Low-carbon steel samples deformed by cold rolling—analysis by the magnetic adaptive testing

Three series of samples of low-carbon steel were investigated by the method of magnetic adaptive testing (MAT). The samples were plastically deformed by cold rolling to five consecutive stages of deformation. Samples in one series were magnetically closed, those in the other two series were magnetically open. The presented results of MAT - typical by its low required magnetization of the samples - show highly sensitive and reliable correlation with plastic deformation and as a consequence also with mechanical embrittlement of the investigated material, regardless of the sample shape.     

Governing conditions of repeatable Barkhausen noise response

The paper is devoted to the establishment of experimental conditions, which ensure the repeatability of magnetic Barkhausen noise testing in practice. For this task, the measurements were performed on open flat samples using different experimental configurations, including: different magnetization frequencies, sampling rates, and filter cut-off frequencies; using a sample-wrapped coil and using attached pick-up coils of various dimensions, with different lift-offs of a single yoke magnet and of the attached coil. The sample magnetization was controlled by a vertical array of three Hall sensors; their readings were extrapolated to the sample surface to precisely define its field. After analysis of the results, a scheme for an optimized sensor with a controlled field waveform was suggested to improve the measurement repeatability. The important issues of signal processing and parameter applicability were also discussed in detail.     

Magnetic characterization of HSLA steel by power-law decay exponents of Barkhausen emission signal

The general trend of magnetic behaviour of materials is that the mechanically hard materials are also magnetically hard. However for the high strength low alloy (HSLA) steel tempered at various aging temperatures, the correlation was reported as negative. The anomaly could not be explained by the magnetic parameters like RMS voltage calculated from the Barkhausen emission signal and the coercivity from the magnetic hysteresis loop. This paper reports another magnetic parameter known as power-law decay exponent which shows excellent correlation with the mechanical properties and thus explains the progressive evolution of the microstructural constituents in HSLA steel.     

Investigation of applicability of extrapolation method for sample field determination in single-yoke measuring setup

In this work the single-yoke measuring technique is proposed to be optimized by extrapolation of a magnetic field profile to the sample surface for determination of the “real” field inside the sample. It has been shown that this approach gives reasonable values of magnetic parameters and allows to solve the well-known problem of considerable fluctuations of the measurement results due to imperfections of the yoke–sample contact. The magnetization process with the single-yoke setup is considered on basis of the surface field measurements around the sample and their extrapolation to the sample surfaces. Advantages as well as drawbacks of the measuring procedure and of the suggested optimization method are discussed.     

Magnetic field computation in a non-oriented sheet cross-section considering the hysteresis phenomenon

The aim of the present work is the numerical computation of the average magnetic induction in the cross-section of a non-oriented 3% Si-Fe sheet by solving the magnetic diffusion equation. Jiles’ dynamic model is used to describe the magnetization law. The obtained results are compared with those of the measurements carried out for frequencies of 0.5, 50, 200 and 500 Hz. A satisfactory agreement is obtained between both types of results.     

Comparison of engineering methods of loss prediction in thin ferromagnetic laminations

The loss predictive methods based on the static and dynamic components of power loss are compared with the methods where the total loss is subdivided into hysteresis, classical and excess components. It is explained why the simplest two-component methods can be preferable. An approach to the characterization of a given steel is outlined.     

Similarity rules of magnetic minor hysteresis loops in Fe and Ni metals

We have studied similarity rules of quasistatic minor hysteresis loops for Fe and Ni single crystals in the wide temperature range from 10 to 600 K. Two similarity rules of M_R*/M_a*∼3/4 and W_R*/W_F*∼1/6, were found in a medium field range where irreversible movement of Bloch walls plays a crucial role for magnetization; M_a*, M_R*, W_F*, and W_R* are magnetization, remanence, hysteresis loss, and remanence work of a minor hysteresis loop. The similarity rules hold true, being almost independent of kinds of ferromagnets, applied stress, and temperature. The origin was discussed from the viewpoint of pinning effects due to dislocations as well as eddy current effects which become predominant at low temperatures for samples with low dislocation density.     

The effect of compound addition Dy2O3 and Sn on the structure and properties of NdFeNbB magnets

NdFeNbB with the additions of Dy₂O₃ and Sn permanent magnets have been attained by means of powder-blending technique, and their magnetic properties, temperature performance and microstructure were studied in this paper. The addition of just 2.0 wt% Dy₂O₃ or 0.3 wt% Sn proved to be very effective in improving the permanent magnetic properties of NdFeNbB magnets. Dy₂O₃ additions result in the increase in the H_ci and temperature dependence due to the increase of T_c, formation of (NdDy)-rich phase and grain refinement of Φ phase. This improvement of the coercivity stability of the magnets from the addition of Sn is attributed to the smoothing effect of the Sn addition at the grain boundaries. The magnetic properties, the temperature dependence and Curie temperature of NdFeNbB with Dy₂O₃ and Sn combined addition were found to be considerably improved. From the X-ray diffraction, SEM-EDAX studies and the thermo-magnetic study, the improved properties due to the solution of Dy and Sn to the Φ phase, the reduced N_eff and the smaller Φ phase.     

Dependence of the power losses of a non-oriented 3% Si-steel on frequency and gauge

The power losses of a non-oriented 3% Si-steel rolled to gauges between 0.05 and 2 mm and heat-treated thereafter have been measured under sinusoidal polarizations at frequencies between 15 Hz and 10 kHz. The losses were analysed using a loss separation model based on statistical theory. For the thick samples the skin effect caused the model to fail above a certain frequency, while for the very thin samples the model seems to describe the losses well at all frequencies studied.     

Lack of magnetoacoustic emission in iron with 6.5% silicon

The phenomenon of magnetoacoustic emission (MAE) has been ascribed usually to one of two origins: either (1) motion of non-180° domain walls or (2) creation or annihilation of domains. In this paper, we present strong evidence for the argument that the only origin for MAE is motion of non-180° domain walls. The proof is evident as a result of measurements of zero MAE for a wide range of stress in the isotropic zero magnetostrictive polycrystalline alloy of iron with 6.5% silicon. We also explain why it was that the alternative origin was proposed and how the data in that same experiment can be reinterpreted to be consistent with the non-180° wall motion origin.     

Optical and magnetic properties of (Ga1−xMnx)N/GaN digital ferromagnetic heterostructures

(Ga_1−xMn_x)N/GaN digital ferromagnetic heterostructures (DFHs) and (Ga_1−xMn_x)N/GaN grown on GaN buffer layers by using molecular beam epitaxy have been investigated. The photoluminescence (PL) spectra showed band-edge exciton transitions. They also showed peaks corresponding to the neutral donor-bound exciton and the exciton transitions between the conduction band and the Mn acceptor, indicative of the Mn atoms acting as substitution. The magnetization curves as functions of the magnetic field at 5 K indicated that the saturation magnetic moment in the (Ga_1−xMn_x)N/GaN DFHs decreased with increasing Mn mole fraction and that the saturation magnetic moment and the coercive field in the (Ga_1−xMn_x)N/GaN DFHs were much larger than those in (Ga_1−xMn_x)N thin films. These results indicate that the (Ga_1−xMn_x)N/GaN DFHs hold promise for potential applications in spintronic devices.     

Determination of the losses induced by irreversible barkhausen jumps of the 180° domain wall moving with a sinusoidal motion in the single domain wall model

The paper presents an analysis of the losses due to irreversible Barkhausen jump of the 180° Bloch wall in the single-domain wall model. The field-intensity and current-density vectors at unit input were determined for a single instantaneous jump. The losses were calculated for the case of the wall moving sinusoidally. The Parseval formula was used at determining the losses, which made it possible to perform calculations. A similar problem was considered by Allia and Vinai who adopted a simplified assumption of the domain of an infinitely long cylindrical shape.     

Preparation and magnetic properties of La-substituted Zn–Cu–Cr ferrites via a rheological phase reaction method

Nanocrystalline La-substituted Zn–Cu–Cr ferrites Zn_0.6Cu_0.4Cr_0.5La_xFe_1.5−xO₄ (x=0.00, 0.02, 0.04, 0.06) were prepared by a rheological phase reaction method. The obtained powders were characterized by X-ray diffractometer, transmission electron microscopy and vibrating sample magnetometer. Permeability of the samples was investigated using an impedance analyzer. The results indicated that ferrite samples had the single spinel phase at low La content. Lattice parameter increased with increasing La content, while particle size calculated from Scherrer's formula decreased with increasing La content in La-substituted ferrite samples. The magnetic properties of La-substituted ferrites were strongly affected by La content. The saturation magnetization decreased, while coercivity increased with increasing La content. The variation of real permeability with La content was investigated in the frequency range of 1 MHz–1 GHz.     

Synthesis and magnetic properties of off stoichiometric Laves phase Fe2Nb amorphous alloy

The formation process and magnetic properties of the distorted Laves phase compound (Fe₂Nb) of the nominal composition Fe₆₀Nb₄₀ in its amorphous phase prepared by mechanical alloying have been investigated. The effect of milling time on the formation of amorphous phase has been studied using the X-ray diffraction technique. Further characterizations were carried out by particle size measurement, energy dispersive X-ray microanalysis (EDAX), dc magnetisation and ferromagnetic resonance (FMR) studies. Magnetisation measurement shows that the deviation from the stoichiometric composition in the Nb rich side enhances the compositional short range order and favours the moment formation responsible for weak ferromagnetic behaviour whereas ferromagnetic resonance spectra show some sort of disorder/strain introduced during the mechanical alloying process. The method of preparation also affects the magnetic behaviour of nominal composition Fe₆₀Nb₄₀.     

A study of the stability of pyrolytic carbon-coated Fe/SiO2 composites in HNO3 and the effect of pyrolysis temperatures on their magnetic properties

Fe/SiO₂ composites prepared by a sol-gel combined hydrogen reduction method were coated with carbon by the pyrolysis of acetylene. Scanning electron microscopy (SEM) showed that the as-prepared sample consists of only carbon-coated Fe/SiO₂ particles without the presence of one-dimensional carbon or any other unwanted carbon formation. The stability of the Fe/SiO₂ composites under a HNO₃ solution at room temperature was greatly enhanced after they were coated with carbon. The effect of pyrolysis temperatures was also studied.     

The effect of aging time and calcination temperature on the magnetic properties of α-Fe/Fe3O4 composite

Composites of α-Fe/Fe₃O₄ having dimensions in the range of 100–150 nm have been prepared by disproportion method. The structure and morphology are investigated by XRD and TEM. XRD shows that the metal has got the BCC structure. TEM shows balls of metallic iron about 100-nm-wide stuck to magnetite grains. Magnetic measurement shows that the sample aged for 3 h and calcined at 200 °C has the maximal saturation magnetization corresponding to the highest concentration of α-Fe in the final sample.