Magnetsotrictive Metals

There are many magnetostrictive metals. The large magnetostriction of terbium, element #65 on the periodic table of elements, makes it preferred for actuators.

Quantum mechanics dictates that the inner, non-bonding 4f electron cloud of the terbium atom is inherently oblate (flattened at its poles) and not spherical. Unbalanced charge in motion yields magnetic moment, an inherently indestructible atomic phenomenon. Labeled magnetostriction, it manifests as the inseparable coupling of magnetic permeability with elastic modulus. Re-stated, both magnetic permeability and elastic modulus vary in step with each other. This is the source of terfenol-d's durable capabilities.

Functional Schematic

Compressing a terfenol-d rod inside a solenoid coil of wire is the simplicity of a magnetostrictive actuator. The diagram shows how it works. Within the solid body of each rod exist magnetic domains, shown here as ovals each with North-South orientation.
From left to right

• No Stress, No Field. The rod is in its free, as-manufactured state. In this state, it is not ferromagnetic -- the effects of magnetic domains cancel each other.
  
  
• Compressed, No Field. A compressive preload has been applied to the rod. Since nothing is infinitely stiff, the rod contracts axially due to this force. The domains rotate as shown but still cancel each other. Compression stores mechanical energy because terfenol-d is a spring.
  
  
• Compressed, Moderate Field. A moderate magnetic field aligns the domains. The magnetic field is the result of electrical current circulating around the rod in a coil (not shown for clarity).
  
  
• Compressed, Strong Field. An increase in current increases the magnetic field, causing the domains to rotate toward closer alignment with the magnetic field. This expands the terfenol-d rod, even against the applied compressive preload. That is, the mechanical energy stored like a spring by the terfenol-d rod is released by the applied magnetic field generated by electrical energy. Terfenol-d force and expansion are akin to thermal expansion, except magnetically fast.