An overview of magnetostriction, its use and methods to measure these properties

Abstract

The paper provides a description of the magnetostriction effects, and an explanation why it occurs. It lists a number of devices where the properties have been put to use, and describes a range of methods which have been used to measure these properties.

Introduction

A magnetostrictive material will change shape when it is placed in a magnetic field. Most ferromagnetic materials exhibit some measurable magnetostriction. Since the phenomenon involves a bidirectional energy exchange between magnetic and elastic states, these materials provide a mechanism both for actuation and sensing applications. They are also successfully employed in a wide variety of devices for vibration control of mechanical equipment.

The nature of this effect is illustrated in Fig. 1. A rod of a magnetic material of length L, is shown surrounded by a coil of wire carrying an electrical current so that a magnetic field, H, is produced along the rod [1]. With the current flowing, the length of the rod increases by a small amount ΔL. The strain ΔL/L is called the magnetostriction (for which the symbol λ is used).

Fig. 1(a) also shows that in addition to an increase in length in the direction of the field, there is also, usually a decrease in length in the perpendicular directions, and as a result of this, the volume of the rod remains nearly, but not quite, constant. Fig. 1(b) also reveals two important features of magnetostriction: firstly for high values of H, λ eventually reaches a constant value λ_sat, indicating saturation, and secondly the sign of λ does not change when the field H becomes negative. The rod increases in length for both positive and negative values of the magnetic field strength. (Magnetostriction is therefore a second order effect [2].)

The principal magnetostrictive effects observed experimentally are: (1) The Joule effect. This can be an extension or a contraction in the same direction as the magnetic field or in some other direction. (2) The volume effect (volumetric expansion), a very weak effect. (3) The Wiedemann effect (a shear strain response to the magnetic field, analogous to the tensile or compressive strain produced in the Joule effect). In addition, inverse effects are also observed, such as the Villari effect. In this case there is a change in magnetic permeability in response to an applied stress. This is also referred to as a magnetostrictive effect or a magnetomechanical effect. For most transducer applications the maximum force or movement are desired as outputs, and so the Joule effect or its inverse, the Villari effect are the most useful in technology. Therefore the discussion that follows will concentrate primarily on these effects. Information on other effects can be found in reference [3].