Die Temperaturabhängigkeit des magnetischen Barkhausen-Effekts. I

The apparatus described allows to measure the size distribution of Barkhausen discontinuities with a magnetic moment greater than 10^?6 e.m.u. in a temperatur range from liquid air up to the Curie point. We used the counting method, developed byTebble and his co-workers, in connection with a multichannel-analyzer. Nickel samples of different purity and of different thermal treatment have been investigated. The results are as follows: 1. The size distribution of two samples, having in practice the same hysteresis loop can show a completely different behaviour as a function of the magnetic field. 2. The number of great discontinuities is reduced more rapid with rising temperature than that of the smaller ones; thus leading to the conclusion, that the Barkhausen component of total magnetization is vanishing much stronger with increasing temperature than that of differential susceptibility. 3. The critical field strength, being characterized by a maximum in the number of discontinuities per unit field, shows the same temperature dependence as the coercive force. This critical field is only slightly dependent on the size of jumps. 4. The “average” magnetic moment of the discontinuities in the measured range of size appears to vary less with temperature than the spontaneous magnetization. 5. Above a temperature of about 270°C spontaneous jumps will be produced, even in a magnetic hard sample, by diminutive vibrations, such as speaking loud or coughing. 6. The Barkhausen part of total magnetization varies with temperature like the coercive force and therefore seems to be a structure-dependent quantity. The results of our measurements are discussed in connection with those of other authors.     

Barkhausen effect in magnetic thin films: General behaviour and stationarity along the hysteresis loop

Several aspects of the Barkhausen effect can be investigated by using a measurement equipment that records the Barkhausen signal as a function of time in digital form on computer tape. Measurements on thin magnetic films containing a small number of domain walls are presented. The behaviour of the wall jumps during a reversal is investigated. During the steep part of the loop the stationarity of the signal is measured with the magnetic field as a parameter. The influence of the coercive fieldH _c and the thicknessd _m is considered and compared with results obtained by others on bulk samples.     

Stress response of Barkhausen noise in a tempered C-Mn steel

The dependence of Barkhausen noise on tensile stress is measured in a high strength vanadium microalloyed C-Mn steel after tempering at 200, 400, 500, 600 and 650°C. Simultaneously, the coercive force and two field parameters related to the magnetic reversal curve are recorded for some of the specimens tempered at 500, 600 and 650°C. Tempering causes a maximum in Barkhausen noise vs. stress curve and minima in the field strength vs. stress curves. An increase in the impedimental force due to stress gradients experienced by domain walls is suggested as the cause of the maximum. Stress gradients with short “wavelenghts” are thought to be produced in lath-or plate-like martensitic/bainitic structures. The effect appears in the course of tempering, as recovery processes reduce the dislocation density, and vanishes as recrystallization leads to an almost complete elimination of these dislocations.     

Stability of the single-domain state in single crystals of triglycine sulphate

The stability of the single-domain state in single crystals of triglycine sulfate is investigated using the Barkhausen effect. Barkhausen jumps are detected upon invariable macroscopic polarization. The parameters of the observed Barkhausen jumps are estimated and compared.     

Traditional,Barkhausen and MAT Magnetic Response to Plastic Deformation of Low-Carbon Steel

Magnetic traditional and adaptive methods as well as Barkhausen noise analysis were used for the testing of material degradation of plastically deformed high quality low-carbon steel. The dependences of the measured magnetic (coercive field, maximum permeability, optimum adapted permeability) and Barkhausen noise (RMS voltage, power spectrum) parameters are understood in terms of the increased dislocation density. The results are discussed in the context of potential application of magnetic testing methods for the indication of plastic deformation levels of ferromagnetic construction steel products.     

Dependences of magnetic Barkhausen emission and magnetoacoustic emission on the microstructure of pearlitic steel

The relationships between microstructure, mechanical behaviour and magnetic properties of completely pearlitic steels have been investigated, with the objective of determining the applicability of magnetic measurements to non-destructive evaluation of the properties of high-strength pearlitic steels. High-carbon steels were heat treated to produce completely pearlitic structures with various interlamellar spacings, and their magnetic properties, including hysteresis loops, Barkhausen emission (BE) and magnetoacoustic emission (MAE) profiles, were measured. MAE profiles were found to have two peaks at the knee of hysteresis loop, while BE profiles show only a single peak at about the coercive field for all samples. The peak height ratio of the MAE profiles and the amplitude of the BE profiles increase monotonically with increasing pearlite spacing, whereas coercivity is inversely proportional to pearlite spacing. These results can be interpreted in the context of magnetic domain structures and magnetization reversal processes observed by Lorentz transmission electron microscopy, which revealed that, in specimens with smaller pearlite spacing, reverse domains nucleated and grew at higher reverse magnetic field, and domain wall jumps across cementite lamellae were smaller than in samples with coarser pearlites. The good correlation observed between the magnetic properties, mechanical strength and microstructures of these steels provides the basis for rapid and effective non-destructive assessment of their properties for industrial applications.     

The investigation of the process of magnetization in FeNiBSi glasses by means of the Barkhausen jumps

The Barkhausen jumps occuring during the magnetization of the amorphous Fe-Ni-B-Si alloys have been studied by the measurements of the derivative of magnetization with respect to time (dM/dt). The peaks in dM/dt vs. applied field (H) are reproducible after averaging over some fifty cycles of magnetization. The investigation of the variations of dM/dt with the applied field (H) and with the instantaneous magnetization (M) accompanied with the same studies, but performed starting from different values of the initial remanence (M_ri), completes our earlier analysis of the process of magnetization in these alloys. It is shown that the model of Chikazumi used for the explanation of the variations of remanent magnetization, coercive field and loss with the maximum magnetization (M_m) is consistent with the observed Barkhausen jumps. The possibility of separation of contributions from particular domain walls to the process of magnetization in soft magnetic materials (which would enable the determination of the locations and strengths of the particular pinning centres) is briefly discussed.     

Nondestructive evaluation of isothermally annealed 12% CrMoV steel by magnetic BN measurement

The effects of microstructural evolution during isothermal annealing on structure-sensitive magnetic properties and Barkhausen noise (BN) characteristics have been investigated for 12% CrMoV steel. A rapid decrease in coercive force, remanence, hysteresis loss and hardness took place in accordance with the release of internal stresses from supersaturated martensite after annealing for only 20 minutes at 923 K. BN energy is correspondingly related with three stages of microstructural evolution such as the recovery of strain energy, the increase of precipitates size as a result of Ostwald ripening and the annihilation of dislocation density during isothermal annealing of the specimen. The linear relation between hardness and BN parameters in the magnetization region of irreversible domain wall displacement of 2.4 kA/m suggests that hardness and microstructural evolution could well be evaluated nondestructively by using BN measurements.     

Model for domain wall avalanches in ferromagnetic thin films

The Barkhausen jumps or avalanches in magnetic domain-walls motion between successive pinned configurations, due the competition among magnetic external driving force and substrum quenched disorder, appear in bulk materials and thin films. We introduce a model based in rules for the domain wall evolution of ferromagnetic media with exchange or short-range interactions, that include disorder and driving force effects. We simulate in 2-dimensions with Monte Carlo dynamics, calculate numerically distributions of sizes and durations of the jumps and find power-law critical behavior. The avalanche-size exponent is in excellent agreement with experimental results for thin films and is close to predictions of the other models, such as like random-field and random-bond disorder, or functional renormalization group. The model allows us to review current issues in the study of avalanches motion of the magnetic domain walls in thin films with ferromagnetic interactions and opens a new approach to describe these materials with dipolar or long-range interactions.     

An investigation of electrodeposited cylindrical iron-nickel films of supercritical thickness IV. Magnetic properties of films

The saturation magnetization in cylindrical nickel and iron-nickel films with a high nickel content is significantly lower than in pyrometallurgical metal. This is due to the presence of a high proportion of amorphous nonferromagnetic phase in the film. The application of an axial magnetic film during the electrodeposition of the films increases the total number of Barkhausen discontinuities in axial reverse magnetization, reduces the coercive force, shifts the field at which discontinuities appear into the region of smaller re verse-magnetization fields, and increases the permeability and the rectangularity of the hysteresis loops. The variation of the magnetic characteristics with the alloy composition is correlated with the spontaneous internal stresses in the films.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 3, pp. 18–22, March, 1972.The authors thank V.M. Rudyak for discussion of the results, and B. S. Zolotkovskii and B. L. Nechaevfor carrying out the test experiments with flat films and for participation in discussion.     

Influence of hydrogenation-dehydrogenation process on magnetic properties and Barkhausen noise of the Fe-W-B amorphous alloy

Internal stresses, as a consequence of structural changes of the Fe_81.4 W_2.6B₁₆ amorphous alloy during hydrogenation-dehydrogenation process, were investigated by structurally sensitive properties as energy of a total and stress induced anisotropy, a coercive force, a demagnetizing factor and Barkhausen noise parameters.     

Bending effects and temperature dependence of magnetic properties in a Fe-rich amorphous wire

Amorphous wires with composition Fe_77.5Si_7.5B₁₅ exhibit a very peculiar magnetization process characterized by a single and quite large Barkhausen jump. This gives rise to a squared hysteresis loop at a critical magnetic field. The bistable behaviour, widely studied in wires with typical length of 10 cm and diameter of 125 μm, appears above a length of about 7 cm in straight wires and disappears for curvature radius within the range 2–12 cm in bent wires. In this work it is shown that bistability occurs in bent wires, whatever their curvature is, provided the wires are long enough. To this purpose spiral-shaped samples with several turns are considered. However, when the wire length is not a integer number of turns the magnetization reverses through many large Barkhausen jumps. In this condition, varying the measuring temperature can activate the energy barriers for the jumps.     

Influence of annealing process on magnetic properties and Barkhausen noise of the Fe-W-B amorphous alloy

The study of structural changes followed by measuring of structure sensitive magnetic properties as the energy of total and stress induced anisotropy, the coercive field, the demagnetizing factor and the Barkhausen noise parameters of the as-cast and annealed Fe_81.4 W_2.6B₁₆ amorphous alloy was performed. The investigated structural changes were connected with the temperature range within which the Fe_81.4W_2.6B₁₆ amorphous alloy was characterized by the soft magnetic properties.     

Delayed demagnetization jumps in (NdDy)(FeCo)B magnets in a steady-state magnetic field

Spontaneous demagnetization jumps are observed in sintered magnets (Nd_0.6Dy_0.4)₁₆(Fe_0.77Co_0.23)₇₈B₆ in a constant magnetic field after a sharp decrease in an external magnetic field from the value corresponding to the saturation to a value close to the coercive force. It is shown that the number of the magnetization jumps is proportional to their amplitudes. A low value of the autocorrelation coefficient between the jump amplitude and the time of its appearance (R < 0.1) demonstrate the stochasticity of the jumps. It is found that the spectral jump density is independent of the frequency, i.e., a white magnetic noise is observed. The distribution of the magnetic field gradient has been obtained near the sample surface that makes it possible to distinguish domains and the grain magnetization in the dependence on the direction of the texturing of the sintered magnet.

     

Magnetic structure of Fe-based amorphous and thermal annealed microwires

The magnetic structure of amorphous and thermal annealed glass coated microwires is studied by thermomagnetic, DSC, and Bitter domain pattern techniques. The long-range dipolar interaction between parallel aligned microwires and the appearance of large Barkhausen jumps steps in the axially magnetized loops are discussed in terms of reversal magnetization process.     

Statistical properties of barkhausen noise in thin Fe films

The statistical properties of Barkhausen noise in an epitaxial Fe film grown on MgO have been characterized with magneto-optical Kerr effect measurements. The data reveal that magnetization reversal takes place via sudden jumps between a discrete number of randomly distributed magnetic configurations. The smallest jumps occur on a scale length of 10 &mgr;m and their amplitude distribution can be fitted with a power law: P(DeltaM) = DeltaM-alpha with alpha = 1.1 and DeltaM spanning over several decades.     

Influence of the inhomogeneity of local magnetic parameters on the curves of magnetization in an ensemble of Fe<Subscript>3</Subscript>C ferromagnetic nanoparticles encapsulated in carbon nanotubes

Methods have been proposed and tested for analyzing local magnetic parameters in a system of single-domain ferromagnetic nanoparticles using their magnetization curves. The magnetic inhomogeneity in ensembles of Fe₃C nanoparticles encapsulated in carbon nanotubes has been investigated. It has been established that the Fe₃C nanoparticles encapsulated in carbon nanotubes are characterized by two-modal distribution functions of the local magnetic anisotropy energy. The particle distribution over the blocking temperature is reconstructed from the experimental temperature dependence of the coercive force. The allowance made for the inhomogeneity of the local magnetic parameters of the Fe₃C nanoparticles, which were studied by the proposed methods, explains the discrepancy between the magnetic anisotropy energy determined by the method of the magnetization approaching saturation and the magnetic anisotropy energy estimated from the coercive force of single-domain nanoparticles.     

Fe-Cr-Co-Si永磁合金的磁性和穆斯堡尔效应研究

本文研究了成分为(重量百分比)65Fe-22Cr-12Co-1Si的可加工永磁合金。对高温淬火样品和经过磁场热处理及时效的样品,研究了饱和磁矩与温度的关系,以及室温和约80K的穆斯堡尔谱,计算了谱参数。实验表明:两种样品的磁性都随测量中热循环而改变。时效样品在室温除出现铁磁性相外,还出现顺磁性相;顺磁性相在低温下转变为铁磁性相。在时效样品中发现了磁矩的不完全的取向效应。最后讨论了这一材料的反磁化机制和提高矫顽力的途径。 关键词：     

Magnetic properties and microstructure of CoCr and CoCrTa thin films sputtered at high pressure on a PET substrate

Thin magnetic films (CoCr/Cr, CoCrTa/Cr and CoCrTa/Cr/SiO) are deposited by DC-magnetron sputtering on a PET substrate. The optimum sputter pressure, giving rise to the highest coercive force, is found to be around 50 mTorr. However, the deposition parameters influence the value of this optimum pressure. The addition of Ta to CoCr leads to phase segregation, resulting in CoCrTa films in which the magnetic interaction between the grains is more dipolar than in CoCr films. The magnetic interactions in CoCrTa films are less dependent on the sputter parameters than those in CoCr films. The evaporation of a seedlayer of, e.g. SiO prior to the deposition of the Cr underlayer increases the coercive force of the CoCrTa thin films, especially when the CoCrTa layer is thinner than 50 nm. This makes these films more suitable for high-density magnetic recording.     

Investigation of the coercive force of thin magnetic films obtained by vacuum sputtering

The investigation of the magnetic properties of thin magnetic films is of great scientific as well as practical interest for two reasons. First, thin magnetic films, being two dimensional (one of the dimensions is much smaller than the others) show novel properties; their study can solve various problems in the theory of ferromagnetism which are difficult to solve using only data obtained from bulk materials [1,2]. Second, scientific and practical interest with respect to thin magnetic films has increased due to their possible practical applications for logic and memory elements in computer technology [3–7].Their coercive force, which is relatively increased with respect to bulk magnetic materials, is one of the basic properties of thin magnetic films. As is known, the coercive force of thin magnetic films is one of the most important design parameters for memory and logic elements in computer technology, since the coercive force defines the current necessary for the reversal of the magnetic polarity. Although this parameter is of great importance and intensive investigations of it have been carried out, precise data do not yet exist. As a matter of fact, the existing data are often contradictory, because basic technological factors were not taken into account in the investigations.To illustrate the above we give the results of a number of experimental studies in which attempts were made to establish the dependence of the coercive force upon the thickness of the magnetic film as shown in Fig. 1 (curves 2,3,4,7,15,17). Curve 16 is a calculated curve, the other curves refer to thin magnetic films obtained by various authors using the electrolytic depositing method.From this data it is evident that the magnitude of the coercive force for a certain thickness of thin magnetic film, prepared from the same magnetic material according to the same method, assumes different values with different authors. This is explained by the fact that the investigation of the dependence of the coercive force upon the film thickness is extremely complex because it is difficult to control the basic technological factors and also because various additional factors come into play which were not taken into account by the authors.