Stoop Ralph L, Straube Arthur V, Tierno Pietro
Departament de Física de la Matèria Condensada , Universitat de Barcelona , Avenida Diagonal 647 , 08028 Barcelona , Spain.
Department of Mathematics and Computer Science , Freie Universität Berlin , Arnimalle 6 , 14195 Berlin , Germany.
Nano Lett. 2019 Jan 9;19(1):433-440. doi: 10.1021/acs.nanolett.8b04248. Epub 2018 Dec 4.
The performance of nanoscale magnetic devices is often limited by the presence of thermal fluctuations, whereas in micro- and nanofluidic applications the same fluctuations may be used to spread reactants or drugs. Here, we demonstrate the controlled motion and the enhancement of diffusion of magnetic nanoparticles that are manipulated and driven across a series of Bloch walls within an epitaxially grown ferrite garnet film. We use a rotating magnetic field to generate a traveling wave potential that unidirectionally transports the nanoparticles at a frequency tunable speed. Strikingly, we find an enhancement of diffusion along the propulsion direction and a frequency-dependent diffusion coefficient that can be precisely controlled by varying the system parameters. To explain the reported phenomena, we develop a theoretical approach that shows a fair agreement with the experimental data enabling an exact analytical expression for the enhanced diffusivity above the magnetically modulated periodic landscape. Our technique to control thermal fluctuations of driven magnetic nanoparticles represents a versatile and powerful way to programmably transport magnetic colloidal matter in a fluid, opening the doors to different fluidic applications based on exploiting magnetic domain wall ratchets.
纳米级磁性器件的性能常常受到热涨落的限制，而在微纳流体应用中，同样的涨落可用于扩散反应物或药物。在此，我们展示了磁性纳米颗粒在沿外延生长的铁氧体石榴石薄膜内一系列布洛赫壁移动和驱动过程中的可控运动及扩散增强。我们使用旋转磁场产生行波势，以频率可调的速度单向传输纳米颗粒。令人惊讶的是，我们发现沿推进方向的扩散增强，且扩散系数与频率相关，可通过改变系统参数精确控制。为解释所报道的现象，我们开发了一种理论方法，该方法与实验数据吻合良好，能够给出在磁调制周期性景观之上增强扩散率的精确解析表达式。我们控制驱动磁性纳米颗粒热涨落的技术代表了一种通用且强大的方式，可对流体中的磁性胶体物质进行可编程传输，为基于利用磁畴壁棘轮效应的不同流体应用打开了大门。
