Magnetic structure | EPFL Graph Search

The term magnetic structure of a material pertains to the ordered arrangement of magnetic spins, typically within an ordered crystallographic lattice. Its study is a branch of solid-state physics. Magnetic structures Most solid materials are non-magnetic, that is, they do not display a magnetic structure. Due to the Pauli exclusion principle, each state is occupied by electrons of opposing spins, so that the charge density is compensated everywhere and the spin degree of freedom is trivial. Still, such materials typically do show a weak magnetic behaviour, e.g. due to Pauli paramagnetism or Langevin or Landau diamagnetism. The more interesting case is when the material's electron spontaneously break above-mentioned symmetry. For ferromagnetism in the ground state, there is a common spin quantization axis and a global excess of electrons of a given spin quantum number, there are more electrons pointing in one direction than in the other, giving a macroscopic magnetization

The effect of Cr composition on the local atomic and magnetic structure of pristine and ion-irradiated FeCr alloys

Andi Idhil Ismail

High Cr (9-14% Cr) ferritic/martensitic steels are considered as one of the most promising candidates for structural materials for advanced nuclear power plants. Therefore, the understanding of the evolution of properties under operation conditions is of primary importance. This work focuses on the effect of Cr composition on the local atomic and magnetic structure of pristine and ion-irradiated FeCr alloys. The materials considered for this study are commercial purity FeCr binary alloys with varying Cr content up to 16% at.Cr, which were also implanted with heavy ions (Fe+) under different conditions. Using a combination of spatially resolved and element specific techniques the following results were obtained: - The dependence of the total (bulk) magnetic moments is linearly decreasing as the Cr-content is increasing - The Fe magnetic moments at the surface reduced by the presence of impurities and defects. The dependence on Cr-content is non-linear, with a tendency of increasing the spin moments above 10% Cr. - The Cr spin moments exhibit a reorientation transition around 9% Cr from ferromagnetically to anti-ferromagnetically aligned with respect to the Fe spin moments. - The effects of ion irradiation are fairly complex, with an overall higher spin magnetic moment after exposure to the heavy ions. The ion dose has a reduced effect on the spin moments while the target temperature shows a more significant, but scattered influence on the Fe spin moments. - All specimens show a decrease of the Cr nearest neighbor distance for Cr contents between 6 – 10% Cr. Also, the atomic disorder around the Cr atoms is enhanced compared to the Fe atoms. - For the Fe9% Cr irradiated alloy the effect of elevated temperatures and dose is clearly seen as a reduction of atomic coordination numbers around the Cr atoms, which can be associated with formation of vacancies. - For the Fe5% Cr and Fe12% Cr alloys, the number of nearest neighbors around the Cr atoms is not sensitive to irradiation.

EPFL2013

Visualisation of magnetic domain structures and magnetisation processes in Goss-oriented, high permeability steels using neutron grating interferometry

Benedikt Karl-Josef Gerhard Betz

Industrial transformers cores are built from stacked sheets of an iron silicon alloy, called laminations. The magnetic domain structures of these highly anisotropic electrical steels with a sharp (110)[001]-texture, the so-called Gosstexture, determines the magnetic properties of transformers. Commonly used investigation techniques for these laminations are inductive B-H-measurements. This technique reveals global magnetic properties such as hysteresis, remanence, saturation and losses. Locally resolved information about the underlying domain structure cannot be obtained. In this thesis, the neutron grating interferometer (nGI) is used to investigate the domain structures in Goss-oriented (GO) electrical steels at the Swiss Spallation Neutron Source using the cold neutron imaging facility ICON. In contrast to the attenuation based transmission image, the dark-field image (DFI) is related to multiple refraction of unpolarised neutrons at magnetic domain walls. Thereby the use of the DFI allows for the visualisation of bulk magnetic domain structures in two and three dimensions with a spatial resolution of down to 70 um in a field of view of 64mm x 64mm. The DFI is used for the visualisation of the locally resolved response of the bulk magnetic domain structures under the influence of externally applied magnetic fields. In the first part, the domain formation and growth along the initial magnetisation curve (0-6000 A/m) up to saturation in static DC magnetic fields was studied. For decreasing field values, the visualisation of the recurrence of the hysteretic domain structure down to the remanent (0 A/m) state is given. A correlation of the grain orientation and the corresponding basic and supplementary domain structure is given. In the second part, the DFI is used to investigate the response of magnetic domain walls to dynamic AC magnetic excitations. The visualisation of the domain wall motion under influence of alternating magnetic fields is performed. In detail, scans combining varying levels of an offset DC (0-30 A/m), oscillation amplitude A (0-1500 A/m), and oscillation frequency f (0-200 Hz) are conducted. By increasing the amplitude while maintaining constant values of DC and f, the transition from a frozen domain wall structure to a mobile one is recorded. Vice versa, increasing f while keeping A and DC constant led to the reverse transition from a mobile domain wall structure into a frozen one. It is shown that varying both, A and f shifts the position of transition region. Higher frequencies require higher oscillation amplitudes to overcome the freezing. The DFI allows for the analysis of the laminations coatings impact to the magnetic structures. To visualise the stress effect of the coating to the underlying domain formation an uncoated lamination is investigated with stepwise increasing applied external tensile stresses up to 20MPa. The domain configurations of the intermediate stress states are imaged and the original domain structure of the coated state is reproduced. To verify the results, complementary experiments using Kerr microscopy, Faraday imaging, small-angle neutron scattering and Laue X-ray diffraction were performed. The here presented findings allow for new insights into macromagnetism and enable new approaches in the field of descriptive models for bulk macromagnetism. Furthermore the results have the potential to further improve the properties of GO-steels used in industrial transformer applications.

EPFL2016

Bootstrap current compensation with electron-cyclotron waves in 3D reactor-size configurations

Sergi Ferrando I Margalet

In magnetic confinement devices, the inhomogeneity of the confining magnetic field along a magnetic field line generates the trapping of particles (with low ratio of parallel to perpendicular velocities) within local magnetic wells. One of the consequences of the trapped particles is the generation of a current, known as the bootstrap current (BC), whose direction depends on the nature of the magnetic trapping. The BC provides an extra contribution to the poloidal component of the confining magnetic field. The variation of the poloidal component produces the alteration of the winding of the magnetic field lines around the flux surfaces quantified by the rotational transform ι. When ι reaches low rational values, it can trigger the generation of ideal MHD instabilities. Therefore, the BC may be responsible for the destabilisation of the configuration. This thesis is divided into two parts. In the first part, we present a self-consistent method to calculate the BC and assess its effect on equilibrium and stability in general 3D configurations. This procedure is applied to two reactor-size prototypes (both with plasma volumes ∼ 1000m3): a quasi-axisymmetric (QAS) system and a quasi helically symmetric (QHS) system with magnetic structures that develop BC in opposite directions. The BC increases with the plasma pressure, therefore its relevance is enhanced when dealing with reactor-level scenarios. The behaviour of both prototypes at reactor level values of β ≡ (kinetic plasma pressure)/(magnetic pressure) is assessed, as well as its alteration of the equilibrium and stability. In the QAS prototype, BC-consistent equilibria have been computed up to β = 6.7% and the configuration is shown to be stable up to β = 6.4%. Convergence of self-consistent BC calculations for the QHS case is achieved only up to β = 3.5%, but the configuration is unstable for β ≥ 0.6%. The relevance of symmetry breaking modes of the Fourier expansion of the confining magnetic field on the generation of BC is studied for each prototype. This proves the close relationship between magnetic structure and BC. Having established the potentially dangerous implication of the BC, principally, in reactor prototypes, a method to compensate its harmful effects is proposed in the second part of the thesis. It consists of the modelling of the current driven by externally launched ECWs within the plasma to compensate the effects of the BC. This method is flexible enough to allow the identification of the appropriate scenarios in which to generate the required CD depending on the nature of the confining magnetic field and the specific plasma parameters of the configuration. Both the BC and the CD calculations are included in a self-consistent scheme which leads to the computation of a stable BC+CD-consistent MHD equilibrium. This procedure is applied in this thesis to simulate the required CD to stabilise the QAS and QHS prototypes introduced in the first part. The estimation of the input power required and the effect of the driven current on the final equilibrium of the system is performed for several relevant scenarios and wave polarisations providing various options of stabilising driven currents. Several scenarios have been devised for each prototype in order to drive current at the appropriate location and with the desired direction. Different polarisations and launching conditions have been employed to this purpose. In particular, a HFS launched X2 ECW with an input power of 1.5MW has been shown to drive sufficient current to maintain the rotational transform below the critical value 2/3 at β = 6.4% for the QAS reactor. Correspondingly, in the QHS reactor, an X3-mode ECW of 100KW was sufficient to drive the current required to push the rotational transform below unity near the magnetic axis at β = 3%. Thus, stabilisation of BC-driven instabilities with externally launched ECWs has been achieved for both contrasting configurations. The method proposed in this thesis allows also the utilisation of EBW in the generation of CD. The possible advantages of EBCD for compensation studies are described as well as their possible application to the two prototypes under consideration. The BC+CD procedure is particularly interesting to investigate new magnetic geometries as potential candidates for fusion reactors. With this numerical tool, it is possible to assess the implications of their consistent BC when operating at reactor level. It also allows to quantify how much power would be required to maintain the system MHD stable in these circumstances. Nevertheless, this method is flexible enough to be applicable to any configuration.

EPFL2006