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Devices based on magnetic films are used in applications ranging as widely as from computer memory to heart pacemakers. An important characteristic of magnetic films is the saturation field, which is a measure of how small an applied magnetic field can be and still completely magnetize that film in the direction of the field. The smaller the saturation field, the more sensitive the device.
The saturation field is often determined by the stress in the film. During film deposition, a common phenomenon is the buildup of stress with increasing thickness. Thick films, however, are often desirable when an application requires a high magnetization. Researchers in the Metallurgy Division in MML have discovered that during deposition, interlayering such a magnetic film with another metal having either a different crystal structure or lattice constant can achieve unprecedented reductions in the saturation field. The interlayers of the other metal are used to force a restart of the growth of the magnetic film. The restart induces the creation of new crystalline grains of the magnetic material. Thus, very thick magnetic films can be made with very low saturation fields.
Measurements of the stress in thick film samples of the ferromagnetic Ni77Fe14Cu5Mo4 alloy were made by depositing them on thin glass cantilevers. A multilayer structure with a 5 nm thick Ag layer interspersed every 100 nm reduced the tensile stress in the film by a factor of 200 and the saturation field by a factor of 400.
This result is also important for magnetic field amplifiers known as flux concentrators, which are widely used in magnetic field sensors. As a consequence, advancements like earlier detection of unexploded ordinance and biological warfare agents, improved sensing of counterfeit currency, earlier crack detection in aircraft wings, and more powerful computers will be possible.
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