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What is a Magnetostrictive
Magnetostrictive materials transduce or convert magnetic energy to mechanical energy and vice versa. As a magnetostrictive material is magnetized, it strains; that is it exhibits a change in length per unit length. Conversely, if an external force produces a strain in a magnetostrictive material, the material's magnetic state will change. This bi-directional coupling between the magnetic and mechanical states of a magnetostrictive material provides a transduction capability that is used for both actuation and sensing devices. Magnetostriction is an inherent material property that will not degrade with time.

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The history of magnetostriction begins in the early 1840s when James Prescott Joule (1818-1889) positively identified the change in length of an iron sample as its magnetization changed. This effect, known as the Joule effect, is the most common magnetostrictive mechanism employed in magnetostrictive actuators. A transverse change in dimensions accompanies the length change produced by the Joule effect. The reciprocal effect, in which applying a stress to the material causes a change in its magnetization, is known as the Villari effect (also referred to as the magnetostrictive effect and magnetomechanical effect). The Villari effect is commonly used in magnetostrictive sensors.

An additional magnetostrictive effect used in devices is the Wiedemann effect, a twisting which results from a helical magnetic field, often generated by passing a current through the magnetostrictive sample. The inverse Wiedemann effect, also known as the Matteuci effect, is used for magnetoelastic torque sensors. (Lee 1955 and Lacheisserie 1993.)

The existence of both direct and reciprocal Joule and Wiedemann effects leads to two modes of operation for magnetostrictive transducers: (1) transferring magnetic energy to mechanical energy and (2) transferring mechanical energy to magnetic energy. The first mode is used in design of actuators for generating motion and/or force, and in design of sensors for detecting magnetic field states. The second mode is used in design of sensors for detecting motion and/or force, in design of passive damping devices, which dissipate mechanical energy as magnetically and/or electrically induced thermal losses, and in design of devices for inducing change in a material's magnetic state.

In many devices, conversion between electrical and magnetic energies facilitates device use. This is most often accomplished by sending a current through a wire conductor to generate a magnetic field or measuring current induced by a magnetic field in a wire conductor to sense the magnetic field strength. Hence, most magnetostrictive devices are in fact electro-magneto-mechanical transducers.

Some of the earliest uses of magnetostrictive materials during the first half of this century include telephone receivers, hydrophones, magnetostrictive oscillators, torque-meters and scanning sonar. These applications were developed with nickel and other magnetostrictive materials that exhibit bulk saturation strains of up to 100 mL/L (units of microlength per unit length). In fact, the first telephonic receiver, tested by Philipp Reis in 1861, was based on magnetostriction [Hunt, 1953]

With the discovery of "giant" magnetostrictive alloys in the 1970s (materials capable of over 1000 m L/L), there is a renewed interest magnetostrictive transducer technologies. Many uses for magnetostrictive actuators, sensors, and dampers have surfaced in the last two decades as more reliable and larger strain and force giant magnetostrictive materials such as Terfenol-D and Metglass have become commercially available (in the mid to late 1980's). Current applications for magnetostrictive devices include ultrasonic cleaners, high force linear motors, positioners for adaptive optics, active vibration or noise control systems, medical and industrial ultrasonics, pumps, and sonar. In addition, magnetostrictive linear motors, reaction mass actuators, and tuned vibration absorbers have been designed, while less obvious applications include high cycle accelerated fatigue test stands, mine detection, hearing aids, razor blade sharpeners, and seismic sources. Ultrasonic magnetostrictive transducers have been developed for surgical tools, underwater sonar, and chemical and material processing.

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How does it work
Figure 3 shows a cartoon to help depict the strain and magnetic induction observed in magnetostrictive materials as they are magnetized and as described by these equations. In Figure 3, the essence of a magnetostrictive device is lumped into discrete mechanical and magnetic attributes that are coupled in their effect on the magnetostrictive core strain and magnetic induction [after Hall]. As shown, the external stress is set to a constant compressive value (s = s0) provided by the mass resting on a stiff spring on top of the magnetostrictive core.  
Figure 3. Cartoon of changing strain, e and magnetic induction, B, in a magnetostrictive element subjected to a constant compressive stress,s0. The applied magnetic field, H, increases from (a) -Hs through (c) H = 0 to (e) Hs. Figure (f) indicates the magnetic flux lines associated with magnetization of the driver shown in (e) (after Hall).

Looking first at the case of no applied field (H = 0) the sample has an initial length (e = 0) and no net axial magnetic induction (B = 0), as depicted in Figure 3c. As the magnitude of the applied field H increases to its saturation limits, Hs, the elliptical magnets rotate to align with the applied field; the axial strain increases to es and magnetization of the element in the axial direction increases to +Bs (Figure 3e) or decreases to - Bs (Figure 3a). At an applied field strength of Hs, the saturation magnitudes of strain and magnetic induction have been reached, that is the strain and magnetization of the sample will not change with further increases in the applied field. Thus, both the magnetically induced strain and the magnetic induction magnitudes increase moving from the center figure outward as the magnitude of the applied field increases to its saturation values.

Alternatively, picture the applied field being set to a constant, like Hs, and then placing an increasing mass load or compressive force on the magnetostrictive element as you move from the outermost figures to the center figure. Both the axial strain and axial magnetization magnitude in the element will decrease with increasingly negative (compressive) stress.

Figure 3f shows the flux lines associated with alignment of the magnetic domains in the magnetostrictive driver shown in Figure 3e. These lines are technically the superposition of the contributions due to the magnetization of the magnetostrictive driver element and the applied field Hs. Flux lines for Figure 3a would be similar in shape but flux would be flowing in the opposite direction. Similarly, Figures 3b and 3d would exhibit the same shapes in the appropriate direction, but would be lower in magnitude. Figure 3c would not exhibit flux lines, as the driver as shown is demagnetized. The flux field is used in sensors to measure the magnetization, strain and/or force on the magnetostrictive driver by monitoring the flux with either with a Hall effect probe or by detecting the voltage induced in a wire conductor perpendicular to the flux lines.

While this cartoon is not to be taken literally (!), it conveys the essence of the behaviors observed in magnetostrictive materials. For instance the cartoon suggests that for ac applications, simply apply a dc magnetic bias to strain the material to one half its saturation length and cycle between initial and saturated lengths. While this does convey the idea behind biasing, in practice the dc bias needed for ac operation, Hc, is based on operation centered in the steepest region of the curve shown in Figure 2 where the strain-field slope is a maximum. This region, called the "burst" region, arises due to reorientation of magnetic moments (produced at the atomic level by electron spins) from an "easy" crystallographic axis perpendicular to the applied field to one more closely aligned with the applied field. Easy axes correspond to orientations for the magnetic moment vectors that satisfy local crystallographic energy minimization states in the magnetostrictive material as the applied stress and magnetic field vary.

Furthermore, an initial compressive stress is often used to increase the alignment of magnetic moments along easy axes perpendicular to the applied field. Although a given sample can not get any longer than its saturation length (Figures 3a and 3e), under a prestress the sample's zero field length can decrease (get shorter than shown in Figure 3c), thereby maximizing the net achievable strain.

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