This is a collaborative work with the group of Prof. A. Yethiraj at Memorial University of Newfoundland, St. John's (Canada). Actually, the experiments have been done in his laboratory. The spincoating technique applied to the colloidal crystallization [P. Jiang et al. J. Am. Chem. Soc. 126 (2004) 13778] [A. Mihi et al. Adv. Mater. 18 (2006) 2244] has two conceptually different steps. Firstly, the colloid is spun in order to form a (relatively thin) layer on the substrate. Secondly, the spinning colloid structure is frozen by means of rapidly evaporating the solvent or by other techniques. In order to get results not dependent on the acceleration rate due to rapid solvent evaporation, we dropped the suspension onto already spinning substrates.

We studied the effect of changing the solvents and the spin speed on the thickness and the structure of the colloidal crystals (showing 4-fold and 6-fold symmetry). Finally, we established that the resulting colloidal crystals can be used to electrodeposit magnetic materials in the interstices of the structure.

Our main contribution was in the experimental part (C. Arcos) and in the correlation analysis of the colloidal crystals (W. González-Viñas).

We obtained (among other interesting things [C. Arcos et al. Phys. Rev. E 77 (2008) 050402(R)]) that the colloidal structure is not monocrystalline as assumed by some authors, but polycrystalline. The grains are oriented, in mean, following an axisymmetric distribution. That is, one of the diffraction peaks (four in the 4-fold symmetry case: f.c.c. (1 0 0) face parallel to substrate) is oriented in the direction determined by the measurement point in the sample with the spin center. In this way, we showed that the maximum of cross-correlation between the peaks function of two diffraction patterns takes place for a lag corresponding to the angle difference between the two measurements points in the sample. Also, we proved that the quality of the square symmetry samples increases as the point of measurement is farther from the center of spinning (along with the decreasing domain orientation dispersion). It is out of discussion that these two quality indicators, in a more general case, should be supplemented by others. Only 4-fold cases are shown below.

• • C. Arcos et al. Phys. Rev. E 77 (2008) 050402(R).

  • C. Arcos, Ph.D. thesis. Universidad de Navarra (2008)

We acknowledge discussions with M. Plumer, N. Li, A. Agarwal and S. Palit. This work was partly supported by the Natural Science and Engineering Research Council of Canada, and by the Spanish MEC (refs. MAT2003-02369 and FIS2007-66004-C02-01). C. Arcos acknowledges partial financial support from the "Asociación de Amigos de la Universidad de Navarra".

Sketch of orientational correlated polycrystal, 4-fold symmetry macroscopic reflections and laser diffraction pattern (inset), and SEM image near the center of spinning:

Domain orientation dispersion (dashed line is only a guide for the eye):