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.     

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 surface crystalline layers on asymmetric off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons

The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons was measured using a pick-up coil wound around the sample. One side of a ribbon was etched in hydrofluoric acid solution during various times in order to change the thickness of the surface crystalline layer appearing after annealing. The asymmetric two-peak field dependence of the off-diagonal impedance was observed for all samples. The evolution of the off-diagonal magnetoimpedance with the change in the ribbon thickness is analyzed.     

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.     

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.     

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.     

Loss analysis of non-oriented soft-magnetic materials associating results from hexagonal single sheet testers and Epstein frame testers

The main subject of this work was to analyze magnetic losses of non-oriented soft magnetic materials, which are used in rotating machines like motors or generators, under the conditions of alternating and rotating magnetic flux. For this purpose a hexagonal rotational single sheet tester (HRSST) and a standardized 25 cm Epstein frame tester (EFT) were used, respectively. Since each HRSST sample represents only one random sample, whereas a stack of EFT samples represents a basic population of a production lot, a simple procedure is introduced to derive characteristic rotational loss data from EFT measurements and previously acquired loss factors.     

Lean non-oriented electrical steel grades

A “Lean Grade Policy” has been adopted involving reduced losses of a given steel composition and supplied grades with typical losses closer to the guarantees. Increased final annealing temperatures and modified hot rolling have resulted in reduced losses and improved permeability. A “lean grade” will have higher heat conductivity and permeability compared to a “standard grade”. The policy could result in less variability and better quality control.     

Polarization-dependent fractional loss improvement of laser scribed GO FeSi steels

Magnetization curves of untreated and laser scribed GO FeSi steels were measured for 19 different frequencies from 0.05 to 500 Hz and for four polarizations from 1.4 to 1.7 T. From hysteresis loops, hysteresis losses were separated and frequency-dependent anomaly factors were calculated. Frequency-dependent anomaly factors for all measured polarizations can be very well described by an empirical equation. This behavior can be explained by the fact that an increase in polarization at a fixed magnetizing frequency corresponds to an increase of magnetizing frequency at a fixed polarization. Both an increase in frequency and an increase in polarization activate a higher number of domain walls in the magnetization process. The power losses can be described only by the frequency dependence of the anomaly factor and by the additional knowledge of hysteresis loss.     

Biased magnetization reversal in bi-phase multilayer microwires

We report on the magnetic behaviour of a novel family of two-magnetic-phase multilayer microwires consisting of: (i) a bistable FeSiB glass-coated amorphous microwire as soft nucleus, and (ii) a polycrystalline CoNi outer microtube as harder layer. Such bi-phase microwires are prepared by combined quenching and drawing plus sputtering and electroplating techniques. The stray field produced by the harder outer layer after premagnetizing it to saturation is used to bias the magnetization reversal process of the soft nucleus via dipolar magnetostatic coupling. A detailed analysis of the asymmetric low-field magnetization reversal process of the soft nucleus is presented together with the study of the fluctuating switching field and its asymmetric behaviour. The study of the domain wall characteristics under the presence of a nucleation coil at one end of the microwire allows us to draw conclusions on the role of the bias field generated by the premagnetized hard outer layer.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

Magnetic circuit of a contactless torque sensor for electric power steering

Modern passenger cars are increasingly equipped with electromechanical steering assist rather than hydraulic systems known for many decades. Major benefits are reduced fuel consumption (up to 0.2l/100 km) and increased functionality. As such a system reacts to the drivers input in terms of steering torque or steering effort, a sensor is required that accurately measures the steering torque. Valeo has adopted a magnetic technology and has improved the performance by adding specially designed flux concentration devices. The magnetic circuit consists of a multi-pole ring magnet and a pair of ring-shaped soft magnetic parts rotating together with the steering shaft and an additional pair of soft magnetic flux concentration devices which are fixed stationary inside the housing. The steering torque causes a relative twist between magnet and the soft magnetic rings, therefore implementing a proportional magnetisation of the latter. A U-shape was chosen for the flux concentration devices in order to compensate mechanical tolerances of the system. The main focus of this paper will be on the tolerance behaviour of the sensor system and the impact of the flux concentration devices. Because of the nonlinear nature of the magnetisation curve of the NiFe alloy used extensive 3D FEM simulation was necessary. Simulation enables us to have a look inside the soft magnetic material and predict the spatial magnetisation distribution with the benefit of avoiding saturation. The result is an optimised sensor, which meets both the harsh environmental conditions inside the motor compartment as well as the cost pressure in the automotive business.     

Magnetic domain wall pinning and coercivity in FePt/MgO and FePt/Pt thin films

FePt thin layers have been epitaxied either on Pt(0 0 1) or on MgO(0 0 1) substrates, and magnetically characterized using extraordinary Hall effect magnetometry and magnetic force microscopy. The coercivity originates in both cases from the pinning of domain walls on structural defects. Whereas the coercivity increases with the FePt layer thickness in FePt/Pt samples, it decreases in FePt/MgO samples. This discrepancy is explained on the basis of structural observations, and of atomistic simulations of magnetic domain wall pinning.     

Measurement of resistivity of soft magnetic laminations at elevated temperatures

The variation of resistivity in soft magnetic materials with temperature is extremely important for applications of electrical machines in aerospace applications. The design of a system for rapid and accurate measurement of the resistivity of Epstein strips under a wide range of temperatures is presented. Two cobalt–iron alloys are assessed up to 200 °C, showing significant differences in their resistivity but almost identical temperature coefficients of resistivity.     

Cobalt antidot arrays on membranes: Fabrication and investigation with transmission X-ray microscopy

We have developed a method to fabricate ferromagnetic antidot arrays on silicon nitride membrane substrates for electron or soft X-ray microscopy with antidot periods ranging from 2 μm down to 200 nm. Observations of cobalt antidot arrays with magnetic soft X-ray microscopy show that for large periods, flux closure states occur between the antidots in the as-grown state and on application of a magnetic field, domain chains are created which show a spin configuration at the chain ends comprising four 90° walls. Pinning of the domain chain ends plays an important role in the magnetization reversal, determining the length of the chains and resulting in preservation of the domain chain configuration on reducing of the applied magnetic field to zero.     

Simple recipe to synthesize single-domain BaFe12O19 with high saturation magnetization

We report a new synthesis route for preparation of single-domain barium hexaferrite (BaFe₁₂O₁₉) particles with high saturation magnetization. Nitric acid, known as a good oxidizer, is used as a mixing medium during the synthesis. It is shown that formation of BaFe₁₂O₁₉ phase starts at 800 °C, which is considerably lower than the typical ceramic process and develops with increasing temperature. Both magnetization measurements and scanning electron microscope micrographs reveal that the particles are single domain up to 1000 °C at which the highest coercive field of 3.6 kOe was obtained. The best saturation magnetization of ≈60 emu/g at 1.5 T was achieved by sintering for 2 h at 1200 °C. Annealing at temperatures higher than 1000 °C increased the saturation magnetization, on the other hand, decreased the coercive field which was due to the formation of multi-domain particles with larger grain sizes. It is shown that the best sintering to obtain fine particles of BaFe₁₂O₁₉ occurs at temperatures 900-1000 °C. Finally, magnetic interactions between the hard BaFe₁₂O₁₉ phase and impurity phases were investigated using the Stoner-Wohlfarth model.     

Controlled domain wall motion by current into patterned-U Ni80Fe20 wires

The current-induced domain wall motion was observed experimentally in the case of the domain wall trapped at the semicircular arc within the U shape Ni₈₀Fe₂₀ wire. The measurement of the current-induced domain wall motion was achieved by adding a biased field before switching field and a critical current density was measured. We found two magnetic domain structures in the U pattern. At zero fields, the vortex domain wall nucleated at the semicircular arc of the U pattern. Continuous magnetic state without wall was investigated in near-switching field.     

Enhancement of Curie temperature and room-temperature magnetoresistance in double perovskite (Sr1.6Ba0.4)FeMoO6

The effects of A-site average cation size 〈r_A〉 and anti-site defects on Curie temperature (T_C) and room-temperature magnetoresistance (MR) in (Sr_2−xBa_x)FeMoO₆ (x=0, 0.4 and 1.6) have been investigated. By Ba doping, not only the room-temperature MR but also the T_C have been enhanced. The larger MR in the Ba-doped samples compared with the prototype Sr₂FeMoO₆ is associated with the lower saturation field. The optimization of T_C and MR in (Sr_1.6Ba_0.4)FeMoO₆ other than in the reported (Sr_0.4Ba_1.6)FeMoO₆ can be understood according to the two competing effects: anti-site defects and chemical pressure.     

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.     

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.