Sections of a continuous 400-
nanometer-thick magnetic film of
alloy; © NIST
This material could lead to greatly improved magnetic sensors for a wide range of applications from weapons detection to medical devices.
Those applications and many others are based on thin films of magnetic materials in which the direction of magnetization can be switched from one orientation to another. An important characteristic of a magnetic film is its saturation field, the magnitude of the applied magnetic field that completely magnetizes the film in the same direction as the applied field - the smaller the saturation field, the more sensitive the device.
The saturation field is often determined by the amount of stress in the film - atoms under stress due to the pull of bonds with neighboring atoms are more resistant to changing their magnetic orientation. Metallic films develop not as a single monolithic crystal, like diamonds, but rather as a random mosaic of microscopic crystals called grains. Atoms on the boundaries between two different grains tend to be more stressed, so films with a lot of fine grains tend to have more internal stress than coarser grained films. Film stress also increases as the film is made thicker, which is unfortunate because thick films are often required for high magnetization applications.
The National Institute of Standards and Technology (NIST) research team discovered that magnetic film stress could be lowered dramatically by periodically adding a layer of a metal, having a different crystal structure or lattice spacing, in between the magnetic layers.
The researchers prepared multilayer films with layers of a nickel-iron-copper-molybdenum magnetic alloy each 100 nanometers (nm) thick, interleaved with 5-nm layers of silver. The structure reduced the tensile stress (over a monolithic film of equivalent thickness) by a factor of 200 and lowered the saturation field by a factor of 400.
The work has particular application in the design of “flux concentrators,” magnetic structures that draw in external magnetic field lines and concentrate them in a small region. Flux concentrators are used to amplify fields in compact magnetic sensors.
COMPAMED.de; Source: National Institute of Standards and Technology