Location	Contact Info	Key Specs	Description
306 Durham Hall 354 Durham Hall	David C. Jiles 351 Durham Hall 515-294-8353	Transcranial Magnetic Stimulation (TMS) Non-Destructive Evaluation (NDE) Hysteresis and Computer Simulations

  Transcranial Magnetic Stimulation (TMS) In the search for non-invasive therapeutic treatment of brain disorders, magnetic methods offer promising alternative opportunities.  Deep brain stimulation using magnetic fields is a method that will enable diagnosis of deeper lying regions of the brain in a similar way to present TMS procedures, which are limited to the outer cortical regions of the brain. Among related methods repetitive transcranial magnetic stimulation (rTMS) is a newer and exciting non-invasive technique for stimulating cortical regions of the brain that can be used for diagnostic purposes and has potential for use in therapeutic purposes in various neurological disorders including strokes, Parkinson’s disease, vascular dementia, and psychiatric disorders including post-traumatic stress disorder, major depressive disorder and chronic depression. These have been reported by Elahi et al, Emara et al., Huang et al., Leung et al., Lipton et al., Minamati et al., Slotema et al., Walpoth et al. and Chen et al..  Other medical applications range from detection of myelopathy, detection of upper motor neuron involvement in amyotrophic lateral sclerosis, atypical parkinsonism, movement disorders and stroke rehabilitation.  There is recent evidence that it can be used as an alternative to conventional electroconvulsive therapy (ECT) with milder post-treatment side effects New types of coil configurations for TMS systems have been studied recently by the PI. The new class of coil configurations comprised two coils with different coil diameters, different numbers of turns, different currents in the two coils, and in some cases non-coaxial and non-planar coils. Some of these showed improved penetration of magnetic field into the brain in which the ratio of field strength far from the coil is increased relative to that close to the coil when compared with conventional coils as shown in Fig.1.  The new configurations offer better prospects as a basis for developing coils for improved deep brain stimulation without over stimulating the outer regions of the brain. Finite element calculations on different coil configurations will be carried out to identify the best designs for deep brain stimulation with calculated field distribution inside the human head. Measurement of the fields produced by different coil configurations will be carried out and compared with predictions of model calculations. The Halo coil configuration was invented by the PI while at the Wolfson Centre for Magnetics with access to various custom-built field profiling equipment for TMS coils and modelling facilities. These are needed for estimating magnetic field generated from different coil systems and for electric field generated inside the human head using the latest heterogeneous head model. The student(s) working on this project will use modelling software such as SEMCAD X, MAGNET and COMSOL to calculate the induced electric fields in the brains of humans and animals when stimulated using new TMS coil designs. The student(s) working on this project will also be expected to use magnetic finite element codes together with a realistic model of the head and brain to help design new coils and then to test these new coils with field measuring equipment in order to  improve the depth of penetration and focality of stimulation in TMS. Fig. 1.  (a) The Halo coil combination consists of a standard single or double coil, shown here on top of the head, together with a larger encircling coil that can increase the field strength in deep brain regions. the presence of the encircling coil in the “Halo coil” configuration increases the field strength at a depth of 10 cm by three to four times, while the field on the surface remains the same. (b) Decay of magnetic field strength with distance from the single circular coil both with and without the encircling coil. It can be seen for example that at a distance of 0.1 m (which corresponds to deep brain stimulation) the presence of the encircling coil in the “Halo coil” configuration increases the field strength from about 25 kA/m to about 90 kA/m, while the field on the surface remained much the same. Non-Destructive Evaluation (NDE) Non-destructive evaluation (NDE) is proving to be increasingly important for evaluating the properties of structures particularly to assess the dangers of failure. The need to determine the condition of structures without causing any damage is of interest to manufacturing, construction, service and maintenance industries. Such methods can be used to evaluate the remaining life-time of structures, the quality of products as they are produced and the stress-state in load bearing parts. All of which can help develop sustainable infrastructures. Magnetic NDE exploits the interaction between magnetic properties of materials and their mechanical conditions. Magnetic properties of materials are known to vary depending on their stress state and this has been extensively investigated by the PI. These results can be exploited in developing magnetic sensors that can show the present stress state in the structure and in some cases predict the useful lifetime left in the structure. In magnetic non-destructive evaluation research, we are working to develop a method of detecting the depth dependence of stress using magnetic Barkhausen signals. The uniqueness of this research is that it offers a more informative approach to magnetic NDE providing not just the stress state in structures but how that varies with depth. Using electromagnetic finite element analysis simulation packages, alongside the magnetic Barkhausen signal NDE measurements offers researchers a way to interpret the results of measurements in terms of the state of materials or components as shown in Fig. 2. Hysteresis and Computer Simulations Magnetic hysteresis is a non-linear, path-dependent response of a ferromagnetic material to the variation of an applied magnetic field. Hysteresis results in energy dissipation leading to inefficient energy conversion. Energetic considerations can be used in order to derive equations describing the hysteresis models. Where possible the materials properties (such as permeability, coercivity, remanence, anisotropy and inter-domain coupling) are included as model parameters. However, additional physical parameters (temperature, stress, density of the pinning sites and shape anisotropy) need to be considered to improve the predictive properties of the models. They are related to the physical properties of the materials or connected to the external influences (stress or temperature, for example). All models have limitations and this is true for the earlier models such as the Rayleigh law, as well as for more advanced models such as the Stoner-Wohlfarth model, the Preisach model, the Jiles-Atherton model, or the Hauser model. The aim of this research is to explore unification of the existing models of ferromagnetic hysteresis into a single, multi-scale, unified theory of hysteresis. This unified theory based on physical principles, will include conditions of its validity and will be able to describe hysteresis behavior of different magnetic materials and systems across different length scales. It will also specify conditions under which the existing models will become special cases of the unified theory of hysteresis. Magnetic measurements under various levels of stress and temperature will be performed on specimens covering a broad range of different magnetic properties. The data obtained together with the experimental data found in the literature will be used in order to (a) establish the relationship between model coefficients and microstructure, (b) establish the connection between existing hysteresis models and (c) develop and test a unified model theory of hysteresis. The project will involve a review and examination of the existing models of ferromagnetic hysteresis and the principles from which they are derived. We will investigate the relationship between the existing models of hysteresis through the established relationships between model coefficients and microstructure using stress and temperature as auxiliary variables. We propose the unification of hysteresis models and relate the existing models to the proposed unified theory. We will test the unified theory against experimental data including both data measured in this research and results found in the literature Novel materials (multiferroic, magnetocaloric, magnetoelastic) Magnetocaloric Materials There is a tremendous benefit to be obtained from improving the efficiency of refrigeration and air conditioning. Energy consumption from domestic refrigeration accounts for 7% of the total energy usage in the United States and 6% in the United Kingdom. Industrial refrigeration and office air conditioning both contribute to an even higher percentage of energy usage. The normal liquid vapor refrigerators have a Carnot efficiency of about 30% which is half the Carnot efficiency of solid state gadolinium based magnetic refrigerators. With the discovery of the giant magnetocaloric effect in Gd₅(Si_xGe_1-x)₄, there has been extensive research on magnetocaloric materials and magnetic refrigeration. Magnetic refrigerators built with the new giant magnetocaloric materials such as Gd₅(Si_xGe_1-x)₄ and NiMnFe could have efficiencies even higher than 60%  of Carnot efficiency. Gd₅(Si_xGe_1-x)₄ exhibits one of the highest giant magnetocaloric effects ever observed. This occurs at the first order phase transition which is close to room temperature. It is important to study the properties of Gd₅(Si_xGe_1-x)₄ in order to understand and improve the performance of magnetic refrigerators. In addition Gd₅(Si_xGe_1-x)₄ exhibits other unusual properties at the first order phase transition such as giant magnetoresistance of ΔR/R= 25%, and colossal magnetostriction of the order of 10,000 ppm.  The bulk material Gd₅(Si_xGe_1-x)₄  has been studied by researchers including the PI and one of the co-PIs  in order to study the transition temperatures of various compositions/phases, magnetostriction, and resistivity recovery.   Fig.3 shows the phase diagram for the material with transition temperatures of different phases determined by modified Arrott plot technique or the inflection point method. While the bulk forms of various magnetocaloric materials have been studied widely, there are until now no reports of successful growth of thin films of Gd₅(Si_xGe_1-x)₄ magnetocaloric materials that can have various applications in micro-cooling such as cooling integrated chips, or maintaining lower temperatures to improve operation of disc drive heads. Recently we have successfully produced thin films of Gd₅(Si_xGe_1-x)₄ that show magnetic behavior and Curie temperature similar to the corresponding bulk materials. Magnetostrictive Materials Highly magnetostrictive materials are useful for developing non-contact stress sensors and actuators. Materials derived from cobalt ferrite have a high strain derivative, dλ/dH; a figure of merit which shows how much strain output is achievable with a given applied magnetic field. Cation substitution into cobalt ferrite has proven to be effective in tailoring its magnetic and magnetostrictive properties for intended applications. A research group led by the PI was the first to systematically synthesize and study the new magnetostrictive cation substituted cobalt ferrite composites under NSF funding (Grant No. DMR 0402716). It was discovered that substituting cations results in lower magnetomechanical hysteresis and can also be used to control the Curie temperature of the material. We have studied a number of cation substituted cobalt ferrite materials including Cr³⁺, Ga³⁺, Al³⁺, Ge⁴⁺/Co²⁺, Mn³⁺, Ti⁴⁺/Co²⁺ and Mg²⁺– substituted cobalt ferrites. These derivatives of cobalt ferrite have shown remarkable enhancement of the strain sensitivity of the material (Fig. 4). It has been shown for example that Ge⁴⁺/Co²⁺ co-substituted cobalt ferrite requires only 7 % of the energy that would have been needed if the non-substituted variant was used in a sensor application. Furthermore Al³⁺ substituted cobalt ferrite will require only 3 %. These materials can therefore be used for energy efficient sensors and actuators. The drive for miniaturization, particularly for nanoscale devices dictates that these materials with high sensitivity be made in thin film form. However, there has not been a commensurate effort to study the magnetostrictive  properties of substituted cobalt ferrite. Multiferroic Materials Magnetoelectric multiferroic materials, exhibit unusual physical properties and promise new device applications such as magnetic sensors, generators, filters and so on. Multifunctional devices call for the coexistence of ferroelectricity and ferromagnetism and requirements of a strong coupling interaction between two ferroic orders. I am currently working on the developing high magnetoelectric coefficient material based on CoFe2O4/BaTiO3 composite. We are trying to manipulate the long-range order magnetic and ferroelectric coupling property and electronic conductivity by introducing dopant in both bulk material and then film. The quality and properties of the samples were monitored by XRD, SEM, etc.