Iron-platinum nanoparticles are ideal for making high-density recording media, high-performance nanocomposite magnets and in integrated circuits. This is because an ordered phase in the material has a high anisotropy constant, which prevents superparamagnetism at room temperature for particles as small as 3 nm.

Lorenzo Castaldi at the Demokritos Research Centre in Athens, Greece, and colleagues have engineered a silver/iron-platinum system in which the hard magnetic properties of the iron-platinum alloy are enhanced. Improvement is seen at temperatures as high as 500 °C, which means that the technique can be used in integrated circuitry technology.

The researchers used a method called thermal evaporation. They first deposited silver with a thickness of 1.5 nm onto a silica substrate at 500 °C and then co-deposited iron-platinum at the same temperature.

100% coercivity enhancement
A coercivity enhancement of as much as 40% was seen and some samples were annealed to further improve this figure. Thinner samples showed an even higher enhancement of more than 100% with and without annealing.

The team is still unsure about the exact mechanism behind the coercivity increases but says that it is related to the formation of an "L10" ordered phase in the iron-platinum. The silver seeds appear to promote the phase transition from a soft A1 phase to the hard L10 phase, which has better magnetic properties.

"Engineering materials at the nanoscale is rather difficult but our study shows that it is possible to integrate nanoparticles with enhanced magnetic properties," Castaldi told The work could be important for perpendicular storage media (the most common application for ferromagnetically hard nanoparticles) or in integrated circuit technology.

The team will now try to understand how exactly the silver clusters enhance the magnetic properties of the iron-platinum nanoparticles. "We will also try to control the crystallographic preferred orientation of the nanoparticles", said Castaldi, "because a (001) preferred orientation would enhance the remanence and thus the maximum energy product of these nanoislands."

Remanence is the magnetization that remains once an external magnetic field has been removed and the coercivity is the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero. The maximum energy product is the figure of merit for a magnet's strength.

The work was reported in Journal of Applied Physics.