Amorphous Ferromagnetic Materials Research Articles

Effects of the viscosity field on the Barkhausen noise of amorphous ferromagnetic materials: 38836 Celasco, M.; Mazzetti, P.; Masoero, A.; Stepanescu, A. Journal of Applied Physics, Vol. 63, No. 8, pp. 2983–2985 (15 Apr. 1988)

Effects of the viscosity field on the Barkhausen noise of amorphous ferromagnetic materials: 38836 Celasco, M.; Mazzetti, P.; Masoero, A.; Stepanescu, A. Journal of Applied Physics, Vol. 63, No. 8, pp. 2983–2985 (15 Apr. 1988)

A new experimental method for measuring differential resistivity and magnetoresistance in amorphous materials

A system for measuring differential resistivity and magnetoresistance has been designed and tested. This system eliminates the various difficulties involved in this type of measurement (thermoelectric and magneto-thermal effects, contact voltages, drifts, etc.) and makes it possible to measure variations in resistivity of about 0.01%, generated by alterations in temperature or the application of a magnetic field (magnetoresistance). This system is computer controlled so that long sequences of measurements for different temperatures and magnetic field values are possible. They can be measured using direct or alternating current. For the elimination of background signal a voltage is used which includes the variations generated by drifts in the measuring current. Experimental results relating to the study of amorphous ferromagnetic materials are presented.

Effects of the viscosity field on the Barkhausen noise of amorphous ferromagnetic materials

Contrary to the case of crystalline or polycrystalline materials, the magnetic aftereffect is almost always present in amorphous ferromagnetic materials. This fact makes the study of its influence very important in order to understand the magnetization dynamics of these materials, which have a growing technological interest. Experimental results prove the existence of a strong correlation between the magnetic aftereffect and the Barkhausen noise. In the present paper a theoretical study of the effect of the viscosity field on the power spectrum of the magnetization noise in amorphous materials will be given. Comparison with the available experimental results will also be made, showing a rather good agreement between theory and experiment.

A Mössbauer investigation of melt-spun Fe60Al28B12

Mössbauer studies on two distinct microstructural phases produced in the same batch of melt-spun Fe60Al28B12 reveal distinctly different spectra and hyperfine field distributions. One phase is essentially an amorphous ferromagnet, whereas the other contains a large paramagnetic component together with amorphous ferromagnetic material.

Anomalous temperature dependence of the effective susceptibility exponent in amorphous Fe-Ni-Zr alloys (abstract)

It is well known that the effective exponent γ(T) for the paramagnetic susceptibility in amorphous ferromagnets exhibits an unusual temperature dependence over a wide range. In sharp contrast to the monotonic behavior observed for ‘‘pure’’ magnetic systems, with increasing temperatures γ(T) for amorphous ferromagnetic materials increases from the critical Heisenberg value of 1.38 near TC to much larger values (≥2) and after a maximum eventually approaches the Curie-Weiss behavior. Such a crossover behavior in χ(T) has been explained as being due to a noncritical interplay between the magnetic correlations and the spatial fluctuations of the exchange interactions. With this point of view a recent Monte Carlo simulation1 of the phase transition in random ferromagnets predicts that site disorder has a much stronger influence on χ(T) than bond disorder. In order to study the consequences of site, and bond disorder and also the possible role of competing exchange interactions on γ(T) behavior we have measured the low-field ac and dc susceptibilities for melt-spun amorphous Fe-rich (Fe-Ni)90Zr10 alloys over an extended range of temperature. In these ‘‘ferroglasses’’ substitution of Fe with Ni is found to suppress the low-temperature ‘‘spin-glass phase’’ completely for Ni conc. above 8 at. %, while at the same time increasing TC. Thus Ni substitution in these alloys results in a material where γ(T) behavior exhibits progressive change from a nonmonotonic to conventional behavior. We find that the dynamics of correlated spins and its sensitivity to local inhomogeneities in an amorphous material can thus be well resolved near the percolation threshold for the loss of long-range ferromagnetic order.

Amorphous Alloys for Distribution Transformers: Design Considerations and Economic Impact

The use of amorphous ferromagnetic materials for the magnetic circuits of distribution transformers could reduce the losses associated with iron cores and result in substantial economic savings to utilities. The value of such savings obviously depends on the loss evaluation. Optimization calculations show that this value varies according to the transformer design. A comparative performance evaluation revealed the advantages and shortcomings of four different transformer designs and of different types of steel used to build the magnetic circuits. The paper finally gives a few design details of a pre-prototype distribution transformer, developed at IREQ, which has an amorphous-alloy magnetic circuit.

Amorphous Alloys for Distribution Transformers: Design Considerations and Economic Impact

The use of amorphous ferromagnetic materials for the magnetic circuits of distribution transformers could reduce the losses associated with iron cores and result in substantial economic savings to utilities. The value of such savings obviously depends on the loss evaluation. Optimization calculations show that this value varies according to the transformer design. A comparative performance evaluation revealed the advantages and shortcomings of four different transformer designs and of different types of steel used to build the magnetic circuits. The paper finally gives a few design details of a pre-prototype distribution transformer, developed at IREQ, which has an amorphous-alloy magnetic circuit.

Magnetic properties of amorphous FeCo base ultrafine particles

RF sputter deposition of (FeCo) 80 B 20 onto a sputter-etched polyimide substrate produces a dense array of elongated finger-like projections of amorphous ferromagnetic material, with diameters less than 1000 A and aspect ratio about 4. These particles, which we call ultrafine amorphous particles, have a magnetic easy axis perpendicular to the substrate. The coercive field and magnetic anisotropy depend strongly on the Fe/Co ratio and also on the annealing conditions. A maximum coercive field of almost 1200 Oe is obtained for (Fe 0.4 Co 0.6 ) 80 B 20 particles annealed at about 500°C. After this treatment the particles are crystalline.

Mean-field analysis of amorphous rare earth-transition metal alloys for thermomagnetic recording

A mean-field model is developed for amorphous ferromagnetic materials with potential applications in thermomagnetic recording/magneto-optical readout systems. The emphasis is on the reduction of the number of adjustable parameters, so that important variables and their effects on magnetic properties can be investigated. The available experimental data on GdCo-, GdFe-, and TbFe-based alloys is compared with the model predictions and good agreement is obtained in all cases. Expressions for the exchange stiffness coefficient and macroscopic anisotropy energy constant are derived and the latter is compared with available experimental data. The results have been used to study domain wall characteristics of the three material systems.

Quasistatic magnetic permeability of an amorphous ferromagnet

Dependence of the differential magnetic permeability μ on the speed of magnetization reversal was found in ribbons of an amorphous ferromagnetic material. When the ribbon thickness is ∼10−3 cm and the initial permeability is ∼10−5, an increase in the frequency of the harmonic magnetization reversal causes a decrease of μ by a factor of 2–3; this takes place in the region around 100 Hz. The effect is explained by the influence of eddy currents on the magnetization distribution.

Structural approach to the permeability aftereffect in zero- magnetostrictive amorphous alloys

Magnetic aftereffects are a general feature of both magnetostrictive and zero-magnetostriction amorphous ferromagnetic materials. It is shown, that the existence of magnetic relaxation effects in zeromagnetostrictive amorphous ferromagnets can be explained within a recently proposed structural approach, by taking into account the nonisotropic character of the local magnetostrictive strain.

Transverse magnetization due to shearing stress on amorphous ferromagnetic materials

It is shown that a shearing stress σ applied to an amorphous ferromagnetic sample magnetized along a longitudinal direction x, gives rise to a transverse magnetization component M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> . A square sheet (25 × 25 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of Metglas 2826 (Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</inf> Ni <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">40</inf> P <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> ) was clamped so as to apply a shear stress, while the material was magnetized by a field H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> parallel to the clamps. The M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> component is almost unaffected by the stress, while a transverse component M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> appears, which depends on H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> and σ. At constant stress M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> increases with H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> up to a maximum, which reaches about 15% of the saturation M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sigma \cong 1.5</tex> kg/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . By minimizing the sum of magnetostatic and magnetoelastic energies, M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> can be calculated as a function of H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> , σ and λ, the magnetostrictive relative elongation, determined by conventional methods. A good fit is found between the M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> vs. H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> experimental and calculated curves, limited to the high field region. Alternatively, the M <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</inf> measurements can be used to calculate λ as a function of H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> . Values of λ vs. H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> consistent for most of the measured curves are found in this way, which agree with the 11.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> saturation value, but deviate from low fields data obtained with conventional magnetostriction measurements. This discrepancy is probably due to the different stress conditions under which λ's are calculated or directly measured, or to some distribution of magnetoelastic anisotropy due to frozen-in stresses.

A method for the calculation of the magnetization of random high anisotropy amorphous materials

We introduce a procedure for the calculation of the magnetization distribution for a random anisotropy amorphous ferromagnetic material using a steepest descent method. The technique is applied to several anisotropy distributions, and the magnetization of each is presented as a function of temperature for realistic values of molecular field constant and anisotropy.

Localized spin excitations in amorphous ferromagnets

Within the framework of a binary collision theory and a random lattice model the linearized equation of motion for the circular spin component S + j = S x j + i S y j was solved for an ensemble of amorphous ferromagnetic materials for two different situations: (i) originally generated, extended spinwaves. (ii) Continuous, local, external excitation of the spin S + 0 at site j = 0 in each sample. In both cases we derive solutions of the ensemble-averaged equation of motion as a function of time. It is shown that we have an exponential damping of the ensemble-averaged spinwave states, exp(-Γ t), and a localization of the ensemble-averaged excitation mode according to exp(-κ∣ R j ∣) in the case of local excitation at j = 0. The ratio Γ/κ is given by the group velocity of spinwaves in amorphous ferromagnets. Analogous results hold for phonons in amorphous materials.

Temperature dependence of the magnetization for a random-uniaxial-anisotropy amorphous ferromagnetic material

An equilibrium thermodynamic approach is used to calculate the temperature dependence of the magnetization for a random-anisotropy amorphous ferromagnetic material. The model assumes a single "lattice" and a classical mean-field theory calculation is done using the method of steepest descents. Several anisotropy distributions are considered in addition to a completely isotropic one. It is shown that the inclusion of texture causes a marked change in the magnetization versus temperature curve and that the magnetization is also sharply dependent on the angle between the applied magnetic field and the texture direction. The several resultant curves and a modified curve for a two-"sublattice" model are compared with each other and with experimental data for Tb${\mathrm{Fe}}_{2}$.

Note on RKKY interactions in nearly-free electron systems

The RKKY interaction between localized magnetic moments via polarization of conduction electrons is calculated for systems with spherical, but non-quadratic conduction electron dispersion. It is shown that even rather small changes in the electron dispersion may produce large modifications of the density of electronic states and therefore deviations from the common RKKY expression derived for free electrons. The results might be important for the discussion of interaction mechanisms in amorphous ferromagnetic materials.

Spontaneous vibrational modes excited in ferromagnetics by reversible magnetostriction

Experimental results for magnetostrictive generation of low-frequency standing elastic waves in polycrystalline and amorphous ferromagnetic materials are given. By exciting magnetized samples with bursts of magnetic impulses, extensional and torsional modes of damped wave propagation were observed. These propagation patterns are qualitatively explained by means of the theory of linear magnetoacoustic coupling taking into account the residual stresses alone. A comparison of the magnetomechanical behavior of the materials investigated has been made by studying the conditions of reversible magnetostriction and the magnetic damping. In particular, in the Ni rod, it was possible to clarify the log decrement behavior on the basis of the reversible magnetomechanical permeability and the ΔE effect measured.

The influence of spatially random magnetostatic, magnetocrystalline, magnetostrictive and exchange fluctuations on the law of approach to ferromagnetic saturation of amorphous ferromagnets

Within the framework of the micromagnetic theory developed by Kronmüller and Ulner we have calculated the influence of spacially random dipolar, magnetocrystalline, magnetostrictive and exchange fluctuations on the law of approach to ferromagnetic saturation of amorphous ferromagnetic materials. The effect of fluctuations of dipolar and exchange interactions turns out to be negligibly small in most materials. The magnetocrystalline and magnetostrictive fluctuations, however, may give rise to a 1/✓ H-term in the saturation magnetization in materials (such as TbFe 2) with huge local magnetocrystalline fields. In materials with non-vanishing volume averages of the magnetocrystalline energy or the magnetostriction the H-dependence of the saturation magnetization reflects the nature of the strained arrangement of atoms.

Spin wave excitations in amorphous itinerant ferromagnet

The Izuyama, Kim and Kubo theory of spin waves in crystalline itinerant ferromagnets is extended to the amorphous itinerant ferromagnetic materials. The qualitative features of this theory are similar to that of the generalized Landau-Lifshitz theory discussed by Henderson and de Graaf.