hard ferrite by ~13%. The presence of the exchange interaction in this nanocomposite is confirmed by the Henkel plot. Moreover, a detailed Reitveld study, magnetization loop and corresponding variation of the magnetic energy product, Henkel plot analysis and First Order Reversal Curve analysis are performed on nanocomposites of hard Strontium Ferrite and soft Cobalt Ferrite. We have proved the exchange spring behaviour in this composite. In addition, we could successfully tailor the magnetization behaviour of the soft Cobalt Ferrite- hard Strontium Ferrite nanocomposite from non exchange spring behaviour to exchange spring behaviour, by tuning the size of the soft Cobalt Ferrite in the Cobalt Ferrite/Strontium Ferrite nanocomposite. The relative strength of the interaction governing the magnetization process in the composites has been studied using Henkel plot and First Order Reversal Curve method. The FORC method has been utilized to understand the magnetization reversal behaviour as well as the extent of the irreversible magnetization present in both the nanocomposites, having smaller and larger particle size of the Cobalt Ferrite. It has been found that for the all the studied composites, the pinning is the dominant process for magnetization reversal. The detailed structural analysis using thin film XRD, angle dependent magnetic hysteresis and remanent coercivity measurement, coercivity mechanism by micromagnetic analysis and First Order Reversal Curve analysis are performed for thin films of Strontium Ferrite which are grown on c-plane alumina using Pulsed Laser Deposition (PLD) at two different oxygen partial pressures. The magnetic easy directions of both the films lie in the out of plane direction where as the in plane direction corresponds to the magnetic hard direction. Depending on the oxygen partial pressure during deposition, the magnetization reversal changes from S-W type reversal to Kondorsky kind of reversal. Thus, the growth parameter for the Strontium Ferrite single layer which will be used further as a hard layer for realizing oxide exchange spring in oxide multilayer, is optimized. The details of the magnetic and structural properties are analyzed for Nickel Zinc Ferrite thin film grown on (100) MgAl2O4. We have obtained an epitaxial growth of Nickel Zinc Ferrite by tuning the growth parameters of PLD deposition. The ferromagnetic resonance and the angle dependent hysteresis loop suggest that, the magnetic easy direction for the soft Nickel Zinc Ferrite lie in the film plane whereas the out of plane direction is the magnetic hard direction. Using the growth condition of respective Nickel Zinc Ferrite and Strontium Ferrite, we have realized the exchange spring behaviour for the first time in the trilayer structure of SrFe12O19 (20 nm)/Ni0.8Zn0.2Fe2O4(20 nm)/ SrFe12O19 (20 nm) grown on c-plane alumina (Al2O3) using PLD. The FORC distribution for this trilayer structure shows the single switching behaviour, corresponding to the exchange spring behaviour. The reversible ridge measurement shows that the reversible and the irreversible part of the magnetizations are not coupled with each other.

Estudamos propriedades magnéticas de ligas nano cristalinas de Sm-F e-Co. As ligas foram preparadas por mecano-síntese e posterior tratamento térmico. Como resultado, obtivemos ímãs nanocristalinos de Sm18 (Fe,Co)82 , com Sm(Fe,Co)7 como fase principal. As ligas apresentaram excelentes propriedades magnéticas: remanências relativas Mn/ Ms ~ 0.6 e coercividades na faixa 5-20 kOe, dependendo do teor de Fe nos materiais. Interações magnéticas nas ligas foram estudadas com base nos gráficos ele Henkel. Interpretamos, nestes gráficos, de maneira consistente a influência elas interações e dos estados desmagnetizados. / We studied the magnetic properties of nanocrystalline Sm-Fe-Co alloys. These alloys were prepared by mechanical alloying and subsequent annealing. We obtained nanocrystalline rnagnets of composition Sm18 (Fe, Co )82 , for which the main hard magnetic phase is Sm(Fe, Co)82. The alloys showed excellent magnetic properties: relative remanence Mn/ Ms :2; 0.6 and coercive fields ranging from 5 to 20 k0e, depending upon the amount of Fe present. Henkel plots were used in order to study magnetic interactions in these alloys. The influence of the interactions and the demagnetized state on the Henkel plots was also studied.

DIAS, Diego Felix. Síntese e caracterizações de nanopartículas de FeCo/(Fe,Co)3O4 com acoplamentos magnéticos sintetizadas pelo método sol-gel proteico. 2015. 84 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2015. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2016-02-05T18:59:30Z No. of bitstreams: 1 2015_dis_dfdias.pdf: 4365302 bytes, checksum: 86e3e995f04f6424f30d8e5ab94293bc (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2016-02-05T18:59:49Z (GMT) No. of bitstreams: 1 2015_dis_dfdias.pdf: 4365302 bytes, checksum: 86e3e995f04f6424f30d8e5ab94293bc (MD5) / Made available in DSpace on 2016-02-05T18:59:49Z (GMT). No. of bitstreams: 1 2015_dis_dfdias.pdf: 4365302 bytes, checksum: 86e3e995f04f6424f30d8e5ab94293bc (MD5) Previous issue date: 2015 / FeCo Nanoparticles coated with (Fe,Co)3O4 (Magnetite doped with cobalt) were synthesized by the chemical route known as Sol-Gel Protein. The synthesized materials were characterized by thermogravimetry (TG), X-Ray Diffraction (XRD), Vibrating Sample Magnetometry (VSM), Mössbauer spectroscopy, Scanning Electronic Miscroscopy (SEM) and Transmission Electronic Microscopy (TEM). The results show that the increase in temperature directly influences the size of the shell (Shell) and increased the ratio of remanent magnetization to the saturation magnetization. The effect of Exchange Spring and Exchange Bias were observed in the hysteresis curves. The Core-Shell structure was formed a non-homogeneous manner, ie not all the magnetite particles were coated. / Nanopartículas de FeCo recobertas com (Fe,Co)3O4 (Magnetita dopadas com cobalto) na estrutura casca caroço (Core-Shell), foram sintetizadas pela rota química conhecida como Sol-Gel Proteica. Os materiais sintetizados foram caracterizados por Termogravimetria (TG), Difração de Raios-X (DRX), Magnetometria de Amostra Vibrante (VSM), Espectroscopia Mössbauer, Microscopia Eletrônica de Varredura (MEV) e Microscopia Eletrônica de Transmissão (MET). Os resultados mostram que o aumento da temperatura de re-oxidação influi diretamente no tamanho da casca (Shell) bem como o aumento da razão entre a magnetização remanente pela magnetização de saturação. O efeito de Exchange Spring e Exchange Bias foram observados nas curvas de histerese. A estrutura Core-Shell foi formada de maneira não homogênea, ou seja, nem todas as partículas foram recobertas pela magnetita.

Estudamos propriedades magnéticas de ligas nano cristalinas de Sm-F e-Co. As ligas foram preparadas por mecano-síntese e posterior tratamento térmico. Como resultado, obtivemos ímãs nanocristalinos de Sm18 (Fe,Co)82 , com Sm(Fe,Co)7 como fase principal. As ligas apresentaram excelentes propriedades magnéticas: remanências relativas Mn/ Ms ~ 0.6 e coercividades na faixa 5-20 kOe, dependendo do teor de Fe nos materiais. Interações magnéticas nas ligas foram estudadas com base nos gráficos ele Henkel. Interpretamos, nestes gráficos, de maneira consistente a influência elas interações e dos estados desmagnetizados. / We studied the magnetic properties of nanocrystalline Sm-Fe-Co alloys. These alloys were prepared by mechanical alloying and subsequent annealing. We obtained nanocrystalline rnagnets of composition Sm18 (Fe, Co )82 , for which the main hard magnetic phase is Sm(Fe, Co)82. The alloys showed excellent magnetic properties: relative remanence Mn/ Ms :2; 0.6 and coercive fields ranging from 5 to 20 k0e, depending upon the amount of Fe present. Henkel plots were used in order to study magnetic interactions in these alloys. The influence of the interactions and the demagnetized state on the Henkel plots was also studied.

Chemically ordered FePt is one of the most promising materials to reach the ultimate limitations in storage density of future magnetic recording devices due to its high uniaxial magnetocrystalline anisotropy and a corrosion resistance superior to rare-earth based magnets. In this study, FePt and FePt/Cu bilayers have been sputter deposited at room temperature onto thermally oxidized silicon wafers, glass substrates and self-assembled arrays of spherical SiO2 particles with diameters down to 10 nm. Millisecond flash lamp annealing, as well as conventional rapid thermal annealing was employed to induce the phase transformation from the chemically disordered A1 phase into the chemically ordered L10 phase. The influence of the annealing temperature, annealing time and the film thickness on the ordering transformation and (001) texture evolution of FePt films with near equiatomic composition was studied. Whereas flash lamp annealed FePt films exhibit a polycrystalline morphology with high chemical L10 order, rapid thermal annealing can lead to the formation of chemically ordered FePt fifilms with (001) texture on amorphous SiO2/Si substrates. The resultant high perpendicular magnetic anisotropy and large coercivities up to 40 kOe are demonstrated. Simultaneuosly to the ordering transformation, rapid thermal annealing to temperatures exceeding 600 °C leads to a break up of the continuous FePt film into separated islands. This dewetting behavior was utilized to create regular arrays of FePt nanostructures on SiO2 particle templates with periods down to 50 nm. The addition of Cu improves the (001) texture formation and chemcial ordering for annealing temperatures T < 600 °C. In addition, the magnetic anisotropy and the coercivity of the ternary FePtCu alloy can be effectively tailored by adjusting the Cu content. The prospects of FePtCu based exchange spring media, as well as the magnetic properties of FePtCu nanostructures fabricated using e-beam and nanoimprint lithography have been investigated.

Chemically ordered FePt is one of the most promising materials to reach the ultimate limitations in storage density of future magnetic recording devices due to its high uniaxial magnetocrystalline anisotropy and a corrosion resistance superior to rare-earth based magnets. In this study, FePt and FePt/Cu bilayers have been sputter deposited at room temperature onto thermally oxidized silicon wafers, glass substrates and self-assembled arrays of spherical SiO2 particles with diameters down to 10 nm. Millisecond flash lamp annealing, as well as conventional rapid thermal annealing was employed to induce the phase transformation from the chemically disordered A1 phase into the chemically ordered L10 phase. The influence of the annealing temperature, annealing time and the film thickness on the ordering transformation and (001) texture evolution of FePt films with near equiatomic composition was studied. Whereas flash lamp annealed FePt films exhibit a polycrystalline morphology with high chemical L10 order, rapid thermal annealing can lead to the formation of chemically ordered FePt fifilms with (001) texture on amorphous SiO2/Si substrates. The resultant high perpendicular magnetic anisotropy and large coercivities up to 40 kOe are demonstrated. Simultaneuosly to the ordering transformation, rapid thermal annealing to temperatures exceeding 600 °C leads to a break up of the continuous FePt film into separated islands. This dewetting behavior was utilized to create regular arrays of FePt nanostructures on SiO2 particle templates with periods down to 50 nm. The addition of Cu improves the (001) texture formation and chemcial ordering for annealing temperatures T < 600 °C. In addition, the magnetic anisotropy and the coercivity of the ternary FePtCu alloy can be effectively tailored by adjusting the Cu content. The prospects of FePtCu based exchange spring media, as well as the magnetic properties of FePtCu nanostructures fabricated using e-beam and nanoimprint lithography have been investigated.

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