PROPAGATION OF ELASTIC WAVES IN NICKEL

An important source of ultrasonic oscillations at the present time is magnetostrictive generators, which make use of the effect of magnetostriction in ferromagnetic materials. In particular, the features of the propagation of ultrasonic waves in ferromagnets are therefore of considerable interest not only for theory but also for technology.

The velocity of propagation of transverse waves in a rod, \(V_t\), is, as is known,

\[ V_t=\left[\frac{G}{\rho}\right]^{\frac{1}{2}} = \left[\frac{E}{2\rho(1+\sigma)}\right]^{\frac{1}{2}}, \]

where \(G\) is the shear modulus, \(E\) is Young’s modulus, and \(\sigma\) is Poisson’s ratio.

Fig. 1.

As has been shown by a number of authors, when a specimen is magnetized its elastic constants change. Therefore, under magnetization one should also expect changes in \(V_t\). Recently Rogers and Johnson*) carried out a corresponding experiment. A rod 45.8 cm long and 1.25 cm in diameter was made from technically pure nickel (99.4%). Short pulses of transverse ultrasonic waves were transmitted along this rod from a piezoquartz plate serving as a generator. The carrier frequency was 1 MHz. The time of propagation of the pulse along the rod was measured; it proved to be 170 μsec. The rod was then heated approximately to \(870^\circ\)C (to red heat), slowly cooled to room temperature (\(21^\circ\)C), and placed wholly in a longitudinal magnetic field. The experiment consisted in measuring the propagation velocity of the pulse along the rod, \(T\), and also the amplitude of the arriving pulse, as functions of the magnetic-field intensity—

) T. F. Rogers and S. J. Johnson, Journ. Appl. Phys. 21*, 1067 (1950).

of the magnetic field applied to the rod. In their measurements the authors used an oscillographic method.

Figure 1 shows the dependence of the relative magnitude of the decrease in the propagation time of the pulse along the rod, \(\Delta T/T\), on the magnetic-field strength. The observed sharp decrease in the propagation time of the pulse is 1000 times greater than that due to magnetostriction. The observed effect is explained, in the authors’ opinion, by a sharp increase in the velocity of propagation of elastic waves in nickel under the action of the magnetic field.

Fig. 2.
Vertical axis: attenuation coefficient.
Horizontal axis: \(H\) (oersteds).

Since the magnetic fields are small, the small change in the density \(\rho\) and the corresponding increment in velocity may be neglected. Therefore the authors explain the change in velocity by an increase in Young’s modulus.

In a magnetic field another interesting phenomenon is also observed—the decrease in the attenuation coefficient of transverse waves.

In Fig. 2, the values of the magnetic field are plotted along the abscissa, and along the ordinate the values of the attenuation coefficient of transverse waves (the relative weakening of the pulse in the absence of a field is taken as unity). At 1000 gauss the attenuation coefficient decreases by more than a factor of 100.

The authors, as they themselves admit, were unable to give an explanation of this second phenomenon.

M. G.