NONLINEARITY ESTIMATION BY USING THE HARMONIC DISTORTION METHOD OF SOUND WAVES PROPAGATION IN A FERROMAGNETIC SAMPLE Nicolae CREŢU, Mihail POP Physics Department, Transilvania University Brasov Nicolae Creţu: Mihail Pop: SISOM 2007 and HOMAGIAL SESSION OF THE COMMISSION OF ACOUSTICS BUCHAREST, May

About the nonlinearity Classical nonlinear acoustics

Obtaining the nonlinear acoustics equations:

For cubic crystalline elastic media For a longitudinal wave which propagate in the direction(100) For a transversal wave which propagate in the direction(100) : For a transversal wave propagating in (110) direction: The density of the elastic energy of the cubic crystall:

The influence of the magnetocrystalline anisotropy  for a ferromagnetic crystall The stable value of the strain  and of the energy will be: magnetostriction

Magnetostriction behavior Magnetostriction shows a nonlinear behavior vs magnetic field Magnetostriction has two regions with strong nonlinearity vs magnetic field The propagation of the elastic waves in ferromagnetic samples becomes a nonlinear acoustic propagation especially in these regions

Experimental setup Calibrated signal generator sine-wave Linear power amplifier Laser Velocity Transducer Preamplifier USB External data acquisition board Computer for Signal Analysis Magnetic coils Calibrated current source Magnetic fluxmeter Magnetization and magnetostriction measurements were made in CNRS Lab Magnetisme, Grenoble.

Matrix method Fourier coefficients of the implied waves: Transfer matrix

Obtaining of transfer matrix For the ferromagnetic sample:  transfer matrix is a nonlinear matrix

Nonlinear considerations Because the ferromagnetic medium has a nonlinear behavior, the matrix is also nonlinear. The process of the sound propagation in such media is very complicated and was solved only in some particular cases [4]. Is possible to estimate the nonlinearity from the sound propagation measurements, by considering the ferromagnetic sample as a nonlinear system. From the theory of the nonlinear systems we know that if we inject a monochromatic signal in such sample, the output signal will contain a number of harmonics and the more the nonlinearity means the higher number of harmonics.

Total Harmonic Distortion Method Total Harmonic Distorsion Analysis (THD) is an analysis in the frequency domain. To determine the amount of nonlinear distorsion we need to measure the amplitude and the frequency of the fundamental and of the harmonics when in the sample is injected a monochromatic signal. By definition : with An the corresponding amplitudes of the output signal:

THD vs magnetic field

An extension of the THD Method, based on the energy conservation. If the input energy is distributed on the harmonics, the decresing of the first amplitude is an indication of the dispersal of the energy on the others

Conlusions By spectral analysis of Fourier components, using THD and first frequency amplitude decreasing, we find that also THD such as first frequency tend to have a minimum value in two regions of the intensity of the electric current from the magnetization coil. By inspection of the value of the magnetic field B(T) from the magnetization coil in which appear the minimum of the THD, is possible to connect these values with the regions where magnetostriction has strong nonlinearity on his variation versus magnetic field.

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