Magnetostriction effect is defined as 'the change in dimensions of a magnetized ferro­magnetic rod (e.g., iron, nickel), on application of an alternating magnetic field paral­lel to its length.

1. The change in length of the rod is of the order of 1 ppm and is independent of the sign of the field, but depends upon the magnitude of the field and the nature of the material.

2. When the rod is placed inside a coil carrying an alternating current, it suffers a change in dimension for each half cycle and therefore, the resulting vibrating frequency is twice that of the alternating current.

3. Since short rods that produce high-frequency ultrasonic waves are difficult to pre­pare, long rods that produce low resonance frequency are used for the production of ultrasonic waves. Therefore, magnetostrictive oscillators are used for producing low- frequency ultrasonic waves.

4. Some examples of magnetostrictive materials are nickel, nickel alloys (invar, monel metal, and perm alloy) and cobalt ferrites.

Experimental Arrangement :

In the experimental arrangement of a magnetostriction oscillator (below figure), a bar of a ferromagnetic material, say iron or nickel, is held at the middle by means of a clamp C. Two coils L1, and L2, are connected (wound) to the ends of the rod. The coil which forms an inductance is connected in parallel with a variable capacitor C. The com­bination of L2 and C form a resonant tank circuit. The tank circuit provides the oscilla­tions and determines the frequency of the circuit. The other inductance coil L1, forms the feedback loop.

Working:

Initially, direct current is passed to magnetize the rod, so that it is ready to undergo the magnetostriction effect. Now when the circuit is switched ON, the capacitor C charges and discharges through the inductance coil L2, setting up vibrations of frequency f1 given by

$f_1=\dfrac1{2\pi\sqrt{L_2C}}$

This produces longitudinal vibrations in the rod. As the rod elongates and con­tracts along its length, the vibrations set up an emf in L1. This emf is fed to the base of a transistor T, which positively feeds the amplified emf back to the coil L2 through the tank circuit. This feedback overcomes the loss of energy in the tank circuit and the oscillations are maintained in the tank circuit. The variable capacitor is now adjusted so that the frequency of the oscillatory circuit is same as that of the natural frequency of the rod. The vibrations of the rod attain a maximum when it is set to resonance. The millimeter shows a maximum current when resonance is achieved. The resonant vibrating frequency f2 is given by the physical dimensions and material of the rod

$f_1=\dfrac1{2l}\sqrt{\dfrac E\rho}$

where l is the length of the rod; E is the modulus of elasticity of the rod; and ρ is the den­sity of the rod.

Some advantages of magnetostriction circuit are:

1. Continuous production of ultrasonic waves with large amplitude.

2. Construction of the circuit is simple and costs less.

3. At low ultrasonic frequencies, large power output is possible without the risk of any damage to the oscillatory circuit.

Some of the limitations of the magnetostriction oscillator circuit are:

1. It cannot produce USW above 3 MHz.

2. The frequency of oscillations is dependent greatly on temperature.

3. The breadth of the resonance curve is large (above fig.) (due to the variations of elas­tic constants of the rod with degree of magnetization), and hence the frequency of oscillation is not constant.