Numerical study of magnetic processes: extending the Landau-Lifshitz-Gilbert approach from nanoscale to microscale

The micromagnetic theory describes the magnetic processes in magnetic materials on a microscopic time and space length. Therefore, micromagnetic models are since long employed in the design of for instants magnetic storage media as magnetic tapes and Random Access Memory elements, used in computers. The use of efficient numerical techniques and the availability of powerful computers now make it possible to apply the same micromagnetic models on larger and more complex material systems with the aim of increasing our insight in the experimentally observed magnetic phenomena. In this PhD research, an efficient numerical micromagnetic model is developed that enables the analysis of magnetic processes starting from the nanometer space scale up to the micrometer space scale. Therefore, efficient algorithms are presented on the one hand to simulate the ultra fast dynamics of the magnetic processes as described by the Landau-Lifshitz-Gilbert equation. On the other hand, powerful numerical techniques are developed to evaluate the magnetic fields, characteristic to the micromagnetic description, in a fast way. The developed micromagnetic model is validated extensively in comparative studies with other micromagnetic and macroscopic magnetic material models. Moreover, the model is successfully applied in different magnetic research domains: magnetic switching processes in classical samples with nanometer dimensions are analysed, magnetic domains are studied in structures with order micrometer dimensions and magnetic hysteresis properties are investigated

Hysteresis, Avalanches, and Disorder Induced Critical Scaling: A Renormalization Group Approach

We study the zero temperature random field Ising model as a model for noise and avalanches in hysteretic systems. Tuning the amount of disorder in the system, we find an ordinary critical point with avalanches on all length scales. Using a mapping to the pure Ising model, we Borel sum the $6-\epsilon$ expansion to $O(\epsilon^5)$ for the correlation length exponent. We sketch a new method for directly calculating avalanche exponents, which we perform to $O(\epsilon)$. Numerical exponents in 3, 4, and 5 dimensions are in good agreement with the analytical predictions.Comment: 134 pages in REVTEX, plus 21 figures. The first two figures can be obtained from the references quoted in their respective figure captions, the remaining 19 figures are supplied separately in uuencoded forma

Scaling in Magnetic Materials

The chapter presents applications of the scaling in several problems of magnetic materials. Soft magnetic materials (SMMs) and soft magnetic composites (SMCs) are considered. Application of scaling in investigations of problems, such as power losses, losses separation, data collapse of the losses characteristics and modelling of the magnetic hysteresis, is presented. The symmetry group generated by scaling and gauge transformations enables us to introduce the classification of the hysteresis loops with respect to the equivalence classes. SMC materials require special treatment in the production process. Therefore, algorithms for optimization of the power losses are created. The algorithm for optimization processes is based on the scaling and the notion of the pseudo-equation of state. The scaling makes modelling and calculations easy; however, the data must obey the scaling. Checking procedure of statistical data to this respect is presented

Development of a tissue-conducting audio transducer and sensor for mobile use

The ever increasing number of mobile devices with a cellular link as well as services associated with them require innovations in audio technologies. Especially problematic are circumstances, in which high background noise level prohibits communication. This thesis studies a tissue-conducting device for audio reproduction and recording. The proposed concept is not based on producing or sensing pressure changes in air, but in soft tissues. The device considered is an in-ear actuator and sensor that couples to tympanic canal walls. A finite element model of ear is developed for simulating the actuator function. The FEmodel includes a novel idea of using a lumped parameter representation for the middle ear bones. The results are compared with respect to the published data and the approach is found valid. The simulations concerning the actuator function show that the mode is unfeasible due to the energy loss in soft tissues. The result is confirmed by subjective tests. The prototype of the actuator is analyzed with a FE-model. It is observed that the linear FEM cannot account for the observed characteristics in the actuator response. Therefore, a time-domain model accounting for hysteresis is developed. The hysteresis prediction is realized with a rate-independent Preisach model with an addition of a scalar product model for the reversible part of hysteresis. It is shown that the rate-independent Preisach model is not sufficient to predict the response and that a dynamic model is required. In the sensor mode the device works up to 2.5-3 kHz, after which the recorded signal drops below the noise floor. The finding is supported by the literature. The transfer function between the speech recorded with a microphone and the device is observed to have a decreasing trend. The study leaves open, whether this effect is due to the tissue transfer characteristics, sensor coupling to the tissue or sensor properties. Moreover, a comprehensive discussion on the theory associated with the Finite Element Method is given. Both, structural and piezoelectric FEM are covered

Development of a tissue-conducting audio transducer and sensor for mobile use

The ever increasing number of mobile devices with a cellular link as well as services associated with them require innovations in audio technologies. Especially problematic are circumstances, in which high background noise level prohibits communication. This thesis studies a tissue-conducting device for audio reproduction and recording. The proposed concept is not based on producing or sensing pressure changes in air, but in soft tissues. The device considered is an in-ear actuator and sensor that couples to tympanic canal walls. A finite element model of ear is developed for simulating the actuator function. The FEmodel includes a novel idea of using a lumped parameter representation for the middle ear bones. The results are compared with respect to the published data and the approach is found valid. The simulations concerning the actuator function show that the mode is unfeasible due to the energy loss in soft tissues. The result is confirmed by subjective tests. The prototype of the actuator is analyzed with a FE-model. It is observed that the linear FEM cannot account for the observed characteristics in the actuator response. Therefore, a time-domain model accounting for hysteresis is developed. The hysteresis prediction is realized with a rate-independent Preisach model with an addition of a scalar product model for the reversible part of hysteresis. It is shown that the rate-independent Preisach model is not sufficient to predict the response and that a dynamic model is required. In the sensor mode the device works up to 2.5-3 kHz, after which the recorded signal drops below the noise floor. The finding is supported by the literature. The transfer function between the speech recorded with a microphone and the device is observed to have a decreasing trend. The study leaves open, whether this effect is due to the tissue transfer characteristics, sensor coupling to the tissue or sensor properties. Moreover, a comprehensive discussion on the theory associated with the Finite Element Method is given. Both, structural and piezoelectric FEM are covered