Mini magnets

Atomic-layer deposition can produce iron oxide nanotubes with highly controllable magnetic properties

Credit: © 2007 ACS

Magnetic nanotube arrays hold great potential for high-density data storage. The magnetic properties of individual nanotubes can be varied by changing their wall thickness, but it is often difficult to control their dimensions during synthesis. Now, Julien Bachmann and co-workers¹ at the Max Planck Institute of Microstructure Physics in Germany have achieved much tighter control over the synthesis of iron oxide nanotubes so that their magnetic data storage applications can be more fully investigated.

The researchers improved the atomic-layer deposition process to apply individual layers of atoms as thin as 0.026 nm to a porous alumina substrate. Using this method, they grew arrays of atomically smooth iron oxide (Fe₂O₃) nanotubes. A hydrogen and argon mixture was applied to convert the gold-coloured Fe₂O₃ to black Fe₃O₄, which behaves as a ferromagnet — that is, the nanotubes remain magnetized after being in a magnetic field.

With the new process, the researchers were able to explore the effects of different wall thicknesses on the magnetic properties of the array while keeping the diameter of individual tubes fixed at 50 nm. Optimum magnetic properties were found in arrays in which the tubes walls were 13 nm thick, but interactions between tubes became problematic for thicker tubes.