FIBER-LASER INTERFEROMETRY: Einstein-de Haas effect measured at nanometer scale
An optical fiber, 125 µm in diameter, at the end of an optical-fiber laser interferometer was positioned less than 10 µm from the surface of a cantilever to measure movement caused by changes in magnetization.
An optical fiber, 125 µm in diameter, at the end of an optical-fiber laser interferometer was positioned less than 10 µm from the surface of a cantilever to measure movement caused by changes in magnetization. A gyromagnetic effect discovered by Albert Einstein and Dutch physicist Wander Johannes de Haas-the rotation of an object caused by a change in magnetization-was first observed in experiments reported in 1915, in which a large iron cylinder suspended by a glass wire was made to rotate by an alternating magnetic field applied along the cylinder’s central axis.
By contrast, recently published experiments at the National Institutes of Standards and Technology (NIST; Boulder, CO) measured the Einstein-de Haas effect in a 50-nm-thick ferromagnetic, nickel-iron thin film deposited on a 200 µm × 20 µm 600 nm, commercial silicon nitride microcantilever. One end of the cantilever was fixed and the other end projected unsupported within a ring of Helmholtz coils.1 An alternating magnetic field applied in the plane of the cantilever and perpendicular to its length induced changes in the magnetic state of the thin film, and the resulting torque bent the cantilever up and down by just a few nanometers
Using a laser interferometer to measure the movements of the cantilever and comparing those data to changes in the magnetic state of the material, researchers were able to determine the magnetomechanical ratio, or the extent to which the material twists in response to changes in its magnetic state. Obtaining the magnetomechanical ratio enabled them to indirectly determine the “g-factor,” a measure of the internal magnetic rotation of the electrons in a material in a magnetic field.
The magnetomechanical ratio and the g-factor are critical in understanding magnetization dynamics and designing magnetic materials for data storage and spintronics applications, but they are extremely difficult to determine. The NIST experiments provide a proof-of-concept for using the Einstein-de Haas effect to determine the magnetomechanical ratio and the related g-factor in thin ferromagnetic films. The researchers note that a number of improvements in the experimental method are possible, such as operating the cantilever system in a vacuum to reduce the effects of any changes in temperature.
Hassaun A. Jones-Bey
1. T.M. Wallis, J. Moreland, P. Kabos, Appl. Phys. Lett. 89, 122502-1 (Sept. 18, 2006).