Die Temperaturabhängigkeit des magnetischen Barkhausen-Effekts IV

The size distribution of Barkhausen discontinuities (magnetic moment > 6.5 · 10^?7 cgs) of very pure nickel was measured between ?195 and 360 °C on the hysteresis loop. The mean value of internal stresses was changed in a well defined manner by stretching the specimens. For different stress values the variation with temperature of the following properties is given in diagrams: The specific number and the “mean volume”² of discontinuities, the maximal value of Barkhausen susceptibility, the mean deviation of the volume of discontinuities along the hysteresis curve, and some characteristic field strengths defined by reversible and irreversible processes. Conclusions are drawn concerning the coupling of individual wall movements composing a Barkhausen jump.     

Der elektromagnetische Aspekt der Energiezufuhrerscheinung von einem longitudinalen Wechselmagnetfeld an ein Torsionspendel mit ferromagnetischer Aufhängung

It is shown that under conditions in which a torsion pendulum with ferromagnetic suspension thread oscillates with torsional oscillation being excited by the energy transferred from an a-c magnetic field, Barkhausen discontinuities are released by the torsional deformations. A study is made of the dependence of the voltage pulse, which induced these Barkhausen discontinuities in the coil in the field of which the ferromagnetic suspension thread is placed, on the intensity of the a-c magnetic field, the amplitude of torsional deformation and the angle of phase shift between these two sine quantities. The influence of an underlying longitudinal d-c magnetic field on these Barkhausen discontinuities is also investigated.

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Wir danken I. Schreck für seinen wertvollen Beitrag zur Herstellung der in dieser Arbeit benützten elektronischen Apparatur.     

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.     

Die Temperaturabhängigkeit des magnetischen Barkhausen-Effekts. II

The size distribution of Barkhausen discontinuities was measured during “thermal idealisation” of a hard drawn nickel specimen. Thermal idealisation means the magnetization by increasing temperature in a constant magnetic field. The sensitivity of the apparatus covered a range of magnetic moment between 1.3 and 17.3·10^?6 e.m.u. The results are in good agreement with investigations ofGerlach andTemesváry on the irreversible part of thermal idealisation. The average volume of Barkhausen discontinuities of the measured size was found to be nearly independent of temperature in a range from ?150 to+200°C, while above 200°C the volume increases markedly with temperature. By means of the average Barkhausen volume the temperature dependence ofNéels thermal fluctuation field was calculated. The results are in good agreement with those ofPescetti andBarbier on the temperature dependence of magnetic after effect.     

Der Einfluß der Probenform auf die Größenverteilung von Barkhausen-Sprüngen

The size distribution of Barkhausen discontinuities of polycristalline nickel wires was measured as a function of specimen length, i.e. of demagnetizing field. The observed behaviour can be represented by a factor (1 +N · χ) in the size interval 1.5?15 Vsm. Size distributions of specimens with different dimensions can be easily converted into one another by means of this factor.     

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.     

Visualization of the Barkhausen effect by magnetic force microscopy

By visualization of the Barkhausen effect using magnetic force microscopy we are able to provide detailed information about the physical principles that govern the magnetization reversal of a granular ferromagnetic thin film with perpendicular anisotropy. Individual Barkhausen volumes are localized and distinguished as either newly nucleated or grown by domain wall propagation. The Gaussian size distribution of nucleated Barkhausen volumes indicates an uncorrelated random process, while grown Barkhausen volumes exhibit an inverse power law distribution, which points towards a critical behavior during domain wall motion.     

Der Einfluß der Kopplung von Barkhausen-Sprüngen auf deren Größenverteilung

The size distribution of Barkhausen discontinuities was investigated in poly-crystalline nickel as a function of coercive force (2.1≦H _c ≦17.0 Oe), demagnetizing factor (0.0316≦N≦0.405) and counting rate (5≦dn/dt≦500 pulses/sec). The comparison by means of the factor (1 +N·χ) of size distributions from specimens with differentN gives much better results, if the distributions are measured at constant counting rate. This rate should be as large as possible because of the blocking mechanism of the demagnetizing field.     

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.     

Direct observation of Barkhausen avalanche in Co thin films

We report direct full-field magneto-optical observations of Barkhausen avalanches in Co polycrystalline thin films at criticality. We provide experimental evidence for the validity of a phenomenological model of the Barkhausen avalanche originally proposed by Cizeau, Zapperi, Durin, and Stanley [Phys. Rev. Lett. 79, 4669 (1997)]], where the model describes a 180 degrees -type flexible domain wall deformed by a localized defect with consideration of long-range dipolar interaction. The Barkhausen jump areas show a power-law scaling distribution with critical exponent tau approximately 1.33 for all the samples having different thickness from 5 to 50 nm, which is in accord with the two-dimensional prediction of the model.     

Evaluation of surface decarburization depth by magnetic Barkhausen noise technique

Industrially unfavorable process of steel surface decarburization was induced by annealing in air. Two methods of after-anneal surface treatment were used: an acid pickling and a sand blasting. The obtained decarburized layers were examined by optical microscope, wave dispersive spectrometer, and surface X-ray diffraction method. Magnetic Barkhausen noise technique was tested for applicability of non-destructive characterization of the decarburized layer depth. A newly introduced parameter, Barkhausen noise coercivity, was proposed for practical use due to its sensitivity to decarburization and stability to measurement conditions. Other magnetic parameters, e.g. number of Barkhausen noise counts, were found to be sensitive to the compressive residual stress caused by the sand blasting.     

Magnetic Barkhausen noise measurement by resonant coil method

This paper describes a powerful new technique for nondestructive evaluation of ferromagnetic material. A method has been developed for measuring magnetic Barkhausen signals under different coil resonance frequencies. The measurements allow one to establish the behavior relating the power spectral density maximum and the resonant coil frequency. Time-frequency analysis of Barkhausen signals puts in evidence the tuning regions for each coil, and allows clear identification of each contribution to the Barkhausen signal spectrum. This concept was used in order to evaluate the relation between the degree of plastic deformation in carbon steel samples, and the power spectral density maximum at different resonance frequencies. This result also makes it possible to the selectively modify measurement sensibility to the magnetic Barkhausen signal by using different resonance frequencies.     

Investigation of the dependence of Barkhausen noise on stress and the angle between the stress and magnetization directions

The dependence of Barkhausen noise amplitude on stress, and on the angle between the stress and magnetization directions, has been investigated in specimens of AISI/SAE 4340, HY-80 and ASTM A-36 steels. In all three materials it was found that the Barkhausen noise amplitude measured with the magnetic field applied at 60° to the stress axis is independent of stress. This result is compared with the theory of elasticity which indicates that under a uniaxial or a pure bending stress, the normal component of a strain in steel is zero along the direction approximately 61° from the direction of the applied stress. It was also found that the Barkhausen noise follows the same angle dependence as that of the normal strain. These findings strongly indicate that the Barkhausen noise amplitude is directly dependent on the normal strain along the direction of the magnetization and not on the shearing strain. An explanation for such a direct normal strain dependence is given based on magnetoelastic energy. Possible application of this finding to quantitative nondestructive stress measurements is also discussed.     

Magnetic Barkhausen emission in lightly deformed AISI 1070 steel

The Magnetic Barkhausen Noise (MBN) technique can evaluate both micro- and macro-residual stresses, and provides indication about the relevance of contribution of these different stress components. MBN measurements were performed in AISI 1070 steel sheet samples, where different strains were applied. The Barkhausen emission is also analyzed when two different sheets, deformed and non-deformed, are evaluated together. This study is useful to understand the effect of a deformed region near the surface on MBN. The low permeability of the deformed region affects MBN, and if the deformed region is below the surface the magnetic Barkhausen signal increases.     

Application of the group pulse processes in the theory of Barkhausen noise

In the paper the 2nd-order periodic group pulse process is considered a statistical model of the Barkhausen voltage. Using this model an analytical formula for the power spectrum is derived and further specified to fit the measured spectrograms of Barkhausen noise. The results are interpreted with respect to the generating physical mechanism and examples of several numerical values are given.     

Effect of the length of action of a steady electric field on the polarization buildup process in triglycene sulfate and barium titanate single crystals

During the investigation of the Barkhausen effect and the reversal current [1] in ferroelectric crystals, it was observed that, together with other factors, the length of time during which the specimens had been kept in a steady electric field has a substantial effect on the polarization reversal process. In this connection, it is of interest to investigate parallely the Barkhausen effect and the reversal current in ferroelectric crystals at various lengths of action of the electric field. In this paper we have investigated the number of Barkhausen jumps, the charge being reversed, the buildup time of the total and irreversible polarization, as well as the starting field for jumps and reversal current as a function of the length of action of the field on specimens of TGS and BaTiO₃ single crystals during the quasistatic polarization reversal process.     

Barkhausen effect in magnetic thin films: Experimental noise spectra

In this paper Barkhausen noise spectra are shown that were measured on thin uniaxial 83-17 Ni?Fe films and these spectra were compared with the known magnetic behaviour of these films. On these types of sample the Barkhausen effect had been investigated by Lambeck [1], but their Barkhausen noise spectra have not been investigated so far. The film thicknessd _m in our experiments ranged between 400 and 2400 Å. In this range a change occurs in the dynamics of the magnetization behaviour caused by transitions of the type of domain wall. The results presented here show the very strong dependence of the Barkhausen noise spectra on the type of domain wall. Films with Bloch walls always show a frequency dependence off ^?1.7 in the higher frequency range. Samples in the thickness range where the transition occurs from the Néel wall via the cross-tie wall to the Bloch wall, have exponents between ?1 and ?2. The frequencyf _c above which the measured noise intensity begins to decrease varies very much for the different films. The curve off _c versus film thicknessd _m has the same form as the curve of the domain wall mobilitym versusd _m.     

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.     

Scaling exponents for barkhausen avalanches in polycrystalline and amorphous ferromagnets

We investigate the scaling properties of the Barkhausen effect by recording the noise in several soft ferromagnetic materials: polycrystals with different grain sizes and amorphous alloys. We measure the Barkhausen avalanche distributions and determine the scaling exponents. In the limit of vanishing external field rate, we can group the samples in two distinct classes, characterized by exponents tau = 1.50+/-0.05 or tau = 1.27+/-0.03, for the avalanche size distributions. We interpret these results in terms of the depinning transition of domain walls and obtain an expression relating the cutoff of the distributions to the demagnetizing factor which is in quantitative agreement with experiments.