Filmes de ligas FePt com uma estrutura quimicamente ordenada Tetragonal de Face Centrada FCT (tipo L1 IND.0) e textura [001] apresentam anisotropia magnética perpendicular, alta coercividade e grande efeito Kerr polar magnetoóptico. Entretanto, filmes completamente ordenados são normalmente obtidos através de um processo de \"sputtering\" com deposições em altas temperaturas do substrato(T IND. S > OU =600 ºC) ou por pós-tratamento térmico em temperaturas maiores que 500 ºC. As pesquisas atualmente em desenvolvimento visam a obtenção dessa estrutura ordenada a temperaturas mais baixas, o que seria mais adequado para aplicações práticas dos filmes. Neste trabalho, procuramos as melhores condições para a preparação de filmes de FePt com a fase ordenada FCT(001) em reduzidas temperaturas. Investigamos a influência do método de deposição, temperatura do substrato, espessura do filme e tipo de substrato sobre o grau de ordem química e textura dos filmes de FePt e, consequentemente, sobre suas propriedades magnéticas. Os filmes finos de ligas Fe IND.xPt IND. x-1(com x = 50 at%) foram preparados por \"Sputtering Magnetron DC\", através de dois diferentes métodos de deposição: pelo usual de codeposição e pelo método de deposiçãO alternada monocamadas atômicas (DAM). Os filmes foram crescidos sobre substratos de SiO IND.2/ Si(100), Si(100) e MgO(100), com e sem uma camada \"buffer\" de Pt ( com espessuras de 50 1 766 nm), sob T IND. s variando de temperatura ambiente a 600 ºC. O \"buffer\" de Pt foi usado para permitir melhores condições para a indução de um crescimento \"epitaxial\" da ordenada fase FCT(001) nos filmes de FePt. Os resultados da caracterização estrutural e magnética realizados através das técnicas de Difração de RAios-X (XRD) e Magnetometria de Amostra Vibrante (VSM) mostram que os substratos de MgO São adequados para induzir um crescimento \"epitaxial\" de filmes de FePt ) com uma estrutura FCT e textura [001]. Entretanto, com o uso de um \"buffer\" de Pt otimizado (com predominante fase FCC(100)), pré-depositado sobre MgO, foram obtifos melhores resultados em termos de \"epitaxia\" conveniente para o ordenamento da estrutura cristalina, para os filmes de FePt preparados por ambos os métodos de deposição. Os resultados mostram também que o efeito de um crescimento \"epitaxial\" induzido varia com a espessura do filme. Em relação ao método de deposição, os resultados revelam a maior eficiência de método DAM em comparação com o métodos de codeposição para a preparação de filmes de FePt com a fase FCT(001), em temperaturas reduzidas (T IND.s < OU = 400ºC). Com o uso do método DAM, foi possível a obtenção da fase ordenanda FCT(001) em filmes de FePt crescidos diretamente sobre o MgO em T IND. s = 400ºC. Para filmes de FePt crescidos sobre Pt/MgO, a formação da fase FCT(001) foi verificada em T IND.s = 200ºC e um alto grau de ordenamento químico (S = 0,88) em T IND. s = 400ºC. Estudos de Espectroscopia de Absorção de Rios-X (XAS) também foram realizados. / FePt alloy films, with a chemically ordered face-centered-tetragonal FCT (type LI IND.0) structure and [001] texture, present perpendicular magnetic anisotropy, high coactivity and large polar magneto-optical Kerr effect. However, completely ordered films are usually obtained either by sputtering process at high substrate temperatures (T IND.s MAIOR IGUAL A 600°C) or by postanneling treatment at temperatures higher than 500°C. For technological purposes the preparation temperatures has to be as low as possible. In this work we have searched the more adequate conditions to preparation of FePt films with ordered FCT(001) phase at reduced temperatures. We have investigated the influence of deposition methods, substrate temperature, thickness of film, and type of substrate on the degree of the chemical ordering and preferred texture of FePt films and, consequently, on their magnetic properties. The Fe IND.xPt IND.1-x alloy thin films (with x SEMELHANTE A 50 at%) were prepared by DC Magnetron Sputtering, via two different deposition methods: a conventional code position method and an alternate monatomic layer (DAM) deposition method. The films were grown on SiO IND.2/Si (100), Si (100) and MgO (100) substrates, with and without a Pt buffer layer (thickness ranging from 50 to 76 nm), at T IND.s varying from room temperature to 600°C. The Pt buffer layer was used for obtaining better conditions to the induction of an epitaxial grown of FePt films with the ordered FCT(001) phase. The results of structural and magnetic characterization performed by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM) show that the MgO substrates are adequate to induce an epitaxial growth of FePt films with a FCT structure and [001] texture. However, with the use of an optimized Pt FCC(100) buffer layer pre-deposited on MgO, the best conditions to an epitaxial induced growth were obtained, by both deposition methods. The results show also that the quality of the epitaxial induced growth depends on the thickness of the film. In relation to the deposition method in comparison to the code position method for preparation of FePt FCT(001) films, at reduced temperatures (T IND.s MENOR IGUAL A 400°C). With the use of the DAM method it was possible to obtain the ordered FCT(001) phase in the FePt film grown directly on MgO, at T IND.s = 400°C. For FePt film grown on Pt/MgO, the formation of the FCT(001) phase was verified at T IND.s = 200°C, and high degree of chemical order (S=0,88) was obtained at T IND.s = 400°C. X-ray Absorption Spectroscopy (XAS) studies also were done.

Areal densities of perpendicular hard-disk drives over 500Gb/in2 have already been demonstrated, with 1Tb/in2 densities being forecasted in the near future. However, at these high areal densities the information bearing units on the magnetic storage medium are magnetically unstable at temperatures expected in hard-disk drives. To extend or bypass this limit, new developments in head and media technologies and understanding of their record, readout and noise performances are necessary. The aim of this project was to study the record, readout and noise properties of conventional and future perpendicular magnetic recording media, heads and their related technologies. The objectives were therefore to develop a flexible and robust experimental recording platform to test the performance of different heads and media, and develop the necessary readout theory to predict the replay performance. In line with the project objectives, a high-precision open contact recording tester was developed with 1nm resolution. The open nature of this system allows different heads and media combinations to be tested. A second, closed system was also developed based on a commercial perpendicular hard-drive, modified to serve as a spin-stand to provide signal and noise measurements in practical drive conditions. The readout process in perpendicular recording was modelled based on the reciprocity principle for a shielded TMR head to study the parameters that affect the readout signal performance, and for comparison with the experimental measurements. Measured signal roll-off curves showed a practical linear density of 450KFCI for the commercial perpendicular disk medium, and indicated that non-linear effects happen at linear densities approaching 550KFCI. These results were in agreement with the theoretical calculations of the replay process. Two-dimensional readout scans were found have similar or higher resolution than Magnetic Force Microscopy (MFM) images of the same recorded regions – indicating the versatility and precision of the developed contact tester. Inverse filtering employing the Wiener filter was used to extract the magnetic transition. The extracted transition profiles and transition extents from the replay signals had much higher resolution than MFM images measured for the same transition region, thus showing the applicability of the developed testers for in situ magnetic characterisation. The developed contact and non-contact testers allowed the investigation of a new proposed recording scheme, Shingled Magnetic Recording (SMR). Measured signal roll-off curves of shingled tracks indicated a rise in the signal amplitude at low densities. At higher linear densities the signal performance was the same as conventionally written tracks with guardbands. It was found that a 30% reduction in track width in SMR, increase the areal density by a factor of 1.58 above that in existing hard drives.

Nanomateriais de fct-FePt, SmCo e Carbeto de Cobalto têm sido bastante estudados para a aplicação em gravação magnética e imãs de elevado desempenho, devido as suas energias magnetocristalinas e coercividades elevadas. Nanopartículas de FePt unidimensionais foram propostas na tentativa de obter melhora no alinhamento magnético das estruturas auto-organizadas. Neste trabalho, a formação de nanobastão e nanofios de FePt foi estudada através da presença de oleilamina e pequena quantidade de monóxido de carbono liberado pelo pentacarbonilferro(0). Estes dois parâmetros foram estudados a fim de analisar a influência no alongamento das nanopartículas e verificou-se que ambos atuam sinergicamente. Foram obtidos tanto nanofios de FePt ramificados de comprimento de 20-100 nm quanto nanobastões de FePt de 20-60 nm de comprimento, ambos com diâmetro de 2-3 nm. Todas as nanopartículas sintetizadas foram obtidas na fase cúbica de face centrada e o processo de tratamento térmico nas temperaturas de 450 oC e 560 oC levou a conversão para a fase tetragonal de face centrada, com custo da sinterização das nanopartículas. Os nanobastões, entretanto, apresentaram maior estabilidade térmica se comparado com o nanofio ramificado, obtendo propriedade ferromagnética na amostra. Alternativamente, têm sido obtidos satisfatoriamente nanobastões de platina para posterior recobrimento com ferro para a formação da liga FePt após o processo de recozimento. Na síntese de SmCo, foi estudada a formação da liga diretamente por via química através do uso de redutores comumente utilizandos em síntese de nanopartículas, porém foi possível observar apenas uma pequena quantidade da liga Sm2Co17 quando se utiliza o boroidreto de sódio. Isto se deve, principalmente, ao alto potencial de redução de Sm3+ e a sua instabilidade química. Entretanto, foram desenvolvidos métodos promissores para a obtenção de nanopartículas de CoO e SmCoO com tamanho e forma controlada. Além destes sistemas, tem sido obtidas diretamente através de síntese química nanopartículas de carbeto de cobalto com coercividade de até 2,3 kOe e magnetização de 45 emu/g, além de desenvolver um método geral de síntese de carbetos de outros metais. / SmCo, fct-FePt and CoC nanomaterials have been studied for application in magnetic recording and permanent magnets due to theirs high coercivity and magnetocrystalline anisotropy. One-dimensional FePt nanoparticles were proposed to improve the magnetic alignment of self-assembled system. In this work, the formation of FePt nanorods and nanowires was studied by using a small amount of carbon monoxide from the precursor pentacarbonyliron(0) and oleylamine. Both parameters of synthesis were studied and was verified that they influence the one-dimensional growth of FePt. In fact, branched FePt nanowires with 20 - 100 nm of length and nanorods with 20 - 60 nm were obtained, both with 2-3 nm of diameters. The FePt nanoparticles were obtained in face centered cubic phase and the transformation to face centered tetragonal phase was carry out in the temperatures of 450 oC and 560 oC, which led the formation of sintered nanoparticles. FePt nanorods have better thermal stability than nanowires according the results obtained. The platinum nanorods covered with iron oxide also were obtained to formation of FePt by thermal treatment. In concern of SmCo syntheses, the formation of SmCo phase directly by chemical synthesis was investigated by using some reduction agent, but was obtained a small amount of smco phase only if the sodium borohydrate was used in the synthesis. This could be occurred due to high reducing potential of Sm3+ and its chemical instability. However, some methods were obtained to obtain CoO and SmCoO nanoparticles with size and shape control. Furthermore, cobalt carbide nanoparticles were well obtained with coercivity of 2,3 kOe and magnetization of 45 emu/g, and a new general method to obtain metals carbides was developed.

No description available.

The area of ultrafast (sub-nanosecond) magnetization dynamics of ferromagnetic elements and thin films, usually driven by a strong femtosecond laser pulse, has experienced intense research interest. In this dissertation, laser-induced demagnetization is theoretically studied by taking into account interactions among electrons, spins, and lattice. We propose a microscopic approach under the three temperature framework and derive the equations that govern the demagnetization at arbitrary temperatures.To address the question of magnetization reversal at high temperatures, the conventional Landau-Lifshitz equation is obviously unsatisfactory, since it fails to describe the longitudinal relaxation. So by using the equation of motion for the quantum density matrix within the instantaneous local relaxation time approximation, we propose an effective equation that is capable of addressing magnetization dynamics for a wide range of temperatures. The longitudinal and transverse relaxations are analyzed, magnetization reversal processes near Curie temperatures is also studied. Furthermore, we compared our derived Self-consistent Bloch equation and Landau-Lifshitz-Bloch equation in detail. Finally, the demagnetzation dynamics for ferromagnetic and ferrimagnetic alloys is studied by solving the Self-consistent Bloch equation.

The hard-disk storage industry is at a critical time as the current technologies are incapable of achieving densities beyond 500 Gb/in2, which will be reached in a few years. Many radically new storage architectures have been proposed, which along with advanced signal processing algorithms are expected to achieve much higher densities. In this dissertation, various signal processing algorithms are developed to improve the performance of current and next-generation magnetic storage systems.Low-density parity-check (LDPC) error correction codes are known to provide excellent performance in magnetic storage systems and are likely to replace or supplement currently used algebraic codes. Two methods are described to improve their performance in such systems. In the first method, the detector is modified to incorporate auxiliary LDPC parity checks. Using graph theoretical algorithms, a method to incorporate maximum number of such checks for a given complexity is provided. In the second method, a joint detection and decoding algorithm is developed that, unlike all other schemes, operates on the non-binary channel output symbols rather than input bits. Though sub-optimal, it is shown to provide the best known decoding performance for channels with memory more than 1, which are practically the most important.This dissertation also proposes a ternary magnetic recording system from a signal processing perspective. The advantage of this novel scheme is that it is capable of making magnetic transitions with two different but predetermined gradients. By developing optimal signal processing components like receivers, equalizers and detectors for this channel, the equivalence of this system to a two-track/two-head system is determined and its performance is analyzed. Consequently, it is shown that it is preferable to store information using this system, than to store using a binary system with inter-track interference. Finally, this dissertation provides a number of insights into the unique characteristics of heat-assisted magnetic recording (HAMR) and two-dimensional magnetic recording (TDMR) channels. For HAMR channels, the effects of laser spot on transition characteristics and non-linear transition shift are investigated. For TDMR channels, a suitable channel model is developed to investigate the two-dimensional nature of the noise.

Shingled Magnetic Recording (SMR) technology is important in the immediate need for expansion of magnetic hard disk beyond the limit of current disk technology. SMR provides a solution with the least change from current technology among contending technologies. Robust easy to implement Digital Signal Processing (DSP) techniques are needed to achieve the potentials of SMR. Current DSP techniques proposed border on the usage of Two Dimensional Magnetic Recording (TDMR) techniques in equalisation and detection, coupled with iterative error correction codes such as Low Density Parity Check (LDPC). Currently, Maximum Likelihood (ML) algorithms are normally used in TDMR detection. The shortcomings of the ML detections used is the exponential complexities with respect to the number of bits. Because of that, reducing the complexity of the processes in SMR Media is very important in order to actualise the deployment of this technology to personal computers in the near future. This research investigated means of reducing the complexities of equalisation and detection techniques. Linear equalisers were found to be adequate for low density situations. Combining ML detector across-track with linear equaliser along-track was found to provide low complexity, better performing alternative as compared to use of linear equaliser across track with ML along track. This is achieved if density is relaxed along track and compressed more across track. A gain of up to 10dB was achieved. In a situation with high density in both dimensions, full two dimensional (2D) detectors provide better performance. Low complexity full 2D detector was formed by serially concatenating two ML detectors, one for each direction, instead of single 2D ML detector used in other literature. This reduces complexity with respect to side interference from exponential to linear. The use of a single bit parity as run length limited code at the same time error correction code is also presented with a small gain of about 1dB at BER of 10^-5 recorded for the situation of high density.

Nanomateriais de fct-FePt, SmCo e Carbeto de Cobalto têm sido bastante estudados para a aplicação em gravação magnética e imãs de elevado desempenho, devido as suas energias magnetocristalinas e coercividades elevadas. Nanopartículas de FePt unidimensionais foram propostas na tentativa de obter melhora no alinhamento magnético das estruturas auto-organizadas. Neste trabalho, a formação de nanobastão e nanofios de FePt foi estudada através da presença de oleilamina e pequena quantidade de monóxido de carbono liberado pelo pentacarbonilferro(0). Estes dois parâmetros foram estudados a fim de analisar a influência no alongamento das nanopartículas e verificou-se que ambos atuam sinergicamente. Foram obtidos tanto nanofios de FePt ramificados de comprimento de 20-100 nm quanto nanobastões de FePt de 20-60 nm de comprimento, ambos com diâmetro de 2-3 nm. Todas as nanopartículas sintetizadas foram obtidas na fase cúbica de face centrada e o processo de tratamento térmico nas temperaturas de 450 oC e 560 oC levou a conversão para a fase tetragonal de face centrada, com custo da sinterização das nanopartículas. Os nanobastões, entretanto, apresentaram maior estabilidade térmica se comparado com o nanofio ramificado, obtendo propriedade ferromagnética na amostra. Alternativamente, têm sido obtidos satisfatoriamente nanobastões de platina para posterior recobrimento com ferro para a formação da liga FePt após o processo de recozimento. Na síntese de SmCo, foi estudada a formação da liga diretamente por via química através do uso de redutores comumente utilizandos em síntese de nanopartículas, porém foi possível observar apenas uma pequena quantidade da liga Sm2Co17 quando se utiliza o boroidreto de sódio. Isto se deve, principalmente, ao alto potencial de redução de Sm3+ e a sua instabilidade química. Entretanto, foram desenvolvidos métodos promissores para a obtenção de nanopartículas de CoO e SmCoO com tamanho e forma controlada. Além destes sistemas, tem sido obtidas diretamente através de síntese química nanopartículas de carbeto de cobalto com coercividade de até 2,3 kOe e magnetização de 45 emu/g, além de desenvolver um método geral de síntese de carbetos de outros metais. / SmCo, fct-FePt and CoC nanomaterials have been studied for application in magnetic recording and permanent magnets due to theirs high coercivity and magnetocrystalline anisotropy. One-dimensional FePt nanoparticles were proposed to improve the magnetic alignment of self-assembled system. In this work, the formation of FePt nanorods and nanowires was studied by using a small amount of carbon monoxide from the precursor pentacarbonyliron(0) and oleylamine. Both parameters of synthesis were studied and was verified that they influence the one-dimensional growth of FePt. In fact, branched FePt nanowires with 20 - 100 nm of length and nanorods with 20 - 60 nm were obtained, both with 2-3 nm of diameters. The FePt nanoparticles were obtained in face centered cubic phase and the transformation to face centered tetragonal phase was carry out in the temperatures of 450 oC and 560 oC, which led the formation of sintered nanoparticles. FePt nanorods have better thermal stability than nanowires according the results obtained. The platinum nanorods covered with iron oxide also were obtained to formation of FePt by thermal treatment. In concern of SmCo syntheses, the formation of SmCo phase directly by chemical synthesis was investigated by using some reduction agent, but was obtained a small amount of smco phase only if the sodium borohydrate was used in the synthesis. This could be occurred due to high reducing potential of Sm3+ and its chemical instability. However, some methods were obtained to obtain CoO and SmCoO nanoparticles with size and shape control. Furthermore, cobalt carbide nanoparticles were well obtained with coercivity of 2,3 kOe and magnetization of 45 emu/g, and a new general method to obtain metals carbides was developed.

In this paper, we use the forward recursion BCJR algorithm to estimate the symmetric information rate for Two Dimensional Magnetic Recording (TDMR) channels. In particular, we consider a TDMR read/write channel whose all components, including recording medium, write and readback processes are modeled in software. Since the primary source of noise in TDMR arises from irregularities in the recording medium and leads to highly colored and data-dependent jitter, the pattern dependent noise predictive (PDNP) algorithm is implemented to improve the accuracy and performance of SIR estimation. Furthermore, we study the performance gain of using the PDNP algorithm in SIR estimation through simulations over the Voronoi based media model for different TDMR channel configurations.

Bit-Patterned Media (BPM) has been recognised as one of the candidate technologies to achieve an areal density beyond 1Tb/in2 by fabricating single-domain islands out of continuous magnetic media. Though much attention has been focused on the fabrication of BPM, existing lithography techniques demonstrate difficulties in producing uniform islands over large areas cost effectively; the resulting fabricated islands often vary in position and size. The primary purpose of the research documented in this thesis is to investigate the issue of island geometry variations on the data recovery process from a perpendicular patterned media with head and media configurations optimised to achieve an areal density of 1Tb/in2. In order to achieve the research aim, a read channel model has been implemented as a platform to evaluate the read channel performance numerically. It can be also altered to investigate new read channel designs. The simulated results demonstrate that island geometry variations have a detrimental effect on read channel performance. It has shown that a BPM system can be tolerant to island position variations, but more effort needs to be paid to the effect that island size variations have on the read channel performance. A new read channel design revolving around the design of a modified trellis has been proposed for use in the Viterbi detector in order to combat the effect of island geometry variations. The modified trellis for island position variations results in extra states and branches compared to the standard trellis, while the modified trellis for island size variations results in only extra branches. The novel read channel designs demonstrate an improved read channel performance in the presence of island geometry variations even with increasing amounts of island position and size variations. There are two ways to obtain the read channel performance in terms of the bit-error-rate (BER): a) by running a numerical Monte-Carlo simulation to count the number of bits in error at the output of the read channel model and b) using an analytical approach to calculate the BER by approximating the noise into a known distribution. It is shown that both ways demonstrate very similar results, which indicates as long as the distribution of the noise present in read channel model is predictable, the analytical approach can evaluate the BER performance more efficiently, especially when the BER is low. However, the Monte-Carlo simulation is still useful for understanding of the correlation of the errors. Novel trellis proposed in this work will contribute to the commercial development of BPM in two ways: a) to improve the data recovery process in BPM systems, b) to allow a tolerance of 10% size variations for the existing fabrication techniques.