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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/102594
Magnetic deflagration in Mn₁₂-ac and Nd₅Ge₃ : new techniques and phenomena
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[eng] The goal of this work is to investigate the magnetic deflagration phenomena in two very different magnetic systems, using two very different experimental techniques. In the first chapter of this dissertation I introduce the concept of magnetic deflagra-tion, together with a description of the state of the art of the field. In the second chapter of this thesis I present the study of magnetic deflagration in single crystals of the prototypical single molecule magnet Mn12—ac, using the magneto-optical imaging method; never done before. In the chapter the reader will find out that, effectively, the deflagration process can be explored using this technique. The second part of the thesis include the other five chapters. Such a difference in length between both parts is due two factors. The first factor is that the Mn12—ac is a very well known system, with few new experiments to study; while on the contrary, the Nd5Ge3 intermetallic compound has been barely studied in its single crystalline form, and therefore there are many experiments to conduct to understand its dynamics. The second factor is the rich phenomena that this intermetallic compound presents directly related with the abrupt magnetic changes that possesses. In this system sudden changes occur in every studied physical property when the magnetization changes in a stepped manner. Two fundamental properties of the magnetic dynamics of the Nd5Ge3 system make it so remarkable. First, it is one of few systems with a large irreversible AFM-+FM transition induced by the magnetic field, and second, the magnetic changes during its magnetization processes occur in a stepped manner. Therefore, the motivation to investigate the possibility of finding for the first time magnetic deflagration phenomena in a ferromagnet, and also for the first time in two different magnetic phases of a given material, was very high. In the first two chapters dedicated to the Nd5Ge3 I explore the magnetic properties, the heat capacity and the electric resistivity. Some of the measurements were not previously reported in a single crystal. In the fifth chapter I explore explicitly the spon-taneous field-induced avalanches in magnetism, heat capacity and resistivity. The reason of this three chapters is to have as much ingredients as possible to be able to answer the questions that the possible magnetic deflagrations would raise. I say possible because it is not until the fifth chapter of this thesis when we get the experimental confirmation that the field-induced spontaneous avalanches that the compound present correspond to magnetic deflagration phenomena. In this chapter the magnetic deflagrations were studied in detail in the AFM-+FM transition and in the FM reversal, as a function of the magnetic field and the temperature. The spontaneous and induced deflagrations are explored, obtaining good estimations of the propagation speed and the temperature of the deflagration flame. Using the theoretical framework of the magnetic deflagrations we fit the speed propagation of the front to the experimental data using only one pa-rameter, the heat diffusivity of the material; obtaining a value within the range of heat diffusivities found in other intermetallic compounds.
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VILLUENDAS PELLICERO, Diego. Magnetic deflagration in Mn₁₂-ac and Nd₅Ge₃ : new techniques and phenomena. [consulta: 5 de desembre de 2025]. [Disponible a: https://hdl.handle.net/2445/102594]