MAGNETISM

Author: SAMPEDRO ROZAS BLANCA.
Year: 2004.
University: COMPLUTENSE DE MADRID [www.ucm.es].
Place of preparation: INSITUTO DE MAGNETISMO APLICADO. FACULTAD DE CC. FÍSICAS. UCM.

Summary: The atom-free PD has a configuration [Kr] 4d10 and it is not magnetic. The crystalline structure of Pd metal is focused on the faces cubic (fcc) with a constant network of a = 3.8898 A. In relation to the structure of bands, the band has a Pd d which is nearly full and a band s paracialmente filled, resulting in a configuration formally designated as (4d) 10 (5S) where ***. Of all the intrinsic properties of palladium, two are in the spotlight especially:-Performance of the magnetic susceptibility versus temperature. The susceptibilida paramagnética of Pauli is due to the different pbolación of spines and electrons in a metal mobile x =***( FE). Virtually is independent of temperature. The Pd is an exception. Presents a very dependent magnetic susceptibility to temperature and exihibe a sharp paramgnetismo with Pauli susceptibility with a markedly higher than the rest of the elements of its grupp. B-density of states at the Fermi level. The density of states of Pd, N (E) shows a sharp peak (f (EF) = 1.23 estadis and V-1 spin-1 atom -1) just below the Fermi level (EF). However, it becomes sufficient to meet the criterion for Stoner to ferromagnetism, g (EF) Imayor 1 (where I is the parameter Stoner typically 0.71 for Pd). These two features have as a consequence that habale of "instability ferrmagnética" Pd. These two features have as a consequence the talk of the "instability ferromagnetic" Pd. This report presents the work done to try to explore small and unknown to the border that separates palladium to be a ferromagnetic material. The exhaustive study of the magnetic behavior of this element, which elaborates on its most outstanding features, will lead us to resutlados relevant. One example is the observation pilot in order ferromagnetic nanoparticles Pd.

Author: DOMINGO MARIMON NEUS.
Year: 2004.
University: BARCELONA [www.ub.es].
Place of defense: FACULTAT DE FÍSICA.
Place of preparation: FACUTLAT DE FÍSICA.

Summary: The objective of this dissertation work is the study of magnetism of various materials in nature metal-orgánica framed in various fields of research. On the one hand, the molecules with high spin magnetic anisotropy negative. This creates a barrier anisotropy energy in these molecules which prevents the magnetic moment rotate freely and to preferentially oriented in a direction of space. The magnetic moment can change the direction of their orientation overcoming the barrier energy classic form by thermal activation or cuánticamente by tunneling. This property can be used for technological applications of these molecules in the field of magnetic recording of ultra-alta density or in the field of quantum computing. In the first block of this thesis presents the studies of the magnetic properties of some of these molecules of Mn12 in different environments (up from monocrystals polymeric films through solutions), as well as new techniques of magnetic characterization of these molecules as is the the magnetic circular dichroism in the same. This method allows us to measure for the first time some basic physical phenomena in these molecules through characterization magneto-óptica. In the second block of this thesis, presents the study of magnetism in a new family of materials nanoporosos highly innovative synthesized from radical trifenilmetílicos polclorados (PTM), funcionalizados with different carboxylic groups. The use of these radicals has led us to obtain not only material nanoporosos with properties very high porosity, but also with some magnetic properties impressive. The autoensamblage of these radicals has enabled us to obtain the first ferromagneto nanoporoso purely organic, with a transition temperature of about 125 mK. The crystallization of combinations of these radicals with transition metal ions in the approximation metal-orgánica has led to several notable systems. The combination deiones Cu (II) with radical MSPs allows us to obtain the first sponge magnetic materials nanoporoso some pores of about 3 nm in diameter and with the loss of absorbent properties and resorption of the same reversible and monitorizables through its magnetic property (magnetic sensor selective to certain solvents). Moreover, the combination of ion Co (II) with the radical PTM (leads to the first structure nanoporosa magnetic systems formed by helical not interpenetrados and that presents some relaxation phenomena slow very unusual, such relaxation presents a strong dependence the applied magnetic field and can be described by the model Davidson-Cole.

Author: RODRIGUEZ ARANDA GLORIA ISIDRA.
Year: 2004.
University: PAÍS VASCO [www.ehu.es].
Place of defense: FACULTAD DE CIENCIAS QUIMICAS.
Place of preparation: FACULTAD DE CIENCIAS QUIMICAS.

Summary: This thesis presents a study on the field of anisotropy on tapes ferromagnéticas amorphous and nanocristalinas. The work consists of a pilot and another one composed of computer simulations. The experimental part deals with tapes produced by cooling speed, composition FINEMET (Fe73.5Cu1Nb3Si13.5B9) and FINEMET with iron hand replaced by aluminum (Fe71.5Cu1Al2Nb3Si13.5B9). In the early treatments are conducted by current and is calculated using the anisotropy field work imanación obtained through adjustment based on the model of random anisotropy values for the constants of anisotropy and magnetostriction and analysis on the evolution of the domains of the tape because of the treatments. The anisotropy of the second composition is analyzed by the method of susceptibility cross Hoffmann, which deepens in chapters of simulations, confirming that the anisotropy field is not that correspond to low coercive fields according to the theory model anisotropy of random. It looks in this case the influence of the replacement of iron by aluminum and the applicability of a two-dimensional model such as the Hoffman to such tapes by bitter study of magnetic domains present. As for the models, the goal is to look deeply into the results that gives us the technical Hoffmann to obtain the anisotropy field, where no restrictions are implemented that he used in his model (it is not infinite and finite samples as implied, and it is not used here its approximate field means, mainly) and putting the results into contact with the microstructure previously established systems. Simulations are carried out both geometric mean of the local anisotropy of the material. This study also includes another field provided by the technique of Hoffmann-called scatter field, which turns out to be not only home bipolar, as Hoffmann course, but able to count on substantial contributions from other energies.

Author: MARTINEZ VECINO EDUARDO.
Year: 2004.
University: SALAMANCA [www.usal.es].
Place of defense: FACULTAD DE FISICAS.
Place of preparation: DPTO FISICA APLICADA.

Summary: The development of nanotechnology with ferromagnetic materials requires knowledge and describe the dynamic processes of the magnetisation at the nano-scale. The micromagnetismo is the theoretical formalism appropriate to describe the behavior of samples ferromagnéticas on this scale. This thesis extends the range of validity of micromagnetismo standard to include some phenomena that are of great importance to the further development of devices nanotecnológicos. The main contributors of the work are: 1. Study of the thermal agitation: Introduces the dynamics of Langevin based on the theory of stochastic processes, as appropriate formalism to describe the evolution of the magnetization in the presence of a thermal bath. It discusses the problems relating to space discretization of the equations describing the evolution of the magnetization. Also, it has been determined the maximum density information storage can be achieved by avoiding the superparamagnetic behavior. 2. Flows loss or eddy currents: a theoretical framework is proposed to include the effect of the flow of losses (eddy currents). It has studied its effect on the dynamics of the magnetization describing and quantifying their impact so semi-analítica from a parameter losses no-fenomenológico. 3. Torque Transfer Spin: We have developed tools for studying the numerical excitement of the magnetization by injecting currents of spin polarized. In particular, it has studied the link magnetostático between different layers that are part of a spin valve. 4. Dynamics walls: It has been studying the dynamics of thin strips walls in the presence of an external field. We study the speed depending on the cross section. Finally, we have studied the dynamics of walls between estrechamientos allowing fix the position in the absence of field.

Author: RIVAS ARDISANA MONTSERRAT.
Year: 2005.
University: OVIEDO [www.uniovi.es].
Place of defense: ESCUELA POLITÉCNICA SUPERIOR.
Place of preparation: FACULTAD DE CIENCIAS.

Summary: Among the wide variety of materials nanocristalinos ferromagnetic there is a small group that presents a behavior histerético very particular, the hysteresis cycle is posted on the horizontal axis and its shape is asymmetric. Normally, the breaking of symmetry hysteresis cycle has been observed in materials that have anisótropía swap, however behavior histerético anomalous that we refer here presents some characteristics that clearly differ from it. In this paper thesis has analyzed the relationship between nanocristalizaicón of the alloy Co66 Fe4 Mo2 Si16 B12 and its magnetic hysteresis. SE has a pilot study to relate the microstructure in volume and surface samples thermally treated with the phenomenon of hysteresis cycle shifted. The origin of this shift has been attributed to dipolar interactions between phases magnetically soft (amorphous matrix) and hard (crystals embedded in the amorphous matrix). Based on this model has been simulated numerically how cycles and the evolution of its main features with the applied fields, time and temperature. Through measures torque magnetometry in samples with a high fraction nanocristalina are estimated constants anisotropy of the nanocrystals existing samples. There is also anomalous behavior of the pair in the samples with a low level of nanocristalización.

Author: Cheng Hao.
Year: 2005.
University: AUTÓNOMA DE MADRID [www.uam.es].
Place of defense: Facultad de ciencias.
Place of preparation: Facultad de ciencias.

Summary: Spintronics is an area of emerging research that deals with the development of a new class of devices, spintronic devices. The devices are based on spintronic effects magnetorresistivos (MR), changes in electrical resistance in the presence of magnetic fields variables, which are to be described in terms of models of the spin-dependent electronic transport. The magnetorresistencia is called ballistics (MRB) was observed in nanocontactos of ferromagnetic materials at room temperature and in the presence of camposa magnetic relatively small by Professor Nicholas Garcia in 1999. The discovery of the magnetorresistencia ballistics (MRB) offers good potential for application in next-generation devices. The high sensitivity of the effect MRB, much higher than that observed for the effect of magnetorresistencia giant (MRG), it is very suitable for the manufacture of warheads readers for systems of ultra-high density information storage. This thesis presents the systematic experimental study of the properties of magnetotransporte in nanocontactos of different materials and configurations. These experiments show that the changes in resistance of nanocontactos changing magnetic fields are applied effects of magnetorresistencia ballistics (MRB). To implement devices based on the effect MRB have been fabricadpo nanoconstricciones of Permalloy of 30nm width on silicon surfaces by electronic lithography. In chapter 1, are briefly described the different types of systems magnetoresistivos that have been found so far, the concepts of nanotechnology and the archetype of devices based on the electron spin has been described as well. The effect of magnetorresistencia ballistics (MRB) nanocontactos manufactured rupture controlled unions matálicas (MCBJ) can be kept stable for several seconds. We have made efforts to produce nanocontactos more stable than those made with MCBJ using electroplating techniques chemistry. In chapter 2, are described in detail nanocontactos nickel produced by electroplating chemical. In addition there have been nanocontactos iron and copper to carry out a comparative study. It has been studied the appearance of roughness on the surface of nanocontactos electrodepositados Ni, Cu and Fe in chapter 2. For nanocontactos nickel, when the electrical potential of cathode is -1000mV/SCE, presents a smooth surface that is not dependent on magnetic fields; When the electric potential of cathode is -1200mv/SCE, the appearance of the surface depends the orientation of the magnetic field perpendicular to the path of ions, being more smooth with perpendicular magnetic fields when the potential elétrico cathode is -1300mV/SCE, the appearance of the surface depends on the applied magnetic fields, in the case of fields parallel to the path of magnetic ions, are observed structures 3D arborescent - Granular - dendrícticas with magnetic fields perpendicular to the path of the ion, nickel surfaces again be smooth. The surface of Fe and Cu electrodepositados are 3D arborescentes- granular -dendríticas (ABDS) and do not depend on the direction of the magnetic field applied. In chapter 3 we study the electronic properties of nanocontactos produced by electroplating chemical macroscopic electrodes. Electrodes macroscopic are Ni-Ni, Au-Au and Cu-Cu. Metals Ni, Fe, Ci and NiFe have been electrodepositados to form nanocontactos. By measuring the magnetorresistencia ballistics (BMR) of nanocontactos obtained variations in the magnetorresistencia up to 4000%. To rule out the possibility that the magnetorresistencia ballistics off due to magnetostrictive effect has been measured distortion of contacts through atomic force microscopy (AFM). The results of atomic force microscopy shows that the distortion of nanocontactos ferromagnetic electrodes between macroscopic Cu-Cu is less than 1nm. All 8 s experi e98 ments show that the effect of magnetorresistrencia ballistics (BMR) is not dependent on the electrodes macroscopic materials. The nanocontactos produced by electroplating chemical can be kept stable for many hours. This stability enables us to perform experiments systematic. However, the level of stability achieved in the development of this thesis is not yet sufuciente so that nanocontactos can be used in commercial devices. Instability problems may be due to oxidation of nanocontactos and high current density which circulates them. To avoid the problems associated with a nanocontactos individually, we conducted experiments in which afluyen flows in a network of nanocontactos. In chapter 4, explores nanocontactos between particles ferromagnéticas Fe. In this type of nanocontactos can reduce current density for each nanocontacto. We started with iron particles with a diameter between 1 micrometer and 6 micrometers; particles were ground with a ball hard in different atmospheres for several hundred hours. These particles were then used to manufacture nanocontactos. We EDX analysis of the degrees of oxidation, we find that for devices made of fine particles of iron with degree of oxidation between 2.5% and 3.3% can be obtained high variations in the magnetorresistencia ballistics (BMR) and keep them stable for over a year . The technique of lithography electrónoca currently used routinely in the semiconductor and microelectronics industries. This technique is the best way to manufacture the devices based MRB. The technique e-beam can be used in the manufacturing process of nanoconstricciones with different dimensions. In chapter 5, explores nanoconstricciones ferromagnéticas of different dimensions manufactured with the technical e-beam. First, nanoconstricciones ferromagnéticas have been fabricated on silicon with a number nanométros silica on the surface, silica is to prevent the flow passing through silicios. But nanoconstricciones ferromagnéticas fabricated on silicon coated with silica can not be maintained stable for a long time due to thermal effects. The surface of the silicon is more smooth silica and the process that emits heat is homogeneous. To produce nanoconstricciones stable, nanocontactos are deposited directly on silicon. So far, we have managed to manufacture nanoconstricciones of 30nm wide, the resistance of these nanoconstricciones is 1000 ohms about changes in resistance around 8 ohms, and some samples can be kept stable until 400K with up to 2 mA current (DC) . The results show that nanoconstricciones here studied may have applications in the electronics industry spin. Finally in Chapter 7 presents the conclusions are derived from work done in this thesis.

Author: DE BLAS DEL HOYO ALFREDO.
Year: 2005.
University: POLITÉCNICA DE CATALUÑA [www.upc.edu].
Place of defense: SALA D'ACTES EUETIB.
Place of preparation: COMTE D'URGELL, 187 DESPATX BA17 BARCELONA.

Summary: The research work focuses on the modeling of the magnetic hysteresis and its application to the numerical calculation. In particular it is to provide a system that allows calculation considering the effect of hysteresis in determining the distribution of the magnetic field in ferromagnetic cores of electrical machines. What is described here extrapolates easily to other systems that employ ferromagnetic materials. We have analyzed and compared the model most widely hysteresis. The result of the comparison, we have chosen the most suitable model for our scope, the model Preisach. This model is analyzed extensively, not only in their bases and properties, but also compares the various methods of characterization and development, identifying what is the most appropriate option. Our proposal is modeled on Preisach characterized by the method of Mayergoyz and developed directly from its definition algebraic or through comprehensive Everett. Based on an idea by G. Bertotti, we developed a model Preisach dynamic. In addition we propose a model Preisach completely reverse necessary in trouble where the diffusion equation of the magnetic field is formulated in AU. To characterize the model Preisach through the method of Mayergoyz we have developed a test based on the ballistic method that allows for an economical way to obtain the necessary experimental data. It also investigates how to obtain these experimental data using the method histeresigráfico, this team developed a computer-controlled automatic also we use to determine the magnetic characteristics of the samples. The combination of model Preisach with numerical methods is performed by an application, determining the field, magnetic flux and disgregadas losses in the nucleus of a single-phase transformer of 220V/380V and 1.3 KVA.

Author: Martí Diaz Zeus.
Year: 2006.
University: AUTÓNOMA DE BARCELONA [www.uab.es].
Place of defense: L.de Llum de Sincrotró.
Place of preparation: Laboratori de Llum de Sincrotró.

Summary: In Chapter 1 presents the inverters within the sincrotrones, in summary are also its main properties. Chapter 2 provides an intensive and exhaustive description of the techniques and methods of characterization and implementation of inverters. Chapter 3 contains a number of different studies on different methods for disenyo an inverter. These methods have been developed by the author in the implementation of dissenyo of 2 of 4 future inverters for the future sincrotron ALBA in Barcelona. Chapter 4 descrive the montage of an inverter with polarization helicoà ¯ dal. This section highlights the problems caused by inhomogeneidades of blocks magnèticos. In chapter 5 proposes a new method of managing blocks for contrarestar the problem identified in the preceding paragraph.

Author: ILLA TORTÓS XAVIER.
Year: 2006.
University: BARCELONA [www.ub.es].
Place of defense: FACULTAD DE FÍSICA.
Place of preparation: FACULTT DE FÍSICA (UNIVERSIDAD DE BARCELONA).

Summary: In this dissertation is a study of the properties and applications of the Ising model with random fields (RFIM). This model combined with a dynamic metastable at zero temperature contains the basic ingredients that enable model the hysteresis phenomenon independent of the pace of change of external field. Calculations have been made of the analytical model networks Bethe, because these networks have a topological properties allowing a treatment analytical model. We have calculated exactly all the terms of hamiltoniano model with a dynamic metastable that considers only the individual spines (1-spin-flip). And using the analytical expressions past has shown equivalence between the model and field half Bethe network connectivity with infinite. It has also obtained an analytical expression for mgnetización model with a more cooperative believes that the relaxation of couples spines (2-spin-flip). We have obtained the statistical properties of lso fields caused by barriers in the energy system (pinning fields), and exploring the scenario of the energy system. The model has been studied diluted with intent to generalize disorder into the system. For this model with the widespread lawlessness we have obtained the same critical exponents demonstrating that the model numerically diluted is dominated by the same critical point that the original model. From RFIM has obtained a new model that can explain the Exchange Bias, a phenomenon used for technological purposes, such as in data storage on magnetic disks. Using this model have been able to obtain the curbas first imantación (FORC), and since these curbas were obtained diagrams FORC's. Finally, it has studied the behavior of systems atérmicos at finite temperature when driven with a variable extension (magnetization, volume, etc.). Rather than a variable intensive (external field, pressure etc..), And it has become a study the RFIM with zero temperature when it is driven by the magnetization.