Jun 14, 2007
Magnetic nanoribbons toughen up
Commercially available iron-based magnetic ribbons have excellent soft magnetic properties, but are notoriously brittle after they have been crystallized. Researchers at the US Naval Laboratory and Clark Atlanta University have now shown that cobalt-rich nanoribbons are significantly tougher than their commercial counterparts, while retaining their good magnetic properties. The nanocomposite ribbons could find use in high-temperature applications such as inductors, power transformers and rotors.
Nanocrystalline magnetic alloys, such as the iron-based "Finemet" and "Nanoperm", are usually made with melt-spun amorphous precursor ribbons that are annealed above the primary crystallization temperature. This results in a microstructure consisting of iron-rich nanocrystallites embedded in an amorphous matrix. Although they have excellent soft magnetic properties, the ribbons become brittle after a crystallization step – something that is needed to improve their performance in many applications.
Matthew Willard and colleagues have shown that cobalt-rich magnetic alloys are more than twice as tough as the Finemet and Nanoperm alloys after annealing, while keeping their good magnetic properties. For example, a sample annealed at 823 K for one hour had a high saturation magnetization of 131 A m2/kg and a low coercivity of 8 A/m. Previous work at the Naval Research Lab found that these alloys have low magnetic losses at temperatures of 300 °C for periods of up to 300 hours, indicating that they are very stable.
The researchers obtained their results by performing bend tests on the samples to measure the strain-at-fracture on a set of six Co-rich compositions. All the samples showed a nearly threefold increase in strain-at-fracture compared to the commercially available compounds. Magnetic properties of the samples were measured using a vibrating sample magnetometer.
The team also examined the fracture surfaces of the samples after the bend tests and found significant microvoid coalescence dimples in the alloys with the largest strain-at-fracture. This also indicates improved fracture strength.
Willard told nanotechweb.org that the Co-rich alloys could be used as inductor core materials for high-frequency-power electronic applications. These include power transformers, choke coils and rotors for electric motors.
The team now plans to continue experimenting with different alloy compositions in an effort to understand the origins of the improved mechanical strength in these Co-rich materials – and hopefully improve it further.
The work was published in Appl. Phys. Lett..
About the author
Belle Dumé is contributing editor at nanotechweb.org