What are magnetostrictive materials? Overview of magnetostrictive materials and various potential applications such as vibration power generators, sensors, and actuators

Offering excellent magnetostrictive performance along with mechanical properties, Fe-Ga magnetostrictive alloys have drawn attention for their potential use for vibration power generation, a form of energy harvesting (*). They also offer potential applications not just for vibration power generators, but for applications in sensors, actuators, and other products. This column provides an overview of Fe-Ga magnetostrictive alloy single crystals, with examples of their applications.

Magnetostrictive effect and magnetostrictive materials

Magnetic materials* such as iron and nickel contain microscopic regions known as magnetic domains, in which magnetic moments are aligned in a common direction. When a magnetic field is applied to such a material, the magnetic moments align with the magnetic field, resulting in a slight deformation of the material. This phenomenon is called the magnetostrictive effect.

Conversely, when an external mechanical force is applied and the magnetic material is slightly deformed, the direction of the magnetic moments inside the material changes according to the direction of the applied force, changing how easily magnetic flux passes through the material (i.e., the magnetic permeability). This phenomenon is called the inverse magnetostrictive effect.
Materials that exhibit a large magnetostrictive effect are referred to as magnetostrictive materials, and because of their properties, they are widely utilized in applications such as magnetostrictive vibration sensors and actuators.
(*Magnetic material: a material that becomes magnetized and exhibits magnetic properties when subjected to an external magnetic field.)

Magnetostrictive effect: When magnetized, shape changes
Inverse magnetostrictive effect: When force is applied, magnetic flux changes

Functions of magnetostrictive materials

1) Generating electricity from vibration  (vibration energy harvesting, vibration sensing)

When vibration is applied to a magnetostrictive material around which a coil is wound, a change occurs in the density of magnetic flux passing through the magnetostrictive material, generating a voltage in the coil via electromagnetic induction. The generated voltage can be used as electric power (vibration energy harvesting). The generated voltage can be also used as a signal to indicate vibration information (vibration sensing).

2) Generating vibration from electricity (vibration actuation)

Electric power is applied to the coil wound around a magnetostrictive material. The current in the coil produces a magnetic field, and by changing this magnetic field (for example, by using an AC current), the magnetostrictive material repeatedly expands and contracts. This expansion and contraction generate　vibration.

3) Converting external changes into electrical signals (e.g., magnetic, stress, strain sensing)

Small changes in the magnetic field, stress, or strain applied to a magnetostrictive material around which a coil is wound results in changes in the density of magnetic flux passing through the magnetostrictive material, generating a voltage in the coil. The generated voltage can be used as a signal to capture small changes in magnetic field, stress, strain, and other parameters.

Fe-Ga magnetostrictive alloy

Fe-Ga magnetostrictive alloy, an iron-based magnetostrictive material, demonstrates both a significant magnetostrictive effect and superior mechanical properties, and has drawn attention in recent years as a core material for magnetostrictive vibration energy harvesting. In addition to vibration energy harvesting, applications as a core material for sensors and actuators are also being studied. The main characteristics of Fe-Ga magnetostrictive alloy are shown below:

Features of Fe-Ga alloy
✓ Magnetostriction: Approximately 300 ppm (about 10 times that of iron)
✓ Young’s modulus: 70 GPa (about the same as aluminum)
✓ Tensile strength: 400 MPa (about the same as iron)
⇒ Attracting attention as a material for magnetostrictive vibration energy harvesting

For many years, Sumitomo Metal Mining has manufactured and sold crystal materials used in various applications. Based on single crystal growth and crystal processing technologies supported by the expertise gained in these efforts, we have developed Fe-Ga magnetostrictive alloy single crystals suitable for magnetostrictive vibration energy harvesting for the IoT (Internet of Things). Applications as sensors or actuators that leverage their excellent properties are also being studied.

Development of Fe-Ga magnetostrictive alloy single crystal growth technology based on the Vertical Bridgman (VB) method is currently underway at Sumitomo Metal Mining (Figure 2). The magnetostrictive performance of Fe-Ga magnetostrictive alloys is significantly affected by variations in the Ga concentration within the alloy, and the VB method makes it possible to minimize variations in the Ga concentration within grown single crystals (Figure 3: left). Additionally, applying special post-processing aligns the directions of the magnetic domains within the material, making it possible to produce magnetostrictive materials characterized by high magnetostriction and small variation in magnetostrictive properties (Figure 3: right).

At present, growth of single crystals in the form of rectangular prisms up to approximately 66 mm × 100 mmL has been achieved. These single crystals can be cut to produce magnetostrictive materials of various sizes and shapes.

VB (Vertical Bridgman) method

Shown below are examples of applications based on Fe-Ga magnetostrictive alloy single crystals for vibration energy harvesting,  vibration sensing, and vibration actuation.

Vibration energy harvesting (Vibrational Power Generation Research Laboratory, Kanazawa University)

Reference: Vibrational Power Generation Research Laboratory, Kanazawa University – Magnetostrictive Vibration Power Generation Device (V-GENERATOR)

Figure 4 shows the structure of the device. A plate-shaped Fe-Ga magnetostrictive alloy is bonded to a U-shaped steel frame; copper wire is wound around that portion; and a permanent magnet is placed under the end of the Fe-Ga magnetostrictive alloy plate. This configuration creates a closed magnetic circuit (indicated by the red line in Figure 4) in the vibration power generation device. When a weight is attached to the tip of the frame and the vibration power generation device is fixed to a vibrating body, the weight vibrates up and down due to inertial forces acting on the weight. This vibration deforms the frame and applies, in alternation, tensile and compressive forces to the Fe-Ga magnetostrictive alloy plate bonded to the frame. Due to the inverse magnetostrictive effect, this changes the ease with which magnetic flux passes through the Fe-Ga alloy plate (i.e., the magnetic permeability), causing magnetic changes within the coil, which in turn generates a voltage. This voltage can be extracted as a direct current through an electric circuit and used as a self-sustaining power source in the field of the IoT (Internet of Things) and other fields.

Self-powered bridge sensor (Laboratory of Machine Design and Tribology, Faculty of Engineering Science, Kansai University)

Reference: Magnetostrictive Bridge Sensor Device, Laboratory of Machine Design and Tribology, Kansai University
Figure 5 shows the device structure. A coil is wound around a cylindrical Fe-Ga magnetostrictive alloy; magnets are arranged around it to apply an external magnetic field; and a yoke (circuit for magnetically coupling magnets) is formed with SS400 (rolled steel plate). These results in a vibration sensor device having a structure covered with SS400. The device is deployed between the bridge girder and bridge pier. When a vehicle passes over the bridge, the bridge girder moves up and down, applying compressive stress in the vertical direction to the cylindrical magnetostrictive material. This in turn changes the magnetic permeability, causing changes in magnetic flux density within the coil, and generating a voltage. This voltage can be rectified, stored, and used as direct current power.

Given the correlation between the magnitude of the generated voltage and the vibration of the bridge, analyzing this voltage as vibration information concerning the bridge provides information used for bridge health diagnosis. The electric power generated by bridge vibrations is stored in a secondary battery or otherwise; bridge vibration information is acquired and analyzed at fixed intervals to diagnose the health of the bridge. The results can be transmitted to a server via wireless communication. Deploying this system should enable automatic bridge inspections and health diagnoses, which have conventionally relied on manual labor.

Active vibration control mechanism for heavy industrial machinery (Panasonic Connect Co., Ltd.)

In electronic component mounting machines, higher operating speeds and an increasing number of nozzles have led to heavier and more complex head assemblies. These changes make residual vibrations more difficult to suppress using stage position control alone, limiting further improvements in head movement speed and positioning time.
To overcome this challenge, Panasonic Connect has developed an active vibration control mechanism that can both support heavy head structures and actively suppress residual vibrations. The system uses Fe–Ga magnetostrictive alloy single crystals to generate vibrations in the opposite phase, effectively damping unwanted motion. The performance of this mechanism was verified through experimental evaluation.
Figure 6 shows an overview of the active vibration control experimental apparatus. The setup consists of an inverted pendulum that represents the head assembly, mounted on a stage motion mechanism that moves in a specified direction. A pair of magnetostrictive fine-motion actuators is installed at the base of the inverted pendulum. These actuators expand and contract vertically in response to an applied electric current, enabling precise vibration control.

During stage motion, the vibration velocity at the tip of the inverted pendulum was measured, and feedback control was applied to drive the magnetostrictive actuators to generate out-of-phase vibrations. As shown in Figure 7, activating the actuators dramatically reduced residual vibrations, achieving vibration damping in a very short time compared with conventional operation. The vibration decay time was reduced by more than 90%, demonstrating that active vibration control and robust support of heavy structures can be achieved simultaneously.
This technology is expected to contribute to higher-speed, higher-precision industrial equipment. Potential applications include electronic component mounting machine heads and other systems that require active vibration control while supporting heavy loads.

Magnetostrictive alloy single crystals have a wide range of potential functions, including vibration energy harvesting (converting vibration into electric power), sensing (converting micro-vibration and other small changes such as those in a magnetic field, stress, strain into voltage signals), and actuation (outputting vibration from electric power). They hold the promise of broad applications, new devices, and the creation of new value.

Even in vibration energy harvesting alone, combining it with the IoT presents unbounded potential. For example, it can be used to operate sensors in remote locations and to collect information on the weather, environment, the movement and position of people and objects, and the integrity of structures and equipment in real time. Reducing or eliminating the need for battery replacement reduces maintenance burdens and waste. We expect vibration energy harvesting to create new value in industry, transportation, logistics, medical care, welfare, infrastructure, environment, daily life, and entertainment.

Sumitomo Metal Mining produces high-quality Fe-Ga magnetostrictive alloy single crystals through proprietary single crystal growth and processing technologies. Working with device manufacturers, universities, and various other parties in cooperative efforts characterized by open innovation, we plan to develop devices and applications that make use of the characteristics of high-quality single crystal magnetostrictive materials to help realize applications that create new value in wide-ranging fields.