Researchers at the US National Institute of Standards and Technology (NIST) have discovered that silver spacers a few nanometres thick between layers in a magnetic alloy dramatically enhance sensitivity, by 400 times in some cases, claims the institute.
"This material could lead to greatly improved magnetic sensors for a wide range of applications from weapons detection and non-destructive testing, to medical devices and high-performance data storage," said NIST.
The silver affects the 'saturation field' - the field required to flip the internal magnetic state stored in the material to the reverse state - 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 neighbouring atoms are more resistant to changing their magnetic orientation," said NIST.
Atoms at the boundaries between metal grains tend to be more stressed, so films with a lot of fine grains tend to have more internal stress than coarser grained films.
According to NIST, stress also increases as the film is made thicker, "which is unfortunate because thick films are often required for high magnetisation applications".
Periodically adding a layer of a metal with a different lattice spacing appears to lower magnetic film stress, although the mechanism isn't completely understood.
Scientist William Egelhoff speculates that the intervening layers disrupt the magnetic film growth and induce the creation of new grains that grow to be larger than they do in the monolithic films.
Films with 100nm layers of a nickel-iron-copper-molybdenum magnetic alloy interleaved with 5nm silver layers have tensile stresses 200 times less than those in a monolithic film of equivalent thickness, and a saturation field less by a factor of 400.
"The work has particular application in the design of flux concentrators," said NIST.
Sections of a continuous 400nm-thick magnetic film of a nickel-iron-copper-molybdenum alloy (below) and a film of the same alloy layered with silver every 100nm (above).
By relieving strain in the film, the silver layers promote the growth of notably larger crystal grains in the layered material as compared to the monolithic film (several are highlighted).
Electron diffraction patterns (inset) indicate that the material with larger crystal grains display sharper, more discrete scattering patterns.
