Magnetic Minor Hysteresis Loops Research Articles

Interlayer antiferromagnetic coupling in Tb3Fe5O12/Y3Fe5O12 bilayers

The interlayer antiferromagnetic (AFM) coupling between thin films plays a significant role in the application of spintronics and magnetic memory devices. Previously, we observed AFM coupling phenomenon at low temperatures in rare-earth iron garnet bilayers epitaxially grown on Y3Al5O12 substrates. Here, we report a detailed study on the impacts of various factors, including temperature, crystallographic orientation, and layer thickness, on the AMF coupling and magnetization reversal behavior of such a bilayer system. A simple energy model qualitatively described the coupling behavior of the two layers during the magnetization reversal process. The interlayer coupling strength was calculated by measuring the minor magnetic hysteresis loops. The current results can serve as a reminder for future research on interlayer AFM coupling phenomena and highlight the potential of manipulating the magnetic properties in rare-earth garnet bilayers for spintronics studies and other applications.

Application of Magnetic Adaptive Testing for Nondestructive Investigation of 2507 Duplex Stainless Steel

Duplex stainless steels are two-phase alloys, which contain ferritic and austenitic phases in their microstructure. Their duplex structure provides exceptional resistance to pitting and chloride stress corrosion cracking, and their strength is about twice that of austenitic stainless steels. Due to their good properties, they are widely used in chemical and petrochemical industries as a base material in pressure vessels, pipelines and containers. Duplex stainless steel samples were nondestructively investigated by measuring sets of magnetic minor hysteresis loops using the method called magnetic adaptive testing (MAT). Several series of heat-treated and cold-rolled 2507 duplex stainless steels were measured, and the magnetic parameters were compared with the results of the DC magnetometry of the samples. It was found that the changes in the material properties that were generated by heat treatment and mechanical deformation could easily be followed by magnetic measurements. In contrast to DC magnetic measurements, good correlation was found with the magnetic parameters determined by MAT method and Vickers hardness. Based on our experiments, MAT seems to be a powerful tool for the nondestructive characterization of duplex stainless steels.

Interpretation of Nondestructive Magnetic Measurements on Irradiated Reactor Steel Material

Neutron irradiation-generated embrittlement of nuclear pressure vessel steel was inspected by a nondestructive magnetic method, called magnetic adaptive testing (MAT). This method is based on systematic measurement and evaluation of minor magnetic hysteresis loops. Result of MAT measurement was compared with the result of the traditional Charpy measurement. Good correlation was found between these parameters. One of the main findings of the present work is that the considerable part of scatter of points obtained by magnetic measurement can be attributed to local material inhomogeneity. Another important conclusion is that the embrittlement highly depends on the initial local material conditions, i.e., the initial microstructure, which are very different even within the same block of reactor steel material. By taking this into account, the magnetic descriptors obtain more precise determination of the local embrittlement than the traditionally used destructive mechanical parameters from Charpy data.

A Sensitivity Mapping Technique for Tensile Force and Case Depth Characterization Based on Magnetic Minor Hysteresis Loops

In the nondestructive testing and evaluation area, magnetic major hysteresis loop measurement technology are widely applied for ferromagnetic material evaluation. However the characterization ability of major hysteresis loop measurement technology greatly varies as the evaluated target properties. To solve this limitation, magnetic minor hysteresis loops, which reflect the responses of ferromagnetic material magnetization in a systematic way, is recommend. Inspired by plenty of information carried by the minor loops, the sensitivity mapping technique was developed to achieve the highest sensitivity of minor-loop parameters to the nondestructively evaluated targets. In this study, for the first time, the sensitivity mapping technique is used to measure the tensile force in a steel strand and evaluate the effective case depth in induction-hardened steel rods. The method and procedures for the sensitivity mapping technique are given before experimental detection. The obtained experimental results indicate that the linear correlation between the induced voltage (or the magnetic induction intensity) and the tensile force (or effective case depth) exists at most of the locations in the cluster of minor loops. The obtained sensitivity maps can be used to optimize the applied magnetic field (or excitation current) and the analyzed locations at the minor loops for achieving the highest sensitivity. For the purpose of tensile force measurement, it is suggested that the strand should be firstly magnetized to the near-saturation state and then restored to the remanent state. In this way, the highest sensitivity is obtained as about 15.26 mV/kN. As for the induction-hardened steel rods, the highest sensitivity of magnetic induction intensity to the effective case depth occurs under low magnetic field conditions and the absolute value of the highest sensitivity is about 0.1110 T/mm. This indicates that if the highest sensitivity is required in the case depth evaluation, the induction-hardened steel rods are only required to be weakly magnetized. The proposed sensitivity mapping technique shows the good performance in the high-sensitivity evaluation of tensile force and case depth in ferromagnetic materials and its application scope can be extended to other nondestructive detection fields.

Detection of neutron irradiaton induced degradation of reactor steel by magnetic method

A novel nondestructive method, based on step by step measurement of minor magnetic hysteresis loops was used to determine the neutron irradiation induced degradation of nuclear reactor pressure vessel steel material. This method is called Magnetic Adaptive Testing (MAT), and two types of reactor steel material were investigated: base material type 22NiMoCr37 and weld material type 18MND5. Charpy geometry samples were irradiated by neutrons at the BR2 in SCK•CEN. The neutron irradiation generated structural changes in the material. As a result of our measurement, it was shown that this degradation could be detected by MAT measurements. Charpy impact tests were also done on the investigated samples and the results of these measurement were compared with the nondestructively determined magnetic quantities. It was shown that a closely linear correlation existed between the ductile-to-brittle transition temperature (DBTT) and MAT parameters.

Nondestructive Investigation of Neutron Irradiation Generated Structural Changes of Reactor Steel Material by Magnetic Hysteresis Method

The neutron irradiation embrittlement of four different types of nuclear pressure vessel materials (three base metals and one weld material) were investigated by a magnetic nondestructive testing method, magnetic adaptive testing (MAT). The method is based on the measurement of minor magnetic hysteresis loops on Charpy specimens irradiated by neutrons in the BR2 reactor. Due to the neutron irradiation, the structure of the material was modified. The Charpy impact method is suitable for destructive characterization of material embrittlement. The results of Charpy impact test measurements at SCK CEN Belgian Nuclear Research Centre were compared with the nondestructively measured magnetic parameters. A definite correlation was found between magnetic descriptors and the ductile-to-brittle transition temperature (DBTT), regardless of the type of material or irradiation condition. The results suggest that this “calibration curve“ can be used to estimate the DBTT from non-destructive measurements.

Inspection of Reactor Steel Degradation by Magnetic Adaptive Testing.

Degradation of nuclear pressure vessel steel materials, 15Kh2NMFA type and A508 Cl2 type (definition is given in the text) were investigated by a novel magnetic nondestructive testing method, so-called Magnetic Adaptive Testing (MAT), which is based on systematic measurement and evaluation of minor magnetic hysteresis loops. The measured samples were thermally treated by a special step cooling procedure, which generated structural changes in the material. It was found that this type of degradation can be easily followed by magnetic measurements. Charpy impact test were also performed and the results were compared with the magnetic parameters. In case of 15Kh2NMFA steel, a good, reliable and closely linear correlation was found between magnetic descriptors and transition temperature.

Investigation of Cast Iron Matrix Constituents by Magnetic Adaptive Testing

Two types of cast iron materials-flake graphite cast iron and ductile cast iron-were investigated by the method of magnetic adaptive testing, which is based on the systematic measurement and the evaluation of minor magnetic hysteresis loops. In all cases, even within the same type of material rather big differences existed considering both the chemical composition and the material structure of the investigated sample series. The matrix structure was determined by traditional metallographic examination, and these values were compared with the non-destructively measured magnetic parameters. It was shown that the relative pearlite volume fraction of the metallic matrix of cast iron samples could be determined by this method with a good reliability. The same is true also for the relative chill content of the matrix, as long as the samples contain chill at all. A general expression was found to characterize those correlations, which do not depend on the actual sample and neither on the applied parameters of the concrete magnetic measurement.

Formation and Perpendicular Magnetic Coupling of A1 and L10 CoPt in CoPt/TiN Films on Glass Substrate

We have studied the structure and the magnetic properties of CoPt films by using TiN spacer layer deposited on a glass substrate. We show that after annealing at 700 °C, a highly (001)-textured L10 CoPt structure has been successfully obtained. However, by increasing the annealing temperature to 800 °C, two magnetic phases A1 CoPt and L10 CoPt are formed despite of the better crystallinity of CoPt than that annealed at 700 °C. By measuring the minor magnetic hysteresis loops of the samples after applying a field cooling in a range from room temperature to 500 °C, we have further studied the magnetic coupling between A1 and L10 CoPt phases in the out-of-plane direction of the films. Our work may provide a piece of information for other studies on the heat-assisted magnetic recording.

Calculation and Analysis of Specific Magnetic-Loss Density for Hysteresis due to Steel-Magnetization Reversal over Minor Magnetic-Hysteresis Loops

An interrelation is shown to be valid and grounded between the density of specific magnetic-hysteresis loss Wh and of steels measured over minor magnetic-hysteresis loops under magnetization reversal, as well as technical saturation magnetization Ms, coercive force Hcs, remanent magnetization Mrs measured over the saturation-hysteresis loop, and maximum strength of the magnetizing field Hm measured over a minor hysteresis loop. A formula is found that allows one to calculate Wh for steels based on their Ms, Hcs, Mrs, and Hm. The features of the effect of the magnetic properties of steels exerted on Wh depending on Hm are analyzed. It is established that, with increasing Hm, the changes in Mrs affect the value of Wh to a lesser extent. At the same time, the increase in Mrs always causes an increase in Wh. On the contrary, the increase in Hcs leads to increasing Wh only in the case of steel magnetization reversal in strong fields. In the case of Hm<Hcs, the dependence of Wh(Hcs) becomes inverse. This phenomenon is a physical property of ferromagnetic materials under magnetization reversal in weak magnetic fields. Another consequence of this property consists in a change in the dependence of Wh for carbonized heat-treated steels on tempering temperature Tt from inverse to direct at small Hm. So, for example, at small Hm, the dependence of Wh(Tt) for steel 30 becomes monotonous for the entire feasible range of Tt. This is a prerequisite for using the Wh parameter measured in a small magnetizing field and for quality control of mid- and high-temperature tempering of medium-carbon steels.

Graphite structure and magnetic parameters of flake graphite cast iron

Different matrix and graphite morphologies were generated by a special heat treatment in three chemically different series of flake graphite cast iron samples. As cast, furnace cooled and air cooled samples were investigated. The length of graphite particles and the pearlite volume of samples were determined by metallographic examination and these parameters were compared with the nondestructively measured magnetic parameters. Magnetic measurements were performed by the method of Magnetic Adaptive Testing, which is based on systematic measurement and evaluation of minor magnetic hysteresis loops. It was shown that linear correlation existed between the magnetic quantities and the graphite length, and also between the magnetic quantities and the relative pearlite content in the investigated cast iron. A numerical expression was also determined between magnetic descriptors and relative pearlite content, which does not depend on the detailed experimental conditions.

Effects of cold rolling on magnetic minor hysteresis loops of duplex stainless steels

We have investigated scaling properties of magnetic minor hysteresis loops for duplex stainless steels with 60% ferrite and 40% austenitic phases, subjected to cold rolling up to 97% reduction. While the Steinmetz law, which is a scaling relation between maximum flux density and hysteresis loss and is generally valid for ferromagnetic steels, fails, that between remanent flux density and hysteresis loss exhibits a power law in the wide low and medium flux density regime, associated with a scaling exponent of . Further, the coefficient drops after rolling reduction of 10%, and then increases with reduction, whereas coercivity monotonically increases. Considering microstructural observations, the coefficient seems to also reflect morphology of the ferritic and austenitic phases in addition to the density of dislocations.

Unexpected Width of Minor Magnetic Hysteresis Loops in Nanostructures

We study the coercive fields and phase stability of magnetic nanostructures in hard axis loops and periodic rotational external magnetic fields over wide ranges of field amplitudes depending on the particle size and shape. For this purpose, we use a finite difference code with a gradient descent energy minimization algorithm. Our numerical simulations show that magnetic nanostructures can, in certain field amplitude regions, exhibit minor magnetic hysteresis loops with higher width than their major counterpart as well as variations in the coercive field for consecutive external field periods. The origin of the variations is chaotic behavior of the underlying magnetization processes leading to different residual domains after each field cycle.

Optimization of fatigue damage indication in ferromagnetic low carbon steel

Fatigue damage was investigated by the method of magnetic adaptive testing (MAT), which is based on the systematic measurement and evaluation of minor magnetic hysteresis loops. A large number of magnetic measurements were performed on a single reference series of low carbon steel flat samples, which were fatigued by cyclic bending in an identical way, up to an increasing level of fatigue damage. The measurements of the magnetic properties of these samples were repeated under varied conditions, including speed of magnetization of the samples, sample temperature during the measurement, choice of the evaluated signal, frequency of the voltage sampling, and range of the applied amplitudes of the magnetizing field/current. Special attention was turned to the influence of the thickness of the non-ferromagnetic spacers positioned between the surface of the samples and the flat fronts of the attached magnetizing yokes. On one hand, the spacers decrease the values of the induced signal and its derivatives, but on the other hand they substantially increase the reproducibility of the measurement and positively influence the shapes of the resulting degradation curves. Optimum conditions for the magnetic measurement of the fatigue damage were searched, found, and recommended. The results indicate the reliable applicability of MAT to detect early stages of the material fatigue, and to predict its residual lifetime.

Nondestructive characterization of flake graphite cast iron by magnetic adaptive testing

Three series of flake graphite cast iron samples having different chemical compositions and different heat treatments within each series were investigated by the method of magnetic adaptive testing. The flat samples were magnetized by an attached yoke, and sensitive descriptors were obtained from the proper evaluation, based on the measurements of series of magnetic minor hysteresis loops, without magnetic saturation of the samples. Results of the non-destructive magnetic tests were compared with the destructive mechanical measurements of Brinell hardness and linear correlation was found between them in all cases, where the influence of chemical composition and influence of heat treatment were considered.

Nondestructive indication of fatigue damage and residual lifetime in ferromagnetic construction materials

A new revolutionary attitude toward investigation of fatigue damage in cyclically loaded steel samples is reported. The measurement is based on the method of magnetic adaptive testing, which-–in contrast to traditional magnetic hysteresis investigations-–picks up the relevant information from systematic measurement and evaluation of whole minor magnetic hysteresis loops and their derivatives. Satisfactory correlations between nondestructively measured magnetic descriptors and actual lifetime of the fatigued material were found. The presented method is able to serve as a powerful tool for indication of changes, which occur in the structure of the inspected objects during their industrial service lifetime, as long as they are manufactured from ferromagnetic materials.

Effect of Neutron Flux on Magnetic Hysteresis in Neutron-Irradiated Pressure Vessel Steels

The effect of neutron flux on magnetic minor hysteresis loops has been investigated on nuclear reactor pressure vessel steels, which were irradiated to a fluence of 3.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A minor-loop coefficient, which is an indicator of internal stress, exhibits a local maximum at a fluence of ~1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> n/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , whose position shifts to a low-fluence regime with decreasing neutron flux. Introducing an effective fluence, used to correct the flux effect of irradiation hardening, the data obtained by different flux were found to almost fall on single curve for some alloys. This implies that the flux effect on magnetic property is dominated by efficiency of radiation-enhanced diffusion of solute atoms, such as Cu, as in the case of irradiation hardening.

Flake Graphite Cast Iron Investigated by a Magnetic Method

The method of magnetic adaptive testing (MAT) was applied for the investigation of flake graphite cast iron samples having various metallic matrix and graphite structures. MAT is typical by its low required magnetization of samples, because it is based on the measurement of families of minor magnetic hysteresis loops. The flat samples were magnetized by an attached yoke and sensitive descriptors of their magnetic/structural state were obtained from the evaluation of the measured data. Metallographic examination of the matrix and graphite structures was performed and results of the nondestructive magnetic tests were compared with these data. A very good correlation was found between the magnetic descriptors and the graphite morphology. With these results, MAT is suggested as a highly promising nondestructive alternative of destructive tests for monitoring structural changes in cast iron and other ferromagnetic objects.

The influence of a combined strain-heat treatment on the features of electromagnetic testing of fatigue degradation of quenched constructional steel

The possibilities of the magnetic and eddy-current methods for testing fatigue degradation during low-cycle loading of quenched steel 50 (0.51% C) that was subjected to a combined strain-heat treatment according to an optimal regime that included friction treatment with subsequent tempering at T = 350°C, were investigated. It is shown that for steel that was subjected to a combined nanostructuring treatment, the accumulation of a plastic strain under “hard” cyclic loading can be tested using the coercimetric method and values of the residual magnetic induction on the major and minor magnetic-hysteresis loops, values of the maximum and initial magnetic permeabilities, and readings of an eddy-current instrument at a low excitation frequency of the eddy-current transducer. The appearance of surface fatigue cracks can be tested via eddy-current measurements at high frequencies, when the contribution of the crack formation in the hardened layer to the eddy-current characteristics is considerable.

Complex Characterization of Degradation of Ferromagnetic Materials by Magnetic Adaptive Testing

Tensile tests of plastically deformed specimens were performed in order to investigate the relation between variation of magnetic characteristics and residual plastic strain. Deformed steel (CSN 12050) specimens were investigated by the method of magnetic adaptive testing (MAT), measuring and evaluating series of minor magnetic hysteresis loops. It was shown that an efficient combination of the MAT parameters yields a reliable and unambiguous correlation with residual strain of the specimens even though all relations between the strain and each of the individual MAT-parameters were nonmonotonous.