Invited Paper TF+NC-MoM7
Ferromagnetic Nanostructures by Atomic Layer Deposition: From Thin Films to Ferrofluids and Core-Shell Nanotubes

Monday, October 20, 2008, 10:20 am, Room 302

Magnetic nanostructures have a broad range of applications, most prominently in the areas of data storage, microelectronics, biosensing, and cell separation. For the preparation of such structures, we use atomic layer deposition (ALD), a thin film technique with the unique ability to coat pore structures of high aspect ratio conformally and uniformly while accurately tuning the layer thickness. We approach the preparation of the magnetic materials Fe₃O₄, Co, and Ni in two steps. ALD is first used to deposit Fe₂O₃,¹ CoO or NiO,² either from the reaction of the corresponding metallocene with ozone or from that between the metal alkoxide and water. The oxide is subsequently reduced to the desired ferromagnetic phase by hydrogen gas. By conformal coating of self-ordered porous anodic alumina membranes, arrays of magnetic nanotubes with diameters down to 20 nm and wall thicknesses of less than 5 nm have been achieved. The magnetic properties of the nanotube arrays, studied by SQUID magnetometry, strongly depend on the geometric parameters. The effects of tube wall thickness and tube diameter have been studied systematically,¹ and the experimental data match the results of our theoretical simulations.³ Dissolution of the alumina matrix yields ferrofluidic suspensions, made out of widely tunable magnetic particles. Finally, ALD also enables us to create core-shell nanoobjects combining different magnetic materials in various, accurately controlled geometries.

¹ J. Bachmann, J. Jing, M. Knez, S. Barth, H. Shen, S. Mathur, U. Gösele, K. Nielsch, J. Am. Chem. Soc. 2007, 129, 9554-9555.
² M. Daub, M. Knez, U. Gösele, K. Nielsch, J. Appl. Phys. 2007, 101, 09J111.
³ J. Escrig, J. Bachmann, J. Jing, M. Daub, K. Nielsch, D. Altbir, Phys. Rev. B 2008, accepted.