magnetostriction

magnetostriction is a property of ferromagnetic materials where their physical dimensions change upon the application of a magnetic field. This reversible effect occurs due to the rotation and alignment of magnetic domains within the material's crystal lattice, converting magnetic energy into mechanical strain. The phenomenon is distinct from related effects like piezoelectricity and is fundamental to several modern technologies. Its magnitude, while typically small, is significant in specific alloys and compounds developed for precision engineering.

Physical principles

The underlying mechanism arises from the coupling between spin-orbit interaction and the exchange interaction within the material's atomic structure. When an external magnetic field is applied, the magnetic moments of domains reorient, causing a shift in the equilibrium positions of atoms and a resultant deformation of the crystal lattice. This Joule effect is characterized by a strain that is generally quadratic with respect to the field strength at lower magnitudes. The inverse effect, where mechanical stress induces a change in magnetic permeability, is known as the Villari effect and is described by Maxwell's equations in electromagnetism. Further complexities include the Wiedemann effect observed in ferromagnetic wires under torsional stress.

Materials and properties

Key materials exhibiting significant magnetostriction include pure nickel, certain cobalt-based alloys, and modern rare-earth compounds like terfenol-D, developed at the Naval Surface Warfare Center. The giant magnetostriction observed in terbium-dysprosium-iron alloys arises from high magnetic anisotropy and large spin-orbit coupling. Properties such as saturation magnetostriction constant, magnetocrystalline anisotropy, and Curie temperature are critical for performance. Other notable materials are galfenol, researched at the University of Maryland, College Park, and metglas alloys produced by AlliedSignal. The magnetostrictive coefficient varies widely, from minimal in silicon steel to substantial in samarium-based compounds.

Measurement techniques

Quantifying the effect employs precise instruments like strain gauges bonded to samples within controlled solenoids or Helmholtz coil systems. Optical methods such as laser Doppler vibrometry and interferometry, often using Michelson interferometer principles, provide non-contact detection of minute displacements. Dynamic characterization utilizes acoustic emission sensors and impedance analysis to study resonant frequency shifts in transducer elements. Standardized testing protocols are maintained by organizations like the American Society for Testing and Materials and the International Electrotechnical Commission. Early quantitative work was advanced by Arthur Ewing at the University of Cambridge.

Applications

Major applications are found in sonar arrays and ultrasonic transducers used by the United States Navy and in medical imaging devices like some magnetic resonance imaging systems. The effect drives precise actuators and sensors in industrial automation, including valve controllers and active vibration control systems. It is crucial in delay line memory devices historically used in early computers like the UNIVAC I. Contemporary research focuses on energy harvesting from ambient vibration and advanced acoustic projectors. Companies such as Etrema Products, Inc. and Cedrat Technologies commercialize these technologies.

History and discovery

The effect was first observed in 1842 by James Prescott Joule while experimenting with iron rods, a finding later termed the Joule effect. Subsequent investigations by Emilio Villari led to the discovery of the inverse effect in 1865. Significant theoretical understanding emerged from the work of Wilhelm Röntgen and Friedrich Heusler on magnetic materials. The 20th century saw accelerated development during World War II for sonar applications, with major contributions from researchers at the Massachusetts Institute of Technology and the Bell Telephone Laboratories. The synthesis of terfenol-D in the 1970s at the Ames Laboratory under Department of Energy funding marked a breakthrough for high-performance devices.

Category:Electromagnetism Category:Condensed matter physics Category:Materials science