Creating a perfect crystal sounds straightforward: Arrange identical building blocks into a repeating, orderly structure. At the microscopic scale, though, even subtle biases in how particles interact can push them in preferred directions, making it harder to form uniform crystalline materials.

A Rice University-led study offers a way around that problem. Tanaka Tatsuya, a visiting Rice research scientist from Kao Corp. in Japan, has developed a theoretical strategy for eliminating an unintended directional bias that can arise when magnetic particles are assembled using rotating magnetic fields. The work, published in Physical Review Research, shows the solution can be surprisingly simple: Rotate the magnetic field slightly more than one full turn before reversing its direction.

“When we rotate the magnetic field, the magnetic response of the particle is always slightly behind,” said Sibani Lisa Biswal, chair of chemical and biomolecular engineering and the William M. McCardell Professor in Chemical Engineering at Rice and the corresponding author of the study. “That tiny delay turns out to matter. It can create a preferred direction for assembly even when the applied field appears perfectly symmetric.”