Fundamental lengths scales of magnetism, namely domain wall width and exchange lengths, are at the nanometer scale. Domain wall width strongly depends on the magnetic properties of the materials and is usually between a few tens and a few hundreds of nanometers. Exchange length for most materials is within the range between 1 and 10 nm. When the size of magnetic structures becomes comparable to one of these length scales or both of them, new magnetic properties are expected from these structures.

Fig.1 SEM image of ferromagnetic dots and corresponding dot size distribution
In addition to electron-beam lithography, we use alternative nanofabrication methods. Using self-assembled nanopores in anodized alumina as a shadow mask [1-5], sub-100 nm magnetic dots covering an area of 1 cm2 are deposited by electron-beam evaporation. This method provides a good control over dot size and separation. Due to the narrow distribution of dot sizes (10-15% standard deviation) we can perform measurements of large macroscopic samples covered with dot arrays and talk about properties of a single nanometer-sized dot.

Fig.2 a) Mz component of dot magnetization corresponding to the vortex as calculated with micromagnetic simulation; b) schematic results of the neutron scattering measurements.
The structural and magnetic properties of these nanodots are studied by variety of experimental techniques: SEM, AFM, MFM, X-ray diffractometry, Magneto-Optical Kerr effect (MOKE), SQUID and VSM magnetometers, polarized neutron scattering, etc. In studies of ferromagnetic nanodots we discovered that for a certain range of dot size, the magnetization can form a vortex state, where the moments form a circular in-plane structure everywhere in the dot, except for the middle portion of the dot, where an out-of-plane magnetization, vortex core, is formed to avoid mathematical singularity (Fig.2a). Using the world’s first neutron scattering on such arrays, we have determined the out-of-plane magnetization of the vortex core (~100 emu/cm3) and core diameter (Fig.2b). Micromagnetic and Monte Carlo simulations [6,7] are performed to study details of the magnetic configurations. Studies of properties of the dots depending on the dot size, material, shape, etc. are in progress.

[1] C.-P. Li, I. V. Roshchin, X. Batlle, M. Viret, F. Ott, and I. K. Schuller, Fabrication and structural characterization of highly ordered sub-100 nm planar magnetic nanodot arrays over 1 cm2 coverage area. Journal of Applied Physics, 100, 074318 (2006).

[2] K. Liu, J. Nogues, C. Leighton, H. Masuda, K. Nishio, I. V. Roshchin, and I. K. Schuller Fabrication and Thermal Stability of Arrays of Fe Nanodots, Applied Physics Letters 81, 4434 (2002).

[3] H. Masuda and K. Fukuda, Science 268, 1466 (1995).

[4] O. Jessensky, F. Muller, U. Gosele, Appl. Phys. Lett. 72, 1173 (1998).

[5] MRS Bulletin 28(7), 530 (2003).

[6] J. Mejia-Lopez, D. Altbir, A. H. Romero, X. Batlle, I. V. Roshchin, C.-P. Li, and I. K. Schuller, Vortex state and effect of anisotropy in sub-100-nm magnetic nanodots Journal of Applied Physics, 100, 104319 (2006).

[7] I. V. Roshchin, C.-P. Li, X. Batlle, S. Roy, S. K. Sinha, S. Park, R. Pynn, M. R. Fitzsimmons, J. Mejia-Lopez, D. Altbir, A. H. Romero, and I. K. Schuller, Euro. Phys. Lett. 86, 67008 (2009).

(c) 2007 Ivan K. Schuller       -       designed by Thomas Gredig