I. Tomas

Optimization of single-yoke magnetic testing by surface fields measurement

The main aim of the work is to discuss ways of optimizing a single-yoke magnetization set-up with respect to fluctuations of the measurement results due to imperfections of the yoke?sample contact. The nondestructive testing method is proposed to be optimized by simultaneous multi-point determination of surface magnetic fields of the investigated sample. The presented measurements of the surface fields are accompanied by computational modelling of the magnetic system by the finite element method.

Direct measurement of domain wall coercive field

Reproducible and well defined small hysteresis loops of rhomboid shape were measured on thin uniaxial magnetic garnet films. Observation of the domain structure showed that the change in the magnetic moment described by these loops was due solely to elementary translations of domain walls. The authors have called the half-width of these loops the domain wall coercive field, Hdw; this parameter differed substantially from the half-width, Hc of the major saturation-to-saturation hysteresis loops. They called the rhomboid small loops the domain wall coercive loops (DWCL) and showed that Hdw obtained from DWCL was identical to the coercive field value measured by the low frequency domain wall oscillation method. The slope of DWCL coincided with the initial susceptibility of stripe domain structures and thus made it possible to calculate the basic parameters of the sample, giving results in agreement with other established methods of measurement of these parameters.

Magnetic response to cyclic fatigue of low carbon Fe-based samples

The Preisach model formalism has been applied to analyse hysteresis measurement results for evaluating fatigue damage in Fe-C alloys caused by cyclic fatigue loading. Hysteresis loops and differential permeability curves were measured at various stages of the fatigue life of the samples. The parameters which were built by means of the PMF and the classical hysteresis magnetic parameters (such as saturation magnetization, coercive field and others) were studied as a function of the fatigue lifetime. The present results show that the Preisach model analysis can be used to improve the sensitivity of magnetic hysteresis measurements for non-destructive evaluation of the accumulation of fatigue damage in steel components.

Investigation of two-phase samples by Preisach modelling

Model samples of iron-based alloys containing two magnetically different phases are used to demonstrate applicability of the Preisach model formalism for non-destructive testing of ferromagnetic materials. The suggested material uniformity parameters show their high sensitivity, provide quantitative relation with the relative volumes of the two phases in the samples and do not require saturation of the samples in the measurement.

Temperature dependence of the exchange parameter and domain-wall properties

The A exchange parameter and its temperature dependence were determined for epitaxially grown magnetic thin garnet films. The method is based on the measurement of saturation magnetization, uniaxial anisotropy field, and zero-field stripe domain period. The temperature dependence of domain-wall energy density and domain-wall width was also determined. A correlation was found between domain-wall pinning field and domain-wall parameters, which can be useful for the experimental verification of theories that describe the coercivity on the basis of domain-wall parameters.

The dependence of coercivity on domain structure and domain wall energy of LPE garnet films

The coercivity of epitaxial magnetic garnet films has been measured on both stripe and bubble domain structures by the domain wall oscillation method. The bubble coercivity versus bubble array density relation has been determined experimentally. It was found that on the array reaching a certain density the apparent coercivity was strongly influenced by the neighbouring domains interaction. Also, the coercivity of bubbles was found to be a function of domain wall structure (domain wall energy); the presence of Bloch lines increased the experimentally measured coercivity value.

Investigation of sensitivity of Preisach analysis for nondestructive testing

We applied the Preisach model formalism as a nondestructive material testing method to theoretical systems described by the Jiles-Atherton model. We investigated 1) the influence of stress in the model with magnetomechanical effects on a Preisach-model-based parameter and 2) the sensitivity of the parameter with respect to variation of all the Jiles-Atherton model constants and of the traditional magnetic hysteresis variables.

Temperature dependence of the uniaxial anisotropy of garnets

The uniaxial anisotropy constant K/sub U/ of epitaxial magnetic garnet films was measured between 10 K and 450 K. An exponential decrease of the uniaxial anisotropy with increasing temperature was found. The steep temperature dependence a of K/sub U/ is supposed to be also responsible for the steep temperature dependence of the coercive field, which was reported earlier. >

Evaluation of fatigue damage in steels using Preisach model analysis of magnetic hysteresis measurements

The Preisach model analysis of magnetic hysteresis measurements has been applied to evaluate the microstructural changes in steels subjected to cyclic loading. Families of hysteresis loops were measured to obtain the Preisach-like functions. Barkhausen effect signals were also measured. The Preisach representation was found to be more sensitive to the increase in the number of stress cycles during the stable fatigue stage than the traditional hysteresis loop properties and Barkhausen effect signals.

Application of Preisach analysis to detection of fatigue damage

Measurements of hysteresis loops, initial magnetization curve and families of differential permeability curves were made on a series of 0.05 and 0.1 wt.% C Fe-C samples subjected to fatigue under load control. Preisach-like functions were calculated using the permeability data. Based on these functions a new parameter was identified which was found to decrease monotonically with increasing number of fatigue cycles whereas the classical magnetic hysteresis loop parameters remained constant. This result indicates that the method may be used as a nondestructive evaluation (NDE) technique for estimating expended fatigue life of ferromagnetic materials.